United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA'600/S3-87/047 Jan. 1988
&EPA Project Summary
Tropospheric Ultraviolet
Radiation: Assessment of
Existing Data and Effects on
Ozone Formation
M.W. Gery, R.D. Edmond, and G.Z. Whitten
This study was performed to
determine the impact that potential
changes in stratospheric ozone
concentrations and surface
temperatures might have on the
chemical processes that create
tropospheric ozone and cloud
acidification precursors. The
investigation consisted of two
distinct parts. First, an assessment
was performed of the ultraviolet
radiation information and molecular
absorption cross section and
quantum yield data currently used in
air quality simulation. This
assessment addressed both the
quality of existing data and
approaches available for utilizing
these data to determine chemical
photolysis rates in the troposphere.
Particular attention was paid to the
photolysis reactions of ozone,
formaldehyde, and acetaldehyde
because these species absorb light
in the spectral region where surface
ultraviolet irradiance could increase
due to decreased stratospheric
absorption by ozone.
The algorithms and data resulting
from this assessment were used in
the second portion of the study to
determine photolysis rates that might
occur in the troposphere under
future conditions of decreased
stratospheric ozone. The sensitivity
of photochemical dynamic
processes was tested for a large
number of urban airshed data sets
under conditions of decreased
stratospheric ozone and increased
surface temperatures. The predicted
surface ozone and hydrogen
peroxide concentrations resulting
from incremental changes in the
assumed future stratospheric ozone
and temperature parameters were
analyzed for each city and for
specific groups of cities. Instances of
greater future oxidant forming
potential were most common for
cities with already high hydrocarbon
control requirements. The increased
energy input during future scenarios
provided more rapid ozone formation
in all cases, indicating the possible
exposure of a larger portion of the
urban population to higher ozone
concentrations nearer to the center
of the urban plume
This Project Summary was
developed by EPA's Atmospheric
Sciences Research Laboratory,
Research Triangle Park, NC, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
In recent years scientists have
become increasingly aware that potential
chemical modification of the upper
atmosphere by trace gases may produce
an overall global warming by blocking the
escape of thermal infrared radiation. This
phenomenon is commonly referred to as
-------
the "Greenhouse Effect." In addition, the
chemical reactions of halocarbons,
methane, afcd NgO, may cause future
decreases in stratospheric ozone levels.
One consequence of this decrease would
be an \increase in the penetration of
ultraviolet radiation to the troposphere,
since stratospheric ozone is a principal
attenuator of radiation in the middle
ultraviolet radiation spectral region. A
higher intensity of ultraviolet radiation at
the earth's surface, combined with higher
global temperatures, might increase the
prevalence and intensity of photo-
chemical smog episodes in populated
areas and result in an unanticipated
deleterious impact on public health.
An increase in the prevalence of
photochemical smog might result
because a greater transmission of
ultraviolet radiation would augment the
principal energy source responsible for
photochemical reactions in the
troposphere. Combined with higher
global surface temperatures, this could
lead to an increase in the reaction rates
of a number of photochemical reactions
critical to smog formation. These
potential increases in specific
photochemical reaction rates have been
largely ignored in studies of future
chemical changes within the planetary
boundary layer. Any enhanced
photochemical reactivity could increase
the prevalence and magnitude of regional
and urban ozone formation, and augment
both the rural and urban contributions of
acid precursors by increasing the
production capacity for oxidized species.
It was our intent in this study to address
the magnitude and chemicai dynamics of
such impacts.
Because the rates of photolysis for
certain chemical species may vary
strongly with increased ultraviolet
transmission, the first part of the study
assessed the uncertainty associated with
the calculation of the tropospheric
photolysis rates of specific chemical
species. We addressed both the
uncertainty in fundamental chemical
parameters and the error associated with
determining surface actinic flux under
present and future conditions. This was
followed by an analysis of the possible
impacts of increased surface
temperature and decreased stratospheric
ozone (and thus, increased near-
surface photolysis rates) on the
photochemistry of various airsheds.
Because the effects of projected global
changes on near-surface photo-
chemistry will vary with the
characteristics of each airshed tested, we
chose to analyze a large number of
different test sets so that a reasonable
variation in input data could be applied.
