Airborne bacterial samples were collected using wet cyclone and cascade impact samplers 2 to 3 times a week at 2 agricultural sites in the mid-Willamette River valley, near Corvallis, OR during 1997. The concentrations of total (TB), culturable (CB), and particulate-associated culturable (PACB) bacteria in the ambient atmosphere were measured using epifluorescence microscopic and culture methods. All three categories of airborne bacteria were found to have major concentration peaks in the summer (e.g., from June to September), especially in July and August. This may reflect the greater summer flux of bacteria from agricultural sources and activities and dry/dusty soil conditions. The PACB had several smaller peaks scattered in the winter and autumn. Size analysis of the PACB showed that the summer PACB peak was composed primarily of larger bacterial particles, whereas the smaller peaks in other seasons were composed primarily of smaller bacterial particles that occur during rainfall or storms events. The ...