Black Carbon Resource Packet
Black Carbon Symposium
November, 14, 2012
U.S. Environmental Protection Agency
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Background
• Report to Congress on Black Carbon, U.S. Environmental
Protection Agency
o Executive Summary
o Excerpt from Chapter 2 on metrics
• Climate Change - Characterization of Black Carbon and Organic
Carbon Air Pollution Emissions and Evaluation of Measurement
Methods Executive Summary, Prepared by Chow et. al Desert
Research Institute and submitted to the California Air Resources
Board
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Chapter 2
Black Carbon and Its Effects on
Climate
2.1 Summary of Key Messages
• Black carbon (BC) is the most strongly light-
absorbing component of particulate matter (PM),
and is formed by the incomplete combustion of
fossil fuels, biofuels, and biomass.
- BC can be defined specifically as a solid
form of mostly pure carbon that absorbs
solar radiation (light) at all wavelengths. BC
is the most effective form of PM, by mass,
at absorbing solar energy BC is a major
component of "soot", a complex light-
absorbing mixture that also contains organic
carbon (OC).
- Other carbon-based PM may also be light-
absorbing, particularly brown carbon (BrC),
which is a class of OC compounds that absorb
light within the visible and ultraviolet range
of solar radiation and that can exist within the
same particles as BC. The net contribution of
BrC to climate is presently unknown.
• BC is always emitted with other particles and
gases, such as sulfur dioxide (S02), nitrogen
oxides (NOJ, and OC. Some of these co-emitted
pollutants exert a cooling effect on climate.
Therefore, estimates of the net effect of BC
emissions sources on climate should include the
offsetting effects of these co-emitted pollutants.
• Atmospheric processes that occur after BC is
emitted, such as mixing, aging, and coating, can
also affect the net influence of BC on climate.
• The short atmospheric lifetime of BC (days to
weeks) and the mechanisms by which it affects
climate distinguish it from long-lived greenhouse
gases (GHGs) like carbon dioxide (C02).
- Targeted strategies to reduce BC emissions
can be expected to provide climate responses
within the next several decades. In contrast,
reductions in GHG emissions will take longer
to influence atmospheric concentrations
and will have less impact on climate on a
short timescale, but deep reductions in GHG
emissions are necessary for limiting climate
change over the long-term.
- Emissions sources and ambient
concentrations of BC vary geographically
and temporally, resulting in climate effects
that are more regionally and seasonally
dependent than the effects of long-lived,
well-mixed GHGs. Likewise, mitigation
actions for BC will produce different climate
results depending on the region, season, and
emissions category.
• BC influences climate through multiple
mechanisms:
- Direct effect BC absorbs both incoming and
outgoing radiation of all wavelengths, which
contributes to warming of the atmosphere
and dimming at the surface. In contrast,
GHGs mainly trap outgoing infrared radiation
from the Earth's surface.
- Snow/ice albedo effect: BC deposited on
snow and ice darkens the surface and
decreases reflectivity (albedo), thereby
increasing absorption and accelerating
melting. GHGs do not directly affect the
Earth's albedo.
- Other effects: BC also alters the properties
and distribution of clouds, affecting
cloud reflectivity and lifetime ("indirect
effects"), stability ("semi-direct effect"), and
precipitation. These impacts are associated
with all ambient particles, but not GHGs.
• The direct and snow/ice albedo effects of BC are
widely understood to lead to climate warming.
Based on the studies surveyed for this report, the
direct and snow/ice albedo effects of BC together
likely contribute more to current warming than
any GHG other than C02 and methane (CH4).
• The climate effects of BC via interaction with
clouds are more uncertain, and their net climate
influence is not yet clear.
Report to Congress on Black Carbon
17
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Chapter 2
- All aerosols (including BC) affect climate
indirectly by changing the reflectivity and
lifetime of clouds. The net indirect effect of
all aerosols combined is very uncertain but
is thought to have a net cooling influence.
The contribution of BC to this cooling has not
been quantified.
- BC has additional effects on clouds—including
changes to cloud stability and enhanced
precipitation from colder clouds—that can
lead to either warming or cooling.
- The net climate influence of these cloud
interaction effects of BC is not yet clear.
There is inconsistency among reported
observational and modeling results, and many
studies do not provide quantitative estimates
of cloud impacts.
• The sign and magnitude of the net climate forcing
from BC emissions are not fully known at present,
largely due to remaining uncertainties regarding
the effects of BC on clouds. Though most
estimates indicate that BC has a net warming
effect, a net cooling influence cannot be ruled
out. Further research and quantitative assessment
are needed to reduce remaining uncertainties.
• Regional climate impacts of BC are highly variable,
and sensitive regions such as the Arctic and
the Himalayas are particularly vulnerable to the
warming and melting effects of BC. Estimates of
snow and ice albedo forcing in key regions also
exceed global averages.
• BC also contributes to the formation of
Atmospheric Brown Clouds (ABCs) and resultant
changes in the pattern and intensity of
precipitation.
• Due in large part to the difference in lifetime
between BC and C02, the relative weight given to
BC as compared to C02 (or other climate forcers)
in terms of its impact on climate is very sensitive
to the formulation of the metric used to make the
comparison.
• There is currently no single metric that is widely
accepted by the science and research community
for this purpose.
• There are several metrics that have been applied
to the well-mixed GHGs with respect to different
types of impacts, especially the global warming
potential (GWP) and global temperature potential
(GTP). These metrics can be applied to BC, but
with difficulty due to important differences
between BC and GHGs. Recently, new metrics
designed specifically for short-lived climate
forcers like BC have been developed, including
the specific forcing pulse (SFP) and the surface
temperature response per unit continuous
emission (STRE).
• There is significant controversy regarding the
use of metrics for direct comparisons between
the long-lived GHGs and the short-lived particles
for policy purposes; however, these comparisons
are less controversial when used for illustrative
purposes.
- There are a number of factors that should
be considered when deciding which metric
to use, or whether comparisons between BC
and C02 are useful given a particular policy
question. These include: the time scale (e.g.,
20 years, 100 years, or more), the nature of
the impact (radiative forcing, temperature,
or more holistic damages), the inclusion of
different processes (indirect effects, snow
albedo changes, co-emissions), and whether
sources and impacts should be calculated
regionally or globally.
- If the primary goal is reducing long-term
change, then a metric like a 100-year GWP
or GTP would be more appropriate. If the
rate of near-term climate change and near-
term damages to sensitive regions like the
Arctic are also a consideration, there is
no single existing metric that adequately
weights impacts over both time periods,
and a multi-metric approach may be more
appropriate than developing a single metric
that attempts to serve all purposes.
2.2 Introduction
There is a general consensus within the scientific
community that BC is contributing to climate change
at both the global and regional levels. Like C02, BC
is produced through the burning of carbon-based
fuels, including fossil fuels, biofuels and biomass.
BC is part of the mix of PM released during the
incomplete combustion of these fuels. BC influences
climate by absorbing sunlight when suspended in
the atmosphere or when deposited on the Earth's
surface. The energy absorbed by BC is then released
as heat and contributes to atmospheric warming
and the accelerated melting of ice and snow. In
addition, BC is capable of altering other atmospheric
processes, such as cloud formation and evaporation,
and precipitation patterns.
18 Report to Congress on Black Carbon
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Black Carbon and Its Effects on Climate
The strong absorption, short atmospheric lifetime,
and other characteristics of BC make its impacts
on climate different from those of long-lived GHGs
like C02 (see Figure 2-1). Because BC is involved
in complex atmospheric physical and chemical
processes, it is difficult to disentangle all associated
impacts and to evaluate its net effect on climate. In
addition, the combustion processes that produce BC
also produce other pollutants, such as S02, NOx, and
OC. Since many of these compounds have a cooling
effect, BC's impacts are mixed with—and sometimes
offset by—these co-emitted substances. This must
be considered when evaluating the net effect of
emissions sources.
This chapter focuses on how and to what extent
BC influences the Earth's climate. Specifically, this
chapter discusses approaches for defining BC
and other light-absorbing particles, highlights the
differences between BC and GHGs, and addresses
the role of co-emitted pollutants. Further, this
chapter summarizes recent scientific findings
regarding the processes by which BC affects climate
and the magnitude of BC's impacts on global and
Reflecting
Particles
o o O o o
O o O O o o
°°0 o o°
©
Snow and/or Ice
©
• _ • •
Black Carbon (BC)
GHGs
© \
Figure 2-1. Effects of BC on Climate, as Compared to GHGs. (Source: U.S. EPA)
1. Sunlight that penetrates to the Earth's surface reflects off bright surfaces, especially snow and ice.
2. Clean clouds and non-light-absorbing (transparent) particles scatter or reflect sunlight, reducing the amount of solar
energy that is absorbed by the surface.
3. BC suspended in the atmosphere absorbs some incoming solar radiation, heating the atmosphere.
4. Clouds containing BC inclusions in drops and BC interstitially between drops can absorb some incoming solar radiation,
reducing the quantity that is reflected. Clouds warmed by the absorbed energy have shorter atmospheric lifetimes and
may be less likely to precipitate compared to clean clouds.
5. BC deposited on snow and/or ice absorbs some of the sunlight that would ordinarily be reflected by clean snow/ice, and
increases the rate of melting.
6. Most solar radiation is absorbed by the Earth's surface and warms it. Part of the absorbed energy is converted into
infrared radiation that is emitted into the atmosphere and back into space.
7. Most of this infrared radiation passes through the atmosphere, but some is absorbed by GHG molecules like C02,
methane, ozone and others. These gases re-emit the absorbed radiation, with half returning to the Earth's surface. This
GHG effect warms the Earth's surface and the lower atmosphere.
Report to Congress on Black Carbon
19
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Chapter 2
Figure 2-2. BC Images, (a) High resolution transmission electron microscopy (TEM) image of a BC spherule
(Posfai and Buseck, 2010). (b) TEM image of a representative soot particle. Freshly emitted soot particles are
aggregates of soot spherules (Alexander et al., 2008).
regional climate, highlighting the effect of BC in
sensitive regions such as the Arctic and other snow-
and ice-covered regions. The chapter discusses the
significant remaining uncertainties about BC's effects
on climate, and the need for further research in key
areas. The final section of this chapter introduces
several metrics that can be used to quantify the
climate impacts of BC and other pollutants (such as
C02 and CH4) relative to a common baseline. The
section highlights the fact that there is no one "best"
metric for comparing BC to other pollutants and
that the utility of each metric depends on the policy
objective.
2.3 Defining Black Carbon and Other
Light-Absorbing PM
All PM in the atmosphere can affect the Earth's
climate by absorbing and scattering light. Sunlight
absorbed by PM increases the energy in the
Earth's climate system, leading to climate warming.
Conversely, light scattered by PM generally leads to
increased reflection of light back to space, leading
to climate cooling (Charlson, 1992; Moosmuller et
al., 2009; Seinfeld and Pandis, 2006; Forster et al.,
2007). Carbonaceous PM, a class of material found in
primary and secondary particles, has typically been
divided into two classes: BC and OC (see text box
on "Terminology"). Neither BC nor OC has a precise
chemical definition. The term BC generally includes
the solid forms of carbon emitted by incomplete
combustion while OC refers to the complex mixtures
of different carbon compounds found in both
primary and secondary carbonaceous particles.
Carbonaceous PM includes an array of organic
compounds that, along with BC, possess radiative
properties that fall along a continuum from light-
absorbing to light-scattering. Both BC and OC are
part of the broader category of suspended particles
and gases known as aerosols, all of which have light-
absorption and light-scattering properties.
In this report, BC is defined as the carbonaceous
component of PM that absorbs all wavelengths
of solar radiation.1 For this reason, among the
many possible forms of PM, BC absorbs the most
solar energy. Per unit of mass in the atmosphere,
BC can absorb a million times more energy than
C02 (Bond and Sun, 2005), making it a significant
climate warming pollutant in regions affected by
combustion emissions.
BC forms during combustion, and is emitted when
there is insufficient oxygen and heat available for the
combustion process to burn the fuel completely (see
text box on "Products of Incomplete Combustion").
BC originates as tiny spherules, ranging in size from
0.001 to 0.005 micrometers (pm), which aggregate
to form particles of larger sizes (0.1 to 1 pm) (Figure
2-2). Particles in this range are similar in size to
the wavelengths emitted by the sun, making them
especially effective in scattering or absorbing these
wavelengths (Horvath, 1993). The characteristic
particle size range in which fresh BC is emitted also
makes it an important constituent of the ultrafine
(<100 nanometers (nm)) subclass of PM2.5.
; The spectrum of solar radiation striking Earth's atmosphere ranges
from high energy UV with wavelengths shorter than 280 nm down
to infrared radiation as long as 1000 nm. However.. UV wavelengths
shorter than 280 nm are substantially absorbed by the stratosphere.
For the purposes of this discussion, the term "all wavelengths of
solar radiation" corresponds to the solar wavelengths present in the
troposphere (e.g., in the range 280 - 2500 nm).
100 rim
20 Report to Congress on Black Carbon
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Black Carbon and Its Effects on Climate
Terminology
Black carbon (BC) is a solid form of mostly pure carbon
that absorbs solar radiation flight) at all wavelengths.
BC is the most effective form of PM, by mass, at
absorbing solar energy, and is produced by incomplete
combustion.
Organic carbon (OC) generally refers to the mix of
compounds containing carbon bound with other
elements like hydrogen or oxygen. OC may be a product
of incomplete combustion, or formed through the
oxidation of VOCs in the atmosphere.2 Both primary and
secondary OC possess radiative properties that fall along
a continuum from light-absorbing to light-scattering.
Brown carbon (BrC) refers to a class of OC compounds
that absorb ultraviolet (UV) and visible solar radiation.
Like BC, BrC is a product of incomplete combustion.3
Carbonaceous PIV! includes BC and OC. Primary
combustion particles are largely composed of these
materials.
Light absorbing carbon (LAC) consists of BC plus BrC.
Soot, a complex mixture of mostly BC and OC, is the
primary light-absorbing pollutant emitted by the
incomplete combustion of fossil fuels, biofuels, and
biomass.
BC is emitted directly from sources, making it a form
of primary PM. This distinguishes it from secondary
PM such as sulfates, nitrates and some forms of OC
that are formed in the atmosphere from gaseous
precursors like S02, NOx and volatile organic
compounds (VOCs).
When BC is emitted directly from sources as a result
of the incomplete combustion of fossil fuels, biofuels
and biomass, it is part of a complex particle mixture
called soot which primarily consists of BC and OC.
This mixture is the light-absorbing component of
these air pollution emissions.
Soot mixtures can vary in composition, having
different ratios of OC to BC,2 and usually include
inorganic materials such as metals and sulfates. For
example, the average OCBC ratio among global
sources of diesel exhaust is approximately 1:1.
For biofuel burning, the ratio is approximately 4:1
and for biomass burning it is approximately 9:1
2When referring to emissions and measurements, OC denotes
the total carbon associated with the organic compounds, while
organic mass (OM) refers to the mass of the entire carbonaceous
material, including hydrogen and oxygen. Similarly, measurements
and emissions reported as elemental carbon (EC) denote the non-
organic, refractory portion of the total carbon and is an indicator for
BC. For more details, see Chapter 5 and Appendix J.
(Lamarque et al., 2010). As expected, very dark soot
indicates the presence of low OCBC ratios. As the
OC fraction begins to dominate, the color of the
soot mixture shifts to brown and yellow. A brown
soot sample is dominated by a form of OC known,
as might be expected, as "brown carbon" (BrC).3 BrC,
another product of incomplete combustion, absorbs
portions of the visible spectrum, but is less effective
in capturing solar energy than BC (Alexander et al.,
2008; Novakov and Corrigan, 1995b). The mixture
shifts in color toward yellow when the emissions
source is no longer producing BC and BrC. Yellow
carbon, another form of OC, is also able to absorb
visible radiation, but to a lesser extent than
BrC (Bond, 2001; Gelencser, 2004; Andreae and
Gelencser, 2006). Figure 2-3 illustrates the variance
in soot composition resulting from different fuels
and stages of fuel combustion. The stages of fuel
combustion responsible for producing BC and the
various forms of OC observed in soot are described
in the text box on this page.
(a) (b) (c)
iiiii .i mum id iiiiimmiiiiiiiuu iiiiiniiimiiiiiiD
BIOTKF068 STRST013
Figure 2-3. Representative Examples of Filter
Samples Collected from Different Sources,
including: (a) Smoldering Biomass, (b) Flaming
Biomass, and (c) Diesel Exhaust. (Photo courtesy of
Desert Research Institute)
In general, light absorption by carbonaceous PM can
be described as a continuum from light-absorbing
to light-scattering with BC at one end, most OC at
the other, and BrC occupying the partially absorbing
3 During solid fuel combustion, BrC forms during the preheating
(pyrolysis) phase, and during both flaming and smoldering
combustion. The light-colored smoke characteristic of the pyrolosis
and smoldering combustion phases is primarily OC, including both
BrC and other forms of OC, and does not include soot. Secondary
BrC can also form during reactions, similar to polymerization, that
take place in primary particles as em issions plumes age. BrC of this
type is known as "humic-like substances" (HULIS).
Report to Congress on Black Carbon
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Chapter 2
2.6.6 Summary of BC Impacts in Key
Regions
As described in the previous sections, the climate-
related effects of BC can vary considerably across
regions. Table 2-7 provides an overview of the
regional variability in terms of BC's effects on
radiative forcing, temperature, precipitation, and
snow and ice across the United States, Asia, and the
Arctic. In addition, Figures 2-13 and 2-15 are useful
for understanding the regional variability of BC's
radiative forcing effects.
2.7 Metrics for Comparing Black
Carbon Impacts to Impacts of Other
Climate Forcers
In response to Congress's request for an assessment
of potential comparative metrics, this section
summarizes a number of different approaches to
comparing the effects of BC to C02 and other GHGs,
but cautions that there is no one "best" metric;
rather, the utility of a metric depends on the desired
environmental outcome and policy objective.
Therefore, this section begins by introducing the
concept of using metrics for comparing BC-related
Radiative Forcing
Effects
• Estimates of direct radiative forcing
of BC over the United States range
from 0.1 to 0.7 W m2.
• South and East Asia have some of
the world's highest estimates of
radiative forcing, but large ABCs
exert a counterbalancing dimming
effect at the surface.
• Average annual snow and ice albedo
forcing in the Tibetan Plateau has
been estimated to be 1.5 W m2, with
local instantaneous forcing up to 20
Wm2.
• Springtime Arctic forcing has
been estimated to be 1.2 W
m2(direct) and 0.53Wm2
(snow albedo).
Temperature
Effects
• No studies were identified for U.S.
temperature effects from BC. All
global modeling studies include the
temperature effects over the U.S.,
but results are difficult to extract.
• Estimates of average warming from
BC in the Northern Hemisphere
range from 0.29°C to 0.54°C.
• Over the Himalayan region,
atmospheric BC was estimated to
result in up to 0.6°C of warming.
• BC deposited on snow results
in warming of roughly 0.4 to
0.5°C, varying by season.
• Atmospheric BC was
estimated to contribute
roughly 0.2°C in spring, 0.1 °C
in summer, and nearly zero in
autumn and winter.
Precipitation
Effects
• One study found little change in
the amount of precipitation in the
western United States as a result of
BC effects.
• Other studies have found that
rainfall patterns in the eastern
United States match PM emissions,
but not specifically those of BC.
• The cooling at the surface leads
to reduced evaporation and
precipitation as well as changes in
sea-land temperature gradients.
• Precipitation and temperature
gradient modifications can lead to
shifts of regional circulation patterns
such as a decrease in the Indian and
Southeast Asian summer monsoon
rainfall and a north-south shift in
eastern China rainfall.
• No studies were identified for
Arctic precipitation effects.
Snow and Ice
Effects
• In the western United States, BC
deposition on mountain glaciers and
snow produces a positive snow and
ice albedo effect, contributing to the
snowmelt earlier in the spring.
• Early snowmelt reduces the amount
of water resources that normally
would be available later in the spring
and summer, and may contribute to
seasonal droughts.
• BC atmospheric warming is believed
to be a significant factor in the
melting of the HKHT glaciers and
snowpack.
• The deposition of BC on glaciers
and snowpack in Asia also has
a strong snow and ice albedo
positive feedback that accelerates
melting of the glaciers and snow,
with implications for freshwater
availability and seasonal droughts.
• BC may increase snowmelt
rates north of 50°N latitude by
as much as 19-28%.
• Soot deposition in the Alaskan
Arctic tundra created snow
free conditions five days
earlier than model runs
without BC deposition.
Table 2-7. Climate Effects of BC in the United States, Asia, and the Arctic (Summary).
Effects U.S. Asia Arctic
56 Report to Congress on Black Carbon
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Black Carbon and Its Effects on Climate
k
Figure 2-24. Cause and Effect Chain from Emissions to Climate Change, Impacts, and Damages.
(Adapted from Fuglestvedt et al., 2003.) The arrows indicate that a policy could focus on different
elements along the causal chain and, depending on whether the policy focuses on the emissions or
damages end of the chain, can determine the certainty of meeting the stated policy target versus the
certainty of reducing damages at issue.
impacts to those of other climate forcers. It explains
some of the approaches to developing metrics and
provides a comparison of common metrics used
for GHGs and for BC. This section concludes with a
discussion of the most salient limitations associated
with specific metrics and with using metrics in
general.
The goal of a metric, as used in this report, is to
quantify the impact of a pollutant relative to a
common baseline. Such metrics can be used to
compare between two or more climate forcers (e.g.,
C02 versus CH4), or to estimate the climate effects of
different emissions sources (or mitigation measures).
Metrics that enable comparisons among pollutants
or sources based on common denominators can also
be used for the implementation of comprehensive
and cost-effective policies in a decentralized manner
(e.g., in a market-based climate program) so that
multi-pollutant emitters can compose mitigation
strategies (Forster et al., 2007).
Climate metrics are often defined relative to a
baseline pollutant (usually C02) and focus on a
particular climate impact (such as radiative forcing
or temperature) that would be altered due to a
change in emissions. For example, in EPA's annual
Inventory of U.S. Greenhouse Gas Emissions and
Sinks, the GWP metric is used to convert all GHGs
into "C02-equivalent" units. Importantly, metrics
such as GWP have been used as an exchange rate in
multi-pollutant emissions policies and frameworks
(IPCC, 2009). The key assumption when developing
a metric is that two or more climate forcers are
comparable or exchangeable given the policy
goal. That is to say, one pound of apples may be
comparable to or exchangeable with one pound of
oranges if the goal is not to overload a truck, but
not if the goal is to make apple cider (Fuglestvedt et
al., 2010). Therefore, when used as an exchange rate
in multi-pollutant emissions framework, a metric
allows substitution between climate forcers which
are presumed to be equivalent for the policy goals
(Forster et al., 2007).
Metrics can also be used to prioritize among
mitigation measures designed to control emissions
of similar compounds from different sources. As
described previously in this chapter, aerosols are
composed of numerous components, and these
different components can contribute to both
warming (BC) and cooling. A metric can aggregate
these effects in order to determine the relative
contribution of a given source or measure.
