Overview of Particulate Matter (PM) Air Quality in the United States
Updated: July 20, 2022
1. Introduction
The overall purpose of this document is to maintain an up-to-date graphical summary of air quality information that
supports the review of the National Ambient Air Quality Standards (NAAQS) for particulate matter (PM). In previous
reviews of the PM NAAQS, this type of information has generally been included in atmospheric sections of the Integrated
Science Assessment (ISA) and Policy Assessment (PA) for PM. This stand-alone document will either replace or complement
the air quality emissions and monitoring data in the atmospheric sections of future PM NAAQS documents, and will be
updated at regular intervals as new data becomes available.
The content of past NAAQS documents' atmospheric sections has included major sections on emissions and concentration
trends utilizing maps and data from the EPA's National Emissions Inventory (NEI) and the EPA's Air Quality System
(AQS) database. In past NAAQS reviews, this often involved adaptation of figures and tables prepared for other reports, or
development of new figures and tables using data analysis and mapping software. Additionally, the release of updated emission
inventories and ambient monitoring data may not coincide with the schedule for the development of NAAQS documents. As a
result, data access and resources can limit the availability of the most recent information for inclusion in NAAQS documents.
This stand-alone document allows the content to be updated as soon as new data becomes available, rather than pulling
from whatever is available at the time of publication. It also ensures that the public will have access to a consistent set of
maps and figures for each NAAQS pollutant that are updated on a routine basis, rather than separated by several years
following the disparate schedules of the various NAAQS reviews for each pollutant. Moreover, a stand-alone document can be
expanded to include new air quality analyses as they are completed, rather than following the timeline for the public release
of the NAAQS documents. Finally, this document takes advantage of a more flexible digital format for the routinely prepared
maps and trends figures, with an end product that more strongly emphasizes visual presentation of data and reduces the
amount text, while also creating a more interactive presentation of the information through the use of external links.
This document follows an organization similar to the structure of the atmospheric sections of past PM NAAQS documents.
The subsequent sections are as follows: 2. Atmospheric Chemistry; 3. Sources and Emissions of PM in Ambient Air; 4.
Ambient Air Monitoring Requirements and Monitoring Networks; 5. Data Handling Conventions and Computations for
Determining Whether the Standards are Met; and 6. PM Concentrations Measured at Ambient Air Monitoring Sites Across
the U.S. These sections are broad enough in scope to handle changes in what is known about PM atmospheric science as it
advances but specific enough that NAAQS-relevant information will be able to be quickly retrieved by users of the document.
2. Atmospheric Chemistry
In ambient air, PM is a mixture of substances suspended as small liquid and/or solid particles. Particle size is an important
consideration for PM, as distinct health and welfare effects have been linked with exposures to particles of different sizes.
Particles in the atmosphere range in size from less than 0.01 to more than 10 micrometers (jum) in diameter. When describing
PM, subscripts are used to denote the aerodynamic diameter1 of the particle size range in micrometers (jum) of 50% cut
points of sampling devices. The EPA defines PM2.5, also referred to as fine particles, as particles with aerodynamic diameters
generally less than or equal to 2.5 jum. The size range for PMio_2.5, also referred to as coarse particles, includes those particles
with aerodynamic diameters generally greater than 2.5 jum and less than or equal to 10 jum. PM10, which is comprised of
both fine and coarse fractions, includes those particles with aerodynamic diameters generally less than or equal to 10 jum.
Figure 1 provides perspective on these particle size fractions. In addition, ultrafine particles (UFP) are often defined as
particles with a diameter of less than 0.1 jum.
Aerodynamic diameter is the size of a sphere of unit density (i.e., 1 g/cm3) that has the same terminal settling velocity as the particle of
interest.
1
-------
^PMio
Dust, pollen, mold, etc.
<10 fim (microns) in diameter
90 Jim (microns) in diameter
FINE BEACH SAND
HUMAN HAIR
50-70 Jim
C- PM2.5
Combustion particles, organic
compounds, metals, etc.
<2.5 Jim (microns) in diameter
Figure 1. Comparisons of PMas .and PMiฎ diameters to human luiir and beach sand. Reproduced from Figure 2-1 of the
2020 PM PA.
Atmospheric distributions of particle size generally exhibit three: distinct modes ("nucleation mode", "accumulation
mode";, and "coarse mode") that roughly align with the; PM size, fractions defined above. Figure 2 below shows an example
of the particle size distribution for each of these three modes. I'lio nucleation mode is made up of freshly generated particles:,
formed either during combustion or by atmospheric reactions of precursor gases. The nucleation mode: is especially prominent
near sources like heavy traffic, industrial emissions, biomass burning, or cooking. While nucleation mode particles are: only a
minor contributor to overall ambient PM mass and surface area, they are the main contributors to ambient particle number.
By number, most nucleation mode particles fall into the EFP size range, though some fraction of the nucleation mode number
distribution can extend above 0.1 /.tin in diameter. Nucleation mode particles can grow rapidly through coagulation or uptake
of gases by particle surfaces, giving rise to the accumulation mode. The: accumulation mode is typically the predominant
Contributor to P VI mass and surface area, though only a minor contributor tฉ: particle number. PMs.s sampling methods
measure most of the accumulation mode mass, although a small fraction Of particles that make up the accumulation mode
are greater than 2.5 /tm in diameter. Coarse mode particles are formed by mechanical generation, and through processes like
dust res.uspension and sea spray formation. Most coarse mode mass is captured by PMxQ-2.51 sampling, but small fractions of
coarse mode: mass can be:smaller than 2.5 //in or greater than 10 /tm in diameter.
2
-------
4.0
! 3.0
o~
n 1ii i i i i
n 1ii i i i
UFP (Dp <100 nm).
"1 1II I I I I |
1 1II I I I I
Zฅ:iiiPo_f. 1
PM,S (D < 2.5 |jm)
i
Nucleation Mode: Arisesfrom
cluster formation and condensation
of semi-volatile gases.
Number
Surface Area
Mass
Accumulation Mode: Develops as small particles
agglomerate or grow through condensation and
gas-to-particle partitioning.
/ A\ \
r-"-T i i ii
Coarse Mode: Dust, particles
reentrained from surfaces,
mechanically-separated aerosols
H 1I Mill
D_ (nm)
1.000
10,000
Figure 2. Comparison of particle size distribution by particle number, surface area, and mass. Ot&tai = total particle
concentration; I), = particle diameter. Reproduced from Figure 2-1 of the 2019 PM ISA.
Most particles are found in the lower troposphere, where they can have residence times, ranging from a few hours to weeks.
Particles are; removed from the atmosphere by wet deposition, such as when they are carried by rain or snow, or by dry
deposition, such as gravitational settling or surface collision. Atmospheric lifetimes are generally longest for PM^g, which
often remains in the atmosphere for days to weeks before being removed by wet or dry deposition. In contrast, atmospheric
lifetimes for I'l l' and PMi{j-2,,8 are shorter. Within hours, I'l l' can undergo coagulation and condensation that lead to
formation of larger particles in the accumulation mode, or can he removed from the atmosphere by evaporation, deposition,
or reactions with other atmospheric: components. PMio^.s are also generally removed from the: atmosphere within hours,
through wet or dry deposition.