Large numbers of data sets are currently
available only for urban areas whose
compliance with the air quality standards
for ozone must be verified. Atmospheric
oxidant formation in urban areas is one of
the most studied atmospheric
photochemical phenomena because of
the potential health effects of high ozone
concentrations in areas of high
population. For this reason, these
systems are also probably the best
understood, and because of high
amounts of anthropogenic emissions of
oxidant precursors, could be the most
immediately sensitive to potential global
changes. Thus, we chose to investigate
the impacts of potential global changes
on this type of system first.
Procedure
Range of Future Values
In such a sensitivity study, it is also
necessary to formulate some
understanding of the possible ranges of
projected surface temperature and
stratospheric ozone changes anticipated.
Our methodology was to analyze
available information and then devise two
sets of values, representing moderate
and extreme future conditions. The
moderate values were selected to be
between projections for the period 2010
to 2030, and the extreme values
represent even higher but possible
conditions, especially for decades later in
the next century. From these
assumptions we formulated a set of
future scenario temperature and
ultraviolet light (photolysis rate)
conditions. It is beyond the scope of this
discussion to consider the complex
perturbations that may alter future
surface temperatures. However, recent
efforts in global climate modeling
indicate that global warming could
increase the average temperature by as
much as 1 to 3 K by the year 2030.
Therefore, in the scenarios considered in
this work, we used surface temperature
increments of 2 and 5 K over the
measured temperature profiles for each
data set.
The potential changes to mid-
latitude ozone column density must be
estimated before attempting to calculate
representative photolysis rates. We
chose a base-case ozone column of
0.300 cm-atm for all simulations. This
represents approximately the monthly
average at North American latitudes for
the July and August period when most
measurements in the test data sets were
performed. Daily meteorologies
conditions specific to each data set ar
assumed to be contained in th
simulation of each base-case data se
As we will discuss later, only base-cas
simulations that accurately predicted th
actual measurements for a specific da
were further utilized for future scenari
sensitivity testing. Such a procedure ac'
as a filter to eliminate days wil
conditions unlike the assumed (month
average) base-case conditions. In th
way, we attempted to focus on futui
photochemical changes that were due
the increases in temperature an
decreases in ozone column relative to tf
base-case values. The moderate ar
extreme overhead ozone colurr
conditions chosen in this study were O.i
and 0.20 cm-atm, representing 16.7 ar
33.3 percent decreases in ozone for tl
future test period. These conditioi
represent current chemical modelit
estimates (15 percent decrease predictt
by the middle of the next century) ai
Nimbus 7 satellite data (possib
decreases of about 0.5 percent/year).
The' most direct way that depletion
stratospheric ozone could induce greai
photochemical reactivity in the low
troposphere would be through
alteration in the magnitude of t
ultraviolet irradiance at the earth
surface. This variation must be account
for prior to simulation, and hence, <
briefly consider the ozone absorpti
process and the uncertainties in t
calculation of tropospheric photoly:
rates.
Below 300 nm, the ability of ozone
absorb radiation increases dramatics
into the Hartley bands (200 to 300 ni
resulting in the cutoff of short waveleni
ultraviolet transmission to the eart
surface. However, in the spectral reg
between 300 to 350 nm (the bands
Huggins) the ability of ozone to absi
light decreases with increasi
wavelength, allowing some transmiss
of ultraviolet radiation to the eart
surface. Hence, the spectral region,
which an increase in ultraviolet irradiai
due to diminished stratospheric oz<
absorption should be manifest,
probably be confined to a rather sr
range between 310 and 280 nm beca
the change depends completely on
absorption characteristics of ozo
which is a much poorer absorber
radiation above 310 nm. A number of
tropospheric trace species photolyze
very reactive products upon absorptio
radiation from this spectral region wf
ultraviolet irradiance is expected
increase. Therefore, even a relath
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small change in total available radiant
energy may induce a relatively large
increase in certain tropospheric
photolysis rates.
