An Evaluation of Current PM2.5 Conditions in Various
Cities in the US
Paper Number: 491
Shao-Hang Chu and Joseph W. Paisie
US Environmental Protection Agency
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
ABSTRACT
In this study, all available PM2.5 sites in the US with more than five years of FRM data
are studied. The critical design values for each site are calculated. The critical design
value concept developed by Chu (2000)1 has proven to be a good predictor of the
likelihood of future violation of the NAAQS based on the existing design values and their
inter-annual variability. Thus, the potential for future violation of PM2.5 standards
among these sites can be estimated and compared. The results suggest that given the
current PM2.5 NAAQS, most of the high-risk areas of potential future violation of the
annual standard are in the East and California. However, only California and a few
isolated areas in the West are at risk of violation of the 24-hr standard in the near future.
The higher risk of violating the PM2.5 annual NAAQS in the East and West is largely
due to the existing high level of average design value concentrations. On the other hand,
the risk of violation of the 24-hour NAAQS in the West is largely attributable to interannual variability, particularly in natural emissions (such as forest fires, dust storms and
volcanic activities), since these phenomena play a significant role in the quick rise of
short-term PM2.5 levels in the West. The more frequent occurrence of wildfires and dust
storms in the West is known to be associated with its much drier climate and
meteorological conditions. On the other hand, higher SO2 emissions in a more humid
East, particularly in the summer, lead to much higher sulfate concentrations which, in
turn, enhance the secondary organic aerosol production. Thus, the much higher annual
PM2.5 design values in the East are largely due to the high level of sulfate and organic
aerosol concentrations. These results may be used by the regulators to identify seriously
polluted areas and prioritize their regional control strategies.
INTRODUCTION
The air quality design value is a mathematically determined concentration at a particular
monitoring site that sets the stage for air quality planning. If this value is above the level
of the national ambient air quality standards (NAAQS), emissions must be reduced in
order to attain the NAAQS2,3,4,5. If the value is below the NAAQS, it is necessary to
maintain emissions at a level necessary to continue to maintain the NAAQS. The
detailed calculation of the design values for various criteria pollutants is described in the
Appendices of the Code of Federal Regulations, which contains the NAAQS6.
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The design value, however, varies from year to year due to both the changes in
anthropogenic pollutant emissions and natural variability, such as meteorological
conditions, wildfires, dust storms, volcanic activities, etc. The critical design value
concept first developed by Chu (2000)1 has proven to be a good statistical measure of the
future violation of the NAAQS with a carefully selected risk factor.
In this paper, an effort was made to analyze the ambient PM2.5 mass data from all
available federal reference method (FRM) monitoring sites nationwide in the last 5 years.
The critical design value and the probability of future violation for each site are
calculated based on its current level and inter-annual variability.
CRITICAL DESIGN VALUE APPROACH
Chu (2000)1 gave a detailed description of the critical design value approach. Here, we
will only briefly discuss the concept and its calculation method. A critical design value
(CDV) is defined as the highest possible average design value (ADV) any site could have
before it risks a future violation of the standard at a certain probability. If we assume that
the design value (DV) is normally distributed and its coefficient of variation (CV) does
not change in the near future, then we can write the relationship as:
CDV = NAAQS/(1+tc*CV)
(1)
Where CDV is the critical design value, CV is the coefficient of variation of the annual
design values adjusted for the long-term trend (the ratio of standard deviation divided by
the mean design value in the past), and tc is the critical t-value corresponding to a
probability,  %, of exceeding the NAAQS in the future. Equation (1) says that the CDV
is the highest ADV any monitoring site could have before it runs a risk of violating the
NAAQS in the future at a probability of  %. The percent probability, , is a risk factor
chosen by the policy maker. One can choose either a more, or less, conservative  value
depending on how much risk one is willing to take.
Using the air quality data collected in the past, one can calculate the annual design values
and their mean and standard deviation. Thus, one can calculate the CDV for any site with
a minimum of five years of data.
Figure 1 is an example that illustrates the relationship among CDV, ADV and NAAQS at
different CV’s. Here we have chosen a risk factor of 10% probability of future violation.
In this example, we see that the CDV depends strongly on the average design value and
its inter-annual variability as measured by the standard deviation. In Figure 1, it shows
that higher risk of violation of the NAAQS can either result from an elevated ADV (as
shown in the left panels) or an increased inter-annual variability (as shown in the right
panels).
Contrasting the upper and lower panels of Figure 1, we also see that whether a site will
have a higher or lower risk of violating the NAAQS in the future depends on the
difference between the ADV and CDV. Thus, unless a drastic change in emissions will
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occur in the future, the CDV can be used as a yardstick to assess the likelihood of a site to
violate the NAAQS in the future at a specific probability (in this example, it is one in
ten). For this reason, this technique and the estimated CDV can be used as a planning
tool for regulatory agencies to decide whether more or less controls are needed in a
specific area.
Figure 1.
A RISK ASSESSMENT OF CURRENT PM2.5 CONDITIONS
Applying this approach to evaluate current fine particulate conditions nationwide and
their future risk of violating the NAAQS, the PM2.5 FRM data collected from 1999 to
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2003 are analyzed. A tolerable risk factor of 10% probability of future violation of the
NAAQS is used to calculate the CDVs for all FRM monitor sites with five years of
complete data. The assessment is discussed and presented in the following figures.
