Carbon in Southeastern Aerosol Particles

摘要

Fine aerosol particles (PM2.5) sampled in the southeastern United States in the Southeastern Aerosol Research and Characterization (SEARCH) air-quality network have shown a large ubiquitous carbon component. The SEARCH network has four urban-rural paired sites at Atlanta (GA), Birmingham (AL), Gulfport (MS) and Pensacola (FL). The 1999 and 2000 measurements indicate mean urban mass concentrations of 12 to 18 mug m~(-3) and mean rural mass concentrations of 11 to 14 mug m~(-3). The average black carbon (BC) fraction tends to be larger (7 to 14 percent) in the cities compared with nearby rural sites (5 to 7 percent), while the mean OC fraction remains about the same (urban—23 to 28 percent and rural—24 to 27 percent). The carbon fraction tends to increase northward from the Gulf of Mexico inland. Recent source attribution analyses of the carbon component, based on composite filters, suggest that primary emissions account for most of the carbon found in the rural and urban samples. However, there is a significant fraction unaccounted for that may be secondary in origin for the summer season. This season is expected to be the time most likely to facilitate OC production with active photochemical oxidation of volatile organic compounds (VOC). The daily 24 hr average concentrations of OC and BC were examined along with pollutant gas data for seasonal changes that would suggest secondary OC production. The gases of interest include CO, O3, SO2 and NO_x, and NO_y; these are used as "indicators" of local urban air or stationary source combustion, and of photochemical activity or atmospheric processing. The OC, BC concentrations and their ratio are interpreted with the assistance of the gas concentration patterns. Using the 1999 and 2000 data, a coarse upper bound for the secondary OC fraction by season is estimated from the data. Further analyses will make use of continuous carbon measurements that are now being made.