Geographic sensitivity of fine particle mass to emissions of SO2 and NOx.

An air quality model, URM-1ATM, was used to investigate tendencies in fine particle (PM2.5) species in response to changes in SO2 and NOx emissions in the eastern United States. The model employed the decoupled direct method (DDM) to estimate sensitivities without the need for multiple model runs for different emissions species and geographic regions. The baseline for sensitivities was emissions projected to 2010. Principal geographic regions investigated were east of the Mississippi River, although the contribution of a region to the immediate west of the river was also included in the study. Sensitivities to emissions changes from point sources (SO2 and NOx) and low-level sources (NOx) were computed. PM2.5 species examined were sulfate, organic carbon, and nitrate as well as total fine mass. Results for the midwest, mid-Atlantic, and southeast regions indicated that those regions affect their own aerosol levels the most. Aerosols in the northeast were most strongly linked to emissions from the midwest and mid-Atlantic regions. In general, midwest emissions had the most influence of any region on other regions. In addition, the southeast was relatively isolated, having the least influence outside itself and being least affected by neighboring regions. Sulfate was the species most sensitive to emission changes. Finally, the largest potential relative sensitivities of sulfate and organic aerosols, along with PM2.5 mass, to emissions changes were usually modeled to occur outside those areas computed to experience the highest aerosol levels.