Secondary PM^s:, which is, derived from both natural and anthropogenic sources, accounts for a substantial fraction
of the I'M-,. - mass. Secondary I'M/.,-. forms through atmospheric photochemical oxidation reactions of both inorganic and
organic gas-phase precursors, primarily sulfur dioxide (SO2), nitrogen oxides (NOx), and ammonia (NHg). Reactions leading
to sulfate (SO42") production from SO2, nitrate (NO3") production from MOx, and t lie gas-to-particla equilibrium between
ammonia (NH$) and ammonium are relatively well understood, while formation of secondary organic: PM, often
referred to. as secondary organic aerosols (SOA), is less well resolved. In contrast, PM3,0-3 5 is mainly primary in origin, as :it
is produced by the abrasion of surfaces or by the suspension of biological material.
Sources: Integrated Science Assessment for Particulate Matter, May 2019
Policy Assessment for the Review of the PM NAAQS, January 2020
3. Sources and Emissions of PM
PM is composed of both primary (directly emitted particles) and secondary chemical components. Primary PM is derived
from direct particle emissions from specific PM sources while secondary PM originates from gas-phase chemical compounds
present in the atmosphere that have participated in new particle formation or condensed onto existing particles. Primary
particles, and gas-phase compounds contributing to secondary formation PM, are emitted from both anthropogenic; and
natural sources.
-------
Anthropogenic sources of PM include both stationary and mobile sources. Stationary sources include fuel combustion
for electricity production and other purposes, industrial processes, agricultural activities, and road and building construction
and demolition. Mobile sources of PM include diesel- and gasoline-powered highway vehicles and other engine-driven sources
(e.g., ships, aircraft, and construction and agricultural equipment). Both stationary and mobile sources directly emit primary
PM to ambient air, along with secondary PM precursors (e.g., SO2, NOx) that contribute to the secondary formation of PM
in the atmosphere.
Natural sources of PM include dust from the wind erosion of natural surfaces, sea salt, wildfires, primary biological
aerosol particles (PBAP) such as bacteria and pollen, oxidation of biogenic hydrocarbons such as isoprene and terpenes to
produce SOA, and geogenic sources such as sulfate formed from volcanic production of SO2. Contributions of natural emission
sources to PM2.5 concentrations can be interconnected with anthropogenic emissions through atmospheric chemistry, such
as the modulation of biogenic SOA production by anthropogenic NOx and SO2 emissions.
Generally, the sources of PM for different size fractions vary. While PM2.5 in ambient air is largely emitted directly by
sources such as those described above or through secondary PM formation in the atmosphere, PM10-2.5 is almost entirely
from primary sources (i.e., directly emitted) and is produced by surface abrasion or by suspension of sea spray or biological
materials such as microorganisms, pollen, and plant and insect debris.
The major components of PM2.5 mass include sulfate, nitrate, elemental or black carbon (EC or BC), organic carbon
(OC), crustal materials, and sea salt. Some of these PM components are emitted directly to the air (e.g., EC/BC) while
others are formed secondarily through reactions by gaseous precursors (e.g., sulfate, nitrate). Anthropogenic SO2 and NOx
are the predominant precursor gases in the formation of secondary PM2.5 sulfate and nitrate, and ammonia is the gas-phase
precursor for PM2.5 ammonium. Atmospheric oxidation of volatile organic compounds (VOCs), both anthropogenic and
biogenic, is an important source of SOA, particularly in summer.
The National Emissions Inventory (NEI) is a comprehensive and detailed estimate of air emissions of criteria pollutants,
precursors to criteria pollutants, and hazardous air pollutants from air emissions sources. The NEI is released every three
years based primarily upon data provided by State, Local, and Tribal air agencies for sources in their jurisdictions and
supplemented by data developed by the US EPA. The NEI is built using the EPA's Emissions Inventory System (EIS) first
to collect the data from State, Local, and Tribal air agencies and then to blend that data with other data sources.
Accuracy in an emissions inventory reflects the extent to which the inventory represents the actual emissions that occurred.
Anthropogenic emissions of air pollutants result from a variety of sources such as power plants, industrial sources, motor
vehicles and agriculture. The emissions from any individual source typically varies in both time and space. For the thousands
of sources that make up the NEI, there is uncertainty in one or both of these factors. For some sources, such as power plants,
direct emission measurements enable the emission factors derived from them to be more certain than sources without such
direct measurements. However, it is not practically possible to directly monitor each of the emission sources individually and,
therefore, emission inventories necessarily contain assumptions, interpolation and extrapolation from a limited set of sample
data.
Figure 3 shows the main sources contributing to primary PM2.5, primary PM10, SO2, and NOx emissions in the U.S.
Fires, which include wildfires, prescribed fires, and agricultural fires, contributed about 44% of primary PM2.5 emissions
and 17% of primary PM10 emissions in 2017. Dust particles from roads, agriculture, and construction contributed 30% of
primary PM2.5 emissions and 69% of primary PM10 emissions, while most of the remaining primary PM emissions came
from stationary fuel combustion (e.g., coal combustion for electricity), industrial and mobile sources. Regarding precursors
to secondary PM formation, the main sources of SO2 and NOx are stationary fuel combustion (64% of total SO2 emissions;
22% of total NOx emissions), industrial processes (19% of total SO2 emissions; 10% of total NOx emissions) and mobile
sources (8% of total SO2 emissions; 52% of total NOx emissions).
4
-------
Figure 4 shows the national trends in U.S. anthropogenic primary PM2.5, primary PM10, SO2, and NOx emissions from
2002 to 2021.2 Primary PM2.5 emissions reached a maximum of 5.2 million tons per year in 2007 and have decreased by
24% to 3.9 million tons per year in 2021. Similarly direct PM10 emissions reached a maximum of 20.7 million tons per year
in 2007 and have decreased by 322% to approximately 14.4 million tons per year in 2021. SO2 emissions have decreased by
88% since 2002, while NOx emissions have decreased by 68% since 2002. The large reductions in NOx and SO2 emissions
are largely due to reductions in the electricity generation and transportation sectors resulting from EPA programs such as
the Glean Air Interstate Rule and the Gross-State Air Pollution Rule for electric generating units, as well as the adoption of
more stringent fuel economy standards and low sulfur diesel fuel standards for mobile sources.
2For the purposes of this document, wildfires are considered to be natural emissions and thus are not included in Figure 4.