Photolysis of stable molecules is the
major source of new radicals in
tropospheric gas-phase chemistry. In
atmospheres capable of sustaining even
a moderate rate of photochemical
reactivity, a greater production rate for
new radicals would tend to increase the
initial and continued oxidation of organic
compounds, resulting in additional
thermochemical radical generation and
sustained production of additional stable
species capable of more photolysis.
Therefore, the increased radical mass
flux that would result from potential
enhancement of some photolysis
reactions could translate into more
reactive tropospheric photochemistry
The result could be increased production
of those oxidized species now regulated
as photochemical oxidants (urban
ozone), or of other compounds
(peroxides) critical to the formation of
acidic precipitation after transport out of
an urban area.
Because our goal was simulation of
present and future urban atmospheric
systems, we felt it was necessary to
estimate the values and uncertainty for
these rates as they changed with
decreasing ozone column densities. We
mainly focused on the uncertainty
associated with the calculation of surface
photolysis rates for ozone [to 0(1D)] and
formaldehyde, since these are the most
significant, radical-generating photolysis
reactions in the spectral region of interest
and in the urban atmosphere in general.
Thus, their associated uncertainty
translates almost directly into uncertainty
concerning the radical production rate of
polluted air masses. We also identified
data and algorithms we felt to be
potentially useful in the development of a
new generation of ultraviolet actinic flux
calculation schemes for current and
developing models. Our predictions were
then compared with actual
measurements and the uncertainty
associated with each step of the process
was examined. Because the calculation
of atmospheric photolysis rates is the
product of actinic flux and specific
molecular properties integrated over
wavelength, we addressed three
separate areas; (1) the methods and data
needed to calculate actinic flux, (2)
uncertainties in experimentally derived
values for molecular properties, and (3)
the integration approach and related
uncertainties. We discuss the results in
the next section.
Simulation Protocol
The goal of the atmospheric
simulation phase of the study was to
investigate the potential changes in
urban oxidant formation caused by
possible future alterations in global
climate. More specifically, we were
interested in (1) additional photochemical
reactivity and (2) the amount of oxidant
formation that could occur as a result of
future increases in surface temperature
and decreases in stratospheric ozone
(increases in surface ultraviolet irradiance
and photolysis rates). Three different
ozone column densities and temperature
ranges were used in the future scenario
calculations. In addition to 57 single-day
city scenarios, we studied multi-day
impacts for two cities-one that had
attained the National Ambient Air Quality
Standard (NAAQS) for ozone, and one in
nonattamment status. A second
photochemical kinetics mechanism was
used in one set of simulations to verify
that the results were not mechanism-
specific. Also, we felt it appropriate to
use the OZIPM photochemical trajectory
model in our investigation of future urban
impacts since, when combined with the
EKMA procedure, this model is most
often employed to determine the amount
of NMOC reduction needed to achieve
compliance with the ozone NAAQS of
0.12 ppm. As we will see, EKMA
calculations can also aid in providing
estimates of atmospheric and emission
conditions expected in future scenarios.
In this study, we felt that a protocol
which merely employed the resimulation
of a present-day scenario using
different photolysis rates or temperature
values would not provide a sufficiently
reasonable estimate of anticipated future
urban conditions because mandated
control requirements must necessarily
alter present conditions. Since the EKMA
procedure is often used to determine the
amount of NMOC reduction that will
ideally be implemented in the future, we
utilized this direct link with the regulatory
process to determine more realistic
future scenarios. Therefore, all present-
day base-case data sets were
implemented in OZIPM-3. For those
with reasonable fits to the observed data,
standard EKMA calculations were
performed to formulate scenarios of
future attainment in each city based on
the alleviation of current smog scenarios.
We felt that this approach would provide
a much better estimate of future base-
case scenarios with which to assess
unanticipated (in the EKMA)
perturbations due to changes in the two
variables (temperature and ultraviolet
radiation) of interest in this study. Of
course, this protocol asaumes "ideal"
performance in the EKMA calculation
method and an "ideal" response of an
urban atmosphere to EKMA-derived
emission controls. As noted, we imposed
the constraint that the simulated ozone
value and the design (measured)
concentration should not vary
excessively In this way, the EKMA
program is not required to compensate
for a poor fit, but only to calculate the
ratio of NMOC reduction needed to
"ideally" produce 0 12 ppm ozone. As
noted above, this procedure also gives
some indication that reasonable
replication of daily meteorological and
ozone column conditions occurred, so
that the future sensitivity tests could
focus on the impacts of changes relative
to the base case conditions
Our initial, single-day modeling
data set consisted of atmospheric
measurements from 45 days in 10 cities.