2.7.1 Metrics Along the Cause and Effect
Chain
For both BC and GHGs, there is a cause and effect
chain starting with anthropogenic emissions and
leading to changes in concentrations, radiative
forcing, physical climatic changes, and impacts on
human and natural systems (Figure 2-24). Some
of the links in this cause and effect chain may be
simultaneous rather than sequential. For example,
the atmospheric loading of aerosols affects
dimming and precipitation directly, rather than
mediated through radiatively induced temperature
Report to Congress on Black Carbon 57
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Chapter 2
changes. Nor is the chain always unidirectional.
Climatic changes can lead to changes in atmospheric
concentrations of climate-forcing pollutants (e.g.,
changes in precipitation will change aerosol lifetimes)
or even emissions of those pollutants (e.g., changes
in temperature affect fossil fuel consumption for
heating and cooling needs, which affects emissions
of particles and precursors). There are uncertainties
at each stage of the cause and effect chain, and
these uncertainties compound over multiple steps
of the chain. The uncertainties for BC are generally
larger at all stages of the causal chain compared to
the long-lived GHGs (for reasons discussed in this
and other chapters of this report).
Within the climate change field, metrics have been
calculated for changes in radiative forcing, global
mean temperature, and monetized damages. The
closer the metric is to the emissions end of the chain,
the less uncertainty there is in how to calculate the
metric; it is easier to determine how a change in
emissions will change concentrations than it is to
determine how a change in emissions will change
temperature (a calculation which requires several
intermediate steps). Additionally, the further along
the chain, the more physical systems (and economic
systems) need to be included in order to calculate
the metric. However, if a reduction in damages is
considered the ultimate objective of the policy,
then a metric that focuses explicitly on impacts or
damages best represents that objective. Since the
economic value of damages (expressed in dollars)
is one of the easiest metrics for the public and
policymakers to place in context, there has been
a great deal of interest recently in calculating the
monetary value of climate change impacts associated
with different pollutants (see Chapter 6). The choice
of a metric can be considered in part a choice about
how to allocate uncertainty between calculation of
the metric and the representativeness of the metric
for the ultimate impacts of interest.
Fuglestvedt (2009) identified the following
considerations for developing a metric for
climate forcers (see Table 2-8 for examples of
how commonly used metrics address these
considerations):
1. What climate impact is of interest for the policy
being considered?
2. What climate forcer will be used as the baseline
for comparison?
3. What is the temporal frame for emissions? Is it
an instantaneous pulse or a sustained change in
emissions?
4. What is the temporal frame for the impact?
10 years, 50 years, 100 years? Is the impact
considered only at the end point of the time
frame, or integrated over the period?
5. Does the metric address the magnitude of
change or the rate of change or both?
6. What is the spatial dimension of the metric
for both emissions and impacts? Is it global or
regional?
7. What economic considerations should be taken
into account? How are damages in the far future
weighed compared to damages in the near term?
Table 2-8. Examples of Commonly Used Metrics for GHGs.
Metric Type
Climate Impact
Baseline
Forcer
Emissions
Type
Spatial
Scale
Includes Rate
of Change?
GWP (Global Warming Potential)
Integrated radiative
forcing
C02
Pulse
Global
No
GTP-pulse (Global Temperature Potential)
Temperature
co2
Pulse
Global
No
GTP-sustained
Temperature
co2
Sustained
Global
No
STRE (Surface Temperature Response per
unit continuous Emission)
Temperature
co2
Sustained
Global
No
SFP (Specific Forcing Pulse)
Energy
Joules/gram
Pulse
Global or
regional
No
Cost-effectiveness Metrics (e.g., Manne
and Richels, 2001, Global Cost Potential)
Mainly temperature
C02or $ value
Optimal
emissions
calculation
Global
Optional
Value of Damages (e.g.. Social Cost of
Carbon, Global Damage Potential)
Range of climate
damages
$ value
Pulse
Global
Limited
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Black Carbon and Its Effects on Climate
First, the climate impact must be identified because
the effectiveness of a given metric is dependent on
the primary policy goal. Considerations 2 through
7 are then framed by the selected climate impact.
This is important because choosing an inappropriate
metric could lead to policy decisions that ultimately
result in undesirable climate or economic impacts.
2.7.2 Commonly-Used Metrics for GHGs
Article 2 of the United Nations Framework
Convention on Climate Change (UNFCCC) calls for
a policy that addresses the magnitude and the rate
of climate change as well as the cost effectiveness
of controlling emissions (IPCC, 2009). Therefore,
appropriate metrics could cover either the physical
or economic dimensions of climate change, or both.
A number of metrics have been developed and
refined for application to C02 and other long-lived
GHGs. These metrics are summarized in Table 2-8
and described further below. Their potential
applicability to BC is considered in the next section.
Note that two of the metrics listed in the table (SFP
and STRE) were developed specifically for application
to short-lived climate forcers like BC, and are
discussed only in section 2.7.3.4 below.
Five considerations are listed in Table 2-8. The first,
climate impact, refers to where the metric falls on
the cause-effect chain shown in Figure 2-24. The
second, baseline forcer, lists whether the metric is
measured in comparison to C02, or in absolute units
(whether dollars or energy). The third column notes
what kind of emissions change is being considered.
A "pulse" of emissions refers to an effectively
instantaneous release of that pollutant (though
sometimes that release is considered to be spread
out over a year). A pulse analysis is appropriate for
a one-time trading of emissions permits, but may
not be as realistic for analyzing investment decisions
which spread reductions out over time (though a
longer term reduction can be approximated as a
series of pulses). Therefore, other analyses consider
the possibility that an emission reduction (or
increase) will be permanent (i.e., sustained overtime).
The third temporal option is to calculate the optimal
emissions path, which is discussed in more detail
in section 2.7.2.3 (cost-effectiveness metrics). The
fourth column shows that most metrics have been
designed to be used on a global scale, though some
of these might be adaptable for regional impacts.
Finally, most metrics consider temperature change or
damages either at a single point in time or summed
over time: only a few consider that there may be
value in limiting the rate of change in addition to
reducing the absolute magnitude of the change.
Table 2-8 is also ordered in a rough approximation
of the transparency of the metric. Metrics which
are transparent and easy to calculate are likely to
be more readily accepted for policy use than those
which require complex modeling. The GWP is in
widespread use and can be calculated based only
on knowing the average lifetime of a molecule
of a gas and the radiative forcing caused by that
molecule. The remaining metrics require the use
of computer models of more or less complexity in
order to calculate, and if the metric is sensitive to
assumptions involved in the modeling then that
could have potential for controversy.
2.7.2.1 Global Warming Potential
To date, the most widely established and well-
defined metric is the GWP. The definition of the GWP
by the IPCC (2007) is
"An index, based upon radiative properties of well-
mixed greenhouse gases, measuring the radiative
forcing of a unit mass of a given well-mixed
greenhouse gas in the present-day atmosphere
integrated over a chosen time horizon, relative
to that of carbon dioxide. The GWP represents
the combined effect of the differing times these
gases remain in the atmosphere and their relative
effectiveness in absorbing outgoing thermal
infrared radiation. The Kyoto Protocol is based on
GWPs from pulse emissions over a 100-year time
frame."
The identified climate impact the GWP addresses is
globally averaged change in radiative forcing and its
baseline climate forcer is C02 (e.g., the GWP of C02 is
defined to be l).15 The temporal frame for emissions
is a pulse. The GWP provides the magnitude, but
not the rate of change, of the integrated radiative
forcing over a given time frame. The time frame is
usually 100 years, but 20-year and 500-year GWPs
are sometimes presented to show how GWPs would
differ if short-term or long-term impacts are given
more weight. Finally, the GWP, which addresses only
radiative forcing, a physical metric, does not take
into account any economic dimension.
As discussed below, there have been a number of
criticisms of the GWP in the peer-reviewed literature
(e.g., O'Neill, 2000; Shine, 2009), mainly focused
on either the inability of the GWP to capture key
differences between gases (such as different
lifetimes) or the failure of the GWP to incorporate
economic considerations. Despite such criticisms, at
the time of the Kyoto Protocol in 1997, the GWP was
15 The GWP is calculated as the ratio of the Absolute Global
Warming Potential (AGWP) of a given gas to the AGWP of
C02. The AGWP has units of W m 2yr g 1.
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adopted as the metric used in climate negotiation.
While acknowledging that there are shortcomings
involved in using GWPs even for comparisons among
the long-lived gases, a recent IPCC Expert Meeting
on the topic found that GWPs were still a useful
measure for these gases (IPCC, 2009). It remains
the most accepted metric due to its simplicity, the
small number of input parameters, the relative ease
of the calculation, and a lower level of uncertainty
compared to some alternatives (Shine et al., 2005).
The GWPs as calculated by the IPCC Second
Assessment Report (Schimel et al., 1996) currently
remain the standard GWPs used for the official U.S.
GHG emissions inventory compiled annually by EPA,
as required by UNFCCC reporting guidelines.16
2.7.2.2 Global Temperature Potential
One alternative metric that has received recent
attention is the GTP. Like the GWP, the GTP is
a physical metric. Whereas the GWP considers
change in globally averaged radiative forcing, the
GTP compares the globally averaged temperature
change at a given point in time resulting from the
emission of two climate forcers of equal mass (Shine
et al., 2005). The GTP moves one step further down
the cause and effect chain and addresses a climate
response to radiative forcing, the global-mean
surface temperature change. The GTP therefore
includes more physical processes, such as the heat
exchange between the atmosphere and ocean, than
the GWP. This introduces more uncertainty to the
metric, and can require the use of more complex
models in order to calculate the GTP value. In
addition, while the GWP represents the integrated
radiative forcing of a pulse of emission over a given
time period, the GTP is evaluated at a given point
in time (IPCC, 2009). Like GWPs, the GTP can be
calculated over a variety of timescales, with 20,100,
and 500 years being the timescales most commonly
presented. There are advantages and disadvantages
to using either the GWP or a GTP, and they may each
address different policy goals and may be more
relevant to different climate forcers and time frames,
depending upon the policy need. To date, however,
the GTP has not been used as a metric for trading
gases in international, national, or regional accords.
There are two versions of the GTP: one that involves
the effects of a pulse of emissions, and another
that involves a sustained reduction of emissions.
The latter version of the GTP results in comparative
values between different gases that are similar
to the values calculated using GWPs. The former
version of the GTP, by contrast, leads to longer-lived
16 See http://www.epa.gov/climatechange/emissions/
usgginventory.html.
gases being given more relative weight because a
pulse of a short-lived gas has very little impact on
temperatures many years in the future.
The GTP can also be calculated as a function of a
future global temperature stabilization target. One
criticism of a number of metrics is that they are not
compatible with a goal of stabilization because the
target is not part of the metric. Manne and Richels
(2001) developed a methodology to calculate a
time-dependent metric (referenced below as a cost-
effectiveness metric) that would change as a target
level was approached. For example, if the target is
not to exceed a 2 degree temperature change above
preindustrial, then when global temperatures are
still only 1 degree above preindustrial, and therefore
the target temperature is still decades away, the
metric will place weight on long-lived gases like
C02. But as the target temperature is approached,
the time to reach that target becomes short, and
the metric places weight on the strong, short-lived
forcers like CH4 and BC.
Shine et al. (2007) used a similar approach to
develop a time-dependent GTP, the GTP(t). Shine
et al. applied the GTP(t) to BC using a target of 2°C,
and found that for a low emissions scenario, GTP(t)
starts at about 2 in 2010, rising to 1,000 by 2080. But
for a high emissions scenario, GTP(t) can start at 200,
reach 1,000 in 2030, and reach 20,000 in about 2045.
While this approach is one of the few approaches
that are truly compatible with a stabilization target,
there are some drawbacks. Drawbacks include
the dependence on assumptions about future
emissions scenarios, the undefined nature of
the metric after reaching the stabilization target,
and the dependence of the metric on computer
modeling, which reduces transparency. In addition,
policymakers might not desire a metric whose value
can change by orders of magnitude over several
decades and without a transparent and predictable
schedule. One advantage of the GTP(t) and related
metrics is that they can easily be adapted to include
a rate of change goal; for example, rather than just
constraining the metric to reach a 2°C target, it is
also possible to value the rate of change as well
by adding on a constraint that the temperature
not increase more than a given amount in any
given decade. Such an additional constraint would
increase the value of short-lived substances like BC.
2.7.2.3 Cost-Effectiveness Metrics
Manne and Richels (2001) examined relative
tradeoffs between different gases that vary over
time and are calculated to optimally achieve a
given target using a computer model that included
economic considerations. Similarly, the Global Cost
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Potential (GCP), compares the relative marginal
abatement costs for two climate forcers when a
given climate change target is achieved at least cost
(IPCC, 2009). These approaches define a temperature
or radiative forcing target and calculate the relative
(or absolute) dollar value that should be imposed on
different gases in order not to exceed that target.
2.7.2.4 GHG Metrics for Measuring Economic
Impacts
Two metrics, the Global Damage Potential (GDP) and
the social cost of a pollutant, involve monetization
of the damages of climate change (see detailed
discussion in Chapter 6). The GDP compares the
relative damage resulting from an equal mass of
emissions of two climate forcers (IPCC, 2009). The
social cost calculation has most commonly been
used for C02 alone, where it is referred to as the
Social Cost of Carbon (SCC). However, even where
risks and impacts can be identified and quantified
with physical metrics, it may be difficult to monetize
these risks and impacts (e.g., such as ecosystem
damage or the potential to increase the probability
of an extreme weather event) such that an accurate
cost-benefit comparison could be undertaken. Both
the GDP and the social cost calculation depend on
the physical aspects of the climate system as well
as the economic linkages between climate change
impacts and the economy (IPCC, 2009). Therefore,
the GDP and the social cost require calculations of
the entire cause and effect chain, but as a result
contain a large amount of uncertainty.
2.7.3 Applicability of Climate Metrics to BC
This section discusses the use of well-established
metrics such as the GWP and GTP as they relate to
BC emissions and identifies alternative metrics that
may be more relevant to BC. As discussed earlier
in this chapter, BC influences the climate differently
than the warming effects of GHGs. These differences
have important implications for identifying
appropriate metrics to compare climate impacts (and
reductions thereof). Table 2-1 compared some of
BC's climate attributes and effects to those of C02.
The implications of these differences with respect to
metrics are discussed here.
As described in detail below, the significant
differences between BC and C02 make applying the
metrics introduced in the previous section difficult
and, for some purposes, inappropriate. One of the
most essential factors to consider is that BC is most
clearly related to short-term climate impacts, and is
principally a regional pollutant. The lifetime of BC
(weeks) is much shorter than the mixing time of the
atmosphere (1 to 2 years), so the climate impacts of
BC depend heavily on where and when it is emitted.
In comparison, the shortest-lived GHG in the Kyoto
basket has a lifetime longer than one year, and the
majority of the Kyoto gases have lifetimes ranging
from decades to millennia. In addition, the variation
in atmospheric concentrations of BC among regions
contrasts with the well-mixed nature of most GHGs.
This distinction has not been captured in most
metrics to date. Thus, focusing on long-term, global
average radiative forcing impacts— the frame
of reference for long-lived GHGs — may lead to
distorted policy decisions about BC. Conversely,
focusing on short-term or regional impacts may
be inappropriate for decisions involving long-lived
GHGs. The following sections discuss how different
physical (GWP, GTP, SFP, and STRE) and economic
metrics have been used to compare BC to other
substances.
2.7.3.1 Global Warming Potential
While a GWP can be calculated for BC, there are
reasons that GWPs may be less applicable for this
purpose due to the different nature of BC compared
to GHGs, in terms of various physical properties and
the fact that unlike GHGs, BC is not well mixed in the
atmosphere. However, because GWPs are the most
commonly used, and only official, metric in climate
policy discussions, many studies have calculated
GWPs for BC. One-hundred-year GWPs for BC in the
literature range from 330 to 2,240. That is to say, 330
to 2,240 tons of C02 would be required to produce
the same integrated radiative effect over 100 years
as one ton of BC. Some of the factors that account
for the range in these estimates include the use of
different and uncertain indirect and snow/ice albedo
effects estimates, use of a different C02 lifetime for
the baseline, and recognition of the dependence of
a GWP for BC on emissions location.
Using time periods shorter than 100 years has also
been explored for determining the GWPs of BC.
Those who are concerned with near-term impacts
(such as Arctic ice retreat) sometimes suggest
20-year GWPs as more appropriate for short-lived
forcers such as BC (CATF, 2009b). Jacobson (2007)
estimates a 20-year GWP for BC of 4,470. However,
for those concerned about the long-term problems
of climate change, even 100-year GWPs may be
considered too short (IPCC, 2009). Because BC is a
short-lived species, the shorter the policy-relevant
time horizon considered, the greater the relative
importance of BC compared to C02 (and vice
versa: the longer the relevant time horizon, the less
important BC is compared to C02). If the focus is on
achieving immediate climate benefits within a 10- to
20-year time period, the 20-year GWP provides a
more realistic picture of the impact of reductions
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in different species in the near-term. On the other
hand, if the concern is to identify measures that will
help avert climate change at a broad scale, over
a longer time frame, as the problem is generally
conceptualized, a 20-year time horizon is insufficient,
and the 100-year GWP is a more relevant metric.
2.7.3.2 Global Temperature Potential
GTPs, as described previously, evaluate the impact
on temperature at a given time. Studies have applied
the GTP using approaches that differ with respect to
how the emissions are reduced and how the impacts
are calculated. Boucher and Reddy (2008) use a
short, pulse-like (1-year) reduction of emissions and
find that the 100-year GTPs are about a factor of 7
smaller than the corresponding GWPs. Berntsen et al.
(2006) reduced BC emissions for a 20year time span
(approximately the lifetime of a given investment
in abatement technology) and found that the 100-
year GTP of BC was about 120 to 230 (i.e., reducing
120 to 230 tons of C02 has the same impact on
temperatures in 100 years as reducing 1 ton of BC).
Several papers have recently summarized different
BC GWP and GTP estimates (Sarofim, 2010; California
Air Resources Board, 2010; Fuglestvedt et al., 2010).
However, of the studies surveyed by these three
papers, only Hansen et al. (2007a) considered
indirect cloud interactions of BC and only a few
included estimates for metrics of co-emitted OC. If
co-emissions are not included, then any metric will
likely overestimate the globally averaged climate
benefits of reducing BC. Inclusion of indirect effects
could either increase or decrease the calculated
value of the metric.
Figure 2-25, based on Fuglestvedt et al. (2010),
summarizes a number of studies that attempted
to develop metrics for comparing C02 and BC. This
figure shows how the GWP metric depends on the
time horizon used (20 years, 100 years, and 500
years). Additionally, for the first four studies, the
range of values results from a dependence of the
GWP on the region in which the emission occurs.
The difference between the studies is the result of
differences in the climate models used to link the
emissions to the temperature change. Figure 2-26
shows a similar analysis from Fuglestvedt et al.
(2010) which evaluates the equivalent GTP for these
different models.
Fuglestvedt et al. (2010) show that the metric for
comparing BC to C02 can range from a ton of BC
being equivalent to 48 tons of C02 based on a
100-year GTP (which measures the temperature
change 100 years after a pulse of emissions) in
one model, to 4,900 tons of C02 based on a 20-
year GWP (which integrates the total radiative
forcing impact of a pulse of emissions over the
20-year time span) in another model. The variation
between GWPs or GTPs for emissions from different
6,000
5,000
4,000
o.
5 3,000
o
2,000
1,000
I
I
Koch et al.
(2007b)
Naik et al.
(2007)
20-year Horizon
Reddy and Berntsen et al.
Boucher(2007) (2006)
¦ 100-year Horizon
Bond and Sun
(2005)
Schulz et al.
(2006)
500-year Horizon
Figure 2-25. Ranges and Point Estimates for Regional Estimates of GWP Values for One-Year Pulse
Emissions of BCfor Different Time Horizons. The GWP values in the Y axis of the figure refer to
the number of tons of C02 emissions which are calculated to be equivalent to one ton of BC
emissions based on the particular metric. (Adapted from Fuglestvedt et al., 2010.) Note that the
first four studies referenced evaluated GWP values for different sets of regions; Bond and Sun
(2005) and Schuiz et al. (2006) produced global estimates only.
62
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1500
1400
1300
1200
1100
1000
900
£L 800
O 700
600
500
400
300
200
100
0
¦
¦
¦
¦
¦
1
¦
¦
¦
¦
1
¦
1
1
¦ ¦
Koch et al.
(2007b)
Naik et al.
(2007)
Reddy and
Boucher(2007)
i
Berntsen et al.
(2006)
Schulz et al.
(2006)
¦ 20-year Horizon ¦ 100-year Horizon ¦ 500-year Horizon
Figure 2-26. Ranges and Point Estimates for Regional Estimates of GTP Values for One-Year Pulse
Emissions of BC for Different Time Horizons. The GTP values in the Y axis of the figure refer to
the number of tons of C02 emissions which are calculated to be equivalent to one ton of BC
emissions based on the particular metric. (Adapted from Fuglestvedt et al., 2010) Note that the
five studies referenced evaluated radiative forcing estimates for different sets of regions (which were
translated into GTP values by Fuglestvedt et al.); Schulz et al. (2006) produced global estimates only.
locations demonstrates how variability in convective
properties, exposure to sunlight, and different
surface albedos can cause the effect of a given unit
of emissions of BC to vary. Given a specific timescale,
metric, and computer model, the two figures show
that this dependence on emissions location can lead
to changes in GWP or GTP by up to factor of three.
Such dependence on emissions location for long-
lived GHGs does not come into play when calculating
their GWPs.
Sarofim (2010) also summarized a number of
studies, and further analyzed how the GWP estimate
depended on inclusion of either fossil fuel OC co-
emissions or snow albedo impacts. Sarofim (2010)
found that inclusion of these processes can change
the value of the metric by about a factor of two.
Other effects that were not quantified in the paper,
but that can lead to significant differences between
model estimates of GWPs, are the inclusion of
indirect effects on clouds and the assessment of
a larger range of sectors and co-emission types.
Additionally, because most metrics use C02 as a
baseline forcer, the use of different carbon cycle
models can significantly influence the metric values
for BC. Some researchers may report metric values
in carbon equivalents, rather than C02 equivalents,
which leads to a factor of 3.7 difference.
2.7.3.3 Specific Forcing Pulse
The SFP is a relatively new metric proposed by
Bond et al. (2011) to quantify climate warming or
cooling from short-lived substances (i.e., substances
with lifetimes of less than four months). This metric
is based on the amount of energy added to the
Earth system by a given mass of the pollutant.
The rationale for developing this new metric was
that short-lived substances contribute energy on
timescales that are short compared to time scales of
mitigation efforts, and therefore can be considered
to be "pulses." Bond et al. (2011) find that the SFP
of the direct effect of BC is 1,03±0.52 GJ/g, and with
the snow albedo effect included is 1.15±0.53 GJ/g.
They also find that the SFP for OC is -0.064 (from
-0.02 to -0.13) GJ/g, which leads to a conclusion that
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for direct forcing only, a ratio of about 15:1 for OC
to BC is close to climate neutral. However, this does
not include cloud indirect effects or co-emissions of
substances other than OC. Bond et al. also find that
the SFP varies by 45% depending on where the BC
is emitted. While the paper notes that fundamental
differences in temporal and spatial scales raise
concerns about equating the impacts of GHGs and
short-lived aerosols, they do use the SFP to calculate
a GWP for the direct effect of BC of 740 (±370), for
both the direct and the snow albedo effect of BC of
830 (±440), and for organic matter of -46 (from -18
to -92).