In Figure 2 the upper panels are longitudinal scatter plots of the paired ADVs and CDVs,
and the lower panels are the inter-annual variability of the DVs at all available sites from
California to Maine. They are plotted to see whether there is a difference from the West
to East. The actual locations of these sites (color-coded by probability of execeedances)
are shown in Figure 3. The probability of future violation of the NAAQS at each site is
Figure 2.
calculated by inverting the t-values using equation (1). Comparing the differences
between these overlaid ADVs and CDVs (upper panels of Figure 2) and the actual
probability of future violation of the NAAQS (Figure 3), we see clearly that, for potential
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Figure 3.
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future violation of the annual NAAQS, most of the high-risk areas are in the Eastern U.S.
and California. However, for future 24-hr NAAQS violations, almost all high-risk areas
are in California. This is a reflection of the fact that the constraining PM2.5 NAAQS is
the annual form of the standard. It is of interest to note the major reasons behind this
risk. From Figures 2 and 3 we see that at a large number of sites the current ADVs are
above the CDVs. Among these sites, quite a few are already at or above the NAAQS.
The lower panels of Figure 2 are longitudinal plots of the variability of design values at
each site. It is worthy noting that, while the inter-annual variability of the annual design
values is about the same in all high risk areas, the inter-annual variability of the 24-hr
design values in the West is double that of the East. Combining the upper and lower
panels of Figure 2 we see clearly that the higher risk of violating the PM2.5 annual
NAAQS in the East and West is largely due to the existing high levels of average design
value concentrations. However, the risk of violation of the 24-hour NAAQS in the West
is largely attributable to inter-annual variability, particularly in natural emissions (such as
forest fires, dust storms and volcanic activities), since these phenomena play a significant
role in the quick rise of short-term PM2.5 levels in the West. The more frequent
occurrence of wildfires and dust storms in the West is known to be associated with its
much drier climate and meteorological conditions. On the other hand, higher SO2
emissions in a more humid East, particularly in the summer, lead to much higher sulfate
concentrations, which, in turn, enhance the secondary organic aerosol production7,8,9,10.
Thus, the much higher annual PM2.5 design values in the East are largely due to the high
level of sulfate and organic aerosol concentrations.
SUMMARY AND CONCLUSIONS
In this paper, five years of PM2.5 FRM data nationwide were studied. The potential for
future NAAQS violation was evaluated using the critical design value approach
developed by Chu (2000)1. These results suggest that given the current PM2.5 NAAQS,
most of the high-risk areas of potential future violation of the annual standard are in the
East and California. However, only California and a few isolated areas in the West are at
risk of future violation of 24-hr standard. The higher risk of violating the PM2.5 annual
NAAQS in the East and West is largely due to the existing high levels of average design
value concentrations. However, the risk of violation of the 24-hour NAAQS in the West,
is largely attributable to inter-annual variability, particularly in natural emissions (such as
forest fires, dust storms and volcanic activities), since these phenomena play a significant
role in the quick rise of short-term PM2.5 levels in the West. The more frequent
occurrence of wildfires and dust storms in the West is known to be associated with its
much drier climate and meteorological conditions. On the other hand, higher SO2
emissions in a more humid East, particularly in the summer, lead to much higher sulfate
concentrations which, in turn, enhance the secondary organic aerosol production. Thus,
the much higher PM2.5 annual design values in the East are largely due to the high level
of sulfate and organic aerosol concentrations. These results may be used by the
regulators to identify seriously polluted areas and prioritize their regional control
strategies.
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ACKNOWLEDGMENTS
The authors would like to give thanks to Dr. Terence Fitz-Simons and Dr. Karen Martin
for reading the draft manuscript and Connie Chu for her help in preparing the manuscript.
REFERENCES
1. Chu, Shao-Hang, Critical Design Value Estimation and Its Applications, Presented at
the 93rd AWMA Annual Meeting, San Diego, CA, 2000.
2. US EPA, PM10 SIP Development Guideline. EPA-450/2-86-001, US Environmental
Protection Agency, Research Triangle Park, NC 27711, 1986.
3. US EPA, Guideline on Air Quality Models (Revised). EPA-450/2-78-27 R. U.S.
Environmental Protection Agency, Research Triangle Park, NC 27711, 1986.
4. US EPA, Guideline for the Interpretation of Ozone Air Quality Standards. EPA450/4-79-003, US Environmental Protection Agency, Research Triangle Park, NC
27711, 1979.
5. Curran, T.C. and W.M. Cox, Data Analysis Procedures for the Ozone NAAQS
Statistical Format. J. Air Pollution Control Association, 1980.
6. Code of Federal Regulations, Protection of Environment, 40 CFR part 50, 1998.
7. Jang, M., Czoschke, N.M., Lee S., Kamens, R., Heterogeneous Atmospheric Aerosol
Production by Acid-catalyzed Particle-phase Reaction. Science 298, 814-817, 2002.
8. Chu, Shao-Hang, PM2.5 Episodes as Observed in the Speciation Trends Network,
Atmospheric Environment 38, 5237-5246, 2004.
9. Chu, Shao-Hang, Paisie, J.W., and Jang, B.W.-L., PM Data Analysis – a Comparison
of Two Urban Areas: Fresno and Atlanta, Atmospheric Environment 38, 5155-3164,
2004.
10. Gao, S., Ng, N. L., Keywood, M., Varutbangkul, V., Bahreini, R., Nenes, A., He, J.,
Yoo, K. Y., Beauchamp, J. L., Hodyss, R. P., Flagan, R. C., Seinfeld, J. H., Particle
Phase Acidity and Oligomer Formation in Secondary Organic Aerosol. Environ. Sci.
Technol. 38(24); 6582-6589, 2004
KEYWORDS
Critical design value, design value, inter-annual variability, PM2.5, risk assessment,
probability
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