Spatial Trends and Urban-Rural Contrasts in Speciated Aerosol Measurements for the United States

The Interagency Monitoring of Protected Visual Environments (IMPROVE) monitoring network measures speciated fine particulate mass and PM10 primarily in remote/rural locations. The EPA PM2.5 chemical Speciation Trends Network (STN) collects similar data, but mostly from urban/suburban locations. Data from these two networks collected from January 2000-December 2004 were integrated to examine the annual rural/urban spatial distributions of PM2.5 mass and its constituents throughout the United States. Collocated IMPROVE and STN sites were used to assess the inter-comparability between the two networks. Annual averages for the primary species were found to have between 10% and 33% (1σ) uncertainty for cross-network comparisons. The highest ammonium sulfate (AS) concentrations were found in the Ohio River Valley, where there are significant SO2 emissions, and the Appalachian Mountains at 6-8 μg/m3. AS concentrations were significantly lower, less than 1 μg/m3, in much of the western US. The urban STN sites had similar AS concentrations to nearby rural sites and rarely exceed two times the nearby rural concentrations. Peak rural organic mass (OMC) concentrations occurred in the Northwest, in the CA mountains, and in the southeastern US. A large band through the interior west from the Mexico border into the upper Midwest had low rural OMC values. The urban STN sites had high OMC concentrations in the Interior West, Northeast and Midwest relative to nearby rural sites. All of the western states with both urban and rural sites had urban concentrations at least 2 times higher than nearby rural concentrations. In the eastern US, this was only true in AL, KS, NC, and NH. Rural and urban ammonium nitrate (AN) concentrations were highest in CA and the Central Great Plains and Great Lakes regions of the midwest where both NOx and ammonia emissions are high. The largest absolute differences between STN sites and neighboring rural sites occurred in CA, the Mountainous West and the Great Lakes regions with excess AN concentrations of 2-4 μg/m3. Similar to OMC, the western states all had urban concentrations at least two times larger than nearby rural concentrations, though this was not universal in the eastern US. The highest rural RCFM concentrations were concentrated in the Ohio River Valley and Appalachian Region. The high RCFM concentrations in this region represent the convergence of the high AS region with the high OMC region to the south and west and the high AN region to the north and west. On a mass basis these sites were almost exclusively dominated by AS. The spatial trends in CM and RCFM were different. The highest CM concentrations, 8-15 μg/m3, were concentrated west of the highest RCFM values in the Central Great Plains and in the arid southwest and southern California. From the Central Great Plains, CM values decreased to 2-6 μg/m3 to the north, east and west until reaching the coasts. The sites along the Pacific, Gulf and Atlantic coasts typically had moderate to high CM values in the 4-8 μg/m3 range. The Midwestern sites added to the IMPROVE network around 2000 allowed for the identification of the high CM region in the Central Great Plains. The spatial patterns in CM also differ from those of fine soil in that the elevated CM regions in the middle of the US and along the coasts were not reflected in the fine soil spatial trends.