AIRBORNE BACTERIA IN THE ATMOSPHERIC SURFACE-LAYER - TEMPORAL DISTRIBUTION ABOVE A GRASS SEED FIELD

Temporal airborne bacterial concentrations and meteorological conditio ns were measured above a grass seed field in the Willamette River Vall ey, near Corvallis, Oreg., in the summer of 1993. The concentration of airborne bacteria had a maximum of 1,368.5 CFU/m(3), with a coefficie nt of variation of 90.5% and a mean of 121.3 CFU/m(3). The lowest conc entration of bacteria occurred during the predawn hours, with an avera ge of 32.2 CFU/m(3), while sunrise and early evening hours had the hig hest averages (164.7 and 158.1 CFU/m(3), respectively). The concentrat ions of bacteria in the atmosphere varied greatly, with a maximum diff erence between two 2-min samples of 1,995 CFU/m(3). The concentrations of bacteria in the atmosphere could be divided into five time periods during the day that were thought to be related to the local diurnal s ea breeze and Pacific Coast monsoon weather conditions as follows: (i) the nighttime minimum concentration, i.e., 2300 to 0600 h; (ii) the s unrise peak concentration, i.e., 0600 to 0800 h; (iii) the midday accu mulating concentration, i.e., 0800 to 1515 h; (iv) the late-afternoon sea breeze trough concentration, i.e., 1515 to 1700 h; and (v) the eve ning decrease to the nighttime minimum concentration, i.e., 1700 to 23 00 h. The sunrise peak concentration (period ii) is thought to be a re latively general phenomenon dependent on ground heating by the sun, wh ile the afternoon trough concentration is thought to be a relatively l ocal phenomenon dependent on the afternoon sea breeze. Meteorological conditions are thought to be an important regulating influence on airb orne bacterial concentrations in the outdoor atmosphere in the Willame tte River Valley.