5
-------
Q_
>ป
cu
E
dL
2000-
1000-
E
dL
8000-
4000-
CN
O
O
CN
ID
O
O
CN
00
O
O
CN
~r
^r
o
CN
o
CN
CN
O
O
CN
LO
O
O
CN
CO
O
O
CN
"T"
Is- O
t- CN
O O
CN CN
Inventory Year
Inventory Year
15000
12000
9000
-------
Primary PM2.5 Emissions Density in tons/year/miA2 (# Counties)
~ 0-0.9 (897) ~ 1-1.9(1169) ~ 2-4.9 (895) ฆ 5-9.9(190) ฆ 10-171(69)
Figure 5. U.S. county-level primary PM2.5 emissions density estimates in tons/year/mi2. Source: 2017 NEI
Primary PM10 Emissions Density in tons/year/miA2 (# Counties)
~ 0-1.9 (390) ~ 2-4.9 (1070) ~ 5-9.9(1135) ฆ 10-19.9 (480) ฆ 20-526(145)
Figure 6. U.S. county-level primary PM10 emissions density estimates in tons/yeax/mi2. Source: 2017 NEI
7
-------
Sulfur Dioxide Emissions Density in tons/year/miA2 (# Counties)
~ 0-0.04(911) ~ 0.05-0.19 (1140) ~ 0.2-0.99 (690) ฆ 1-4.99 (304) ฆ 5-288 (175)
Figure 7. U.S. county-level SO2 emissions density estimates in tons/year/mi2. Source: 2017 NEI
Nitrogen Oxides Emissions Density in tons/year/miA2 (# Counties)
~ 0-1.9 (1024) ~ 2-4.9(1264) ~ 5-9.9 (499) ฆ 10-19.9(251) ฆ 20-826(182)
Figure 8. U.S. county-level NOx emissions density estimates in tons/year/mi2. Source: 2017 NEI
8
-------
4. Ambient Air Monitoring Requirements and Monitoring Networks
The EPA and its partners at state, local, and tribal monitoring agencies manage and operate the nation's ambient air
monitoring networks. The EPA provides minimum monitoring requirements for PM and other pollutants in 40 CFR Part
58. Monitoring agencies carry out and perform ambient air monitoring in accordance with the EPA's requirements and
guidance. Federal Reference Methods (FRMs) and Federal Equivalence Methods (FEMs) are monitoring methods that have
been approved for use by states and other monitoring organizations to assess NAAQS compliance and implementation. The
FRMs for measuring PMio, PM2.5, and PM10-2.5 are specified in CFR 40 Part 50, Appendices J, L, and O, respectively, while
performance requirements for the approval of FRM and FEMs are in 40 CFR Part 53.
The EPA and monitoring agencies manage and operate robust national monitoring networks for both PM10 and PM2.5, as
these are the two measurement programs directly supporting the PM NAAQS. PM10 measurements are based on gravimetric
mass, while PM2.5 measurements include gravimetric mass and chemical speciation. A smaller network of stations is operating
and reporting data for PM10-2.5 gravimetric mass and a few monitors are operated to support special projects, including pilot
studies, for continuous speciation and particle count data.
The EPA first established NAAQS for PM in 1971 based on total suspended particulates, or TSP. The TSP NAAQS was
replaced by the PM10 NAAQS in 1987. TSP sampling remains in operation at a limited number of locations primarily to
provide measurements for the Lead (Pb) NAAQS as well as for instances where a state may continue to have state standards
for TSP. The size of the TSP network peaked in the mid-1970s when over 4,300 TSP samplers were in operation. There were
133 monitoring sites reporting Pb TSP data to EPA during the 2019-2021 period.
To support the 1987 PM10 NAAQS, the EPA and its state and local partners implemented the first size-selective PM
monitoring network in 1990 with the establishment of a PM10 network consisting of mainly high-volume samplers. The PM10
monitoring network peaked in size in 1995 with 1,665 stations reporting data. There were 725 monitoring sites reporting
PM10 data to EPA during the 2019-2021 period. Figure 9 shows the locations of these monitoring sites. Approximately 61%
of these monitoring sites operate FEMs which report continuous PM10 data while the remaining sites operate FRMs which
typically collect samples every day, every 3rd day, or every 6th day.
To support the 1997 PM NAAQS, the first PM NAAQS with PM2.5 as an indicator, the EPA and states implemented
a PM2.5 monitoring network consisting of ambient air monitoring sites with PM2.5 mass and/or chemical speciation mea-
surements. Network operation began in 1999 with nearly 1,000 monitoring stations operating FRMs to measure fine particle
mass. The PM2.5 monitoring program remains one of the largest ambient air monitoring programs in the U.S. There were
1069 monitoring sites reporting PM2.5 data to EPA during the 2019-2021 period. Figure 10 shows the locations of these
monitoring sites. Approximately 50% of these monitoring sites operate FEMs which report continuous PM2.5 data while the
remaining sites operate FRMs which typically collect samples every day, every 3rd day, or every 6th day.
The main network of monitors providing ambient data for use in implementation activities related to the NAAQS is
the State and Local Air Monitoring Stations (SLAMS) network, which comprises about 86% of PM2.5 and 75% of PM10
monitoring sites. Two important subset of SLAMS sites are the National Core (NCore) multipollutant monitoring network and
the near-road monitoring network. The NCore network was designed to collect consistent measurements of criteria pollutants
for trends and NAAQS compliance purposes. NCore was fully operational as of 2011 and consists of approximately 60 urban
monitoring stations and 20 rural monitoring stations. Each state is required to have at least one NCore station. PM2.5
monitoring was required for near-road network sites as part of the 2012 PM2.5 NAAQS review and these sites monitors were
phased into the network between 2015 and 2017. Near-road sites are required in each metropolitan statistical area (MSA)
with a population of 1,000,000 or greater. There were 54 sites reporting PM2.5 data to EPA during the 2019-2021 period.
9
-------
SLAMS (521) NCORE(23) O INDUSTRIAL (91 ) SPM/OTHER (90)
Figure 9: Map of U.S. PMio monitoring sites reporting data to the EPA during the 2019-2021 period. Source: AQS.
SLAMS (801) NCORE (70) O NEAR ROAD (59) SPM/OTHER (139)
Figure 10: Map of U.S. PM2.5 monitoring sites reporting data to the EPA during the 2019-2021 period. Source: AQS.
10
-------
As a result of the 2006 PM NAAQS review, the EPA promulgated a new FRM for the measurement of PMio_2.5 mass in
ambient air. Although the standard for coarse particles uses a PMio indicator, a new FRM for PMiq_2.5 mass was developed
to provide a basis for approving FEMs and to promote the gathering of scientific data to support future reviews of the
PM NAAQS. PMio_2.5 measurements are currently reported at NCore stations, IMPROVE monitoring stations, and at a
few additional locations where state or local agencies choose to operate a PM10-2.5 method. There were 286 monitoring
sites reporting PMio_2.5 data to EPA during the 2019-2021 period. Figure 11 shows the locations of these monitoring sites.
Additionally, some sites that operate both PM10 and PM2.5 monitors also report PMiq_2.5 concentrations by taking the
difference of the two measurements.
Due to the complex nature of fine particles, the EPA and states implemented the Chemical Speciation Network (GSN) to
better understand the components of fine particle mass at selected locations across the country. The GSN was first piloted
at 13 sites in 2000, and after the pilot phase, the program continued with deployment of the Speciation Trends Network
(STN) later that year. The GSN ultimately grew to 54 trends sites and peaked in operation in 2005 with 252 stations: the 54
trends stations and nearly 200 supplemental stations. There were 105 GSN sites reporting data to EPA during the 2019-2021
period. The locations of these sites are shown in Figure 12. Additionally, PM2.5 speciation measurements are collected at
NCore stations, which are also shown in Figure 12.
Specific components of fine particles are also measured through the Interagency Monitoring of Protected Visual Environ-
ments (IMPROVE) monitoring program, which supports the regional haze program and tracks changes in visibility in Glass
I areas as well as many other rural and some urban areas. GSN and IMPROVE data can also be used to better understand
visibility through calculation of light extinction using the IMPROVE algorithm3 to support reviews of the secondary PM
NAAQS. There were 152 IMPROVE sites reporting data to EPA during the 2019-2021 period. The locations of these sites
are shown in Figure 12.
Figure 11: Map of U.S. PM10-2.5 monitoring sites reporting data to the EPA during the 2019-2021 period. Source: AQS.
3Tlie IMPROVE algorithm is ail equation to estimate light extinction based 011 the measured concentration of several PM components and is
used to track visibility progress in the Regional Haze Rule. More information about the IMPROVE algorithm is at available at the IMPROVE
website.
