Spatial and temporal variations of PM2.5 mass and composition in Atlanta: ASACA 1999 2006
Cobb, Charles Evan
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Starting in March of 1999, the ASACA study has measured PM2.5 mass and composition using 24-hr integrated and continuous measurement techniques. The ASACA network has one rural (Fort Yargo) and three urban (Fort McPherson, South Dekalb, and Tucker) monitoring sites located in the metropolitan Atlanta area. Supplementary data from the SEARCH and STN monitoring networks is also used where applicable. Yearly-averaged TEOM measurements recorded violations of the annual PM2.5 NAAQS (>15 μg/m3) every year of the study, and the daily NAAQS (>65 μg/m3) was exceeded on five separate occasions. Seven-year PM2.5 averages for the sites ranged from 18.8 – 19.8 μg/m3. PCMs were employed to collect PM2.5 composition data, detect spatial variations of PM species, and compare results with the continuous mass measurements. From 2004 – 2005, approximately 28% of the mass was OC, 24% was sulfate, 10% was ammonium, 6% was nitrate, and 3% was EC. Lesser ions contribute less than 3% to the total PM2.5 mass. Spatial variation of the major species was minimal, especially for species formed from secondary processes. South Dekalb did exhibit elevated levels of EC compared to the other sites, most likely due to its proximity to an interstate heavily used by diesel vehicles. PCM averages were found to be less than the averaged TEOM data due to the presence of unidentified matter (UM). Depending on the season, UM can contribute as little as 5% and as much as 50+% of the total mass. Secondary organic aerosol (SOA) concentrations from 2004 – 2005 were predicted using the EC-tracer method. Peak SOA occurs in mid-summer, and winter concentrations are significant due to biomass burning increasing the estimated OC/EC ratios. PCM, TEOM, and aethalometer data was also subjected to seasonal, day-of-the-week, and diurnal temporal variations. Active photochemistry plays an important role, as most species exhibit higher concentrations during summer months. The lone exception was nitrate, whose peak occurs in winter. Daily-averaged PM2.5 concentrations tend to peak late in the work-week and reach their low point on Sundays. Morning and afternoon rush-hour spikes in one-hour averaged PM2.5 are visible most days.