There were actually 57 initial base-case
simulations because some days were
modeled with a multiple number of
trajectories Individual NMOC, NOX, and
design 03 values, elevation, location,
regional albedo, temperature, and mixing
height profiles, were used for each city.
The number of overall test sets was
reduced to 15 by testing of the
goodn^ss-of-fit between measured
and simulated hourly maximum ozone
concentrations for present base-case
simulations. EKMA calculations were then
performed to determine the "ideal"
NAAQS compliance conditions for each
base-case data set. From the EKMA
results, a future base-case data set (still
with normal temperature and ozone
column conditions) was derived and
simulated to verify that those conditions
produced ozone at the level of the
NAAQS. Then, using this future base
case as the "ideal" anticipated result of
prescribed NMOC reductions, we were
able to estimate the extent of
unanticipated (by the EKMA) impacts
resulting from the photochemical
changes caused by increased
temperature and ultraviolet irradiance.
This was done by the incremental
increase of these values and the analysis
of resulting changes in the concentrations
of ozone and other chemical species.
Since the city-specific data sets
span a wide range of urban oxidant
production capacities, we consolidated
our analysis and the following discussion
by creating three general group
classifications:
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Group 1: High ozone NAAQS
exceedence days. In this group
we include all days with design
values greater than 0.17 ppm.
There are ten such days in the 45
days simulated. These data sets
are from Los Angeles, Chicago,
New York, Boston, Philadelphia,
and Washington, representing
oxidant production episodes in
regions where severe ozone
exceedences are common
Group 2: Less extreme nonattamment
days, often representing cities
that require moderate control (30
to 50 percent) of organic
precursors to achieve the ozone
NAAQS. Hourly maximum ozone
concentrations are between 0.17
and 0.14 ppm in this group. Local
meteorological conditions can
influence the magnitude of the
ozone production in many of
these cases
Group 3. Days that are nearly in
compliance with the NAAQS for
ozone. These data sets provide
future sensitivity tests with current
ozone production at 0.13 ppm or
below. The data are scattered
among a few test cities, including
Boston, Nashville, and Tulsa.
We have used these groupings
because common group characteristics
facilitate later discussion of chemical
dynamics. Although these data could be
considered typical of each individual
city's ozone formation profile, we will
usually name specific cities only to
identify the source of input data for an
example data set. One notable exception
in the following discussion concerns the
two data sets chosen from the Seattle
data. As with many data sets collected
for use in the 1982 SIP process in
smaller cities, some measurements
needed for OZIPM input parameters
appear to be rather uncertain. In this
case, we refer to both the precursor
concentrations (which appear to be
rather high) and the morning mixing
height (which may have been less than
the 250-m values used in the SIP
calculations). The calculated results for
those days do not easily fit into any of
the grouping schemes. However, their
results do demonstrate some important
atmospheric processes; so, although
they are uncertain, we discuss the
conditions and predictions from those
two tests to demonstrate certain aspects
of chemical dynamics. Therefore, our
discussion focuses on the three general
groups, and a fourth test based on the
Seattle data (denoted as Group 4 in the
following discussion). We feel that this
grouping scheme is as specific as the
current data will allow. We stress that
though it is based on data from one city,
even the Seattle data should be
considered a general test case since a
much larger and better defined set of
city-specific measurement must be
considered before the unique
characteristics of any individual city can
actually be discussed.