O IMPROVE (156) NCORE (54) O SLAMS (47) O SPM/OTHER (29)
11
-------
CSN (107) IMPROVE (150) O NCORE (49) OTHER (9)
Figure 12: Map of U.S. PM2.5 speciation monitoring sites reporting data to the EPA during the 2019-2021 period. Source:
AQS.
5. Data Handling Conventions and Computations for Determining Whether the Standards
are Met
To assess whether a monitoring site or geographic area (usually a county or urban area) meets or exceeds a NAAQS, the
monitoring data are analyzed consistent with the established regulatory requirements for the handling of monitoring data
for the purposes of deriving a design value. A design value summarizes ambient air concentrations for an area in terms of
the indicator, averaging time and form for a given standard such that its comparison to the level of the standard indicates
whether the area meets or exceeds the standard. The procedures for calculating design values for the current PM NAAQS
(established in 2012) are detailed in Appendix K to 40 GFR Part 50 for PMi0 and in Appendix N to 40 GFR Part 50 for
PM2.5.
Daily 24-hour PM10 samples collected at an ambient air monitoring site using Federal Reference or Equivalent Methods,
meeting all applicable requirements in 40 GFR Part 58, and reported to AQS in micrograms per meter cubed (/tg/m3) with
decimal digits truncated are used in design value calculations. If there are multiple monitors at a site, a separate design
value is calculated for each monitor. First, the number of exceedances of the NAAQS is determined for each calendar quarter
(i.e., Jan/Feb/Mar, Apr/May/Jun, Jul/Aug/Sep, Oct/Nov/Dec) over a 3-yeax period. The level of the PM10 NAAQS is 150
/.tg/m3, but monitored concentrations are rounded to the nearest 10 /.tg/m3 when compared to the NAAQS, so an exceedance
occurs when measured concentrations are 155 /tg/m3 or greater.
To correct for missing data, the observed number of exceedances in each calendar quarter is adjusted by dividing it
by the data completeness rate during that quarter and rounded to the nearest hundredth, which is the expected number of
exceedances for that quarter. This adjustment is performed regardless of sampling schedule, for example, a monitoring site
that has an every 3rd day sampling schedule will have a minimum of 3 expected exceedances for each observed exceedance even
if the data completeness rate is 100%. The annual number of expected exceedances is the sum of the expected exceedances
over the four calendar quarters, and the design value is the average of the annual expected exceedances over three consecutive
years, rounded to the nearest tenth. The PM10 NAAQS are met when the design value is less or equal to 1.0.
A PM10 design value meeting the NAAQS must meet minimum data completeness requirements in order to be considered
valid. Specifically, a monitor must have reported concentrations for a minimum of 75% of the scheduled sampled days in
12
-------
each calendar quarter of the 3-year period in order to be considered valid. A PMio design value greater than the NAAQS is
always considered valid. Appendix K to 40 CFR Part 50 has additional language describing situations where a valid design
value may be derived for a monitor which does not meet these minimum data completeness criteria.
Daily 24-hour PM2.5 samples collected at an ambient air monitoring site using Federal Reference or Equivalent Methods,
meeting all applicable requirements in 40 CFR Part 58, and reported to AQS in (/ig/m3) with decimal digits after the
first decimal place truncated are used in design value calculations. If hourly samples are reported from a continuous PM2.5
monitor, 24-hour average concentrations will be calculated from the hourly data. A calculated 24-hour average concentration
is considered valid if hourly concentrations are available for at least 18 of the 24 hours in a given calendar day, or, if after
substituting zero for the missing hourly concentrations, the resulting average is greater than the level of the 24-hour PM2.5
NAAQS. If multiple monitors are operating at a site, one monitor is designated as the primary monitor. Daily values from
collocated monitors are substituted on days where data is missing for the primary monitor to create a site-level data record.
For the annual PM2.5 NAAQS, the 24-hour concentrations from the site-level data record are averaged over each calendar
quarter for a consecutive 3-year period. The four quarterly averages are then averaged over each year to calculate an annual
average, and finally the annual PM2.5 design value is the average of the three annual average values, rounded to the nearest
tenth. The annual PM2.5 NAAQS are met when the design value is less than or equal to 12.0 /ig/m3. Annual PM2.5 design
values must have a minimum of 75% data completeness in each calendar quarter (according to the sampling schedule for the
site) in order to be considered valid. In addition, for sites which fail to meet the 75% quarterly minimum data completeness,
there are two data substitution tests in Appendix N to 40 CFR Part 50 by which an annual design value above or below the
NAAQS, respectively, may be considered valid.
For the 24-hour PM2.5 NAAQS, the 98th percentile of the 24-hour concentrations from the site-level data record is
calculated for each of the three years. The 24-hour PM2.5 design value is the average of the three 98th percentile values,
rounded to the nearest integer. The 24-hour PM2.5 NAAQS are met when the design value is less than or equal to 35 /ig/m3.
Similar to the annual design values, 24-hour design values must have a minimum of 75% data completeness in each calendar
quarter to be considered valid. In addition, a site with a design value meeting the NAAQS may also be considered valid if it
is able to pass the 24-hour NAAQS data substitution test in Appendix N to 40 CFR Part 50.
6. PM Concentrations Measured at Ambient Air Monitoring Sites Across the U.S.
Table 1 below presents summary statistics based on daily PM10, PM2.5, and PMio_2.5 monitoring data reported to AQS
for 2019 to 2021 for the full year and for each calendar quarter. There are two daily metrics for PM10 and PM2.5: the
daily 24-hour average (DA24) metric, which is available for both filter-based and continuous monitoring instruments, and the
maximum daily 1-hour average (MDA1) metric, which is available only for continuous monitoring instruments. For PMio_2.5,
only filter-based measurements are available, thus only the DA24 metric is shown. Table 2 presents summary statistics for
the same daily metrics based on 2019-2021 PM10, PM2.5, and PMio_2.5 monitoring data for each NOAA Climate Region4.
Figure 13 and Figure 14 show maps of the annual and 24-hour PM2.5 design values, respectively, at U.S. ambient air
monitoring sites based on monitoring data from the 2019-2021 period. All sites in the eastern U.S. were meeting both the
2012 Annual PM2.5 NAAQS of 12 /xg/m3 and the 2006 24-hour PM2.5 NAAQS of 35 /xg/m3 during this period. Many sites
in the western U.S. were still violating the 24-hour NAAQS in 2019-2021, while a smaller number of sites were also violating
the annual NAAQS. Large areas of the Western U.S. were impacted by smoke from wildfires in 2020 and 2021. The highest
annual design values are located in the San Joaquin Valley of California, while the highest 24-hour design values are located
in Mono County, California, which was heavily impacted by wildfire smoke in 2020.
The PM10 NAAQS is unique in that the form of the standard is expressed in terms of expressed in terms of expected
exceedances rather than a concentration-based value. Alternatively, a "design concentration" can be used to show PM10
concentrations that would be expected at each site based on the averaging time and form of the NAAQS. The design
concentration for PM10 is determined using a table lookup procedure5. For example, for a PM10 monitor with 3 years of
complete daily sampling data, the design concentration is the 4th highest 24-hour average concentration measured during
the 3-year period. Figure 15 shows a map of the PM10 design concentrations based on monitoring data from the 2019-2021
period. The overall pattern appears similar to Figure 14, with generally low design concentrations in the eastern U.S. and
higher concentrations in parts of the western U.S. One notable difference is the presence of several sites with high design
concentrations in the central U.S., which is likely due to higher emissions of coarse particulates in those regions. This is
corroborated by Figure 16, which shows the average of the PMio_2.5 concentrations measured at U.S. monitoring sites during
the 2019-2021 period.