This metric is mathematically similar to the Absolute
Global Warming potential (see footnote in GWP
section), but is applied somewhat differently.
Additionally, the use of this metric for regional
impacts is interesting, though as discussed earlier,
the regional pattern of radiative forcing (or energy
input) is not necessarily the same as the regional
pattern of temperature response to that forcing.
2.7.3.4 Surface Temperature Response per Unit
Continuous Emission
Another new metric, the STRE has been proposed by
Jacobson (2010). The STRE is similar to the sustained
version of the GTP. Jacobson found that the STRE
(which he compares to GWPs) for BC on the 100 year
time scale is 2,900 to 4,600 for BC in fossil fuel soot
and 1,060 to 2,020 for BC in solid-biofuel soot. The
uncertainty ranges presented by Jacobson depend
on his assumption that C02 will decay exponentially
with either a 30- or a 50-year lifetime. The use of
a more sophisticated carbon cycle model or the
Bern carbon cycle approximation from the IPPC
(which is a sum of 4 exponentials rather than a
single exponential as in the Jacobson calculations)
would result in a lower STRE and would be more
comparable with other approaches. Jacobson also
presents estimates of the combined BC plus OC
STRE, finding that the STRE for emissions of BC plus
OC from fossil fuel soot ranges from 1,200 to 1,900
and for emissions from biofuel soot the STRE ranges
from 190 to 360.
2.7.3.5 Economic Valuation Metrics
Economic valuation approaches for BC that focus
on valuing climate damages from a comprehensive,
societal standpoint are discussed in detail in Chapter
6. For reasons discussed in that chapter, techniques
used to value the climate damages associated with
long-lived GHGs are not directly transferrable to
BC or other short-lived forcers. In fact, most such
approaches have focused exclusively on valuing
the climate impacts of C02, and may not even be
transferrable to other GHGs. Additional work is
needed to design approaches for valuing the climate
impacts of BC directly, and to incorporate those
approaches into metrics comparable to the SCC.
2.7.4 Using Metrics in the Context of
Climate Policy Decisions
There is currently no single metric widely accepted
by the research and policy community for
comparing BC and long-lived GHGs. In fact, some
question whether and when such comparisons are
useful. For example, there are concerns that some
such comparisons may not capture the different
weights placed on near-term and long-term climate
change. However, there are multiple reasons to
compare BC to other short-lived and long-lived
climate substances, including offsets, credit trading,
evaluation of net effects of a mitigation option, or
illustrative analyses.
The choice of a metric depends greatly on the policy
goal. No single metric will accurately address all
the consequences of emissions of all the different
climate forcers, and all of the differences between
BC and the well-mixed gases must be considered.
The appropriate metric to use depends on a range
of factors, such as: the time scale (20 years, 100
years, or more), the nature of the impact (radiative
forcing, temperature, or more holistic damages),
concern over different processes (indirect effects,
snow albedo changes, co-emissions), and whether
sources and impacts should be calculated regionally
or globally. It is important to note that different
climate models will yield different results even if
the same metric definition is chosen. Taking several
of these factors into account, especially the use of
different time scales, a ton of BC has been calculated
to be equivalent to anywhere from 48 tons of C02 to
4,600 tons of C02. For comparison, the UNEP/WMO
assessment, looking only at the 100 year timescale,
estimated that BC could be 100 to 2,000 times as
potent as C02 per ton (UNEP and WMO, 2011a).
Certainly, the appropriateness of the comparison
depends on the policy question at hand, and the
differences in lifetime, uncertainties, co-emissions,
modes of interaction with the climate system, and
non-climatic effects such as human health should
be evaluated when choosing a metric. This section
highlights how these differences affect the metric
choice.
The tradeoff between capturing short-term
and long-term impacts is not strictly a scientific
consideration but also a policy question. Much
like the original choice of 100 years for the GWP
was a policy compromise between long-term and
short-term impacts; policymakers may consider
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whether using a GWP or GTP metric is an acceptable
compromise given a desire to compare BC and the
long-lived GHGs. A key question is how the metric
is used to inform the policy decision. The NRC has
warned against delaying C02 reductions in favor of
short-lived forcer mitigation, suggesting that C02
emissions control and control of short-term forcing
agents could be thought of instead as "two separate
control knobs that affect entirely distinct aspects
of the Earth's climate" (National Research Council,
2011). The results of the UNEP/WMO assessment
suggest that the two strategies are complementary
and should be pursued simultaneously, with BC
reductions forming part of a larger strategy for
near-term climate change and C02 programs
influencing climate over the longer term (UNEP and
WMO, 2011a). Such an approach could incorporate
separate metrics for short-lived and long-lived
species. One metric would be appropriate for
guiding global emissions of climate forcers to
achieve stabilization of GHG concentrations in the
long-term, while another metric would focus on
mitigating near-term warming and could be used to
guide regional emissions reductions in short-lived
climate forcers to reduce the impacts on regional
forcing, precipitation, and ice/snow melt. It is
important to recognize that long-term stabilization
of C02 concentrations requires limiting total
cumulative emissions of C02 and that C02 reductions
today are necessary to achieve climate goals decades
and centuries from now (National Research Council,
2011).
Reductions of BC today do little to achieve climate
goals in the next century: however, they are
important for climate goals in the near future, which
can include reducing impacts on vulnerable regions
such as the Arctic and reducing the rate of near-term
climate change. In addition, if and when we
approach climate stabilization, sustained reductions
in emissions of BC will be important to keep those
peak temperatures lower than they would otherwise
be. Along these lines, the IPCC found that the
complexity of climate change may indicate that a
basket of metrics approach would best capture the
variety of spatial, temporal and uncertain features
(IPCC, 2009). Such a basket approach to addressing
short-lived and long-lived forcers separately (though
not BC specifically) has also been supported by
Jackson (2009) and Daniel et al. (2011).
Outside of the policy context, the use of multiple
metrics can be valuable for illustrative purposes. For
example, Figure 2-26 shows the impact of BC relative
to C02 on different timescales. Such a figure could
be combined with an analysis such as the Linger et
al. (2010) figure replicated in Figure 2-19 to show the
GTP (or GWP) weighted impact of a set of proposed
mitigation options at 20 years and 100 years (or
some other timescale).
2.7.4.1 Considering the Full Range of BC Effects
As discussed in section 2.6, BC is associated
with complex indirect effects and a number of
hydrological effects that are unrelated to radiative
forcing and that—along with the health effects
discussed in Chapter 3—distinguish it from long-
lived GHGs. These effects include impacts on the
water cycle, inhibition of photosynthesis due to
deposition on plants (Kozlowski and Keller, 1966),
enhancement of soil productivity due to deposition
on soil (Laird, 2008), and effects such as surface
dimming. Capturing these additional effects in
a single global metric is challenging. Even the
current GWP metric continues to see widespread
use despite not capturing the ecosystem effects
of C02-driven ocean acidification or the health
and agricultural impacts of CH4-induced ozone
production.
For most GHGs, relative radiative forcing is a
reasonable approximation of temperature impacts:
a given W m 2 of C02 has similar impacts to a
W m 2 of N20. By contrast, BC forcing includes a
combination of surface dimming and absorption
of both incoming and outgoing radiation at many
wavelengths, while GHGs mainly absorb outgoing
thermal infrared radiation. As discussed in section
2.6.1.4, the temperature change resulting from a
given W rrr2 of forcing from the snow albedo effect
may be much greater than the temperature change
resulting from a W rrr2 of C02 forcing, whereas the
result of forcing from BC-related direct effects may
depend on the pattern of BC loading. Inclusion of
the cloud effects of BC makes this metric even more
uncertain.
Further complicating the use of existing metrics
for BC are the significant remaining uncertainties
in estimates of BC forcing, especially regarding the
indirect cloud effects [which can be compared to
the uncertainty in forcing from changes in well-
mixed GHG concentrations, estimated to be only
10% of 2.63 W rrr2 (Forster et al., 2007)]. However,
even if BC forcing is at the low end of the range,
a consequence of the globally averaged nature of
common metrics is that the right mix of BC and
OC emissions might have little net global radiative
forcing impact and yet still have significant impacts
on regional precipitation, dimming, and snow melt
as well as possibly on regional patterns of warming
and cooling.
Report to Congress on Black Carbon 65
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Chapter 2
2.8 Key Gaps in Understanding and
Expressing the Climate Impacts of BC
This chapter has summarized key findings from a
wide range of peer-reviewed studies related to BC
and its effects on climate. The complex atmospheric
chemistry of BC and its regional nature make it a
challenging subject for study The chapter attempts
to identify where the strength of the evidence
suggests that reasonable conclusions can be drawn
(such as for BC's direct forcing impact, which is
widely understood to lead to warming), and also
highlights those areas where such conclusions
may be premature (such as the net effect of BC,
considering its impacts on clouds and also the
impact of co-emitted pollutants). Despite rapidly
advancing science, there is clearly the need for
additional research, particularly with regard to
BC's effects on clouds and its impacts on radiative
forcing, melting and precipitation in specific regions.
Recent studies have begun to apply more rigorous
modeling and estimation approaches to try to
provide better centralized estimates of BC's direct
forcing impact, its impacts on snow and ice, and its
effects on clouds. Further work is needed to improve
these quantitative estimates and to ensure that the
full range of BC effects on climate is considered.
Key research needs include continued investigation
of basic microphysical and atmospheric processes
affecting BC and other co-pollutants, particularly
with regard to the climate effects of BC-cloud
interactions and aerosol mixing state. In addition,
there is a dearth of research on other types of
light-absorbing carbon, such as BrC, which may
also contribute to climate impacts especially in
sensitive regions such as the Arctic. In general,
further investigation of impacts of aerosols in snow-
and ice-covered regions would be fruitful, along
with additional research on the climate impacts
of emissions mixtures from particular source
categories.
It is also difficult to compare BC directly to C02 or
other long-lived GHGs. This chapter has explored
some of the metrics that are currently available to
determine how well they perform for purposes of
expressing the climate effects of BC and comparing
BC to C02. However, there are clear limitations to
using these metrics. In general, there is a strong
need for further refinement of policy-relevant
metrics for BC and other short-lived climate forcers.
Appropriately tailored metrics for BC are needed in
order to quantify and communicate BC's impacts
and properly characterize the costs and benefits of
BC mitigation.
66 Report to Congress on Black Carbon
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Climate Change - Characterization of Black Carbon and Organic
Carbon Air Pollution Emissions and Evaluation of Measurement
Methods
Phase II: Characterization of Black Carbon and Organic Carbon Source Emissions
DRI Contract Number: 04-307
Prepared by:
Judith C. Chow, Sc.D.
John G. Watson, Ph.D.
Douglas H. Lowenthal, Ph.D.
L.-W. Antony Chen, Ph.D.
Desert Research Institute
Nevada System of Higher Education
2215 Raggio Parkway
Reno, NV 89512
Submitted to:
Nehzat Motallebi, Ph.D.
California Air Resources Board
Research Division
10011 Street
Sacramento, CA 95812
Submitted
December 2, 2008
Revised and Resubmitted
February 12, 2009
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Executive Summary
Background: Direct and indirect radiative effects of suspended particulate matter (PM) are
major sources of uncertainty in current climate models. While aerosol organic carbon (OC)
contributes to cooling through light scattering, black or elemental carbon (BC or EC) absorbs
light, producing a forcing of +0.2 to +1 W/m2 that leads to global warming. BC and OC nearly
always accompany each other in PM emissions from incomplete combustion of carbon-
containing fossil and biomass fuels. Including the direct and indirect effects of BC into the
global- and regional-scale climate models requires accurate BC emission inventories and
conversion factors (i.e., mass absorption efficiencies, oabs [X]) that translate BC concentration
into light absorption coefficients (babs) for different wavelengths. The overall objective of this
study is to improve BC/EC and OC emission inventories by understanding what is currently
available, by better characterizing BC and EC measurement methods, and by measuring emission
rates and profiles from BC-emitting sources. One of the major issues is that there is no single,
universally accepted standard for BC or EC measurement, and the available thermal and optical
methods vary by more than two to three orders of magnitude. Neither are there widely accepted
methods to connect BC or EC to babs, the relevant observable for radiative transfer. Simplified
optical theory for calculating oabs (X) and single scattering albedo of BC may not be applied to
BC from various sources featuring different size, morphology, and internal mixing.
To meet the overall goal of the study, the Desert Research Institute has completed a
comprehensive study on BC measurements and emissions. The first phase of this study evaluated
methods for measuring BC and light absorption (babS). The goals of Phase I include: 1) critically
review the literature on carbon analysis methods and comparisons; 2) create carbon analysis
QA/QC methods and plans; 3) conduct laboratory inter-comparison experiments of organic
carbon (OC), EC, BC, and light absorption (babs) measurement methods; and 4) perform a field
comparison of different measurement methods for babs, BC, EC, and OC at the Fresno Supersite.
The Phase I report was reviewed by the California Air Resources Board (ARB), accepted on
October 6, 2006, and is included as an appendix to this report. The second phase of the study
evaluated global and regional BC inventories and approaches for constructing a BC inventory for
California. The goals of Phase II include: 1) review emission inventory methodology and current
inventories for BC, EC, and OC; 2) review BC/EC and OC emission factors used in the ARB
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emission inventory and compare with emission factors for mobile and biomass burning sources
from recent studies; 3) review models and source profiles used to convert estimated particulate
matter (PM) emissions to BC/EC and OC emissions; 4) apply a model using the ARB PM2.5
emission inventory to estimate BC/EC and OC emissions in California; 5) evaluate uncertainties
in estimated BC/EC and OC emissions; 6) summarize database availability and quality
assurance/quality control; and 7) develop recommendations for constructing BC/EC and OC
emission inventories for California. Study outline and major findings of each Phase are
summarized below.
Phase 1:Phase I of this study was carried out through four major tasks: 1) the first task is a
critical review of literature on 19 different carbon-analysis methods and 80 carbon
intercomparison studies published between 1981 and 2005 was conducted; 2) the second task
focused on developing carbon analysis quality assurance and quality control (QA/QC) plans; 3)
for the third task, pure and externally mixed (with sodium chloride [NaCl]) aerosols from diesel
engine, acetylene flame, electric arc, and wood-combustion aerosols were generated and sampled
in the laboratory under controlled conditions. Continuous babs and BC measurements were made
using the photoacoustic analyzer (PA, 1047 nm) and a seven-color aethalometer (7-AE, 370,
470, 520, 590, 660, 880, 950 nm), along with sample collection on Teflon-membrane and quartz-
fiber filters. In addition, carbon black and graphite powders were resuspended and collected on
quartz-fiber filters for carbon analysis; and 4) the fourth task completed an intensive
measurement campaign at the Fresno Supersite between 8/18/05 and 9/17/05, which included six
continuous light absorption instruments (two wavelength [2-AE, 370, 880 nm] and 7-AE
aethalometers, two PA [532 and 1047 nm], one particle soot absorption photometer [PSAP; 467,
530, 660 nm], and one multi-angle absorption photometer [MAAP; 670 nm]), along with 24-hr
sample collection using integrated samplers. This complemented measurements taken during a
winter intensive operating period (IOP, 12/1/03 to 12/22/03). Findings from the laboratory
intercomparisons were applied in understanding the differences observed at Fresno.
The literature review identified possible biases in thermal and optical methods. For filter-
based thermal/optical analyses, the charring correction followed by early EC evolution in an inert
atmosphere (due to trace oxidants) represented the most important uncertainty in thermal/optical
methods (Chow et al., 2004a), biasing the OC/EC split. For the DRI Model 2001 carbon
analyzer, QA/QC procedures were developed including: 1) multi-point temperature calibrations;
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2) characterization of analysis atmosphere; 3) carbon analyzer calibration; and 4) calibration of
laser intensity using neutral density filters. These procedures have been shown to improve the
precision of OC/EC and carbon fraction measurements. For instance, without temperature
calibration, the sample temperature is typically biased high by 14 to 22 °C, causing up to 30%
change in carbon fraction concentrations. This does not affect the OC/EC split, however. The
review indicated that babs measurements by the PA compared well (within ±3%) with the
difference between light extinction by optical extinction cell (OEC) and scattering by
nephelometer for pure soot sample or soot mixed with salts (Sheridan et al., 2005). The studies
also pointed out the need for correcting filter-based absorption methods for particle light
scattering (bscat), the uncertainty involved in oabs estimates and its effect on babs measurements,
the influence of organic aerosols on babs, and its influence on the Angstrom absorption exponent
(a).
In terms of total carbon (TC), diesel, acetylene flame, and electric arc samples were
generated typically within 15% variability. Wood smoke samples showed as much as 50%
variability. EC/TC ratios measured by thermal/optical methods showed consistency within each
source type, as well as diversity between source types. The STN and French two-step protocols
yielded EC/TC ratio similar to (within ±5%) those of the IMPROVEA protocol for diesel soot
(EC/TC -60%), acetylene flame soot (-96%), and electric arc soot (-50%). The French two-step
and STN protocols were lower for EC (86% and 46%, respectively) in wood smoke compared to
the IMPROVE A protocol. The presence of NaCl caused EC to be released at lower
temperatures, and was limited by the presence of oxygen (02) and charring correction. While it
affected the abundance in the EC fractions, it did not affect the OC/EC split in the IMPROVE A
and STN protocols. The French two-step protocol that operates in pure O2, without charring
corrections, reported >60 to 90% lower EC than IMPROVE A TOR for all 19 samples. When
comparing the IMPROVE_A EC to PA (1047 nm) babs, the EC oabs (1047 nm) varied by -50% in
the range of 2.7 to 5.3 m2/g among the different source types. There is no universal conversion
factor that can be applied to convert babs to BC/EC concentrations. The ratio of AE babs to PA
babs was influenced by BC concentrations; lower ratios were found to be associated with higher
BC concentrations.
Using the IMPROVE A protocol, the EC/TC ratios at the Fresno Supersite were 0.22 ±
0.04 and 0.26 ± 0.05 for summer and winter IOPs, respectively. The EC/TC ratio during winter
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was close to the EC fraction in wood smoke (0.26 ± 0.12). The oabs (1047 nm) of EC during the
winter IOP (2.5 m2/g) was also similar to that of wood smoke EC (2.7 m2/g). The value of a in
the Angstrom Power Law, determined by 7-AE during the summer IOP (0.95 ± 0.04) was 10-
20% higher than that observed for diesel and acetylene flame soot (0.79 ± 0.09 to 0.86 ± 0.12),
from both pure source aerosol and when mixed with NaCl. This indicates that the summer
aerosol at Fresno, while being influenced by diesel emissions, might be mixed with aged or
secondary aerosols. The a during the winter period (1.2 ± 0.11) was closer to that observed for
emissions from wood combustion (1.2 ± 0.51). Despite the potential bias in the aethalometer,
this study confirms a higher a for wood smoke than for diesel soot.
Results suggest that the IMPROVEA and STN protocols estimate similar EC for the
source samples (except wood smoke). The presence of a catalyst such as NaCl changes the
abundances in EC fractions, but not the OC/EC split in IMPROVE A and STN protocols. The
French two-step protocol was influenced greatly by the aerosol matrix. A single value of oabs
does not exist. Moreover, a = 1 in the Angstrom Power Law that is commonly used to scale babs
to different wavelengths varied from 0.5 to 1.4. These observations may be explained by more
complex aerosol optical models that consider particle size distributions, morphology, and
internal/external mixing characteristics.
Phase II: Phase II of this study describes and evaluates state-of-the-science BC/EC and OC
emission inventories and provides a framework for creating inventories for California. Global
BC and OC emission estimates range from 8-24 and 33 - 62 Tg/yr, respectively. North
American BC emissions accounted for -6% of the global total, and California BC emissions
accounted for <~0.4% of global emissions. Global inventories are based on fuel use estimates
and emission factors taken from published articles and reports. These emission factors vary
regionally and depend on the degree of economic development. They do not represent
California's special mixture of fuels, combustion technology, operating conditions, and
aggressive emission controls.
The most accurate inventories use a bottom-up approach where emission factors and
activities are specified for all stationary, area, and mobile sources. Examples include the
California Air Resources Board (ARB) inventory for criteria pollutants in California and the U.S.
National Emissions Inventory (NEI). Because such inventories estimate PM emissions, they
provide a basis for estimating BC and OC emissions when the BC and OC PM fractions are
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measured in specific source types. EC and OC mass fractions are included in source profiles that
are used to produce speciated PM inventories and for receptor-oriented source apportionment
modeling.
ARB PM emission factors are based on emission models such as EMFAC2007 and
OFFROAD for mobile sources and the Emission Estimation System (EES) model for biomass
burning sources. In this study, EMFAC2007 produced reasonable agreement with recently-
measured values for heavy-duty diesel-fueled vehicles, but it did not capture the large variability
in measured gasoline-fueled vehicle emissions. EES provided reasonable estimates for dry litter
burning, but it underestimated PM emissions from wet herb and shrub, regen, and wet needles
from Ponderosa and Lodgepole Pine trees. ARB emission factors overestimated Chemise
(Chaparral) but underestimated rice straw and grass (Grassland) burning. The EES emission
model can be updated with more recent emission factor measurements, provide flexibility for
estimating specific fire events, and estimate the uncertainty of the emission factor estimates.
Recently measured source profiles were compiled into a database to supplement the U.S.
EPA SPECIATE version 4.0 and ARB source profile libraries. Many of the recent studies lack
EC and OC measurements, or they applied EC and OC analysis methods that are not compatible
with ambient data. A set of the assembled source profiles was applied to the ARB 2006 PM2.5
emission inventory to estimate BC/EC and OC emissions in California. Total BC/EC emissions
were 52,084 tons/yr. Major sources included biomass burning (wildfires, managed burning, and
residential fuel combustion), and off-road and on-road engine exhaust. Statewide OC emissions
(107,979 tons/yr) were twice BC/EC emissions (52,084 tons/yr). BC/EC emissions derived from
the 1995 ARB inventory (38,781 tons/yr) were in reasonable agreement (33,281 tons/yr) with
those extracted from California's grid squares from a 1996 global inventory. However, there
were large differences for fuel categories (e.g., fossil fuels and biofuels) and source types,
indicating that the overall agreement may have been fortuitous.
California BC/EC and OC emission estimates are sensitive to the choice of source
profiles used to convert PM2.5 to BC/EC and OC. Recently measured gasoline- and diesel-fueled
vehicle exhaust source profiles from the U.S. EPA's SPECIATE version 4.0 source profile
library resulted in twice the mobile on-road BC/EC emissions compared with the profiles drawn
from the ARB source profile library. Using ARB gasoline- and diesel-fueled vehicle source
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profiles provided 17% lower statewide BC emissions. Source profile documentation in both
libraries is limited, making selection of appropriate profiles difficult to justify.
ARB can build on its current PM2.5 emission inventory effort by coupling relevant source
profiles containing BC/EC and OC abundances with its inventory system. Existing profiles were
assembled into a database and documented, and this can serve as a starting point. Examination
of these existing profiles indicates that they are insufficient to represent all of the major source
types, especially for biomass burning and non-road engine exhaust. More systematic testing of
these emissions, using diluted plumes and a common carbon analysis method, are needed to fill
in the gaps.
Further traceability is also needed for emission factors and activity databases, especially
those used by the local air districts to construct the emissions they submit to the state inventory.
