MICROANALYSIS OF THE AEROSOL COLLECTED OVER SOUTH-CENTRAL NEW-MEXICO DURING THE ALIVE FIELD EXPERIMENT, MAY DECEMBER 1989

Thirty-eight size-segregated aerosol samples were collected in the low er troposphere over the high desert of south-central New Mexico, using cascade impactors mounted onboard two research aircraft. Four of thes e samples were collected in early May, sixteen in mid-July, and the re maining ones in December 1989, during three segments of the ALIVE fiel d initiative. Analytical electron microscope analyses of aerosol depos its and individual particles from these samples were performed to phys ically and chemically characterize the major particulate species prese nt in the aerosol. Air-mass trajectories arriving at the sampling area in the May program were quite different from those calculated for the July period. In general, the May trajectories showed strong westerly winds, while the July winds were weaker and southerly, consistently pa ssing over or very near the border cities of El Paso, Texas, and Ciuda d Juarez, Mexico. Aerosol samples collected during the May period were predominantly fine (0.1-0.5 mum dia.), liquid H2SO4 droplets. Samples from the July experiment were comprised mostly of fine, solid (NH4)2S O4 or mostly neutralized sulfate particles. In both sampling periods, numerous other particle classes were observed, including many types wi th probable terrestrial or anthropogenic sources. The numbers of these particles, however, were small when compared with the sulfates. Compo site particle types, including sulfate/crustal and sulfate/carbonaceou s, were also found to be present. The major differences in aerosol com position between the May and July samples (i.e. the extensive neutrali zation of sulfates in the July samples) can be explained by considerin g the different aerosol transport pathways and the proximity of the Ju ly aerosol to the El Paso/Juarez urban plume. Winds during the Decembe r experiment were quite variable, and may have contributed to the wide ly varying aerosol compositions observed in these samples. When the ai rcraft sampled the El Paso/Juarez urban plume, high concentrations of carbonaceous particles were collected. These C-rich particles were of three distinct types, two of which showed combustion morphologies and the third an irregular morphology. Concurrent aethalometer measurement s of aerosol black carbon concentration were well correlated (r = 0.83 ) with the total carbonaceous particle fraction in the aerosol samples . Carbonaceous particles were not observed in abundance in any of the May or July samples (even when the winds passed over El Paso), and we attribute the high concentrations in December to increased wintertime burning of wood, fossil fuels and other combustibles in the urban area .