4 For Table 2, monitoring sites in Alaska were assigned to the Northwest Region and monitoring sites in Hawaii were assigned to the West region.
BThe table lookup procedure is documented in Section 6.3 of the 1987 EPA guidance document PMw SIP Development Guideline.
13
-------
Table 1. National distribution of PM concentrations in /ig/m3 by quarter based on monitoring data from 2019 to 2021. Source: AQS.
pollutant
metric
quarter
N.sites
N.obs
mean
SD
min
Pi
p5
plO
p25
p50
p75
p90
p95
p98
p99
max
max.site
PM10
DA24
all
725
488,628
21.0
25.0
-48.0
2.0
4.0
6.0
10.0
16.0
26.0
41.0
54.0
75.0
97.0
3,956.0
060510011
PM10
DA24
1st quarter
706
121,978
16.0
21.0
-3.0
1.0
3.0
4.0
7.0
12.0
20.0
30.0
39.0
54.0
66.0
3,427.0
060510011
PM10
DA24
2nd quarter
699
120,695
20.0
24.0
-48.0
2.0
5.0
6.0
10.0
16.0
25.0
38.0
47.0
63.0
79.0
3,956.0
060510011
PM10
DA24
3rd quarter
693
123,021
27.0
29.0
-1.0
4.0
7.0
10.0
14.0
20.0
32.0
49.0
64.0
92.0
120.0
2,310.0
060510011
PM10
DA24
4th quarter
697
122,934
22.0
24.0
-3.0
1.0
4.0
5.0
9.0
15.0
26.0
44.0
60.0
85.0
111.0
1,343.0
060510011
PM10
MDA1
all
440
396,413
59.0
161.0
-6.0
6.0
10.0
14.0
21.0
34.0
61.0
111.0
166.0
280.0
417.0
35,585.0
060510011
PM10
MDA1
1st quarter
422
98,667
45.0
158.0
-1.0
5.0
8.0
11.0
17.0
28.0
48.0
83.0
120.0
200.0
292.0
26,803.0
060510011
PM10
MDA1
2nd quarter
424
98,220
58.0
185.0
1.0
7.0
11.0
15.0
22.0
35.0
60.0
105.0
154.0
269.0
402.0
35,585.0
060510011
PM10
MDA1
3rd quarter
422
99,620
71.0
171.0
0.0
10.0
15.0
18.0
26.0
42.0
73.0
133.0
200.0
344.0
511.0
28,161.0
040213014
PM10
MDA1
4th quarter
424
99,906
61.0
121.0
-6.0
6.0
9.0
13.0
20.0
34.0
66.0
120.0
179.0
295.0
440.0
11,508.0
060510011
PM2.5
DA24
all
1,067
801,756
8.1
8.1
-6.7
0.5
2.0
2.8
4.5
6.7
9.9
14.1
17.8
24.1
31.0
824.1
060510005
PM2.5
DA24
1st quarter
1,034
197,097
7.7
5.4
-6.7
0.4
1.8
2.6
4.2
6.5
9.8
14.0
17.5
22.4
26.4
222.4
040130019
PM2.5
DA24
2nd quarter
1,037
198,683
6.8
4.1
-4.8
0.5
1.8
2.6
4.1
6.0
8.6
11.7
13.9
17.2
20.1
134.2
350130022
PM2.5
DA24
3rd quarter
1,027
202,940
9.7
12.8
-5.1
1.0
2.6
3.5
5.2
7.6
10.9
15.7
21.0
33.7
49.3
824.1
060510005
PM2.5
DA24
4th quarter
1,033
203,036
8.2
6.8
-4.0
0.5
1.9
2.7
4.4
6.8
10.3
15.0
19.0
25.2
30.9
487.4
060510001
PM2.5
MDA1
all
781
695,394
16.3
18.6
-6.4
3.0
5.0
6.1
9.0
13.0
18.9
27.4
36.0
53.0
72.8
1,467.2
060510001
PM2.5
MDA1
1st quarter
737
169,510
16.0
13.1
-3.0
3.0
5.0
6.0
9.0
13.1
19.5
27.9
35.0
46.4
57.9
726.1
040130019
PM2.5
MDA1
2nd quarter
750
172,135
13.8
11.7
-5.0
3.0
5.0
6.0
8.0
11.5
16.3
22.9
28.9
39.6
51.0
896.0
230030014
PM2.5
MDA1
3rd quarter
750
176,262
18.6
27.8
-5.0
4.0
6.0
7.0
9.3
13.2
19.0
29.0
43.0
78.0
122.1
1,467.2
060510001
PM2.5
MDA1
4th quarter
761
177,487
16.9
16.8
-6.4
3.0
5.0
6.1
9.0
13.5
20.0
29.9
38.7
54.0
69.0
1,464.4
060510001
PM10-2.5
DA24
all
202
54,632
5.3
6.2
-3.8
0.0
0.2
0.5
1.4
3.4
6.9
12.2
16.7
22.9
28.9
162.0
060190011
PM10-2.5
DA24
1st quarter
202
15,377
3.5
4.8
-3.0
-0.1
0.1
0.2
0.7
1.8
4.3
8.6
12.4
17.8
22.5
108.1
230190002
PM10-2.5
DA24
2nd quarter
202
15,553
5.7
5.6
-2.6
0.1
0.5
0.9
2.0
4.1
7.4
12.3
16.3
21.3
25.8
96.0
530390003
PM10-2.5
DA24
3rd quarter
199
12,081
7.1
6.8
-3.8
0.4
1.1
1.6
2.9
5.2
9.0
15.0
19.3
25.8
31.3
162.0
060190011
PM10-2.5
DA24
4th quarter
198
11,621
5.2
7.4
-2.6
0.0
0.1
0.3
1.0
2.7
6.4
12.8
19.0
27.3
33.9
106.9
060190011
N.sites = number of sites; N.obs = number of observations; SD = standard deviation; min = minimum; pi, p5, plO, p25, p50, p90, p95, p98, p99 = 1st,
5th, 10th, 25th, 50th, 90th, 95th, 98th, 99th percentiles; max = maximum; max.site = AQS ID number for the monitoring site corresponding to the obser-
vation in the max column. 1st quarter = January/February/March; 2nd quarter = April/May/June; 3rd quarter = July/August/September; 4th quarter =
October/November/December.
Table 2. National distribution of PM concentrations in /ig/m3 by climate region based on monitoring data from 2019 to 2021. Source: AQS. N.sites = number
of sites; N.obs = number of observations; SD = standard deviation; min = minimum; pi, p5, plO, p25, p50, p90, p95, p98, p99 = 1st, 5th, 10th, 25th, 50th, 90th,
95th, 98th, 99th percentiles; max = maximum; max.site = AQS ID number for the monitoring site corresponding to the observation in the max column. Central
= Illinois, Indiana, Kentucky, Missouri, Ohio, Tennessee, West Virginia; East North Central = Iowa, Minnesota, Michigan, Wisconsin; Northeast = Connecticut,
Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont; Northwest = Alaska, Idaho, Oregon,
Washington; South = Arkansas, Kansas, Louisiana, Mississippi, Oklahoma, Texas; Southeast = Alabama, Florida, Georgia, North Carolina, South Carolina,
Virginia; Southwest = Arizona, Colorado, New Mexico, Utah; West = California, Hawaii, Nevada; West North Central = Montana, Nebraska, North Dakota,
South Dakota, Wyoming.