Such data are currently not always available but would enable studies to evaluate the sensitivity
of BC/EC and OC emissions to variability and uncertainty in these parameters. California has a
wealth of speciated PM2.5 measurements from the long-term IMPROVE network operated in its
national parks and wilderness areas and numerous special studies conducted in central
California, the San Francisco Bay Area, and the South Coast Air Basin. Estimated PM2.5, EC,
and OC emission inventories can be evaluated by comparing measured concentrations with those
estimated in air quality models.
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Example State and Local Policy/Initiatives
• Low Emission Vehicle III Technical Support Document, Appendix
U: LEV III Climate Change Impacts of Black Carbon Particles,
Colifornio Air Resources Board
• Understanding Particulate Matter, Section 1-C PM and Climate
Change, Boy Area Air Quality Management District
• Multiple Air Toxics Exposure Study III Final Report Executive
Summary, South Coast Air Quality Management District
• Forest Resource Sustainability in Placer County California, Placer
County Air Pollution Control District
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APPENDIX U
PROPOSED
LEV III CLIMATE CHANGE IMPACTS
OF BLACK CARBON PARTICLES
TECHNICAL SUPPORT DOCUMENT
This report has been reviewed by the staff of the California Air Resources Board and
approved for publication. Approval does not signify that the contents necessarily reflect
the views and policies of the Air Resources Board, nor does the mention of trade names or
commercial products constitute endorsement or recommendation for use.
Date of Release: December 7, 2011
Scheduled for Consideration: January 26, 2012
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Table of Contents
I. Introduction 3
II. Overview of relevant sources of BC 6
III. Climate impact of black carbon in California 9
IV. Global warming potential of BC 11
V. Light-Duty Vehicle BC Climate Impact 14
VI. Summary 17
VII. References 19
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I. Introduction
Airborne particles in the ambient air play an important role in the overall energy
balance of the atmosphere by scattering and absorbing incoming and outgoing solar
and terrestrial radiation (the "direct effect") and by modifying the microphysical
properties of clouds (the "indirect effects") through their role as cloud condensation
nuclei and/or ice nuclei. Direct and indirect effects on the climate by atmospheric
particles remain one of the principal uncertainties in estimates of total anthropogenic
radiative forcing. But recent advances in the understanding of the role of particles in the
global energy balance are closing the existing knowledge gaps. Changes to the Earth's
radiation balance can impact the climate at both global and regional scales.
The chemical composition and climate change impacts of particles vary with their
sources; for example, particles emitted into the air as urban industrial pollution influence
the climate vary differently from windblown desert dust or sea salt particles. To further
complicate this picture, particles tend to remain in the air for only a few days to about a
week, resulting in extreme spatial and temporal variability over the surface of the Earth.
Major components of fine particles such as sulfate, nitrate, organic compounds, dust,
and sea salts have reflective properties that scatter radiation (negative radiative forcing
or cooling impact). Carbonaceous particles (those that contain organic and black
carbon) are particularly important because of their abundance in the atmosphere, and
the characteristics of the carbon vary significantly depending on their origin. Black
carbon (BC) is the principal absorber of visible solar radiation in the atmosphere while
organic carbon (OC) is often described as light-reflecting compounds. Recent studies
show that certain fractions of organic carbon can also absorb solar radiation efficiently
but differ from typical BC, and they are referred to as "brown carbon"1. Its sources are
known to be low-temperature biomass and biofuel burning as well as heterogeneous or
multiphase processes that are not clearly determined yet. In addition, optical and
chemical properties of brown carbon have not been determined consistently2. Further
work is necessary through both observations and improved model simulation of brown
carbon particle for better assessing its effects on climate.
The major anthropogenic sources of BC are fossil fuels and biofuels (biomass
burning for domestic energy). The atmospheric fate and climate impacts of BC from
different regions differ considerably. Atmospheric processes that occur after BC is
emitted, such as mixing, aging, and coating, also affect the net influence of BC on
climate. Because the climate effects of BC aerosol depend strongly on its physical and
chemical properties, as well as on its residence time and distribution in the
1 Lukacs, H., et al., (2007). Seasonal trends and possible sources of brown carbon based on 2-year aerosol measurements at six
sites in Europe. Journal of Geophysical Research e Atmospheres 112. http://publik.tuwien.ac.at/files/pub-tch 7878.pdf
2 Yang, M.; Howell, S.G.; Zhuang, J.; Huebert, B.J. (2009). Attribution of aerosol light absorption to black carbon, brown carbon,
and dust in China -interpretations of atmospheric measurements during EAST-AIRE. Atmos. Chem. Phys., 9(6): 2035-
2050.http://www.atmos-chem-phvs.Org/9/2035/2009/acp-9-2035-2009.pdf
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atmosphere3, a thorough understanding of these properties and accurate techniques for
the determination of BC in the atmosphere and from sources are deemed essential.
In principle, the relatively strong light absorption properties of BC can be used to
infer BC from an optical measurement and knowledge of the mass specific absorption of
BC. Estimates of BC are made with a variety of instrumentation and measurement
techniques. This has also resulted in a variety of definitions related to chemical and/or
physical particle properties, intended applications, and the different measurement and
estimation approaches, and has given rise to an array of descriptive terms such as
"graphitic carbon", "elemental carbon", "black carbon", and "soot" which are used in the
literature as interchangeable with BC. Light absorption, BC, and elemental carbon
derived from different measurement methods and in different environments have been
compared in more than 100 reports and publications4 Among commonly used methods,
the results are highly correlated, but the absolute values can differ by factors of two or
more. Nevertheless, a high correlation among values for different methods suggests
that empirical relationships might be established that would allow some predictability of
one type of measurement from another.
The large interest in airborne particles and their radiative impact is derived in part
from the Intergovernmental Panel on Climate Change (IPCC)'s conclusion that human-
caused climate change has resulted primarily from changes in the amounts of
greenhouse gases (GHGs) in the atmosphere, but also from changes in small particles.
In recent years there has been increased attention in the particle research community
about the potential of BC to cause global warming. The ability of BC to absorb light
energy and its role in key atmospheric processes link it to a range of climate impacts,
including increased temperatures, accelerated ice and snow melt, and disruptions to
precipitation patterns. It has been proposed that light absorbing particles in the
atmosphere act as a greenhouse pollutant whose net forcing is warming and is second
only to carbon dioxide (CO2). Ramanathan and Carmichael 6 estimate a BC forcing of
0.9 watts per square meter (W/m2) or more than half of the 1.6 W/m2 for CO2. This
estimate of the forcing due to BC is larger than most prior estimates including those of
the IPCC 4th assessment report.
Numerous national and international reports highlight the critical role of BC in climate
change. EPA, in consultation with other Federal agencies, prepared a comprehensive
report7 to Congress on the climate effects of BC. The report synthesized available
information on sources of BC, its impacts on global and regional climate, and the
3
Jacobson, M. Z., (2001) Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols Nature, 409,
695-697. http://www.stanford.edU/qroup/efmh/iacobson/ArticlesA/l/nature.pdf
4 Watson, J.G., Chow, J.C., Chen, L.-W.A., (2005). Summary of Organic and Elemental Carbon/Black Carbon Analysis Methods
and Inter-comparisons. AAQR5, 65-102. http://aaar.ora/VOL5 No1 June2005/6 AAQR-05-06-QA-0006 65-102.pdf
5 IPCC (2007), Working Group I: The Physical Science Basis, Chapter 2: Changes in Atmospheric Constituents and in Radiative
Forcing, available at http://www.ipcc.ch/pdf/assessment-report/ar4/wa1/ar4-wg1-chapter2.pdf
6. Ramanathan, V. and Carmichael, G. (2008) Global and regional climate changes due to black carbon. Nature Geoscience
156, 221-227.
7 EPA (2011). Report to Congress on Black Carbon External Peer Review Draft. EPA-450/D-11-001, March 2011.
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potential utility and cost-effectiveness of mitigation options for reducing climate and
public health impacts of BC. The United Nations Environment Program (UNEP)8
recently released a report which summarizes findings and conclusions of an
assessment of BC and tropospheric ozone. The assessment looks into all aspects of
anthropogenic emissions of BC and tropospheric ozone precursors, such as methane. It
examines a large number of potential measures to reduce harmful emissions, identifying
a small set of specific measures that would likely produce the greatest benefits, and
which could be implemented with currently available technology. In May 2011, a task
force, convened by the Arctic Council, produced a comprehensive technical document 9
on assessment of emissions and mitigation options for black carbon. The task forces
has compiled and compared national and global BC emissions inventories, examined
emission trends and projections, synthesized existing policies and programs, and
identified additional emission mitigation opportunities for BC.
The heightened interest in BC mitigation today is built on the well-recognized
association of these emissions with localized air pollution and their severe negative
health impacts while also achieving significant climate co-benefits. Understanding the
role played by BC is therefore critical for three reasons: from the perspective of
understanding climate change, distinguishing between those particles that are
exacerbating the GHG impacts from those that are masking it (through their cooling
effect) is important. That understanding will allow better characterization of the source
impacts and identifying mitigation options. The second reason is that unlike carbon
dioxide, BC's effects are immediate, lasting only a matter of weeks, and are regional in
their effect. Thus, a mitigation strategy of reducing BC emissions in certain locations
would create an immediate relative cooling. This means buying some time and avoiding
going beyond some of the irreversible tipping points such as the melting of arctic ice
and mountain snowpacks. Third, BC, as a key component of fine particulate matter
(PM2.5), contributes to harmful health effects, including premature death. If BC can
indeed provide a double win from both a global climate and local air pollution
perspective, it makes the strong case for climate action that much more compelling.
The following is a summary of the current scientific knowledge on black carbon,
including where it comes from, its atmospheric effects, the overall impact on
environment, and the need for motor vehicle control.
8 UNEP (2011). Integrated Assessment of Black Carbon and Tropospheric Ozone: Summary for Decision Makers.
http://www.unep.org/dewa/Portals/67/pdf/Black Carbon.pdf
9 Arctic Council (2011). Technical report of the Arctic Council Task Force on Short-Lived Climate Forcers. http://arctic-
council.org/filearchive/ACTF Report 22Julv2011.pdf).
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II. Overview of relevant sources of BC
To curb the warming caused by BC, two questions must be answered: where did the
BC come from, and what created it. In general, sources of BC include either (a)
activities or technologies that emit BC, and (b) geographic areas or regions from where
BC is transported to elsewhere. Neither of these is susceptible to easy analysis, mostly
because BC is an air pollutant that is regulated indirectly, if at all. For example, some
nations and states regulate particulate matter, which includes BC, but other constituents
as well. No nation, however, has yet adopted a requirement that directly and explicitly
regulates BC, whether for the purpose of protecting human health or curbing global
warming.
BC comes from both open biomass burning and from energy-related burning. In
developing countries, biomass burning and residential sources are the dominant
sources of BC, while in developed countries; emissions of BC are lower and are often
dominated by transportation and industry. The use of increasingly clean technologies
such as better cook stoves and emission controls on transportation and industrial
emissions reduces the particle and BC emissions for a given activity. However, the
increase in total energy use that accompanies development, as well as changing
industrial development can offset some of the gains from these improvements.
On the global scale, fossil fuels and biofuels account for 66% and 34% of energy-
related BC emissions, respectively. East and South Asia account for more than 50% of
global energy related BC emissions. In their respective regions China and India account
for most of the atmospheric burden. These emissions are from the transport sector and
from biofuels which suffer from inefficient combustion. In addition, domestic coal
combustion in East Asia accounts for a considerable fraction of BC emissions in this
region. The contribution of different regions to the global burden follows the
corresponding contributions to emissions. The largest contribution to the burden is from
East Asia (37%) followed by South Asia (16%), Africa (14%), Europe (12%), North
America (10%), South America (7%), west and central Asia (4%), Australia (<1%) and
Oceanic regions (<1 %). The relatively longer atmospheric residence time for African
emissions results in a contribution of 14% to the global BC burden, compared to a
contribution of 10% to emissions10.
Global inventories are important for providing information on the distribution of BC
emissions world-wide and for identifying key differences between regions, both in terms
of total quantity of emissions and major sources. There are a few global BC inventories
available currently, and the one from Bond et al.11 is the most widely used and
referenced. Compiling a global BC inventory is difficult for several reasons: varying
emissions among similar sources, varying measurement techniques, and different PM
10 Reddy, M. S., and 0. Boucher (2007), Climate impact of black carbon emitted from energy consumption in the world's regions,
Geophys. Res. Lett., 34, L11802, doi: 10.1029/2006GL028904.
11 Bond, T.C.; Streets, D.G.; Yarber, K.F.; Nelson, S.M.; Woo, J.H.; Klimont, Z (2004). A Technology-Based Global Inventory of
Black and Organic Carbon Emissions from Combustion; J. Geophys. Res. Atmos 109, D14203; doi: 10.1029/2003JD003697.
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size cut points used in the measurements, and the definition of BC itself used in the
inventories. Global BC and OC emission inventories are also complicated by a lack of
detailed information on source types, emission factors, activities, and controls,
especially in the developing world. Recent estimates of global emissions of BC and OC
range from 8 to 24 terragrams (Tg) and 33 to 62 Tg per year, respectively. Thus,
uncertainty is the hallmark of these studies, with emission estimates varying by a factor
of 2 or more12. Bond et al estimated that the United States accounted for 5.6% (0.45
Tg/yr) of the global total.
In 2005, emissions of BC from U.S. sources total about 0.65 million tons (0.58 Tg),
which represents about 8% of the global total. Mobile sources account for a little more
than half (52%) of the domestic BC emissions. Nearly 90% of the mobile source total is
from diesel sources. Open biomass burning is the next largest sources in the U.S.,
accounting for about 35% of the total. In general, BC is concentrated in urban areas,
where populations are largest, making health an important issue in addition to climate in
BC mitigation strategies (EPA, 2011). The degree of difference between the
EPA inventory and Bond et al (2004) inventory for U.S. emissions is driven by EPA
estimates for open burning and (to a lesser extent) for mobile sources in the U.S. that
are higher than those from the global inventories. Wildfire emissions can vary greatly
from year to year, and this may explain some of the difference between the estimates
for open burning. Also, EPA estimates include all non-road and on-road emissions in the
transportation source category, while global inventories group emissions from some of
the smaller non-road sources into the Industry category. This could account for global
inventory estimates of U.S. emissions being lower for transport and higher for industry
compared to the EPA estimates.
California's emissions inventory for criteria pollutants and the U.S. National
Emissions Inventory include PM emissions, which provide a basis for building a bottom-
up BC and OC emissions inventory. BC and OC emissions can be estimated from
source-specific PM2.5 emissions and the relative BC and OC fractions in the emitted
PM. For an ARB-sponsored study by the Desert Research Institute13, Chow et al.
evaluated global and regional BC inventories and approaches for constructing a BC
inventory for California. A black carbon inventory for California of 38,730 t/yr was
comparable to the 33,280 t/yr estimated from a bottom-up global BC inventory.
However, further examination showed substantial differences among subcategories.
Most of the discrepancy was due to differences in open biomass burning (wildfires and
agricultural waste) for which carbon emissions are highly variable. BC and OC
emissions are sensitive to the availability and variability of existing source profiles, and
profiles more specific to fuels and operating conditions are needed to increase emission
accuracy.
12 Chow JC, Watson JG, Lowenthal DH, Chen LW, Motallebi N., (2010). Black and organic carbon emission inventories: review
and application to California. J. Air Waite Manag Assoc. Apr;60(4):497-507.
13 Chow, J.C., J.G. Watson, D.H. Lowenthal, L.W.A. Chen, (2009). Climate Change—Characterization of Black Carbon and
Organic Carbon Air Pollution Emissions and Evaluation of Measurement Methods. Phase II: Characterization of Black Carbon
and Organic Carbon Source Emissions. California Air Resources Board (Contract No.04-307).
http://www.arb.ca.qov/research/apr/past/04-307 v2.pdf
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Using a bottom-up approach, California emitted 52,0841 BC/yr during 2006. Among
the sources, the largest single source was wildfires, 29% of total BC emission. The next
two largest single sources were off-road mobile sources, 23% of the total BC emission,
and on-road mobile sources, 20% of total BC emission (Table 1). Statewide OC
emissions of about 107,979 tons/yr were twice BC emissions.
Table 1. BC and OC emissions (tons/year) for California in 2006 (Source: C
how et al.).
Source
BC
OC
%BC
%OC
Mobile (On-Road)
Gasoline Powered
2,644
4,657
5%
4%
Diesel Powered
7,840
5,047
15%
5%
On-Road Total
10,484
9,704
20%
9%
Mobile (Off-Road)
12,158
13,890
23%
13%
Residential fuel combustion
4,004
17,422
8%
16%
Managed burning and disposal
7,374
19,652
14%
18%
Wildfires
15,161
29,530
29%
27%
Miscellaneous
2,903
17,781
6%
16%
Total
52,084
107,979
100%
100%
Note: percentage shows the variability of BC and OC fractions for major source types relative to the
total BC and OC emissions, respectively.
California BC emission estimates were sensitive to the choice of source profiles.
When a base-case diesel profile with a 50% BC abundance was replaced by one with
an BC content of 26%, BC emissions from mobile on-road and mobile other (off-road)
sources resulted in a 17% decrease in total BC emissions. The efficacy of the approach
used to estimate BC and OC emissions in California depends on the accuracy and
comprehensiveness of available PM emission factors, source activities, and BC and OC
source profiles. Measurement techniques for BC and OC should also be standardized.
These limitations need to be overcome to improve BC and OC inventories.
The starting point of all climate modeling studies of BC forcing is the emission
inventory. However, Bond et al.14 estimate that the global (and regional) emission
inventory is subject to an uncertainty of about a factor of two, because of a lack of
proper knowledge of emission factors, activity data, and technology splits. The regional
contributions to atmospheric burden largely follow emissions from the respective
regions. The emission of BC is quite dependent on the combustion process and
emissions can vary significantly even among apparently similar sources. The need for a
robust BC emission inventory that can capture some of these fine details is paramount.
Studies are underway to test the emission estimates against currently available field
observations; and we expect that iteration among emissions, atmospheric
measurements, model results, and combustion tests will result in improved
understanding of the magnitude of carbonaceous particle emissions.
14 Bond, T.C. et al. (2007). Historical emissions of black and organic carbon aerosol from energy-related combustion, 1850—
2000, Global Biogeochem. Cycles, 21, GB2018, doi:2010.1029/2006GB002840.
http://www.saqe.wisc.edu/pubs/articles/M-Z/Trautmann/BondetalGBC07.pdf
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III. Climate impact of black carbon in California
Recent work on climate change impacts in the western USA has focused attention
on the shift in snowmelt timing toward earlier dates (Stewart et al.15), the shift from snow
to rain (Knowles et al.16), the earlier onset of spring, and the effect that these changes
will have on water supply in California and throughout the western USA. Pierce et al.17
showed that about half of the observed decline in western USA springtime snowpack
(1950-1999) results from climate changes forced by anthropogenic GHGs, ozone and
particles. Although these trends in climate are largely attributable to increasing
atmospheric concentrations of GHGs, recent modeling work has drawn attention to the
role of soot (which is mostly BC) in modifying climate by reducing snow albedo. Hansen
and Nazarenk18 showed that soot may reduce snow and ice albedo in Northern
Hemisphere land areas by as much as 3%, resulting in a climate forcing of +0.3 W/m2
They found that due to positive feedbacks, the "efficacy" (change in air temperature per
unit forcing) of soot is about twice that of CO2. This indirect soot forcing may have
contributed to global warming of the past century, including the trend toward early
springs in the Northern Hemisphere, thinning Arctic sea ice, and melting land ice and
permafrost. However, Hansen and Nazarenk (2004) also states that the substantial role
inferred for soot in global climate does not alter the fact that greenhouse gases are the
primary cause of global warming in the past century and are expected to be the largest
climate forcing for the rest of this century.
The snowpack in the Sierra Nevada region is important to California's water
resources. The high elevation snowpack serves as a natural reservoir that stores fresh
water during the wet, cold season and releases it gradually during the dry, warm
season. About 60% of the water supply for Southern California comes from melting
Sierra Nevada snowpack. Snowmelt also affects hydropower generation in California
(Vicuna et al.19). Snow albedo is among the most important local parameters in shaping
the spatio-temporal variations in snowpack. Surface insolation and more specifically the
portion of insolation absorbed by the snowpack is the leading energy source in the
evolution of snowpack, especially during the melting period. Thus, variations in snow
albedo can exert significant impact on snowpack during the course of accumulation and
ablation. The surface albedo of sufficiently deep snowpack, and in turn the amount of
15 Stewart IT, Cayan D.R, Dettinger.M.(2005) Changes toward earlier streamflow timing across western North America. J Clim
18:1136-1155. http://tenava.ucsd.edu/~dettinqe/stewart timinq.pdf
16 Knowles N, Dettinger M, Cayan D (2006) Trends in snowfall versus rainfall in the western United States. J Clim 19:4545-4559.
http://tenava.ucsd.edu/~dettinae/iclim rain v snow.pdf
17 Pierce DW, Barnett TP, Hidalgo H, Das T, Bonfils C, Santer B, Bala G, Dettinger M, Cayan D, Mirin A, Wood A, Nozawa T
(2008) Attribution of declining western U.S. snowpack to human effects. J Clim 21:6425—6444.
http://tenava.ucsd.edu/~dettinqe/swe over p attribution.pdf
18 Hansen J, Nazarenk L (2004) Soot forcing via snow and ice albedos. Proc Natl Acad Sci 101:423—428.
http://www.pnas.orq/content/101/2/423.full.pdf+html
19 Vicuna, S., R. Leonardson, M. Hanemann, L. Dale, and J. Dracup. 2008. "Climate change impact on high elevation
hydropower generation in California's Sierra Nevada: A case study in the upper American River." Climatic Change 87:S123-
S137.
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the insolation absorbed by the snowpack, depends largely on the ice grain size and
impurities within or at the surface of ice grains (Waliser et al.20). There exist only a
limited number of studies on the alteration in snow albedo and its impact on surface
hydrology due to dust and BC particles deposited on snowpack. This is an important
concern because the amount of BC deposition on snowpack is closely related with
anthropogenic emissions. Thus, anthropogenic emissions that have bearing on the
causes and characteristics of global climate change include an influence on local
snowpack by altering snow albedo.
Hadley et al.21 examined the concentration of BC particles in snow in California and
the potential of these particles to reduce albedo and increase melt. Samples of falling
snow and rain were collected at three locations in California: Central Sierra Snow
Laboratory in the Sierra Nevada, Lassen Volcano National Park in the Southern
Cascades, and at Trindidad Head on the Northern California coast. This study provides
one of the first direct measurements for the efficient removal of black carbon from the
atmosphere by snow and its subsequent deposition to the snow packs of California. The
data reveal that BC concentrations in the Sierra Nevada snowpack are sufficient to
perturb both snow melt and surface temperatures. The concentration of BC measured in
the snow is consistent with recent model predictions for BC concentration in California
mountain snow.
The associated reduction in snow albedo and reduced snow packs in early spring
snowpack has been shown by regional climate models to be significant. All three
stations reveal large BC concentrations in precipitation, ranging from 1.7 ng/g to
12.9 ng/g. The BC concentrations in the air after the snowfall were negligible suggesting
an extremely efficient removal of BC by snow. The data suggest that below cloud
scavenging, rather than ice nuclei, was the dominant source of BC in the snow. A five-
year comparison of BC, dust, and total fine particle mass concentrations at multiple
sites reveals that the measurements made at the sampling sites were representative of
large- scale deposition in the Sierra Nevada. The relative concentration of iron and
calcium in the mountain particle indicates that one-quarter to one-third of the BC may
have been transported from Asia.