Results and Discussion
Assessment of Photolytic Rate
Calculations
We initially analyzed the methods
and data needed to calculate actinic flux
at the earth's surface. The nature of such
calculations is complicated because the
determination of actinic flux for differing
conditions and locations requires
consideration of a wide range of complex
atmospheric interactions. Rigorous
attempts were made a decade ago,
resulting in the creation of a number of
actinic flux data sets for various
conditions. These data sets have been
used extensively over the last decade in
almost all photochemical modeling
studies of tropospheric air masses. We
believe that an update of this work is now
in order, if for no other reason than to
include more recent extraterrestrial flux
and atmospheric aerosol data in these
calculations and to provide actinic flux
results for conditions not addressed in
the original work (e g., variable ozone
column densities), but now becoming
more important. These new calculations
must also be carried out at smaller
integration intervals to provide flux data
resolved to a magnitude comparable with
that of molecular data, so that
mathematical averaging errors in the
photolysis rate calculations can be
limited. This new data would also be very
useful in improving the mathematical
formulations used to estimate actinic
fluxes for conditions not directly included
in the data.
Our analysis of fundamental
molecular data focused on the short
wavelength photolysis reactions of ozone
and formaldehyde. The greatest
individual area of uncertainty in those
reactions is found in the absorption
cross-section formaldehyde data. The
two key studies in this area obtained
results that differed by about 30 percent.
Since formaldehyde is a very important
photolytic species in the troposphere,
these numbers translate into a large
uncertainty in the radical production
capacity of organic oxidation products.
We believe that this discrepancy should
be alleviated, either through reevaluaton
of existing data sets or through additiona
experimental work to develop more data
Absorption • cross-section and quanturr
yield data for ozone are less uncertain
primarily because they have been the
objects of experimental investigation for e
longer period of time. A somewhat largei
associated error develops for ozone
photolysis to form 0(1D), however
because this process occurs at the
surface ultraviolet cutoff, where actinic
flux calculations are less certain. Ar
experimental program designed tc
measure surface flux distributions in th<
middle ultraviolet range (particularly n
the region near the solar cutoff) woul(
provide information with which the erro
of JoiD calculations might be diminishei
and could also yield important data fo
evaluating and improving actinic flu
calculation schemes.
We also investigated thu
methodology involved in calculatim
photolysis rates. In cases where two c
more of the product terms (actinic flu>
quantum yield, and absorption cros
section) were highly wavelength
resolved, errors resulted if th
wavelength intervals used in numeric;
integration were much larger than th
significant resolved features in th
individual functions. This averaging errc
was only on the order of ten percent, bi
could be easily eliminated if calculation
were performed at 1 or 2 nm interval:
Again, this finding points to the need fc
a new set of more highly resolved actini
flux calculations.
Evaluation of the Impact of
Global Changes on
Tropospheric Photochemistry
This phase of the report focused c
simulating the photochemistry of urbc
air parcels and evaluating the effects
potential changes in ozone colurr
density and surface temperature. Tf
modeling protocol and input data se
were discussed above. For discussic
purposes, the 15 single-day test case
were grouped into four gener
categories also discussed earlier.
Of primary importance to the impa
that future temperature increases
ozone column decreases might have i
the photochemical dynamics of an urb.
system is the method by which tl
additional energy associated with tho
changes is input into that systei
Increased surface ultraviolet irradiati
caused by a diminished ozone absorpti
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capacity in the stratosphere follows a
diurnal curve dependent on the elevation
of the sun. On the other hand, energy
input to the earth's surface through
Greenhouse warming takes a less direct
route. Regarding impacts on ozone
concentrations and other oxidized
species, two distinct types of effects can
be seen in the future scenarios; these
effects can be linked to the differences in
energy input dynamics. Because global
surface temperature increases were
simulated by the addition of 2 and 5 K to
the original, city-specific temperature
profiles evenly across the day, the
effects of temperature increases were
slight increases in reaction rates and
somewhat greater formation of ozone
and oxidized products across the entire
simulation period. This is to be expected
since most temperature-dependent
reaction rates increase with increasing
temperature The second type of impact,
resulting from the enhanced (due to
depleted stratospheric ozone) ultraviolet
irradiation function, was an increase in
the rate of photochemical reactivity
centered around midday.
