Source apportionment of fine particulate matter over the Eastern U.S. Part II: source apportionment simulations using CAMx/PSAT and comparisons with CMAQ source sensitivity simulations

This Part II paper describes source apportionment (SA) for 10 source categories at a 12 km horizontal grid resolution over the eastern U.S. for January and July of 2002 using the Comprehensive Air Quality Model with extensions/Particulate Source Apportionment Technology (CAMx/PSAT). SA results from CAMx/PSAT are contrasted with those from CMAQ/BFM in Part I. The top three sources domainwide in January are identified to be coal combustion (with a monthly–mean contribution of 14.0%, or 1.1 µg m–3), biomass burning (11.3%, 0.9 µg m–3) and other mobile sources (6.8%, 0.6 µg m–3) by CAMx/PSAT but biomass burning (13.7%, 1.1 µg m–3), miscellaneous area sources (11.8%, 0.9 µg m–3), and coal combustion (10.8%, 0.9 µg m–3) by CMAQ/BFM. Both agree that coal combustion, industrial processes, and miscellaneous area sources are the top three sources in July, though they differ in the magnitude of contributions. Both give similar contributions for primary PM, but they differ substantially in SA results for secondary PM, due primarily to their treatments for oxidant–limiting and indirect effects. While CMAQ/BFM inherently accounts for these effects and can provide useful information for primary and secondary PM species, CAMx/PSAT neglects them by linking each PM species only with its direct primary emission precursor. These effects are enhanced in January due to the increased importance of NO3– and lower concentrations of oxidants relative to July. CAMx/PSAT is thus accurate for SA of the primary PM species but incorrect in its SA for secondary PM species. For a highly non–linear system studied here, the true SA cannot be obtained with current SA methods. Policy–makers must be mindful of relative strengths and weaknesses of each method, as well as the limitation of current SA methods in the SA of secondary PM species, when using such information in support of state implementation plan and epidemiological studies.