-------
pollutant metric region
N.sites N.obs mean SD min pi p5 plO p25 p50 p75 p90 p95 p98 p99 max max.site
PM10
DA 24
PM10
DA 24
PM10
DA 24
PM10
DA 24
PM10
DA 24
PM10
DA 24
PM10
DA 24
PM10
DA 24
PM10
DA 24
PM10
DA 24
PM10
MDA1
PM10
MDA1
PM10
MDA1
PM10
MDA1
PM10
MDA1
PM10
MDA1
PM10
MDA1
PM10
MDA1
PM10
MDA1
PM10
MDA1
PM2.5
DA 24
PM2.5
DA 24
PM2.5
DA 24
PM2.5
DA 24
PM2.5
DA 24
PM2.5
DA 24
PM2.5
DA 24
PM2.5
DA 24
PM2.5
DA 24
PM2.5
DA 24
PM2.5
MDA1
PM2.5
MDA1
PM2.5
MDA1
PM2.5
MDA1
PM2.5
MDA1
PM2.5
MDA1
PM2.5
MDA1
PM2.5
MDA1
PM2.5
MDA1
PM2.5
MDA1
PM10-2.5
DA 24
PM10-2.5
DA 24
PM10-2.5
DA 24
PM10-2.5
DA 24
PM10-2.5
DA 24
PM10-2.5
DA 24
PM10-2.5
DA 24
PM10-2.5
DA 24
PM10-2.5
DA 24
PM10-2.5
DA 24
all 725 488,628 21.0 25.0 -48.0 2.0 4.0 6.0 10.0 16.0 26.0 41.0 54.0 75.0 97.0 3,956.0 060510011
Central 66 33,203 20.0 13.0 0.0 3.0 6.0 7.0 11.0 16.0 24.0 35.0 44.0 58.0 68.0 205.0 295100093
East North Central 41 22,683 19.0 13.0 0.0 3.0 6.0 8.0 11.0 16.0 24.0 34.0 43.0 56.0 68.0 169.0 191630017
Northeast 57 30,335 14.0 9.0 0.0 2.0 4.0 6.0 8.0 12.0 18.0 24.0 30.0 38.0 45.0 161.0 230031011
Northwest 35 22,647 18.0 27.0 -1.0 1.0 3.0 4.0 8.0 13.0 21.0 34.0 46.0 68.0 94.0 1,012.0 530050002
South 51 23,779 20.0 15.0 0.0 3.0 6.0 8.0 12.0 17.0 24.0 35.0 44.0 57.0 71.0 440.0 201950001
Southeast 75 49,264 17.0 9.0 0.0 3.0 6.0 8.0 11.0 15.0 20.0 26.0 32.0 42.0 51.0 453.0 720330004
Southwest 97 85,641 28.0 26.0 -2.0 3.0 5.0 8.0 13.0 22.0 35.0 51.0 65.0 88.0 110.0 1,228.0 040213014
West 167 120,598 26.0 38.0 -48.0 2.0 4.0 6.0 11.0 19.0 32.0 50.0 68.0 102.0 133.0 3,956.0 060510011
West North Central 136 100,478 16.0 17.0 -3.0 1.0 3.0 4.0 6.0 11.0 20.0 33.0 45.0 62.0 76.0 1,354.0 560370013
all 440 396,413 59.0 161.0 -6.0 6.0 10.0 14.0 21.0 34.0 61.0 111.0 166.0 280.0 417.0 35,585.0 060510011
Central 30 27,088 47.0 60.0 2.0 8.0 12.0 15.0 21.0 31.0 51.0 87.0 124.0 196.0 272.0 2,000.0 390170020
East North Central 24 18,655 44.0 49.0 2.0 8.0 12.0 14.0 20.0 30.0 49.0 84.0 119.0 181.0 234.0 985.0 270531909
Northeast 27 25,438 31.0 30.0 1.0 6.0 10.0 12.0 16.0 24.0 36.0 57.0 79.0 108.0 139.0 1,278.0 090092123
Northwest 26 21,924 50.0 82.0 0.0 5.0 8.0 11.0 18.0 31.0 53.0 95.0 139.0 244.0 365.0 4,366.0 530050002
South 24 19,706 42.0 66.0 1.0 8.0 12.0 15.0 22.0 31.0 47.0 72.0 98.0 154.0 217.0 4,205.0 201950001
Southeast 56 46,479 34.0 28.0 -6.0 9.0 13.0 15.0 20.0 27.0 38.0 57.0 76.0 108.0 137.0 1,140.0 120952002
Southwest 70 72,099 94.0 198.0 0.0 8.0 14.0 19.0 32.0 55.0 97.0 173.0 271.0 495.0 766.0 28,161.0 040213014
West 125 114,882 70.0 240.0 -1.0 7.0 11.0 15.0 24.0 41.0 72.0 127.0 191.0 314.0 475.0 35,585.0 060510011
West North Central 58 50,142 44.0 65.0 0.0 4.0 6.0 8.0 14.0 26.0 49.0 93.0 141.0 229.0 313.0 1,719.0 560010800
all 1,067 801,756 8.1 8.1 -6.7 0.5 2.0 2.8 4.5 6.7 9.9 14.1 17.8 24.1 31.0 824.1 060510005
Central 164 110,189 8.9 4.7 -4.9 1.9 3.3 4.1 5.7 8.0 11.1 14.8 17.7 21.6 24.8 89.5 180890022
East North Central 92 71,955 7.7 5.6 -5.1 0.2 1.8 2.7 4.2 6.5 9.7 13.9 17.3 22.0 25.8 208.7 270072304
Northeast 171 141,143 7.5 4.6 -4.0 1.0 2.3 3.1 4.6 6.6 9.3 12.7 15.5 19.8 24.0 129.2 230030014
Northwest 51 37,484 8.6 17.3 -3.0 0.4 1.5 2.1 3.4 5.4 9.0 15.5 21.9 34.1 55.4 593.0 410130100
South 107 76,755 8.8 5.0 -1.0 1.8 3.2 4.0 5.6 7.8 10.9 14.6 17.6 21.8 25.5 110.6 482010024
Southeast 163 109,669 7.9 4.1 -4.5 1.5 3.0 3.8 5.2 7.1 9.7 12.9 15.3 18.8 21.7 60.7 132150012
Southwest 77 66,204 7.6 6.2 -3.6 1.0 2.0 2.7 4.0 6.0 9.0 14.0 18.6 25.5 32.3 222.4 040130019
West 165 134,561 9.1 13.4 -6.7 0.3 1.5 2.2 3.8 6.4 10.4 16.6 23.7 38.3 55.0 824.1 060510005
West North Central 75 53,294 5.8 7.2 -4.8 -0.7 0.5 1.1 2.4 4.2 6.9 11.5 16.5 24.9 34.4 276.5 300530018
all 781 695,394 16.3 18.6 -6.4 3.0 5.0 6.1 9.0 13.0 18.9 27.4 36.0 53.0 72.8 1,467.2 060510001
Central 93 87,972 17.4 13.0 -0.4 4.8 6.8 8.1 10.9 15.0 20.7 28.1 34.5 45.2 55.9 622.4 180890022
East North Central 71 61,418 14.9 15.2 -2.0 3.0 4.4 5.7 8.0 12.0 18.0 26.1 33.0 44.0 56.3 876.0 191530030
Northeast 134 132,871 13.9 10.7 -1.8 3.4 5.0 6.0 8.3 12.0 16.9 23.0 28.1 37.0 46.0 916.0 230030014
Northwest 33 27,264 19.1 29.2 -1.0 3.0 5.0 6.0 8.0 12.0 20.0 35.0 51.0 83.9 128.3 943.0 530330080
South 72 65,132 17.3 14.1 -1.0 4.7 6.8 8.0 10.7 14.4 20.0 28.0 35.0 48.9 64.5 621.4 482011039
Southeast 104 86,880 15.1 10.9 -5.0 4.7 6.2 7.2 9.4 13.0 17.7 24.0 29.9 40.7 52.2 727.0 371230001
Southwest 68 63,106 17.1 19.6 -1.1 2.7 4.7 6.0 8.3 12.5 19.5 31.0 41.9 63.0 86.4 836.0 350130022
West 140 120,137 19.3 30.2 -5.0 3.0 5.0 6.0 9.0 13.0 20.7 34.0 48.5 80.0 124.0 1,467.2 060510001
West North Central 65 50,265 14.4 16.3 -6.4 2.0 3.9 4.8 6.7 10.1 17.0 27.0 36.9 55.1 73.0 673.5 301110087