Coats22 quantified the decadal-scale time trends in air temperature, precipitation
phase and intensity, spring snowmelt timing, and lake temperature in the Tahoe basin,
and related the trends to large-scale regional climatic trends in the western USA. It
states that the Tahoe basin has abundant winter-time emission sources of BC. Many
homes are heated with wood-burning stoves, and traffic during the ski season is heavy
20 Waliser, D.et al. (2009) Simulating the Sierra Nevada snowpack: The impact of snow albedo and multi-layer snow physics.
California Environmental Protection Agency and California Energy Commission Report CEC-500-2009-030-F.
http://www.enerav.ca.aov/2009publications/CEC-500-2009-030/CEC-500-2009-030-F.PDF
21 Hadley, 0. L., Corrigan, C. E., Kirchstetter, T. W., Cliff, S. S., and Ramanathan, V. (2010). Measured black carbon deposition
on the Sierra Nevada snow pack and implication for snow pack retreat, Atmos. Chem. Phys., 10, 7505-7513, doi: 10.5194/acp-
10-7505.
22 Coats, R., (2010). Climate change in the Tahoe basin: regional trends, impacts and drivers. Climatic Change 102:435-466,
DOI 10.1007/s10584-010-9828-3.
http://escholarship.orq/uc/item/6d8945fb:isessionid=2E82C49ECFD9B1B34014A86494A7FA1F
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at times. Air quality data are available for a station at South Lake Tahoe (SLT) and at
Bliss State Park (BSP). SLT is the most urbanized area of the basin and had the highest
elemental carbon concentrations, averaging 2.02 |jg/m3. BSP is less influenced by local
sources, but like the rest of the basin is down-wind from major metropolitan areas in
Sacramento Valley and Bay Area. The study indicates that atmospheric deposition of
black carbon in the Tahoe basin may be implicated in the shift in snowmelt timing,
increasing air temperature and the shift from snowfall to rain. Snowpack energy budget
studies together with analysis of snowpack black carbon concentrations are needed to
test this "snow albedo perturbation" hypothesis. Monitoring of black carbon in snow
should be added to routine water quality monitoring in the Tahoe basin.
IV. Global warming potential of BC
Multi-gas mitigation strategies require metrics to compare the effect of emissions of
different GHGs. Following its endorsement by the IPCC and its adoption within the
Kyoto Protocol, the Global Warming Potential (GWP) has established itself as the metric
of choice for the derivation of CC^-equivalent emissions. GWP is a well-defined metric
based on radiative forcing that continues to be useful in a multi-gas approach.
Shortcomings have been identified; however, the scientific basis has not been fully
established to address these shortcomings comprehensively in any currently discussed
metric.
GWPs were meant to compare emissions of long-lived, well-mixed GHGs. Short-
lived species, like BC, vary spatially and, consequently, it is very difficult to quantify their
global warming forcing. Due in large part to the difference in lifetime between BC and
CO2, the relative weight given to BC as compared to CO2 (or other climate forcers) is
very sensitive to the formulation of the metric used to make the comparison. There is
currently no single metric that is widely accepted by the science and research
community for this purpose. The choice of a metric depends greatly on the policy goal.
No single metric can be used to accurately address all the consequences of emissions
of all the different climate forcers. The appropriate metric to use depends on factors
such as: the time scale (20 years, 100 years, or longer), the nature of the impact
(radiative forcing, temperature, or damages), concern over different processes (indirect
effects, snow albedo changes, co-emissions), and whether sources and impacts should
be calculated regionally or globally. The assessment of metrics will be included in the
IPCC 5th assessment report (AR5; to be released around September 2013) process in
an integrated manner with participation from all three working groups and the IPCC
Task Force on GHG inventories. This process will likely include an assessment of
numerical values for metrics that have been proposed in the literature.
It is important to note that different climate models could yield different results even if
the same metric definition is chosen. Climate models are recently beginning to address
the full effects of light-absorbing BC. However, estimates of the importance of
carbonaceous particles as global warming agents vary greatly. Part of the variability
between models is due to the inclusion or exclusion of certain physical effects, including
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the nature of the mixing of absorbing BC aerosols with other, scattering aerosols, the
multiple effects of aerosols on clouds, and the effects of BC on surface albedo.
Differences also arise between models based purely on physical theory and those that
attempt to fit observational data. An additional challenge in comparing model results is
that different authors often present their results in different terms: some use the net
radiative forcing (RF) of carbonaceous particles, others the RF of BC alone, and still
others the net temperature change due to carbonaceous aerosols.
However, within bounded limits, there is no question that BC alters the Earth's
energy balance, and is a net warming species. What is clear is that as scientists are
able to disaggregate the effects of different particles, BC dominates so clearly and with
such magnitude that it can no longer be ignored. The balance has now tipped in favor of
taking action despite the remaining uncertainties on the exact metric to use.
While the IPCC 4th assessment report did not publish a GWP estimate for BC in its
most recent report, independent estimates have been published in the peer-reviewed
literature, including estimates drawn from IPCC report itself. Several leading scientists
have reported estimates of the GWP for BC emissions from different sources. Most of
the regional differences in GWP are caused by differences in the lifetime of BC. In
general, we find in the published literature there are significant variations in the GWP
values for BC emissions assigned to different regions. This indicates that BC emissions
cause warming primarily in the region they are emitted, and that the role of BC in
warming requires close attention to the geography of emissions.
Table 2 shows values from the literature for 100-year and 20-year GWP values of
BC. As the table shows, BC's climate-forcing effect is generally very high (compared to
CO2, which by definition has a GWP of 1), and it is much greater when considered on a
shorter time horizon like 20 years instead of 100 years. The table shows that generally
the 20-year time horizon for BC is about 2000, whereas the 100-year GWP for BC is
generally found to be more like 500 to 800. Overall, Table 2 indicates that BC is capable
of generating warming that is two orders of magnitude greater than carbon dioxide. On
a 20-year horizon, which places greater emphasis on rapid, near-term climate impacts,
this BC warming is three orders of magnitude greater than the CO2 warming.
Hansen et al. (2007) has calculated a GWP for fossil-fuel derived BC of 500, which
includes both positive forcing from soot particles as well as the negative forcing from
co-emitted OC. This value compares well with other published values: Bond and Sun
(2005) calculated a GWP for BC of 680 for the same time horizon, but they did not
include the effect of co-emitted species and cloud effects. The direct forcing of BC is
significantly dependent on assumptions about how BC particles are mixed with other
components of the particle population, with internal mixing tending to accentuate the
positive forcing. BC also causes a complex set of impacts on cloudiness, including the
so-called semi-direct effect, whereby changes in temperature and humidity structure
due to the absorption of solar radiation by BC, alter the structure of clouds.
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Table 2. Black carbon global warming potential values from research literature.
Source
Black carbon globa
warming potential
Indirect (cloud
change) forcing
100-yr
20-yr
Hansen et al, 200723
-500
-2000
Yes
Bond and Sun, 200524
680
2200
No
Jacobson, 200725
840-2240
2530
Yes
Reddy and Boucher (2007)26
480
--
No
Rypdahl et al, 20092/
830
2900
No
Fuglestvedt et al, 201028
460
1600
No
Reddy and Boucher (2007) simulated the atmospheric cycle of energy-related BC
from different regions and estimated the regional contributions to the global BC burdens
and direct radiative forcing. The regional contributions of BC to global mean forcings
closely follow the respective contributions to atmospheric burden in the form of PM. The
GWP of BC for different regions ranges from 374 to 677 with a global mean of 480.
Another variable that plays a role in the overall estimation of BC impacts is the albedo
effect. The global mean indirect GWP due to the BC effect on snow albedo is estimated
at 281. The indirect GWP due to the BC effect on snow albedo is estimated to be
largest for Europe, suggesting that BC emission reductions from this region are more
efficient to mitigate climate change. Rypdal et al (2009) also examined the regional
differences in BC climate impact. The regions considered are EU17, Rest of Europe,
Russia, North America, Latin America, East Asia, Centrally Planned Asia, South Asia,
Japan, the Pacific OECD, Africa and the Middle East. The regional GWP values range
from 640 to 1130 for the direct effect of BC. Most of the regional differences in GWP are
caused by differences in the lifetime of BC, although up to 20% can be explained by
differences in the RF per unit mass BC.
In general, we find in the published literature there are significant variations in the
GWP values for BC emissions assigned to different regions. This indicates that BC
emissions cause warming primary in the region they are emitted, and that the role of BC
in warming requires close attention to the geography of emissions. We find Hansen et
al. (2007) 100-year GWP of 500, which has been estimated globally, provides a
reasonable estimate for use in calculating CO2 equivalent benefits. Hansen's work is
23 Hansen, J., M. Sato, P. Kharecha, G. Russell, D. Lea, and M. Sidall.(2007). "Climate change and trace gases." Trans. R. Soc..
1925,1942. http://pubs.giss.nasa.gov/docs/2007/2007_Hansen_etal_2.pdf
24 Bond, T. C. and H. Sun. 2005. "Can reducing black carbon emissions counteract global warming?" Environmental Science and
Technology 39:5921-5926.
25 Jacobson, M. Z. (2007). Testimony for the Hearing on Black Carbon and Global warming, House Committee on Oversight and
Government Reform. See: http://oversiaht.house.aov/documents/20071018110606.pdf.
26 Reddy, M. S., and 0. Boucher (2007), Climate impact of black carbon emitted from energy consumption in the world's regions,
Geophys. Res. Lett., 34, L11802, doi: 10.1029/2006GL028904. http://www.lmd.iussieu.fr/~obolmd/PDF/2006GL028904.pdf
27 Rypdal, K., N. Rive, T. Berntsen, Z. Klimont, T. Mideksa, G. Myhre, and Ragnhild Skeie (2009). "Costs and Global impacts of
black carbon abatement strategies." Tellus 61B, 625-641. http://folk.uio.no/torbenm/central/Tellus 2009.pdf
28 Fuglestvedt, J.S. and Shine, K.P. and Berntsen, T. and Cook, J. and Lee, D.S. and Stenke, A. and Skeie, R. and Velders,
G.J.M. and Waitz, I.A. (2010) Transport impacts on atmosphere and climate: Metrics. Atmospheric Environment, 44, pp. 4648-
4677. http://elib.dlr.de/68051/1/fual-2010-4648.pdf
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widely recognized and used in IPCC. Although, this should be considered a
conservative estimate for fossil fuel BC forcing, as discussed by Hansen et al. (2007),
because it assumes a high OC/BC ratio for fossil fuel emissions. Also it assigns 50% of
the particle indirect effect (which causes cooling) to soot (BC/OC).
Hansen et al (2007) and Bond and Sun (2005) provide a 20-year GWP value for BC
of 2000. Because BC particles produce a rapid, short-term, localized impact on the
climate, we should also examine the climate effects using GWPs for a shorter time
horizon. The 20-year GWP can be considered a means to control the rate of warming,
while the 100-year GWP can be considered a means to control committed warming.
Hence, reductions in short-lived pollutants like BC will make a significant contribution
toward reducing the rate of warming, which is the basis for arguments that such
reductions can buy time for future climate mitigation measures to be effective.
V. Light-Duty Vehicle BC Climate Impact
In developed countries, the BC problem is primarily associated with the high volume
of fossil fuel use in transportation, particularly diesel. Chow et al. (2010) assembled data
from about 800 PM2.5 source profile libraries and recent studies, and found that the
highest BC abundances were found for on-road diesel vehicles and off-road diesel
engines. In a study by Strawa et al (2010)29, particulate emissions from motor vehicles
inside a San Francisco Bay Area roadway tunnel (Caldecott) were characterized in
2004 and 2006. The amount of absorbing aerosol emitted and the low values of single
scattering albedo indicate that particulate matter from motor vehicles exerts a positive
(i.e., warming) radiative forcing and that the impact of medium/heavy duty diesel trucks
is greater than light-duty vehicles. Another study by Kirchstetter et al (2008)30 indicates
that annual average BC concentrations in the Bay Area decreased by a factor of 3 over
the 1967-2003 period, while diesel fuel use, the main source of BC emissions,
increased by a factor of 6. The study also states that the contrast in the trends in BC
concentration and diesel fuel use is striking, especially beginning in the early 1990s
when BC concentrations began markedly decreasing despite sharply rising diesel fuel
consumption. This contrast suggests that technology and fuel changes to reduce BC
emissions have been successful.
Using data from the Interagency Monitoring of Protected Visual Environments
(IMPROVE) program, Bahadur et al. (2011 )31 examined the temporal and the spatial
trends in the concentrations of BC for the past 20 years in California. Annual average
BC concentrations in California have decreased by about 50% from 0.46 ng m"3 in 1989
29 Strawa, A.W.; Kirchstetter, T.W.; Hallar, A.G.; Ban-Weiss, G.A.; McLaughlin, J.P.; Hariey, R.A.; Lunden, M.M. (2010). Optical
and Physical Properties of Primary On-Road Vehicle Particle Emissions and Their Implications for Climate Change. Journal of
Aerosol Science 41, 36-50.
30 Kirchstetter, et al. (2008) Black carbon concentrations and diesel vehicle emission factors derived from coefficient of haze
measurements in California, AE, 42(3): 480-491.
31 Bahadur R., Feng Y., Russell L. M., Ramanathan V. (2011). Impact of California's air pollution laws on black carbon and their
implications for direct radiative forcing. Atmospheric Environment 45,1162-1167.
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to 0.24 ngm in 2008 compared to a corresponding reductions in diesel BC emissions
(also about 50%) from a peak of 0.013 Tg Yr"1 in 1990 to 0.006 Tg Yr"1 by 2008.
Bahadur et al. attribute the observed negative trends to the reduction in vehicular
emissions due to stringent statewide regulations. Their conclusion that the reduction in
diesel emissions is a primary cause of the observed BC reduction is also substantiated
by a significant decrease in the ratio of BC to non-BC aerosols. The absorption
efficiency of aerosols at visible wavelengths - determined from the observed scattering
coefficient and the observed BC - also decreased by about 50% leading to a model-
inferred negative direct radiative forcing (a cooling effect) of -1.4 Wm"2 over California.
California along with national and international climate-change policies have
embraced a multi-gas approach where a "basket" of GHG emissions are considered
together. In such a plan, emissions of each gas are given a weight relative to CO2 so
that multiple gases can be considered together. Sufficient scientific consensus exists on
BC to provide appropriately accurate metrics on how to assess the climate value of
actions to reduce BC emissions. Table 3 presents the climate impact of BC as
compared with the other GHGs, and shows the product of the GWP and GHG and BC
emissions. 2020 BC emissions are based on 3 mg/mi PM emissions, 66% BC fraction
(500 GWP -|oo-yr; 2000 GWP20-yr).
Table 3. Approximate illustration of equivalent CO2 emissions from different GHGs
for 2009 and for future (2020 and beyond) year new vehicles.
GHG
Global warming
potential3
100-year gC02e/mile
20-year gC02e/mile
emission
100-yr
20-yr
2009
Future (2020+)
2009
Future
(2020+)
CM
O
O
1
1
337
<200
337
<200
AC
refrigerant
1430
3830
6
0
16
0
ch4
25
72
1.8
0.5
1.8
0.5
n2o
298
289
0.1
0.03
0.3
0.05
BCd
500
2000
0.77
0.5
3.08
1.98
From IPCC 2007 fourth assessment review (AR4), except BC global warming potential estimate
is based on ARB review of scientific literature. Pavley I used IPCC 2001 TAR GWPs values (e.g.
1300 for HFC-134a, 23 for CH4, 296 for N20).
100-yr GWPs are IPCC 2007 AR4 (1,430 for HFC-134a, 25 for CH4, 298 for N20) as used by
U.S. EPA. 20-yr GWPs are IPCC 2007 AR4 (3,830 for HFC-134a, 72 for CH4, 289 for N20.
b Based on ARB's recent estimates: i.e., for the four vehicle types LDA, LT1, LT2, and LT3 the
2009 fleet average gasoline exhaust PM is 7.0 mg/mi; for 2020 it is 4.5 mg/mi assuming phase-in
of the 3 mg/mi standard from 2017 to 2020 - with 0.22 BC/PM fraction for on-road gasoline from
Chow et al 2
32 Chow, J.C., Watson, J.G., Lowenthal, D.H., Antony Chen, L.-W., Motallebi, N. (2011) PM2.5 source profiles for black and
organic carbon emission inventories. Atmospheric Environment 45, 5407-5414.
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We use two time horizons to estimate the climate impact: the short-term scenario is
based on GWP for 20 years, and the long-term scenario is based on the more
conventional 100-year GWP. Because BC aerosol exerts a rapid warming effect in the
vicinity of the source, we think a 100-year weighted GWP for BC is less appropriate
than a GWP based on a 20-year time. This shorter time horizon is better reflective of the
speed at which control of, for example, soot emissions can benefit the climate relative to
controlling carbon dioxide emissions. The two time frames can lead to a four-fold
difference in CC^eq emissions. Hence, a 20-year time frame gives a better perspective
of the speed at which BC controls could benefit the atmosphere relative to CO2
emission controls.
There is a wide range of BC and OC abundances in PM2.5 source profiles
representing the same source type. The median PM2.5-BC abundance of 22% was
used in Table 3 for on-road gasoline emission. As discussed earlier, Chow et al (2011)
examined BC and OC abundances in source profiles from the U.S. EPA SPECIATE
data base along with additional profiles obtained by the authors to evaluate their
variability within and between source-types and to assess the effect of this variability on
BC and OC emission rates. For profiles compiled in their study, BC and OC ranged
6-38% and 24-75% for on-road gasoline vehicles, and 33-74% and 20-47% for on-road
heavy-duty diesel vehicles, respectively.
It should be noted that the introduction of new engine technologies (e.g., some types
of gasoline direct injection) in recent model years has increased BC/PM ratios in some
new gasoline-powered motor vehicles which may change the warming profile of
emissions from these vehicles. More testing is needed to determine the typical ratios of
EC/PM in LDV exhaust, as these ratios seem to be changing over time. These data can
then be used, in combination with other vehicle fleet information, to more accurately
estimate the atmospheric contribution of BC by LDV in climate models. The influence of
BC on climate and public health in the future, and the need to pinpoint more precisely
the effectiveness of various mitigation strategies for reducing BC, depend in large part
on the magnitude of future emissions.
While uncertainties remain, emerging research suggests that targeting emission
reductions from key sectors can have measurable benefits for both climate and public
health climate. Mitigation of BC thus offers a clear opportunity: carefully designed
programs that consider the full air pollution mixture (including BC, OC, and other
co-pollutants) can slow near-term climate change while simultaneously achieving lasting
public health benefits. Furthermore, currently available control technologies and
mitigation approaches have already been shown to be effective in reducing BC
emissions, often at quite reasonable costs. These mitigation approaches could be
utilized to achieve further BC reductions.
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VI. Summary
BC is the light-absorbing carbonaceous fraction of PM that results from incomplete
combustion of fossil fuels and biomass. BC causes warming primarily in the regions
where it is emitted, and therefore merits analysis and solutions at the local scale. The
ability of BC to absorb light energy and its role in key atmospheric processes link it to a
range of climate impacts, including increased temperatures, accelerated ice and snow
melt, and disruptions to precipitation patterns. Numerous national and international
reports highlight the critical role of BC in climate change. The heightened interest in BC
mitigation today is built on the well-recognized association of these emissions with
localized air pollution and their severe negative health impacts. Any climate strategy for
reducing BC emissions offers these important co-benefits.
BC is different from long-lived GHGs like CO2 both in the variety of mechanisms by
which it affects climate and its short atmospheric lifetime of days to weeks. This short
lifetime, combined with the strong warming potential of BC, means that the climate
benefits of reductions in current emissions of BC will be nearly immediate. It also makes
reductions in BC emissions a potential near-term opportunity to postpone the effects of
rising GHG levels on the global climate. In contrast, long-lived GHGs persist in the
atmosphere for centuries. Therefore, reductions in GHG emissions will take longer to
influence atmospheric concentrations and will have less impact on climate on a short
timescale.
Because BC particles produce a rapid, short-term, localized impact on the climate,
we should also examine the climate effects using GWPs for a shorter time horizon. The
20-year GWP can be considered a means to control the rate of warming, while the 100-
year GWP can be considered a means to control committed warming. Hence,
reductions in short-lived pollutants like BC will make a significant contribution toward
reducing the rate of warming, which is the basis for arguments that such reductions can
buy time for future climate mitigation measures to be effective. However, since GHGs
are by far the largest contributor to current and future climate change, BC reductions
cannot substitute for reductions in long-lived GHGs and that deep reductions in these
pollutants are essential for mitigating climate change in the long run.
In conclusion, although there remains considerable uncertainty as to the magnitude
of the effect of BC on the climate, mounting scientific evidence suggests that reducing
current emissions of BC can provide near-term climate benefits, particularly for sensitive
regions such as the Arctic. Because of its strong warming potential and short
atmospheric lifetime, BC mitigation offers an opportunity to address key climate effects
and slow the rate of climate change (i.e., reducing the risk of crossing thresholds with
dramatic climate changes). Furthermore, currently available control technologies and
mitigation approaches have already been shown to be effective in reducing BC
emissions, often at quite reasonable costs. These mitigation approaches could be
utilized to achieve further BC reductions. LDVs are currently a minor source of BC
emissions compared to heavy-duty diesel engines. CARB anticipates that the stringency
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of the proposed amendments to the existing PM mass standard will result in reduced
BC emissions that can yield significant local and regional climate and health benefit.
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VII. References
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2-year aerosol measurements at six sites in Europe. Journal of Geophysical Research e
Atmospheres 112. http://publik.tuwien.ac.at/files/pub-tch 7878.pdf.
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absorption to black carbon, brown carbon, and dust in China -interpretations of atmospheric
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atmospheric aerosols Nature, 409, 695-697.
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Atmospheric Constituents and in Radiative Forcing, available at
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consumption in the world's regions, Geophys. Res. Lett., 34, L11802, doi:10.
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Technology-Based Global Inventory of Black and Organic Carbon Emissions from
Combustion; J. Geophys. Res. Atmos 109, D14203; doi: 10.1029/2003JD003697.
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13. Chow, J.C., J.G. Watson, D.H. Lowenthal, L.W.A. Chen, (2009). Climate Change—
Characterization of Black Carbon and Organic Carbon Air Pollution Emissions and
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Organic Carbon Source Emissions. California Air Resources Board (Contract No.04-307).
http://www.arb.ca.gov/research/apr/past/04-307 v2.pdf.
14. Bond, T.C. et al. (2007). Historical emissions of black and organic carbon aerosol from
energy-related combustion, 1850-2000, Global Biogeochem. Cycles, 21, GB2018,
doi:2010.1029/2006GB002840. http://www.saqe.wisc.edu/pubs/articles/M-
Z/Trautmann/BondetalGBC07.pdf
15. Stewart IT, Cayan D.R, Dettinger.M.(2005) Changes toward earlier streamflow timing across
western North America. J Clim 18:1136-1155.
http://tenaya.ucsd.edu/~dettinge/stewart timing.pdf
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United States. J Clim 19:4545-4559.
