Ambient aerosol and its carbon content in Gainesville, a mid-scale city in Florida

Ambient aerosols were collected during 2000-2001 in Gainesville, Florida, using a micro-orifice uniform deposit impactor (MOUDI) to study mass size distribution and carbon composition. A bimodal mass distribution was found in every sample with major peaks for aerosols ranging from 0.32 to 0.56 microm, and 3.2 to 5.6 microm in diameter. The two distributions represent the fine mode (<2.5 microm) and the coarse mode (>2.5 microm) of particle size. Averaged over all sites and seasons, coarse particles consisted of 15% carbon while fine particles consisted of 22% carbon. Considerable variation was noted between winter and summer seasons. Smoke from fireplaces in winter appeared to be an important factor for the carbon, especially the elemental carbon contribution. In summer, organic carbon was more abundant. The maximum secondary organic carbon was also found in this season (7.0 microg m(-3)), and the concentration is between those observed in urban areas (15-20 microg m(-3)) and in rural areas (4-5 microg m(-3)). However, unlike in large cities where photochemical activity of anthropogenic emissions are determinants of carbon composition, biogenic sources were likely the key factor in Gainesville. Other critical factors that affect the distribution, shape and concentration were precipitation, brushfire and wind.