all 202 54,632 5.3 6.2 -3.8 0.0 0.2 0.5 1.4 3.4 6.9 12.2 16.7 22.9 28.9 162.0 060190011
Central 13 3,437 5.4 4.7 -0.5 0.2 0.9 1.3 2.3 4.3 6.9 10.8 14.0 19.5 23.3 42.9 390350060
East North Central 15 4,547 7.4 7.5 -2.0 0.0 0.3 0.6 1.9 5.0 10.3 17.7 22.3 27.9 31.6 74.4 191550009
Northeast 26 6,624 3.9 3.8 -1.0 0.1 0.4 0.8 1.5 3.1 5.3 7.9 10.0 13.2 16.0 108.1 230190002
Northwest 25 7,103 3.2 5.4 -3.8 -0.1 0.0 0.2 0.6 1.6 3.7 7.2 11.1 19.3 28.4 96.0 530390003
South 14 3,420 6.7 5.8 -2.0 0.2 0.8 1.4 2.8 5.3 8.8 13.5 16.9 22.3 27.4 69.8 481090101
Southeast 21 5,435 5.4 5.0 -3.0 0.1 0.6 1.1 2.3 4.3 6.9 10.6 13.6 19.6 24.2 56.8 130499000
Southwest 38 10,863 5.8 6.7 -2.6 0.0 0.2 0.5 1.5 3.5 7.8 14.3 18.6 24.9 30.7 100.3 040139997
West 29 7,746 6.7 8.3 -0.3 0.0 0.3 0.6 1.5 4.2 9.2 15.6 20.2 28.8 36.5 162.0 060190011
West North Central 20 5,201 3.0 3.8 -0.9 0.0 0.1 0.2 0.6 1.7 3.9 7.5 10.2 15.0 18.2 48.0 300779000
-------
1.8 - 6.0 ug/mA3 (102 sites) O 9.1 - 12.0 ug/mA3 (144 sites) 15.1 - 17.8 ug/mA3 (9 sites)
O 6.1 - 9.0 ug/mA3 (468 sites) O 12.1 - 15.0 ug/mA3 (18 sites)
Figure 13: Annual PM2.5 design values in /.tg/m3 for the 2019-2021 period. Source: AQS.
5 - 15 ug/mA3 (96 sites) O 26 - 35 ug/mA3 (87 sites) 51-100 ug/mA3 (44 sites)
O 16-25 ug/mA3 (507 sites) O 36 - 50 ug/mA3 (32 sites) 101-181 ug/mA3 (2 sites)
Figure 14: 24-hour PM2.5 design values in /.tg/m3 for the 2019-2021 period. Source: AQS.
16
-------
18 - 50 ug/mA3 (55 sites) O 101
O 51-100 ug/mA3 (152 sites) O 151
Figure 15: PMio design concentrations in /.tg/m3 for
- 150 ug/mA3 (89 sites) 201 - 500 ug/mA3 (54 sites)
- 200 ug/mA3 (63 sites) 501 - 2310 ug/mA3 (7 sites)
the 2019-2021 period. Source: AQS.
0 - 1.9 ug/mA3 (33 sites) o 4 - 5.9 ug/mA3 (44 sites) 10 - 22 ug/mA3 (23 sites)
o 2-3.9 ug/mA3 (58 sites) o 6-9.9 ug/mA3 (42 sites)
Figure 16: Average PM10-2.5 concentrations in /.tg/m3 for the 2019-2021 period. Source: AQS.
17
-------
Figure 17 and Figure 18 show site-level trends in the annual and 24-hour PM2.5 design values, respectively, for sites having
valid design values in at least 15 of the 20 3-year periods from 2000-2002 through 2019- 2021. The trends were computed
using the Thiel-Sen estimator, and tests for significance were computed using the Mann-Kendall test. From this figure it is
apparent that most of the U.S. has experienced significant decreasing trends in both the annual and 24-hour PM2.5 design
values over the past two decades, especially in the eastern U.S., where regional control programs such as the Clean Air
Interstate Rule (CAIR) and the Cross-State Air Pollution Rule (CSAPR) have enabled large reductions in NOx and SO2
emissions, which led to long-term reductions in secondary PM2.5 components. There has been less progress in the western
U.S., where most controls to-date have focused on local reductions, and emissions from wildfires in recent years have caused
increases in PM2.5 concentrations in some areas.
Figure 19 shows site-level trends in PM10 design concentrations for sites having valid design values in at least 17 of
the 22 3-year periods from 1998-2000 through 2019-2021, while Figure 20 shows site-level trends in annual average PM10-2.5
concentrations for sites having data for at least 13 of the 17 years from 2005 to 2021. The trends in PM10 design concentrations
are much more variable than those for PM2.5. While trends in the eastern U.S. are decreasing in most locations, there is
no clear pattern in the western U.S., with sites even in close proximity sometimes having trends in opposite directions.
Nationally, over half of the sites had no significant trend. The reason for this is apparent from Figure 20, which shows no
clear trend in coarse particulate concentrations at the vast majority of U.S. monitoring sites.
Figure 21 shows the national trends in the annual and 24-hour PM2.5 design values based on the 381 sites in Figure 17
and the 419 sites in Figure 18. Both the annual and 24-hour design values exhibited steady decreases from 2002 to 2016. In
recent years, the median annual design value has remained relatively constant at about 8 /ig/m3 while the 10th and 90th
percentile trends have also remained relatively flat at about 6 /ig/m3 and 10 /ig/m3, respectively. The 10th percentile and
median of the 24-hour design values, which are based on the annual 98th percentile, have also remained relatively constant
at about 15 /ig/m3 and 20 /ig/m3, respectively, since 2016. However, the 90th percentile of the 24-hour design values has
increased substantially in the past 5 years largely as a result of increased wildfire activity in the western U.S.