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17. Pierce DW, BarnettTP, Hidalgo H, Das T, Bonfils C, Santer B, Bala G, Dettinger M, Cayan
D, Mirin A, Wood A, Nozawa T (2008) Attribution of declining western U.S. snowpack to
human effects. J Clim 21:6425-6444.
http://tenava.ucsd.edu/~dettinge/swe over p attribution.pdf
18. Hansen J, Nazarenk L (2004) Soot forcing via snow and ice albedos. Proc Natl Acad Sci
101:423-428. http://www.pnas.Org/content/101/2/423.full.pdf+html
19. Vicuna, S., R. Leonardson, M. Hanemann, L. Dale, and J. Dracup. 2008. "Climate change
impact on high elevation hydropower generation in California's Sierra Nevada: A case study
in the upper American River." Climatic Change 87:S123-S137.
20. Waliser, D.et al. (2009) Simulating the Sierra Nevada snowpack: The impact of snow albedo
and multi-layer snow physics. California Environmental Protection Agency and California
Energy Commission Report CEC-500-2009-030-F.
http://www.energv.ca.gov/2009publications/CEC-500-2009-030/CEC-500-2009-03Q-F.PDF
21. Hadley, O. L., Corrigan, C. E., Kirchstetter, T. W., Cliff, S. S., and Ramanathan, V. (2010).
Measured black carbon deposition on the Sierra Nevada snow pack and implication for
snow pack retreat, Atmos. Chem. Phys., 10, 7505-7513, doi: 10.5194/acp-10-7505.
22. Coats, R., (2010). Climate change in the Tahoe basin: regional trends, impacts and drivers.
Climatic Change 102:435-466, DOI 10.1007/s10584-010-9828-3.
http://escholarship.org/uc/item/6d8945fb:isessionid=2E82C49ECFD9B1B34014A86494A7F
A1F
23. Hansen, J., M. Sato, P. Kharecha, G. Russell, D. Lea, and M. Sidall.(2007). "Climate change
and trace gases." Trans. R. Soc.. 1925, 1942.
http://pubs.giss.nasa.gov/docs/2007/2007 Hansen etal 2.pdf.
24. Bond, T. C. and H. Sun. 2005. "Can reducing black carbon emissions counteract global
warming?" Environmental Science and Technology 39:5921-5926.
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25. Jacobson, M. Z. (2007). Testimony for the Hearing on Black Carbon and Global warming,
House Committee on Oversight and Government Reform. See:
http://oversight.house.gov/documents/20071018110606.pdf.
26. Reddy, M. S., and O. Boucher (2007), Climate impact of black carbon emitted from energy
consumption in the world's regions, Geophys. Res. Lett., 34, L11802,
doi: 10.1029/2006GL028904. http://www.lmd.iussieu.fr/~obolmd/PDF/2006GL028904.pdf
27. Rypdal, K., N. Rive, T. Berntsen, Z. Klimont, T. Mideksa, G. Myhre, and Ragnhild Skeie
(2009). "Costs and Global impacts of black carbon abatement strategies." Tellus 61B, 625-
641. http://folk.uio.no/torbenm/central/Tellus 2009.pdf
28. Fuglestvedt, J.S. and Shine, K.P. and Berntsen, T. and Cook, J. and Lee, D.S. and Stenke,
A. and Skeie, R. and Velders, G.J.M. and Waitz, I.A. (2010) Transport impacts on
atmosphere and climate: Metrics. Atmospheric Environment, 44, pp. 4648-4677.
http://elib.dlr.de/68051/1/fugl-2010-4648.pdf.
29. Strawa, A.W.; Kirchstetter, T.W.; Hallar, A.G.; Ban-Weiss, G.A.; McLaughlin, J.P.; Harley,
R.A.; Lunden, M.M. (2010). Optical and Physical Properties of Primary On-Road Vehicle
Particle Emissions and Their Implications for Climate Change. Journal of Aerosol Science
41, 36-50.
30. Kirchstetter, et al. (2008) Black carbon concentrations and diesel vehicle emission factors
derived from coefficient of haze measurements in California, AE, 42(3): 480-491.
31. Bahadur R., Feng Y., Russell L. M., Ramanathan V. (2011). Impact of California's air
pollution laws on black carbon and their implications for direct radiative forcing. Atmospheric
Environment 45, 1162-1167.
32. Chow, J.C., Watson, J.G., Lowenthal, D.H., Antony Chen, L.-W., Motallebi, N. (2011) PM2.5
source profiles for black and organic carbon emission inventories. Atmospheric Environment
45, 5407-5414.
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The spatial distribution and extent of roadway emissions may vary based upon temporal factors, such
as time of day and season of the year. A study (Zhu et al. 2006) which compared ultrafine particle
numbers for daytime and nighttime conditions near a major freeway (1-405) in Los Angeles found
that the rate of decrease in ultrafine particles downwind of the freeway was much less at night than
during the day. Although traffic volume on 1-405 at night was only 25% of the daytime volume, the
particle count 30 meters downwind of the freeway was about 80% of the daytime value. The authors
attribute the higher ratio of particles to traffic volume at night to a combination of lower wind speed
and weaker atmospheric dilution, as well as cooler temperatures which cause increased particle
formation in the vehicle exhaust. The study also found that particle counts near the freeway were
higher in winter than in summer, for similar reasons to the factors that lead to higher particle counts
at night.
Dispersion is key to reducing ambient concentrations and exposure to PM. However, it is important to
note that some urban environments, such as tunnels and "urban street canyons", are not conducive
to dispersion of air pollutants. When emissions are trapped in enclosed areas, this can lead to much
higher local concentrations, and thus much higher population exposure. One study (Morwaska et
al. 2008) found that ultrafine particle numbers in the near-roadway environment were roughly 18
times higher than in a non-urban background environment, while measured concentrations in street
canyons and tunnels were 27 and 64 times higher, respectively, than background. Another study
(Zhou et al. 2008) found that, due to high population density, combined with the lack of dispersion,
the intake fraction of emissions in urban street canyons is very high, similar in magnitude to the
intake fraction associated with indoor tobacco smoke.
In-Vehicle Exposure
Concerns about elevated exposure to PM near major roadways also apply to drivers and
passengers traveling in vehicles on high-volume roads. In fact, the evidence suggests that in-
vehicle exposure may be a leading source of exposure to PM and other air pollutants for people
who drive on freeways or major arterials on a regular basis. In-vehicle exposure depends on the
volume and mix of vehicles on a given road, as well as the type of ventilation system used in the
vehicle. Moving vehicles typically have high air exchange rates, allowing emissions from the stream
of traffic to penetrate into vehicles. One study (Fruin et al. 2008) found that 36% of total daily
exposure to ultrafine particles occurred during a daily commute of 1.5 hours round trip (6% of the
day) in Los Angeles, and that 22% of total exposure occurred during 0.5 hours (just 2% of the day)
that was spend on freeways. This indicates that exposure rates may be 5 to 10 times higher than
average when driving on busy roadways. Thus, even limited time on a freeway can account for a
significant portion of total daily exposure to ultrafine particles.
Freeways are also where people are most likely to experience higher exposure to diesel PM, which
has been classified by the Air Resources Board as a toxic air contaminant. The 2008 Fruin study
found that on freeways in Los Angeles, concentrations of ultrafine PM, black carbon, nitric oxide, and
polycyclic aromatic hydrocarbons (PAH) bound to small particles are generated primarily by diesel-
powered vehicles, even though diesel vehicles account for only a small fraction (6%) of the traffic
on LA freeways. This study also found, however, that on arterial roads concentrations of ultrafine
DRAFT - UNDERSTANDING PARTICULATE MATTER I 2012 I Bay Area Air Quality Management
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particles appear to be emitted primarily by gasoline-powered vehicle undergoing hard accelerations.
Concentrations of ultrafine particles on arterials were roughly one-third those of freeways.
"Self-pollution", which occurs when the exhaust from a vehicle infiltrates its own passenger cabin,
may also contribute to in-vehicle exposure. This has raised concern about risks to children who ride
to school on diesel-powered buses. One study (Adar et al. 2008) found that PM2.5 on school buses
was double the on-road levels, and that 35% of PM2.5 measured in school buses came from self-
pollution. (See description of the Lower-Emission School Bus Program in Section 4 regarding actions
to address this issue.)
Aircraft and Airports
Studies conducted by the South Coast
AQMD suggest that jet aircraft may be
major emitters of ultrafine particles.
Typical ultrafine particle concentrations
are on the order of 50,000-200,000
particles per cm3 near freeways; by
contrast, ultrafine particle concentrations
near jet exhaust can reach 6,000,000
particles per cm3. As shown in Table 1-3.
Table 1-3 Comparison of Ultrafine Particle Concentrations4
Environment
Ultrafine Particle Concentration
Clean background
Typical urban air
Freeway
Jet exhaust
500 - 2,000 particles per cubic centimeter
5,000 - 30,000 particles per cc
50,000 - 200,000 particles per cc
Up to 6,000,000 particles per cc
A study (Hu et al. 2009) that measured ultrafine particles near the Santa Monica Airport, at the
residence closest to the airport, and at a nearby school showed correlations of ultrafine particle
concentrations from jet exhaust at all three locations. Aircraft operations resulted in average ultrafine
particle concentrations elevated by a factor of 10 at 100 meters downwind and by a factor of 2.5 at
660 meters downwind. In fact, the area impacted by elevated UFPM concentrations was found to
extend beyond 660m downwind and 250m perpendicular to the wind on the downwind side of the
4 Presentation by Dr. Philip Fine of South Coast AQMD to BAAQMD Advisory Council Meeting: Ultrafine Particles 2012 Atmospheric
Monitoring of Ultrafine Particles, February 2012.
DRAFT - UNDERSTANDING PARTICULATE MATTER I 2012 I Bay Area Air Quality Management
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Santa Monica Airport. This study demonstrated that there may be health implications for residences
living in proximity to jetports, especially in the downwind direction.
A study by Carnegie Mellon University researchers (Miracolo etal. 2011) evaluated the effects of
photo-oxidation on ultrafine PM emissions from a gas turbine engine designed to mimic a jet aircraft
engine. The study found that photo-oxidation created substantial secondary PM, suggesting that it
is also important to consider potential secondary PM formation when assessing the influence of jet
aircraft emissions.
Back-up Generators
Back-up generators (BUGs), also known as stationary engines and emergency generators, are
used frequently by hospitals, office buildings, schools, grocery stories, and government facilities to
supply power to a building during a power failure. While power failures are generally rare, BUGs are
operated several times a year for testing. Diesel BUGs emit diesel particulate matter and other toxic
air contaminants and may contribute significantly to people's exposure to toxics and health risks. In
addition, BUGs tend to be concentrated in populated areas, where high numbers of people may be
already exposed to high levels of pollution.
A new BUG installed today in the Bay Area poses little health risk during its operating testing hours
due to the Air District's and ARB's regulations. However, old BUGs that were installed prior to
regulations and continue to be in use today generate high levels of toxics and pose a serious health
risk challenge. Even though these BUGs may be used as little as 100-50 hours a year, they can emit
enormous amounts of diesel PM since their engines do not comply with any emission standards or
contain retrofit technologies. In the Air District's general screening of health risks for BUGs in the
Bay Area, the cancer risk for grandfathered BUGs ranges from 20 to 200 in a million in some cases.
There are close to 3,000 BUGs in the Bay Area, approximately 1,500 of which may have cancer risks
over 10 in a million. The majority of these BUGs are located in Bay Area urban centers. These BUGs
contribute heavily to health risks already experienced by people living near roadways and other
mobile emissions of diesel PM. The Air District's general health risk screening for stationary sources
indicates that addressing emissions from grandfathered back-up generators could significantly
reduce exposure to diesel PM, especially in urban areas with already high exposure rates.
Indoor Exposure to PM
Studies have found that most people experience a major portion of
their total PM exposure when they are indoors. This is not surprising,
since people spend the majority of their time indoors, in the home,
office, school, stores, restaurants, etc. According to one study (Qing
Yu Meng et al. 2005), adults typically spent 87% of their time indoors,
7% in vehicles, and just 6% outside. The PM that we breathe indoors
is a combination of ambient (outdoor) PM that penetrates to the
indoor environment, as well as PM emissions produced by indoor
sources.
to PM when they
major portion of
their exposure
Most people
experience a
are indoors.
DRAFT - UNDERSTANDING PARTICULATE MATTER I 2012 I Bay Area Air Quality Management
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Studies to date to measure indoor PM levels and population exposure have generally been limited
to small numbers of sites, because gaining access to suitable sites (private homes, schools, etc.),
installing monitors, and analyzing data requires substantial time and resources. Analyzing indoor
concentrations and exposures in multi-unit buildings, such as apartment buildings, is complicated
by the fact that PM created indoors can move between units, as well as the fact that heating and
ventilation systems, if not properly designed and maintained, can transfer pollutants between units.
Nonetheless, the findings of existing studies suggest that indoor exposure to PM is a serious issue
that merits more attention.
Factors that determine indoor exposure to PM include (1) the ambient (outdoor) PM concentration
in the vicinity of the building, 2) the infiltration rate: i.e., how much of the ambient outdoor PM
penetrates indoors, 3) the air exchange rate: how quickly indoor air is replaced by outdoor air, and
(4) the amount of primary PM emissions and PM precursors produced in the indoor environment from
sources such as cooking, wood-burning, and cigarette smoking. These factors can vary considerably
depending upon building type and location, the type of heating and ventilation system, and
meteorological conditions.
The infiltration rate of ambient (outdoor) PM to the indoor environment depends upon building
materials, characteristics, and design, such as the type of ventilation system, the location of air
intake units, whether windows are open or closed, and whether a building has air conditioning or an
air filtration system. The PM infiltration rate also varies upon the size and composition of the particles
present in the ambient PM. Because different sizes and types of particles have different infiltration
rates, the composition of PM in the indoor environment generally differs from the ambient outdoor
PM. Ammonium nitrate levels, for example, are generally higher outside than indoors. Ammonium
nitrate can exist in either particle or gaseous form in the atmosphere, depending upon temperature.
In colder weather, ammonium nitrate particles account for a sizable portion of total ambient PM2.5
in the Bay Area. However, when they encounter warmer air in the indoor environment, ammonium
nitrate particles generally volatize (convert to the gaseous form), such that they no longer exist in
particle form.
Ultrafine particles are less likely to penetrate through a building envelope because they deposit
more rapidly on building surfaces due to Brownian motion at the molecular level. Whereas typical
infiltration factors for PM10 and PM2.5 are in the range of 50%, (Ott et al. 2000), infiltration factors
for ultrafine particles are on the order of 30% (Wallace & Howard-Reed, 2002). Since ultrafine
particles do not easily penetrate to the indoors, this suggests that indoor sources of ultrafine particles
play an important role in determining total personal exposure to UFPM.
DRAFT - UNDERSTANDING PARTICULATE MATTER I 2012 I Bay Area Air Quality Management
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Chemicals Released from
Modern Building ft
Furnishing Materials
Cumbustion Gases
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Sources of PM in the
Home Environment
Although PM in outdoor air does
penetrate to the indoor environment,
particles generated within the home
often account for a substantial share
of indoor PM levels and exposure..
Indoor sources of PM include
fireplaces and wood stoves, cooking,
gas stoves, cleaning products,
cigarette smoking, candles, and
incense, laser printers, as well as
human activities that may re-suspend PM2.5, Indoor PM may also include a mixture of dander from
pets, other types of allergens, chemical substances, mineral particulate, mold spores, viruses and
bacteria. The RIOPA study (Polidari, A. et al. 2006) found that fine organic particles dominates indoor-
generated PM2.5 in the homes that were studied. Indoor sources of PM can cause PM levels to
spike, especially because the emissions are often retained within a confined area. Several of the key
sources of PM generated in the home environment are briefly described below.
Animal Hair & Dander
Carbon Monoxide Fumes
from attached garage
Cooking: Studies have found that cooking is a leading source
of ultrafine particles in many homes. Indoor monitors show
that ultrafine particle counts spike whenever cooking occurs.
Studies suggest that emissions of UFPM are higher from natural
gas stoves than from electric stoves, but the particle emission
rates are high in both cases. Ultrafine particle levels tend to be
significantly higher in homes with gas stoves that use a pilot
light (compared to pilot-less
stoves). Emission rates when
the oven is in use may be
greater than for stove-top
cooking.. One study found that the indoor concentration of ultrafine
particles jumped from 5,000 particles per cubic centimeter to 1
million particles - a 200-fold increase - within a few minutes after
the oven in a residential kitchen was turned on.5
Fine particle emissions
in one hour.
15-30
grams
2-7
grams
Certified sieves are 50% more energy
efficient than n on-certified stoves
March 9, 2011.
Old, ineffioent stove
EPA certified stove
Wood-burning devices: People are exposed to wood smoke in
both indoor and outdoor environments, in addition to its negative
impact on outdoor air quality, residential wood-burning can be a
major source of indoor PM, especially if the chimney or stovepipe
does not vent smoke to the outdoors effectively. This problem
occurs most commonly when a fire is first ignited and the fireplace
flue is not warmed up, thus failing to draw smoke efficiently. One
5 Presentation by Susanne Hering, Ph. D., of Aerosol Dynamics to BAAQMD Advisory Council on
DRAFT - UNDERSTANDING PARTICULATE MATTER I 2012 I Bay Area Air Quality Management
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study (Pierson et al. 1989) found that 70% of smoke from chimneys can reenter the home where it
originated and/or neighboring dwellings.
Appliances: Common household appliances, such as clothes dryers, toaster ovens, irons, and laser
printers can also produce ultrafine particles, especially appliances that operate by heating metal
surfaces.
Cleaning products: Household cleaning products can also produce ultrafine and fine particles in
the indoor environment. Scented cleaning products contain terpenes such as pinene (pine scent) and
limonene (citrus scent); these terpenes can react with ozone to form ultrafine particles.
Contribution of Indoor Exposure to Total PM Exposure
Lance Wallace and Wayne Ott have done pioneering work using portable particle counters to measure
personal exposure to ultrafine particles. In one of their recent studies (Wallace & Ott, 2010) using
personal monitors to measure exposures in environments such as homes, cars, and restaurants, they
estimated that, on average, 47% of daily personal exposure to ultrafine particles for the participants
in the study can be attributed to indoor sources, 36% to outdoor sources, and 17% to in-vehicle
exposure. Consistent with the SHEDS-PM estimates for PM2.5 described below, cooking and cigarette
smokingwere the dominant sources of indoor emission of UFPM. In households with one or more
smokers, the cigarette smoke more than doubled the exposure from all other sources. By measuring
the particle count per cubic centimeter (cm3) and multiplying this by the size of the impacted indoor
area, this study estimates that smoking a single cigarette emits approximately 2 trillion (2 x 1012)
ultra-fine particles.
Lynn Hildebrand at Stanford University and William Nazaroff at UC Berkeley have also done important
research to advance our understanding of exposure to PM in various micro-environments. A recent
study directed by Professor Nazaroff (Bhangar at al. 2011) monitored ultrafine particle concentrations
and exposures in seven residences (with non-smoking inhabitants) in urban and suburban Alameda
County. This study provides several findings of interest:
• Ultrafine particle concentrations in the home environment are heavily
impacted by episodic indoor source events that cause sharp spikes in
particle counts. These events are triggered by activities such as cooking on
the stove; uses of appliances such as toaster ovens, steam irons, or clothes
dryers; burning candles; and use of the furnace.
• Frequency of use of the cooking range (either gas or electric) is the single
most important determinant of exposure from episodic indoor sources.
• Gas stoves with pilot lights are a key source of indoor emissions and
exposures to ultrafine particles.
• Indoor particle counts are much higher when occupants are at home and
active (thus generating particles via indoor source events), compared to
when they are away from home, or at home but asleep.
DRAFT - UNDERSTANDING PARTICULATE MATTER I 2012 I Bay Area Air Quality Management
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• Emissions from indoor sources of ultrafine particles accounted for roughly
60% of the indoor particles; the remaining 40% represent particles that
infiltrated from outdoor air.
• Active particle removal systems can reduce indoor particle levels (of both
particles generated indoors, as well as particles that infiltrate from outdoors)
by a factor of 2 to 4.
The papers cited above analyzed personal exposure to PM at the individual level. Efforts have also
been made to estimate the major sources of aggregate population exposure to PM in various urban
areas. Many of these studies have employed the Stochastic Human Exposure & Dosage Simulation
for PM (SHEDS-PM) model developed by the US EPA National Exposure Research Laboratory.
Synthesizing data from many sources, including personal activity logs, ambient PM2.5 concentrations
for outdoor air, and results from studies of indoor PM, the SHEDS-PM model has been used to
estimate the contribution of outdoor exposure and indoor exposure to total population exposure, and
to examine the role of key indoor sources of PM2.5 such as cigarette smoking and cooking.
An analysis (Burke et al. 2001) using SHEDS-PM for Philadelphia found that, on average, ambient
(outdoor) PM2.5 accounted for only 37.5% of total exposure; however, this percentage varied greatly
within the population. The study found relatively low variation in personal exposure to ambient
(outdoor) PM2.5; however, exposure to PM in the indoor environment varied greatly, with high
levels of indoor exposure caused primarily by emissions from cigarette smoking and/or cooking.
Another study (Cao & Frey, 2011) had similar findings, using SHEDS-PM to analyze and compare
PM exposures in three different areas and climate zones (New York City; Harris County, Texas; and
six counties along the 1-40 corridor in North Carolina). This study found that ambient exposure
accounted for approximately 40% of the estimated total daily average PM2.5 exposure in each of the
three areas. As in the case of the Burke study of Philadelphia, the Cao study also found that some
individuals have extremely high PM exposures, primarily due to indoor emissions from cigarette
smoking and/or cooking.
The Relationship of Indoor, Outdoor and Personal Air (RIOPA) study (Polidari et al. 2006) investigated
residential indoor, outdoor and personal exposures to PM2.5 in three cities with different climates:
Houston, TX; Los Angeles, CA; and Elizabeth, NJ. The study found that the median contribution of
ambient (outdoor) sources to indoor PM2.5 concentrations was 56% for all study homes (63%, 52%
and 33% for California, New Jersey and Texas study homes, respectively).
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Exposure to PM in Schools
Another recent study directed by Professor Nazaroff (Muilen et al. 2011) measured PM concentrations
in six elementary school classrooms in Alameda County; measurements were performed for a total
of 18 days (from 2-4 days in each classroom). None of the schools was in close proximity to a major
freeway; distance from the nearest freeway was 0.5 km or greater in all cases. Two of the classrooms
were equipped with mechanical ventilation systems; the other four used natural ventilation (windows
and doors that open). The study found that (1) indoor particle counts were typically about half of the
outdoor concentrations, and (2) roughly 90% of the ultrafine particles measured in the classrooms
originated outdoors. The authors compared exposure in the classrooms to exposure in the homes
(per Bhangar 2011), noting that the results suggest that elementary school students are subject
to much greater overall exposure to ultrafine particles in the home environment, because in-home
particle counts are higher and because the students spend more time at home than at school. The
authors attribute the difference in concentrations to the fact that fewer ultrafine particles are emitted
in classrooms than in homes. In particular, indoor source events, such as cooking, that lead to sharp
spikes in IJF particle levels, are common in the home, but much less prevalent in the school setting.