In all test cases studied, an increase
in photolysis rates due to decreased
stratospheric ozone caused a more rapid
formation rate for ozone and other
oxidized products Because most
additional energy input was concentrated
in an already photochemically reactive
period of the day, the impact was often
more dramatic than that of the
temperature change. Levels of ozone at
or near the NAAQS (0.12 ppm) were
achieved much earlier, sometimes hours
earlier, at the time when an air parcel
might be over more populated areas
earlier in the trajectory This would occur
because the midday enhancement of
photolysis rates provides a greater
radical production rate and radical
concentrations, thereby increasing
short-term reactivity so that these
photochemical systems can convert
precursors to oxidant more efficiently.
However, this enhanced reactivity may
not always result in greater maximum
ozone concentrations because that
measure of oxidant-forming potential is
also a function of available precursors
and meteorological conditions. Hence,
while increased ultraviolet irradiance from
depleted stratospheric ozone is predicted
to increase short-term reactivity in an
urban air parcel, these conditions will not
always result in greater maximum
concentrations because such long-term
measures are more a function of the
specific system. Air parcels with low
precursor emissions and beneficial
meteorological conditions may actually
produce lower maximum concentrations
of oxidized products because the
enhanced reactivity may consume a
large fraction of the precursor species
under conditions earlier in the day that
are less favorable for oxidant formation.
For the general group of scenarios
just described, our simulations predict
increases in maximum hourly ozone
(over the 0.12 ppm of the future base-
case scenarios) at about 1.4 ± 0.5
percent per degree Kelvin increase for
the first three groups, with the Group 1
cities at the more reactive extreme. With
respect to changes in ozone column
density, we predicted a 1.1 percent
increase in maximum hourly ozone
concentration for each percent decrease
in ozone column for the Group 1 cities,
while the rates of increase for the Groups
2 and 3 cities were very near zero. Some
of this variability was seen as an artifact
of the EKMA calculation procedure, since
the EKMA procedure terminates its
calculations at 1800 hours. In this way,
additional oxidant-forming potential,
which is only realized after 1800 hours,
cannot be accounted for. For the Group 1
EKMA attainment and future base-case
simulations, 0.12 ppm of ozone was
formed at 1800 hours, but there was an
increasing slope at that time, indicating
additional oxidant-forming potential.
Future conditions of enhanced ultraviolet
irradiance utilized this potential more
efficiently to produce higher levels of
ozone prior to 1800 hours. Therefore,
those test cases (Group 1) showed more
extreme sensitivity to future changes in
ozone column densities.
We recognize that simulations in
which some of the rates in the chemical
mechanism vary between present and
future scenarios was never an intended
application of the EKMA. On the other
hand, when it is possible to account for a
large fraction of the oxidant-forming
potential, as in the base-case
simulations for Groups 2 and 3 where the
ozone maximum concentration occurred
prior to 1800 hours, very little additional
ozone formation was predicted with
changes in future parameters because a
large fraction of the oxidant-forming
potential was already accounted for.
Using our estimates of moderate (+ 2 K
and -16.67 percent ozone column
density) and extreme ( + 5 K and -33.3
percent ozone column density)
conditions, we predicted group-
average ozone concentrations of 0.132
and 0.120 ppm for Groups 2 and 3 at the
extreme conditions. Hence, though
ozone forms more rapidly in these future
scenarios, the concentrations do not
significantly exceed the NAAQS for even
the most extreme conditions tested.
Conversely, for Groups 1 and 4 average
concentration results were 0.148 and
0.150 ppm for moderate conditions and
0.174 and 0.207 ppm for extreme tests.
Because of the rather significant changes
predicted for the Group 1 and 4 test
cases, we also analyzed the data for an
indication of whether synergistic
interaction between the two perturbations
would occur. We found that, for the cases
available, the combined effects of
coincident increases in both parameters
were sometimes additive, but not
synergistic. Such a finding is consistent
with our description of the urban
photochemical processes, assuming that
there is a limit to the oxidant-forming
potential of an air mass.