Figure 22 shows the national trend in the PM10 design concentration based on the 438 sites in Figure 19. The national
median of the design concentrations has remained flat over the past two decades, though there has been an increase of about
20 /ig/m3 since 2016. The 10th percentile design concentration has decreased slowly over the full trends period, while the
90th percentile has been highly variable, most likely as a result of year-to-year fluctuations in weather conditions and wildfire
emissions.
18
-------
^ Decreasing > 0.5 ug/mA3/yr (69 sites) o No Significant Trend (30 sites)
V Decreasing < 0.5 ug/mA3/yr (279 sites) A Increasing < 0.5 ug/mA3/yr (3 sites)
Figure 17: Site-level trends in annual PM2.5 design values based on data from 2002 through 2021. Source: AQS, trends
computed using R statistical software.
^ Decreasing > 1 ug/mA3/yr (212 sites) A Increasing < 1 ug/mA3/yr(7 sites)
V Decreasing < 1 ug/mA3/yr (130 sites) A Increasing > 1 ug/mA3/yr (2 sites)
o No Significant Trend (68 sites)
Figure 18: Site-level trends in 24-hour PM2.5 design values based on data from 2002 through 2021. Source: AQS, trends
computed using R statistical software.
19
-------
o No Significant Trend (249 sites)
Figure 19: Site-level trends in PMio design concentrations based on data from 2000 through 2021. Source: AQS, trends
computed using R statistical software.
^ Decreasing > 0.2 ug/mA3/yr (2 sites) o No Significant Trend (114 sites)
V Decreasing < 0.2 ug/mA3/yr (14 sites) A Increasing < 0.2 ug/mA3/yr (7 sites)
Figure 20: Site-level trends in annual average PM10-2.5 concentrations based on data from 2005 through 2021. Source:
AQS, trends computed using R statistical software.
20
-------
Figure 21: National trend in PM2.5 design values in /xg/m3, 2002 to 2021. Source: AQS.
o
O^C\JCO^fUOCQh-COCT)O^C\JCO^fLroCQh-COCT)0-<-
OOOOOOOOOOt-t-t-t-t-t-t-t-t-t-C\|C\I
0000000000000000000000
C\IC\IC\IC\IC\IC\IC\ICMCMCMC\IC\IC\IC\IC\IC\IC\IC\IC\IC\IC\ICM
Figure 22: National trend in PM10 design concentrations in /xg/m3, 2000 to 2021. Source: AQS.
21
-------
Figure 23 has a map with pie charts showing the major PM2.5 species as a fraction of total mass as measured by selected
NCore, CSN, and IMPROVE sites during the 2019 to 2021 period. The six species shown are sulfates (SO4), nitrates (NO3),
elemental carbon (EC), organic carbon (OC), crustal material, and sea salt. The pie charts are located at each monitoring
site on the map. This figure portrays several aspects of regional variability in PM2.5, for example, large portions of total
PM2.5 can be attributed to sulfates in the Appalachian region, nitrates in the upper Midwest, OC in the Pacific Northwest,
crustal material in the southwest, and sea salt in coastal areas.
Figure 24 shows the average concentrations for four PM2.5 components (sulfates, nitrates, EC, and OC) based on data
collected during the 2019 to 2021 period. From this figure it is apparent that sulfate concentrations are highest in the Ohio
River valley and along the Gulf of Mexico, while nitrate concentrations are highest in the upper Midwest, along the mid-
atlantic urban corridor, and in parts of California. EC and OC are spatially more variable, with the highest sites scattered
across the country. EC concentrations tend to be higher near urban areas, especially those with large industrial sources,
while OC tends to be more concentrated in rural areas, with impacts from prescribed burns, wildfires, and residential wood
smoke.
Figure 25 shows trends in annual average concentrations for sulfates, nitrates, EC, and OC based on sites that collected
data for at least 12 out of 16 years from 2006 to 20216. Broad national reductions in SO2 emissions have resulted in significant
reductions in sulfate concentrations nationally and especially in the eastern U.S. Similarly, reductions in NOx emissions have
resulted in significant decreasing trends in nitrates in most of the U.S., especially in areas where nitrate concentrations were
historically highest. EC and OC concentrations were more variable, with some sites showing significant decreases and the
remaining sites having no clear trend.
6Although PM2.5 speciation monitoring has been conducted since 2000, the trends in Figure 25 begin in 2006 to avoid losing CSN sites, which
experienced a change in EC and OC sampling methods between 2007 and 2010.
22
-------
to
CO
ฆNitrates
Sulfates
CrustaP
Figure 23: Map showing pie charts of PM2.5 component species at selected U.S. monitoring sites based on 2019-2021 data. Source: AQS.
-------
to
0-0.49 ug/mA3 (155 sites) o 1-1.49 ug/mA3 (49 sites) 2-3.42 ug/mA3 (8 sites) 0-0.99 ug/mA3 (92 sites) O 2-2.99 ug/mA3 (88 sites) 4-6.72 ug/mA3 (6 sites)
O 0.5 - 0.99 ug/mA3 (66 sites) o 1.5-1.99 ug/mA3 (25 sites) o 1-1.99 ug/mA3 (106 sites) 3 - 3.99 ug/mA3 (11 sites)
Figure 24: Average concentrations for sulfates (top left), nitrates (bottom left), elemental carbon (top right), and organic carbon (bottom right) at U.S. monitoring
sites based on 2019-2021 data. Source: AQS.
0.49 ug/mA3 (92 sites) o 1 - 1.49 ug/mA3 (75 sites)
- 0.99 ug/mA3 (133 sites) o 1.5 - 1.99 ug/mA3 (4 sites)
2-2.37 ug/mA3 (2 sites)
0-0.24 ug/mA3 (129 sites) o 0.5-0.74 ug/mA3 (59 sites) 1-1.61 ug/mA3 (22 sites)
o 0.25- 0.49 ug/mA3 (59 sites) o 0.75- 0.99 ug/mA3 (34 sites)
Nitrates
-------
^ Decreasing > 0.1 ug/mA3/yr (109 sites) o No Significant Trend (1 sites)
V Decreasing < 0.1 ug/mA3/yr (144 sites)
^ Decreasing > 0.1 ug/mA3/yr (3 sites) o No Significant Trend (164 sites)
V Decreasing < 0.1 ug/mA3/yr (69 sites) A Increasing < 0.1 ug/mA3/yr (3 sites)
to
Cn
^ Decreasing > 0.1 ug/mA3/yr (11 sites) o No Significant Trend (116 sites)
V Decreasing < 0.1 ug/mA3/yr (124 sites)
^ Decreasing > 0.1 ug/mA3/yr (5 sites) o No Significant Trend (199 sites)
V Decreasing <0.1 ug/mA3/yr (32 sites) A Increasing <0.1 ug/mA3/yr (3 sites)
Figure 25: Site-level trends in annual average concentrations for sulfates (top left), nitrates (bottom left), elemental carbon (top right), and organic carbon
(bottom right) based on data from 2006 through 2021. Source: AQS, trends computed using R statistical software.
-------
Additional Resources
Particulate Matter (PM) Pollution
Reviewing National Ambient Air Quality Standards (NAAQS): Scientific and Technical Information
Air Emissions Inventories
Ambient Monitoring Technology Information Center (AMTIC)
Air Quality Design Values
National Air Quality: Status and Trends of Key Air Pollutants
Air Data: Air Quality Data Collected at Outdoor Monitors Across the U.S.
26
------- |