Summary of Indoor Population Exposure to PM
Key findings regarding indoor exposure to PM can be summarized as follows:
• Ambient contribution to indoor PM exposure depends on outdoor
concentrations in combination with the infiltration rate.
• When indoor sources are present, indoor PM concentrations can be
substantially higher than outdoor PM concentrations.
• Indoor PM emissions are generated primarily by specific activities and
sources: cooking, cleaning, ironing clothes, burning candles, use of forced-air
furnaces, fireplaces, etc.
• PM levels in the home are characterized by sharp spikes triggered by the
types of activities mentioned above.
• Ventilation to control PM spikes can greatly reduce indoor concentrations
and population exposure.
• PM concentrations in the home are generally much lower at night (when
people are sleeping, and PM-generating activities are not occurring) than
when people are at home and active.
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Occupational Exposure
Exposure to PM and other pollutants on the
job is regulated by the Occupational Safety &
Health Administration (OSHA). Occupational
exposure to PM may differ from ambient
exposure in terms of particle type and
composition, as well as the intensity, frequency,
and duration of exposure. Certain job types
may expose workers to significant occupational
exposures. For example, truck drivers and
other people who drive a lot on the job may be
exposed to higher levels of PM from both diesel and gasoline vehicles. Restaurant workers may
be exposed to PM from cooking and wood smoke from charbroilers. Construction workers and
quarry workers may be exposed to diesel PM, as well as to geologic dust particles from mechanical
processes. Firefighters, especially those who combat wildfires, may be subject to extremely high
acute exposures to PM. Janitorial workers may be exposed to high levels of PM in the indoor
environment when they use cleaning products that contain chemicals which react with ambient
ozone to form PM. Researchers (Morawska et al. 2007) have founds that people who work in office
buildings may be exposed to PM (as well as VOCs) from printers.
Brigham and Woman's Health Hospital conducted a study (Laden etal. 2007) of mortality patterns
associated with job-specific exposure to fine particulate and especially particulate matter from
vehicle exhaust. They examined rates of cause-specific mortality and compared this to the general
population. This study concluded that in the U.S. trucking industry there was an excess of mortality
due to lung cancer and heart disease particularly among drivers.
Summary
Population exposure to PM is heavily dependent on individual activity patterns and the types of PM
emissions sources that people are exposed to in the course of their day-to-day activities. PM levels,
and population exposure to PM, may be greatly elevated in certain micro-environments, such as in-
vehicle, near-roadway, and in the home.
The key to avoiding negative health impacts from PM is to reduce population exposure to PM among
Bay Area residents. Recognizing the importance of reducing population exposure to air pollutants, the
Air District has been working to identify areas that are disproportionately impacted and implementing
policies and programs to protect these communities, as described in Section 4.
But to better protect public health, we need to improve our understanding of population exposure to
PM in the Bay Area. Future steps to enhance our understanding of population exposure to PM are
discussed in Section 5.
Simple steps that Bay Area residents can take to reduce their exposure to PM in the course of their
day-to-day activities are also described in Section 5.
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SECTION 1-C: PM AND CLIMATE CHANGE
This section describes the complex interplay between particulate matter and climate change,
including how PM affects climate, as well as how higher temperatures due to climate change
may impact local PM levels.
Although more work is needed to fully discern the connections, research reveals a two-way
relationship in which air pollutants impact the climate at both the local and global scale, while
changes in climate impact air quality. Most discussion has focused on the need to reduce emissions
of carbon dioxide and other greenhouse gases, but researchers have found that particulate matter
also affects the climate, especially the type of PM known as black carbon.
How PM Affects Climate Change
The thin atmosphere that surrounds the Earth enables our planet to support
ecosystems that sustain us. There is irrefutable scientific evidence
that the Earth's atmosphere is getting hotter, and that a wide range
of human activities, such as combustion of fossil fuels, emit carbon
dioxide (C02) and other greenhouse gases (GHG) that are building up
in the atmosphere and changing the climate at the global scale. The
effects of this man-made global heating are already being experienced
in California and on a global basis in terms of temperature trends,
extreme weather events (e.g., drought, frequency and intensity of
hurricanes and cyclones), sea-level rise, changes in precipitation
patterns, the frequency and intensity of wildfires, changes in habitat for
flora and fauna, etc.
life and the complex
Certain types of
PM, especially
black carbon, can
have a potent
effect in heating
the climate.
Efforts to date to protect the climate have focused primarily on reducing
man-made emissions of GHGs that trap solar radiation (heat) that would
normally escape back into space. Reducing emissions of C02 has been the main focus of climate
protection efforts to date, because on a mass basis emissions of C02 dwarf the other GHGs, and
because C02 remains in the atmosphere for a very longtime.
However, in recent years researchers have discovered that other short-lived air pollutants, including
particulate matter and tropospheric ozone, also affect the climate. Although the effects are complex,
there is evidence that certain types of particulate matter, especially black carbon, can have a
potent effect in heatingthe climate at both the local scale (in the area where PM is emitted) and the
global scale. In response to this research, there is a growing recognition that we need to incorporate
strategies to reduce emissions of black carbon into climate protections efforts. Reducing black
carbon can help to slow the rate of atmospheric heating in the near-term, while also protecting
air quality and public health. Emission control opportunities that provide co-benefits in terms of
protecting both air quality and the climate are highly desirable from the policy perspective.
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Climate Forcing
Climate change is primarily caused by man-made activities that impact the Earth's energy balance
(Denman et al. IPCC, 2007). Energy constantly flows to the Earth in the form of sunlight and other
forms of solar radiation. Some of this solar energy is reflected back into space, and the rest is
absorbed by the planet and stored in the atmosphere, as well as in oceans, forests, etc. Factors
external to the natural energy system - so-called external forcings - can disturb the Earth's energy
balance. These external forcings can be positive or negative. Positive forcers, such as carbon dioxide,
methane, and other greenhouse gases, cause more of the sun's energy to be retained by the planet.
In contrast, negative forcers, such as volcanic dust that reflects sunlight back into space, cause less
of the sun's energy to be retained by the planet. The overall impact of human activities on the climate
depends upon the net sum of positive and negative forcings caused by a wide spectrum of man-made
activities, including emissions of GHGs and other air pollutants, agriculture and forestry practices,
land development and road-paving that affect the reflectivity (albedo) of the Earth's surface.
Climate Forcing Effects of Particulate Matter (PM)
Particulate matter is composed of solid or liquid particles that are suspended in the air; these
particles are sometimes referred to as atmospheric aerosols. Fine particles affect the climate
by means of several direct and indirect processes, some of which heat, and others of which cool,
the climate. All PM in the atmosphere can affect the Earth's climate either by absorbing light or by
scattering light. Particles that absorb sunlight add energy to the earth's system; they act as positive
forcers that lead to climate heating. Particles that scatter light increase the reflection of incoming
sunlight back to space; they serve as negative forcers that cool the climate. In addition to the direct
effect caused by absorbing or scattering incoming sunlight, fine particles may also have indirect
effects on the climate by altering the properties of clouds in various ways.6 More analysis is needed
to fully define the impacts of particles on clouds, but researchers have noted various different
processes by which aerosols can affect the reflectivity and lifespan of clouds, in ways that can have
both heating and cooling effects, as further describe below. (The 2007 IPCC report discusses five
processes; Jacobson 2002 lists 12 processes.)
For purposes of analyzing the impacts of PM on climate, scientists have identified several types of
carbon: black carbon, brown carbon, and organic carbon. The effect of primary (directly-emitted)
PM on sunlight spans a continuum from light-absorbing to light-scattering, with black carbon at the
light-absorbing end of the spectrum, most organic carbon at the opposite, light-scattering end of
the spectrum, and brown carbon (a subset of organic carbon) somewhere in the middle. The ratio
of black carbon, brown carbon and organic carbon produced by fuel combustion depends upon the
specific fuel being burned and the type of combustion conditions. PM emitted by diesel engines is
primarily black carbon, whereas the PM emitted by gasoline engines is mostly organic carbon.
Table 1-4 lists the most significant types of anthropogenic (man-made) aerosol particles in terms of
impact on the climate, and their most common sources. At the global scale, the dominant negative
6 "Atmospheric Aerosol Properties and Climate Impacts" U.S. Climate Change Science Program Synthesis and Assessment Product 2.3;
January 2009.
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forcing agent is sulfate,7 whereas the dominant particle as a positive forcing agent is black carbon.
But organic carbon, brown carbon, and ammonium nitrate also affect the climate in various ways
that can have both heating and cooling effects. In the Bay Area, ammonium nitrate levels are
greater than sulfates.
Table 1-4 Climate-Forcing Properties of PM Components
Negative
Forcer
(Cooling
Agent)
Positive
Forcer
(Heating
Agent)
Direct Effect
Sulfates
Reflects
sunlight
Increases reflectivity of clouds
Secondary PM formed
by S02 emissions from
fossil fuel-burning
Ammonium nitrate
Reflects
sunlight
Increase reflectivity of clouds
Secondary PM formed
by combination of
NOx and ammonia
emissions.
Black carbon
1) Reduces reflectivity of clouds;
Absorbs impacts cloud formation,
sunlight 2) Heats snow & ice by reducing
their reflectivity in polar regions.
Incomplete combustion
of fossil fuels, biofuels,
and biomass (wood-
burning)
Brown carbon
Incomplete combustion
of fossil fuels, biofuels,
and biomass (wood-
burning)
Organic carbon
Absorbs
some
wavelengths
of sunlight
Incomplete combustion
Mildly
. of fossil fuels, biofuels,
absorbs ...
and biomass (wood-
sunlight ,
burning)
The various particle types are never emitted into the atmosphere in isolation. The emissions
produced by a given combustion process or event contain a mixture of black carbon, brown carbon,
7 Text from NASA Fact Sheet "While a large fraction of human-made aerosols come in the form of smoke from burning tropical forests, the
major component comes in the form of sulfate aerosols created by the burning of coal and oil. The concentration of human-made sulfate
aerosols in the atmosphere has grown rapidly since the start of the industrial revolution. At current production levels, human-made sulfate
aerosols are thought to outweigh the naturally produced sulfate aerosols."
http://www.nasa.gov/centers/langley/news/factsheets/Aerosols.html
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San Francisco Health Code Article 38
Model Ordinance for Cities: San Francisco is the first jurisdiction in the country to create a law,
known as Article 38, to protect future residents from exposure to roadway air pollution. The law will
prevent avoidable lung disease and premature death in residents living near busy roadways, as well
as prevent avoidable health care spending, for example, on hospital charges for prevented asthma
attacks.
San Francisco Health Code Article 38, adopted in 2008, requires residential projects with more than
10 units located in "Potential Roadway Exposure Zones" (as defined according to maps provided
by the San Francisco Department of Public Health) to prepare an air quality assessment, using
modeling tools, to determine whether residents would be exposed to unhealthy levels of PM2.5. The
Department of Public Health has defined "unhealthy" levels of PM2.5 as roadway concentrations
greater than 0.2 |jg/m3. If the air quality assessment indicates that the roadway-attributable PM2.5
would be less than 0.2 |Jg/m3, then no further action is required. If the air quality assessment for
the residential project indicates that concentrations would be unhealthy, then the project is required
to mitigate the traffic-related PM2.5 pollutants, using available technology and design features, to
reduce or remove at least 80% of the ambient PM2.5 from indoor spaces.
Meeting the performance standard can be accomplished in several ways, including:
1. Designating lower floors for commercial use and upper for residential use;
2. Setback of buildings from roadway air pollution sources;
3. Locating the intake for fresh air ventilation sources at a non-polluted site;
4. Filtration of fresh air ventilation sources; and/or
5. Recirculation and filtration of indoor air.
Economic Impacts: The City/County of San Francisco's Office of the Controller has determined
that the economic impacts of Article 38 on the San Francisco economy, the development
community, and future residents of the City are neutral to positive. Although there is a cost
associated with implementation of the mitigation measures described above, Article 38 will
also prevent avoidable health care spending (for example, hospital charges for emergency room
visits for asthma attack) and help to prevent premature mortality associated with exposure
to PM. If using a filtration system, the City estimates that costs to install and maintain the
system will range from approximately $50-700 per year per unit, while the monetary benefit
of the reduction of premature death is estimated to be approximately $2,100 per unit per
year. On the basis of this analysis, if installation of a filtration system is required in order
to comply with the requirements of Article 38, then the Controller has determined that the
net economic benefit of Article 38 would be approximately $1,400 per unit per year.
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The Air District has also been providing technical assistance to help the City of San Jose develop
a CRRP. The Air District is preparing city-wide emissions inventory for on-road mobile sources on
freeways and surface streets, permitted stationary sources, and railroads, airports, and construction
projects. Initial air dispersion modeling is underway. The City has also engaged in public outreach, in
partnership with the Air District. As a first step on the policy side, the City included several policies
in its 2011 General Plan update to analyze and mitigate population exposure from major emissions
sources. For example, the air quality section of the General Plan includes policies which (1) require
completion of air quality modeling for sensitive land uses such as new residential developments
located near emission sources such as freeways and industrial uses; (2) require new residential
development projects and projects characterized as sensitive receptors to incorporate effective
mitigation into project designs or to be located an adequate distance from sources of toxic air
contaminants to avoid significant health risks; and (3) require projects that would emit toxic air
contaminants to prepare health risk assessments as part of environmental review and employ
effective mitigation to reduce possible health risks to a less-than-significant level. In addition, the
General Plan policies mentioned above encourage the use of air filtration devices in existingschools,
houses and other sensitive land uses; re-designation of truck routes; and the use of vegetative
buffers between emission sources and sensitive receptors.
Promoting Healthy Focused Development
Continued growth in motor vehicle travel could erode the air quality benefits from the ARB and Air
District programs described above. We need to better integrate land use, transportation, and air
quality planning in order to constrain future increases in vehicle travel and emissions. Therefore,
the Air District supports the effort to focus future development in the Bay Area in areas where public
transit, biking and walking are viable transportation options. At the same time, however, many of
the areas identified as good sites for focused growth already experience high concentrations of air
pollutants due to emissions from existing local sources. In fact, a comparison of areas that have been
designated as Priority Development Areas (PDAs) to date and the impacted communities identified
by the Air District's CARE program shows that there is considerable overlap. This emphasizes that
we need to plan for focused growth in a way that protects people from exposure to air pollutants,
especially local pollutants such as PM and air toxics. To address this issue, the Air District is
committing its resources to help planning agencies (cities, counties, MTC, and ABAG) identify,
evaluate and mitigate these impacts through the planning and design processes.
The Air District is working actively with partners at both regional and local agencies to support
focused development to reduce motor vehicle emissions, while ensuring that development is planned
and designed so as to minimize public exposure to air pollutants and protect public health.
At the regional scale, the Air District is engaged with its regional agency partners in the effort to
develop Plan Bay Area. Plan Bay Area, scheduled for adoption in 2013, will update the Regional
Transportation Plan (RTP) and incorporate a Sustainable Communities Strategy to better integrate
land use and transportation planning, in response to the requirements of Senate Bill (SB) 375.
Although SB 375 requirements focus on the need to reduce emissions of greenhouse gases, the Air
District worked with its regional agency partners to make sure that the performance targets for Plan
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EXECUTIVE SUMMARY
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MATES III
Final Report
Executive Summary
The Multiple Air Toxics Exposure Study III (MATES III) is a monitoring and evaluation study
conducted in the South Coast Air Basin (Basin). The study is a follow on to previous air toxics
studies in the Basin and is part of the South Coast Air Quality Management District (SCAQMD)
Governing Board Environmental Justice Initiative.
The MATES III Study consists of several elements. These include a monitoring program, an
updated emissions inventory of toxic air contaminants, and a modeling effort to characterize risk
across the Basin. The study focuses on the carcinogenic risk from exposure to air toxics. It does
not estimate mortality or other health effects from particulate exposures. The latter analysis was
conducted as part of the 2007 Air Quality Management Plan and is not included here.
A network of ten fixed sites was used to monitor toxic air contaminants once every three days for
two years. The location of the sites was the same as in the previous MATES II Study to provide
comparisons over time. The one exception is the West Long Beach site, which was about 2.5
miles east of the Wilmington location used in MATES II. The locations of the sites are shown in
Figure ES-1.
The initial scope of the monitoring was for a one-year period from April 2004 through March
2005. Due to the heavy rains in the Basin in the fall and winter of this period, there was concern
that the measurements may not be reflective of typical meteorology. The study was thus
extended for a second year from April 2005 through March 2006.
In addition to the fixed sites, five additional locations were monitored for periods of several
months using moveable monitoring platforms. These microscale sites were chosen to determine
if there were gradients between communities that would not be picked up by the fixed locations.
The study also included an update of the toxics emissions inventories for the Basin and computer
modeling to estimate toxics levels throughout the Basin. This allows estimates of air toxics risks
in all areas of the Basin, as it is not feasible to conduct monitoring in all areas.
To provide technical guidance in the design of the study, a Technical Advisory Group was
formed. The panel of experts from academia, environmental groups, industry, and public
agencies provided valuable insights on the study design. Components of the study recommended
by the Advisory Group included monitoring for longer periods at the microscale sites, including
naphthalene in the monitoring program, and including more up-to-date methods to estimate the
contribution of diesel exhaust to ambient particulate levels. In the monitoring program, over 30
air pollutants were measured. These are listed in Table ES-1. These included both gaseous and
particulate air toxics.
The monitored and modeled concentrations of air toxics were then used to estimate the
carcinogenic risks from ambient levels. Annual average concentrations were used to estimate a
lifetime risk from exposure to these levels, consistent with guidelines established by the Office
of Environmental Health Hazard Assessment (OEHHA) of the California Environmental
Protection Agency (EPA).
ES-1
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MATES III
Final Report
Table ES-1 Substances Measured in MATES III
Benzene
1,3-Butadiene
Carbon Tetrachloride
Chloroform
Dichlorobenzene
Methylene Chloride
MTBE
Perchl oroethyl ene
(T etrachl oroethyl ene)
Dichloroethane
Dibromoethane
Ethyl Benzene
Toluene
T ri chl oroethyl ene
Xylene
Styrene
Vinyl Chloride
Acetaldehyde
Formaldehyde
Acetone
Methyl ethyl ketone
Arsenic
Cadmium
Hexavalent Chromium
Copper
Lead
Manganese
Nickel
Selenium
Zinc
Elemental Carbon
Organic Carbon
Naphthalene
PAHs
PMio
PM2.5
To assess the potential carcinogenic risk, at least one full year of data is preferred to represent
exposure potential. Thus, the fixed site data was used to calculate risk estimates and the
microscale sites used solely to determine any gradients compared to the nearest fixed monitoring
site. To estimate the risks from the fixed sites, the concentrations measured over each of the two
years were averaged to estimate exposure. The Huntington Park and Pico Rivera sites did not
have a full year of data for the second year of the study; thus, only the first year of data was used
for these two sites.
In the MATES II Study, elemental carbon (EC) was used as a surrogate for diesel particulate
levels, as staff determined that this was the best method available during the MATES II Study.
For the present study, staff used the Chemical Mass Balance (CMB) source apportionment
technique to estimate the contribution from diesel, as well as from other major source categories,
to the measured particulate levels.
Key results of the study are presented below.
Fixed Site Monitoring
The carcinogenic risk from air toxics in the Basin, based on the average concentrations at the
fixed monitoring sites, is about 1,200 per million. This risk refers to the expected number of
additional cancers in a population of one million individuals that are exposed over a 70-year
lifetime. Using the MATES III methodology, about 94% of the risk is attributed to emissions
associated with mobile sources, and about 6% of the risk is attributed to toxics emitted from
stationary sources, which include industries, and businesses such as dry cleaners and chrome
plating operations. The average risks from the annual average levels of air toxics calculated
from the fixed monitoring sites data are shown in Figure ES-2.
The air toxics risk at the fixed sites ranged from 870 to 1,400 per million. The risk by site
averaged over the two study years is depicted in Figure ES-3. For the second year of the study, a
full year of data was not collected at two of the sites (the Huntington Park site access was not
ES-2
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MATES III
Final Report
extended for the second year; and the Pico Rivera site was moved during the second year
resulting in several months without data). The second year data include results for only eight
sites. Sites with higher levels of risk include Burbank, Central Los Angeles, Inland Valley San
Bernardino, Huntington Park, and West Long Beach. The site with the lowest risk is Anaheim.
The results indicate that diesel exhaust is the major contributor to air toxics risk, accounting on
average for about 84% of the total.
To compare different methods used to estimate diesel particulate levels, the method used in
MATES II, which was based on the emissions ratios of diesel particulate and elemental carbon
from a study conducted in the South Coast in the 1980's, and a method based on the ratio of
PM2.5 emissions from the 2005 emissions inventory were both calculated. For MATES II, the
PMio elemental carbon levels were multiplied by 1.04 to estimate diesel particulate. The 2005
PM2.5 inventory finds a ratio of diesel particulate to elemental carbon emissions of 1.95.
Multiplying the PM2.5 elemental carbon levels by the 1.95 ratio gives another estimate of diesel
particulate. The estimates using these methods compared to using the CMB model are shown in
Table ES-2. Should one use the same diesel particulate estimation methodology as MATES II,
there is about a 30% reduction in ambient levels between the two studies. Based on comparisons
of the three methods to estimate diesel particulate, the method used for MATES II gives the
lowest estimates of ambient diesel particulate.
For the CMB model, the estimates were sensitive to the species profile used for gasoline
vehicles. Table ES-2 shows the range of values using two different gasoline profiles. The
estimates used for the risk calculations were the midpoint of the range. As shown in the table,
both the CMB model and the PM2.5 emissions ratio from the 2005 emissions inventory method
give similar estimates, and both are higher than the MATES II method. Thus the MATES II
Study method is likely underestimating the levels of diesel particulate.
Table ES-2 CMB Estimate of Diesel Particulate Compared to Emissions Inventory Ratio
Methods.
Estimation Method
MATES III
Diesel PM
|lg/m3
MATES II Method:
PM10 EC x 1.04
2.16
2005 Inventory Method:
PM2.5 EC x 1.95
3.5
CMB Method
3.20-3.49
Note: Year 2 includes data for eight sites only. The MATES II diesel particulate was estimated
at 3.4 |lg/m3.
ES-3
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MATES III
Final Report
Modeling
Several updates to the modeling platform were included in this study compared to MATES II.
The model used was the Comprehensive Air Quality Model with Extensions (CAMx). This
model is consistent with that used in the 2007 Air Quality Management Plan. A grid size of 2
kilometers was used.
In addition to using an updated air toxics emissions inventory, an improved geographical
allocation of diesel emissions was employed.
The modeling results are shown in Figure ES-4. The grid cell with the highest air toxics risk was
at the ports. The grid cells near the ports ranged from about 1,100 to 3,700 in a million. In
addition to the ports, an area of elevated risk is shown near the Central Los Angeles area with
grid cells ranging from about 1,400 to 1,900 per million. There are also higher levels of risk that
track transportation corridors and freeways.
Since the modeling platform and emissions inventory methods are different in MATES III than
those used in MATES II, the CAMx model was applied to the MATES II time frame for a more
"apples to apples" assessment. The MATES III methodology was also used to back-cast the
estimates of air toxics emissions for the MATES II timeframe. Comparing the results, a lesser
level of carcinogenic risk was estimated across the Basin for MATES III compared to the
MATES II time period. The model also shows the dominant contribution from mobile sources
and diesel emissions to air toxics risk in the MATES II timeframe as well.
For comparison purposes, Table ES-3 shows the estimated population weighted risk across the
Basin for the MATES III and MATES II periods. The population weighted risk was about 8%
lower compared to the MATES II period.
The MATES III modeling analysis represents several improvements over that used in MATES II
and represents the state-of-science application of regional modeling tools and chemistry applied
to an updated set of meteorological and emissions data input.
Table ES-3 Modeled Air Toxics Risk Comparisons Using the CAMx Model
MATES III
MATES II
Change
Population
weighted risk
(per million)
853
931
-8%
Figure ES-5 depicts the 1998-99 to 2005 change in air toxics risk for each model grid cell
estimated from the CAMx simulations. Overall, air toxics risk improves to varying levels in
most of the Basin with the exceptions of the areas directly downwind of the ports and those areas
heavily impacted by activities associated with goods movement. The model comparison shows
an increase in air toxics risk occurred in the immediate areas encompassing the ports of more
ES-4
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MATES III
Final Report
than 800 in a million between the two periods. This increase correlates with the increased
container cargo moving through the ports and increases in goods movement that occurred
between the MATES II and MATES III time periods.
Noncancer Assessment
To assess the potential for noncancer health risks, the monitored average levels were compared
to the Chronic Reference Exposure Levels (RELs) established by OEHHA. The chronic REL is
the air concentration at or below which adverse noncancer health effects would not be expected
in the general population with exposure for at least a significant fraction of a lifetime. In
general, the measured concentrations of air toxics were below the RELs.
The exception is formaldehyde. The chronic REL is 3 |ig/m3 (2ppb). All of the fixed site
annual averages were above this concentration, ranging from 2.9 ppb for Anaheim to 4.5 ppb at
Los Angeles. Formaldehyde effects include eye irritation, injury to nasal tissue, and respiratory
discomfort. OEHHA, however, is proposing revisions to the RELs for several toxic air
contaminants. For formaldehyde, the proposed chronic REL is 9 |ig/m3 (7 ppb). If the proposed
level is promulgated, then all sites would be under the chronic REL.
Caveats and Uncertainty
One source of uncertainty is that currently there is no technique to directly measure diesel
particulates, the major contributor to risk in this study, so indirect estimates based on
components of diesel exhaust must be used. The method chosen to estimate diesel particulate is
the CMB source apportionment model. This method is a weighted multiple linear regression
model based on mass balance of each chemical species applied to apportion contributions to
ambient particulates using measured source profiles. The CMB method accounts for major
source categories and geographic differences in source contributions and was recommended by
the Technical Advisory Group. It is staff s judgment that this is the most appropriate method to
estimate the ambient levels of diesel particulate matter.
The MATES II Study used elemental carbon as a surrogate for diesel particulate. Elemental
carbon, however, is not a unique tracer for diesel, as there are additional emission sources of
elemental carbon. Using the CMB model takes advantage of the specific profile of chemical
species emitted from different particulate matter sources. Twenty-three species were used in the
CMB model to reconcile source contributions to observed ambient concentrations. This results
in a more robust apportionment of source contributions to ambient particulate matter levels, since
all major sources of particulate matter and elemental carbon are considered.
The CMB model uses the profile of chemical tracer chemical species from different source
categories to estimate the contribution to ambient particulates. Some tracers are unique to a
given source, such as levoglucosan from biomass burning, whereas other sources show specific
chemical profiles that can be used to apportion these sources, such as gasoline and diesel
combustion. The advantage of the CMB model is that it can apportion several sources to
ambient levels. Additional discussion is provided in Chapter 2 and Appendix VII on the CMB
methodology.
ES-5
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MATES III
Final Report
The Positive Matrix Factorization (PMF) model was also evaluated for estimation of diesel
particulate. The PMF model is an alternating least squares method that estimates source profiles
and source contributions from the ambient data. Since possible solutions to this model can be
negative, the procedure uses restrictive functions so that no sample can have a negative source
contribution and no species can have a negative fraction in any source profile. Estimated source
profiles are then attributed to specific sources using experienced judgment. However, using the
MATES III data, the initial attempts at source apportionment found that some source profiles
could not be interpreted, and some profiles could not be confirmed with confidence.
Additionally, the statistical parameters of the PMF model performance were outside of the
bounds used to determine adequate performance of the model. Also, in perusing the literature of
applications of PMF approach, it was found that substantial amounts of measured data were
sometimes excluded from the analyses, and an uncertainty parameter for some variables was
altered to improve the model performance. Staff did not censor any data or alter certain
parameters in the model in an attempt to improve the model performance statistics. The
uncertainties used for the ambient measurements were those provided by the laboratory analyses.
Thus, the PMF method was not pursued.
When compared to the MATES II method, the CMB model available from the U.S. EPA gives
higher estimates of diesel particulates. The CMB model estimate for diesel particulate was
found to be sensitive to the gasoline emissions profile used. To account for this, the midpoint of
a range of estimates using two different gasoline profiles was used.
There are also uncertainties in the risk potency values used to estimate lifetime risk of cancer.
This study used the unit risks for cancer potency established by OEHHA and the annual average
concentration measured or modeled to calculate risk. This methodology has long been used to
estimate the relative risks from exposure to air toxics in California and is useful as a yardstick to
compare potential risks from varied sources and emissions and to assess any changes in risks
over time that may be associated with changing air quality.
The estimates of health risks are based on the state of current knowledge, and the process has
undergone extensive scientific and public review. However, there is uncertainty associated with
the processes of risk assessment. This uncertainty stems from the lack of data in many areas
necessitating the use of assumptions. The assumptions are consistent with current scientific
knowledge, but are often designed to be conservative and on the side of health protection in
order to avoid underestimation of public health risks.
As noted in the OEHHA risk assessment guidelines, sources of uncertainty, which may either
overestimate or underestimate risk, include: (1) extrapolation of toxicity data in animals to
humans, (2) uncertainty in the estimation of emissions, (3) uncertainty in the air dispersion
models, and (4) uncertainty in the exposure estimates. Uncertainty may be defined as what is not
known and may be reduced with further scientific studies. In addition to uncertainty, there is a
natural range or variability in the human population in such properties as height, weight, and
susceptibility to chemical toxicants.
Thus, the risk estimates should not be interpreted as actual rates of disease in the exposed
population, but rather as estimates of potential risk, based on current knowledge and a number of
ES-6
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MATES III
Final Report
assumptions. However, a consistent approach to risk assessment is useful to compare different
sources and different substances to prioritize public health concerns.
Conclusion
Compared to previous studies of air toxics in the Basin, this study found a decreasing risk for air
toxics exposure, with the estimated Basin-wide population-weighted risk down by 8% from the
analysis done for the MATES II time period. The ambient air toxics data from the ten fixed
monitoring locations also demonstrated a reduction in air toxic levels and risks.
Policy Implications
While there has been improvement in air quality regarding air toxics, the risks are still
unacceptable and are higher near sources of emissions such as ports and transportation corridors.
Diesel particulate continues to dominate the risk from air toxics, and the portion of air toxic risk
attributable to diesel exhaust is increased compared to the MATES II Study.
The highest air toxics risks are found near the port area, an area near Central Los Angeles, and
near transportation corridors. The results from this study underscore that a continued focus on
reduction of toxic emissions, particularly from diesel engines, is needed to reduce air toxics
exposure.
ES-7
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MATES III
Final Report
ASun Valley
• Burbank
Los Angeles
Commerce A "Pico Rivera
•Huntington Park
•Compton
N.Long Beach .• • Anaheim
W. Long Beach
a Santa Ana
Inland Valley S.B.
A San Bernardino
3 Ruhidoux
A
Indio
Fixed Sites A Temporary Sites
Figure ES-1 Map of MATES III Monitoring Sites
ES-8
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MATES III
Final Report
Basinwide Risk
¦ Diesel PM
~ Benzene
[] 1,3 Butadiene
~ Carbonyls
¦ Other
83.6%
Basinwide Risk: 1194 Per Million
Based on Average at Fixed Monitoring Sites
Risk per Million
1600
rv
Figure ES-2
MATES III Air Toxics Risk
„
\o
X X
^
^ ^
c#° y
4* * vo^
J1
¦ Other
~ Carbonyls
¦ 1,3 Butadiene
~ Benzene
¦ Diesel PM
* Note: One year of data at Huntington Park and Pico Rivera
Figure ES-3
ES-9
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MATES III
Final Report
Maximum Value - 3692.55
Minimum Volue = 51.61
NORTH
300 320 340 350 360 100 420 440
T T T 1~ I ~1—]—T VI I | 1 1 T 1 -1-1—1 T'—[ T 1 " I I || I
PACIFIC OCEAN
60
SOUTH
§
% % % % % % \ %
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MATES III
Final Report
Figure ES-5
Change in CAMx RTRAC Air Toxics Simulated Risk (per million) from 1998-99 to 2005
Using Back-Cast 1998 Emissions and 1998-99 MM5 Generated Meteorological Data Fields
ES-11
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Forest Resource Sustainability
in Placer County California
-------
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FOREST RESOURCE SUSTAINABILITY IN PLACER COUNTY, CALIFORNIA
PAGE 1
The partners
specifically are focused
on projects that
support hazardous
fuels reduction for
mitigating catastrophic
wildfire behavior,
and processing and
transporting excess
biomass material -
limbs, tops and brush -
for energy production
as an alternative to
open pile burning.
Forest Resource Sustainability
in Placer County, California
Millions of acres of forested land in the Sierra Nevada foothills and mountains are at significant
risk for wildfire. Placer County alone encompasses approximately 550,000 acres of forested land
stretching from Auburn to Lake Tahoe, including parts of two national forests and 60 percent of
Lake Tahoe's west shore.
After decades of successful fire suppression, unnaturally dense vegetation presents a significant
wildfire hazard. Just since 2001, Placer County has experienced six major wildfires that burned
more than 55,000 acres, including important upland watersheds. Also in the last few decades,
many homes and business have been developed within these forests in high fire hazard areas.
Between 2006 and 2010, state and federal fire agencies spent an average of $1.2 billion annually
fighting wildfires in California. Fire restoration costs average in the tens of millions, but more
importantly, fire jeopardizes ecological integrity by creating unacceptable impacts to forest
resources like water and wildlife habitat, and generate additional tons of harmful emissions into
the air.
in response, the Placer County Biomass Partners - including the Placer County Air Pollution
Control District, Placer County, and U.S. Forest Service - are implementing cost effective projects
that help promote the ecological, economic and social sustainability of forests and forest-
dependent communities. These projects focus on improving forest health, reducing the negative
effects of catastrophic wildfires by utilizing excess biomass from forest management projects to
produce renewable power and create family wage jobs.
The partners specifically are focused on projects that support hazardous fuels reduction and
processing and transporting excess biomass material - limbs, tops and brush - for energy
production as an alternative to open pile burning. These efforts improve air and watershed
quality, protect soil productivity, lower fire suppression costs, and produce renewable energy
that reduces regional reliance on fossil fuels.
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PAGE 2 FOREST RESOURCE SUSTAIN ABILITY IN PLACER COUNTY, CALIFORNIA
Sustainable Forestry Practices in Support
of Wildfire Mitigation
Stewardship Projects on Pubiic Lands
We are implementing innovative agreements and contracts with the U.S. Forest Service and
other partners aimed at facilitating cost-effective removal and utilization of forest biomass
material for energy. Placer County has also entered into a master stewardship agreement for
fuels treatment and biomass material recovery with the U.S. Forest Services Lake Tahoe Basin
Management Unit.
These forest fuels reduction projects to reduce the potential for catastrophic wildfire events
include selective thinning and removal of trees and brush to return forest ecosystems to more
natural fuel stocking levels, resulting in more fire-resilient and healthy forests.
These forest fuels
reduction projects
return forest
ecosystems to more
natural fuel stocking
levels, resulting in
more fire-resilient and
healthy forests.
Left: Biomass collection box. Right: Biomass chipping and transport operations.
Regional Biomass Collection
We are currently implementing a program to cost effectively collect biomass material from
forestland management and defensible space clearing. Centralized collection yards have been
located and advertised for convenient biomass material drop-off. Biomass material processing
and transport for utilization at energy recovery facilities then becomes cost effective.
Left: Roadside biomass pile. Right: Biomass pile loading operation.
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FOREST RESOURCE SUSTAINABILITY IN PLACER COUNTY, CALIFORNIA
PAGE 3
Forest Management Benefits
We are sponsoring research efforts to determine the benefits of forest management projects.
Models are being developed to quantify the effects of various levels of thinning and hazard
reduction treatments on wildfire size and intensity. Initial results indicate three types of carbon
benefits that are realized from forest management in the Sierra Nevada:
• Fuels treatments in the study area were shown to reduce the GHG and criteria air pollutant
emissions from wildfires by decreasing the probability, extent, and severity of fires and the
corresponding loss in forest carbon stocks.
• Utilizing biornass from forest management projects for energy production was shown to reduce
GHG and criteria air pollutant emissions over the duration of the fuels treatment project
compared to fossil fuel energy.
• Specific management and thinning of forests stimulates growth, resulting in more rapid uptake
of atmospheric carbon - including reductions in air pollutant and greenhouse gas emissions,
tree mortality, and improved forest growth and vigor.
Far Left: Effect of typical
high intensity wildfire.
Left: Thinning and
hazardous fuel treatments
can reduce the effect of
wildfires.
Current projects
are exploring the
valuation of the
benefits of sustainable
forest management
strategies on key
related resources
such as carbon, water
quantity and quality
and wildlife habitat.
Ecosystem Services
Fire, like development, can dramatically
alter forested landscapes and significantly
reduce or eliminate the flow of ecosystem
services. Project partners are exploring
the valuation ofthe benefits of sustainable
forest management strategies on key
resources such as carbon, water quantity
and quality, and wildlife habitat.
Average Estimated Ecosystem Service Value
per acre by Tributary Basin for
Humboldt County Study Area
Source: Spatial Informatics Group (www.sig-gis.com)
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PAGE 4 FOREST RESOURCE SUSTAIN ABILITY IN PLACER COUNTY, CALIFORNIA
Air Quality
Wildfires and Open Burning
of Biomass
Catastrophic wildfires with high burn intensity and landscape-
scale footprints in the densely forested Sierra Nevada
mountain range have significant adverse effects on air quality.
The September 2006 Ralston Fire in the Tahoe National Forest
east of Foresthill, Calif., for example, burned a total of 8,423
acres and generated massive amounts of harmful emissions.
Forest management thinning and defensible space clearing
generate biomass material in the form of limbs, tops, and brush
from a wide variety of land ownerships, including federal, state,
large and small private forestlands, conservation organizations
and residences.
These forest biomass materials are frequently disposed of
through open pile burning, due to economics - the cost to
process and transport biomass is usually higher than the value
paid by renewable power generation facilities.
Utilization of Biomass
We are sponsoring projects which collect, process, transport,
and utilize woody biomass material for renewable energy
generation in controlled .conversion units as an alternative to
open pile burning. Placer County has worked with the Forest
Services Lake Tahoe Basin Management Unit and the Tahoe
National Forest to implement pilot projects that analyzed
processes for collection, processing and transportation of
excess woody biomass from forest management and hazardous
fuels reduction projects. To date the projects have processed
over 15,000 tons of biomass, produced 15,000 MWh of electricity
and provided insights into improving the overall economics of
forest management and biomass utilization.
Life cycle analysis, has shown significant reductions in air
pollutants bydiverting biomass material awayfrom open burning
to renewable energy generation in biomass conversion facilities
that are equipped with Best Available Control Technology.
There are significant benefits from offsetting fossil fuel use and
few emissions from the processing and transportation of the
biomass-even if transporting for long distances.
Particulate Matter During Ralston Wildfire
90
80
70
60
50
¦ Meadow Vista
¦ Roseville
federal standard
/
9/1/2006
9/2/2006
9/3/2006
9/4/2006
9/5/2006
9/6/2006
9/7/2006
9/8/2006
O vO sO 0 V
15 o o o c
D O O O
M CM (N CM f
£ O CM
N \ \ \ 1
o o o
y/ io/^uuo
9/14/2006
9/15/2006
9/16/2006
9/17/2006
9/18/2006
Disposing
forest biomass
materials
through open
pile burning.
IT)
^ 40
51 30
20
10
0
Emission Benefits of Biomass Energy Project
Open Pile Burn
Unrealized Grid Electricity
Biomass Boiler
Biomass Chipping & Transport
Particulate
Matter
96%
Reduction
|B
Nitrogen
Oxides
54%
Reduction
Carbon
Monoxide
97%
Reduction
Volatile
Organics
99%
Reduction
Greenhouse
Gases
17%
Reduction
Results based on work by TSS Consultants (www.tssconsultants.com)
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FOREST RESOURCE SUSTAINABILITY IN PLACER COUNTY, CALIFORNIA
PAGE 5
The monetary value
secured from the sale
of GHG offset credits
is proposed to be
re-invested in forest
management projects
like reforestation and
thinning that provide
carbon benefits.
Open pile burn.
Cogeneration facility.
Benefits of Hazardous Fuels Reduction Treatments
Hazardous fuels treatment activities can reduce air pollutant emissions through the reduction
of wildfire intensity and size. We are supporting ongoing research efforts to develop models to
quantify air benefits. Initial results from these research efforts confirm that proactive treatment of
hazardous forest fuels mitigates both wildfire behavior and air emissions, including greenhouse gas.
Forest Wildfire Carbon Benefits of Fuel Treatment
Baseline
Treatment 1 (SNAMP)
Treatment 2 (USFS)
Treatment 3 (Private)
Time (Years)
Emissions Accounting
We have developed a greenhouse gas (GHG) offset protocol. This protocol provides a rigorous
accounting framework for obtaining monetary value for the emissions benefits (in the form of
emission offset credits) from the diversion of biomass material away from pile and burn activities
and into renewable energy facilities. The monetary value secured from the sale of emission offset
credits will be re-invested in forest management projects like reforestation and thinning that
provide GHG benefits. Thinning helps reduce the effects of wildfires that emit large quantities of
GHG's and negatively impact key forest resources like water and wildlife habitat.
Biomass for
Energy Project
Excess Biomass
Baseline,
Business as Usual
f
Biomass Processing
Fossil Fuel Fired Engines
Biomass Transport
Fossil Fuel Fired Engines
Uncontrolled
Open Burning
In-field
Decay
Energy Recovery
Fossil Fuel Fired Engines
Biomass Conversion Process
Baseline Energy Supply
Fossil Fuel Combustion
GHG Reduction = GHG Open Burn + GHG Decay + GHG Baseline Energy
— GHG Biomass Energy — GHG Biomass Processing — GHG Biomass Transport
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PAGE 6 FOREST RESOURCE SUSTAIN ABILITY IN PLACER COUNTY, CALIFORNIA
Renewable Bioenergy
Sustainable Distributed Bioenergy in the Lake Tahoe Area
Placer County is. proposing a Lake Tahoe Region biomass facility
through a public-private partnership. The distributed generation
renewable energy facility is scheduled to commence operation in FY
2014. Innovative features of the proposed facility include:
• Conservatively sized to produce 2 MW of electricity through use
of excess woody biomass materials produced from nearby forest
thinning and community defensible space fuels reduction projects.
• Lower energy facility fuel cost by developing innovative woody
biomass supply agreements with local land managers. Funds
normally used to dispose of excess biomass through open burning
or mastication can be used instead to help fund the processing and
transport of forest biomass to an energy production facility.
• Lower fuel cost by efficiently integrating forest management with
biomass production.
Loading chipped biomass for transport
• Utilize state-of-the art gasification energy Conversion technology
that provides high electricity production efficiency and low air
emissions that meet stringent local and federal limits.
• Fully realize the economic value of renewable energy and air emis-
sion benefits in support of community protection, healthy forests
and local employment.
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FOREST RESOURCE SUSTAINABILITY IN PLACER COUNTY, CALIFORNIA
PAGE 7
We are also developing a guidebook that wiil provide guidance, and lessons learned, in support
of small distributed-generation biomass power facility development. The guidebook will assist
communities that are interested in the proactive treatment of hazardous forest fuels and utilization
of resulting biomass material for the production of power and/or thermal energy.
Advanced Biofuels for Transportation
Placer County is a participant in a pilot study grant from the California Energy Commission to
demonstrate and evaluate the production of biomethane (also known as renewable natural gas)
from forest-sou reed woody biomass materials for use as an alternative transportation fuel.
Placer County and
the Placer County
Air Pollution Control
District have been
strong and visible
advocates and
participants, at
both the state and
national level, for
development and
adoption of policies
that favor protection
of communities, wise
forest management,
utilization of excess
biomass for energy,
and long-term
sustainability of forests,
forest resources and
forest-dependent
communities.
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PAGE 8 FOREST RESOURCE SUSTAIN ABILITY IN PLACER COUNTY, CALIFORNIA
Policy Recommendations
We have been strong and visible advocates and participants, at both the state and national
level, for development and adoption of policies that favor protection of communities, wise forest
management, utilization of excess biomass for renewable energy, and long-term sustainability of
forests, forest resources and forest-dependent communities. These policies could include:
• Feed-in tariff renewable energy pricing that provides market-based incentives that facilitate
private sector investment in community-scale for bioenergy facilities
• Greenhouse gases from excess biomass materials need to be recognized as at least carbon
neutral, and beyond carbon neutral if appropriate
• Streamlining of interconnection, siting requirements for community-scale bioenergy facilities
• Electricity contract pricing that is not based on short run avoided costs or market
price referent
• Federal tax credits for biomass energy generation that is commensurate with other forms of
renewable energy (wind, solar)
• Continued funding for proactive forest fuels treatment on public lands
• Continued use of long-term stewardship Contracts and agreements to facilitate landscape
level fuels treatment activities
• Federal tax credit for private forest landowners that treat hazardous forest fuels
• Development and implementation of ecosystem services/payments
Select Project Publications
Bruce Springsteen, Tom Christofk, Steve Eubanks, Tad Mason, Chris Clavin, and Brett Storey, "Emission Reductions from Woody Biomass Waste
for Energy as an Alternative to Open Burning" Journal of the Air and Waste Management Association, Volume 61, pages 63 - 68, January 2011.
"Forest Biomass Removal on National Forest Lands", Prepared by the Placer County Executive Office and TSS Consultant for the Sierra Nevada
Conservancy, November 17, 2008, available at http://www.tssconsultants.com/presentations.php
"Assessment of Small-Scale Biomass Combined Heat and Power Technologies For Deployment in The Lake Tahoe Basin" Prepared by TSS
Consultants for the Placer County Executive Office, High Sierra Resource Conservation and Development Council, and U.S. Forest Service,
December 2008, available at http://www.placer.ca.gov/Departments/CommunityDevelopment/Planning/Biomass/Grants.aspx
Placel GautUy
AIR POLLUTION CONTROL DISTRICT
Contacts
Tom Christofk, Air Pollution Control Officer,
Placer County Air Pollution Control District
530-745-2330 tchristo@placer.ca.gov
Brett Storey, Biomass Program Coordinator, Placer County
530-745-3011 bstorey@placer.ca.gov
Bruce Springsteen, Placer County Air Pollution Control District
530-745-2337 bsprings@placer.ca.gov
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