Bacterial flux from chaparral into the atmosphere in mid-summer at a high desert location

Estimates of the bacterial flux for a daylight cycle were observed at the Hanford Nuclear Reservation, Richland, WA, during June 1992, using a modified Bowen ratio method. The upward daytime bacterial flux was coupled with the solar radiation/sensible heat cycle, but commenced 2 h later in the morning and ceased 3 h earlier in the afternoon. During this period, the maximum flux was ca. 17,000 Colony Forming Units (CFU) m-2 h−1 occurring at solar noon, resulting in a total upward bacterial flux of ca. 76,000 CFU m−2 for the time period. During this same period, the integrated total viable bacterial concentration in the atmosphere was only 0.81 % of total upward bacterial flux. The high temperatures (e.g. 45 C), low relative humidity (e.g. 10%), and particularly high solar radiation (e.g. 910 W m−2) are thought to be the lethal agents in the high desert atmosphere. The decreasing concentration of bacteria in the atmosphere was found to slow within 30 min of the initiation of the upward flux of bacteria. Even though the upward flux of bacteria decreased after solar noon, the bacterial concentration in the atmosphere continued to increase. Presumably, this is due to reduced solar radiation in the afternoon allowing greater survival of the released bacteria and smaller dilution of entering bacteria into the shallow nocturnal mixed layer.