FINE AEROSOLS: A STUDY OF THE VERTICAL DISTRIBUTION BY PARTICLE SIZE AND ELEMENTAL COMPOSITION IN CALIFORNIA'S CENTRAL VALLEY

A study of the concentration of fine particulate aerosols by size and elemental composition collected at the ground and at several altitudes in the planetary boundary layer with an instrumented aircraft was conducted from August 7-17, 1979 in the Central Valley of California near the city of Modesto. Cascade type multistage impactors were used to collect aerosols in five particle size ranges. Particle induced x-ray emission analysis was used to identify the elemental content of the collected aerosols. Data on boundary layer meteorology including the vertical profiles of temperature, moisture, and winds was collected with the aircraft and with the National Center for Atmospheric Research (NCAR) boundary layer profiler system. These data revealed that during the study, two distinct meteorological conditions occurred. The first period (Aug. 7-10) was characteristic of typical summertime conditions in the valley. Maximum temperatures were near 100 degrees F, relative humidities were less than 63%, boundary layer depths wer 250 m in the morning and 800 m in the afternoon. By contrast, the second period (Aug. 13-15) was not typical with maximum temperatures less than 90 degress F, relative humidities generally greater than 63%, mixing depths of 560 m in the morning, and 1000 m in the afternoon. Meteorological conditions returned to normal on August 16 and 17.;Data on particle size distributions indicate that three major groups of elements are present in the valley. These include elements occurring principally in: (1) the fine (submicrometer) size fraction--e.g. sulfur derived from oxidation of sulfur dioxide; (2) the coarse particle size fraction--e.g. iron and calcium derived from wind blown soils; and (3) both coarse and fine sizes--e.g. silicon and potassium with a minimum concentration near 0.5 (mu)m and a distinctly bimodal distribution indicating that both wind blown dust and smoke from agricultural burning are important sources. Both sulfates and smoke (silicon + potassium) contribute significantly to the total elemental mass measured.;Analysis of the vertical distribution of aerosols using dimensionless parameters indicate that for soil particles, differences in the vertical mixing of aerosols were related to particle size. That is, large particle concentrations were reduced as a function of altitude, while fine particle concentrations were nearly constant with altitude. For fine sulfur and silicon aerosols, the analysis technique failed because the method in which these aerosols are generated and dispersed seriously violate the basic assumptions used in the analysis.;The plume from the city of Modesto was not readily discernible in the data. This was due primarily to the detection limits of the aerosol-sampler analysis system which were too high for the low ambient concentrations of the trace materials produced in the study area.;Based on experience gained in laboratory and field tests, the following general remarks can be made about the reliability, accuracy, and utility of the airborne aerosol sampler. The system is very reliable: over 800 samples have been collected in a variety of field conditions with only minor problems in sampler operation. Tests of sampler collection efficiency versus total filters indicate that the system is nearly 100% efficient for particles less than 15 (mu)m. An examination of SEM photographs of representative samples collected in the study indicate that particle sizing is being performed correctly for both high and low particle loading conditions.