Sulfur dioxide oxidation in clouds at Whiteface Mountain as a function of drop size

In situ oxidation of SO2 has been determined in clouds as a function of droplet size using a trace element technique during July 1998 at Whiteface Mountain, New York. The pH of the cloud water ranged from 2.8 to 4.7 with a mean of 3.4, and therefore SO2 oxidation was dominated by hydrogen peroxide. Size-fractioned cloud samples were collected from six different events at the mountain's summit (1.5 km above mean sea level) using a size-fractionating California Institute of Technology Active Strand Cloudwater Collector. Bulk samples were collected using both passive and active collectors. During each event, below-cloud and interstitial aerosols were collected every 2 hours. Cloud water and aerosol samples were analyzed for major ions and selected trace elements. