HETEROGENEOUS CONVERSION CONTRIBUTION TO THE SULFATE OBSERVED OVER LAKE-MICHIGAN

It has been shown that sulfur dioxide (SO2) to sulfate (SO42-) heterog eneous conversion occurs by ozone (O3) oxidation in the high pH water associated with coarse sea-salt particles over the oceans. To further investigate this conversion mechanism, aircraft-based coarse and fine particle filter pack sampling, O3 and SO2 real-time monitoring, and ae rosol optical probe particle size spectra determinations were incorpor ated into measurements over the alkaline fresh waters of Lake Michigan during June and July 1991. Ion chromatography analysis of Teflon filt ers showed that one-third of the observed total SO42-, with a mean val ue of 55 nmol SO42- m-3; was present in coarse-mode, predominantly lak e-derived particles. The sum of lake and soil particle sources contrib uted <6 nmol SO42- m-3 to the coarse-mode 55 nmol SO42- m-3; cloud con version and coagulation contributed < 5 nmol SO42- m-3. The total of t hese sources contributed < 20% of the coarse-mode SO42-, whereas O3-ox idized SO42- conversion in the water associated with coarse particles produced from 10 to > 55 nmol SO42- m-3. Heterogeneous conversion of S O2, when a few mug m-3 (or more) of buffered water is present in large particles, may produce sufficient SO42- that it must be considered in boundary layer sulfur mass balance calculations and models.