The formation of other oxidized
products such as nitric acid and
hydrogen peroxide was found to be
specifically dependent on the types of
processes by which they form in an
urban atmosphere. For instance, the
formation rate and eventual yield of nitric
acid is related to the hydroxyl radical
concentration and the amount of NOX
available, These two parameters are
closely linked to ozone concentration,
since NOX is an ozone precursor and
ozone photolysis is the key source of
hydroxyl radicals in these systems.
Hence, the impacts of potential global
changes on nitric acid formation parallel
those of ozone and are limited by
available NOX. On the other hand,
hydrogen peroxide is formed by the
combination of hydroperoxy radicals,
which only accumulate after NOX is
depleted An increase in photochemical
reactivity will deplete NOX faster and
allow hydroperoxy radical (and therefore,
hydrogen peroxide) to form for longer
periods and at higher concentrations. In
this study, the predicted future increases
in photochemical reactivity stem mainly
from projected future decreases in ozone
column density; thus, hydrogen peroxide
was found to be very sensitive to this
parameter.
We have also found that NMOC
emission control designed to attain the
NAAQS for ozone appears to be a very
effective hydrogen peroxide control since
one net effect of NMOC control is to
reduce the reactivity of a system and
thereby reduce the consumption rate of
NOX. Since hydroperoxy radical
concentrations are highly sensitive to
NOX concentration, the additional
remaining NOX holds down the hydrogen
peroxide formation rate. Conversely, as
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projected conditions of future global
change were implemented in our test
cases, NOX was again depleted more
rapidly and hydrogen peroxide
concentrations began to increase
significantly. For our moderate test
conditions, control of NMOC to attain the
ozone NAAQS was predicted to also limit
hydrogen peroxide to about 70 to 80
percent of its original base-case
concentration. However, as ozone
column conditions were changed to the
extreme case, more hydrogen peroxide
was formed than in the original base
case, even with the added NMOC
control.
Conclusions and
Recommendations
We believe that the current actinic
flux, absorption cross section, and
quantum yield data sets result in
calculations of ozone [to form 0(1D)] and
formaldehyde photolysis rates within an
uncertainty of about 50 percent. These
are the most important photolysis rates
in the simulation of tropospheric
photochemistry, and more confidence in
model results would be gained if this
uncertainty could be limited to more
acceptable bounds. We feel future efforts
to limit this uncertainty would provide
additional confidence in all air quality
simulation. These efforts should (1) use
currently existing information to develop
improved actinic flux data sets and
computer formulations, and (2) make
additional experimental measurements,
particularly of formaldehyde absorption
cross sections, short wave-length
actinic flux distributions, and
actmometrically determined j-values, to
provide critical information in the most
uncertain areas.
Our simulations of urban
photochemical systems indicate that
some areas, predominantly those rich in
emissions of oxidant precursor species,
exhibit strong tendencies to be more
reactive given additional energy input
from increased temperatures and
increased ultraviolet irradiation. This
added reactivity was evident from both
higher concentrations of oxidants and
radical species and more rapid formation
of these species. We have established
some limits to the extremes of change
that could occur in urban scenarios and
suggest that continuation of such an
effort should focus on four aspects: (1)
Use of the newest and more extensive
1987 SIP data along with the recently
developed Carbon-Bond Mechanism-
IV/OZIPM-4 model to obtain more
specific information related to the extent
of the impacts found in this study; (2)
Use of such models and data to study
parameters not varied in this study, but
now predicted by GCMs to be variable
(including water vapor, mixing height and
cloud cover); (3) Use of regional models
such as the ROM or RTM-III, to analyze
linked, multiple, urban trajectories in a
unified regional domain; and (4) Use of a
more complex gridded urban model,
such as the Urban Airshed Model, in
conjunction with demographic data to
evaluate the exposure-related impacts
of reactivity changes predicted to occur
with smaller ozone column densities.
M. W. Gary, R. D. Edmond, and G Z. Whitten are with Systems Applications.
Inc., San Rafael, CA 94903.
Bruce W. Gay, Jr., is the EPA Project Officer (see below).
The complete report, entitled "Tropospheric Ultraviolet Radiation' Assessment
of Existing Data and Effects on Ozone Formation," (Order No. PB 88-130
133/AS; Cost: $25.95) will be available only from.
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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