1. Correction of Particulate Matter Concentrations to Reference Temperature and Pressure Conditions Stefan R. Falke and Rudolf B. Husar Center for Air Pollution Impact and Trend Analysis Washington University St. Louis, Missouri

2. Overview Pollutant concentration data contained in EPA's Aerometric Information Retrieval System (AIRS) prior to the 1997 revisions had been required to be reported in units corrected to standard temperature and pressure (25  C, 760 mm Hg). This requirement was removed so that, in the new regulations, the particulate matter data will be reported to AIRS at local temperature and pressure. This work analyzes the impact of this revision to the spatial and temporal pattern of PM10 concentrations. The influence of pressure and temperature individually on the correction of U.S. PM10 concentrations was first examined over a seasonal time scale. The two correction factors were then combined to produce a total correction factor and, subsequently, uncorrected PM10 concentration maps at local conditions were derived.

3. Correction of PM10 to Local Conditions C L, C S are local and standard concentrations f(P,T) is the correction factor

4. National Weather Service 1992 Average Temperature for Quarters 1 and 3

5. Temperature Correction Factors Correction of 5-10% in the northern half of U.S. Correction factor ~ 1 because average Q3 temperatures are ~ 25° C

6. Pressure Correction Factors

7. Combined Pressure and Temperature Correction Factors Mountainous regions of the West have correction factors between 0.7-0.8. The East has a relatively uniform correction of 1.0 except in areas with low temperatures during Q1.

8. 1995 AIRS PM10 Concentrations at Standard Temperature and Pressure

9. 1995 AIRS PM10 Concentrations Corrected to Local Temperature and Pressure

10. Difference between Local and Standard Temperature and Pressure PM10 PM10 concentrations decreased by 2-6 ppb in many areas of the West while PM10 in the East remained generally unchanged.

11. Local Condition PM10 vs. Standard Condition PM10

12. Conclusions PM10 concentrations expressed in terms of local pressure may be between 10 and 25 percent lower than those reported at standard pressure with the largest decreases occurring in the high elevation areas of the western U.S. The largest changes in PM10 concentrations in the West occur due to the pressure correction factor. The temperature correction is most influential in the Northeast and Upper Midwest during the cold months with PM concentrations up to 10% higher than those reported at standard temperature.