Heterogeneous conversion contribution to the sulfate observed over Lake Michigan

Abstract

It has been shown that sulfur dioxide (SO 2) to sulfate (SO 4 2−) heterogeneous conversion occurs by ozone (O 3) 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, O 3 and SO 2 real-time monitoring, and aerosol optical probe particle size spectra determinations were incorporated into measurements over the alkaline fresh waters of Lake Michigan during June and July 1991. Ion chromatography analysis of Teflon filters showed that one-third of the observed total SO 4 2−, with a mean value of 55 nmol SO 4 2− m −3; was present in coarse-mode, predominantly lake-derived particles. The sum of lake and soil particle sources contributed < 6 nmol SO 4 2− m −3 to the coarse-mode 55 nmol SO 4 2− m −3; cloud conversion and coagulation contributed < 5 nmol SO 4 2− m −3. The total of these sources contributed < 20% of the coarse-mode SO 4 2−, whereas O 3-oxidized SO 2 conversion in the water associated with coarse particles produced from 10 to > 55 nmol SO 4 2− m −3. Heterogeneous conversion of SO 2, when a few μg m −3 (or more) of buffered water is present in large particles, may produce sufficient SO 4 2− that it must be considered in boundary layer sulfur mass balance calculations and models.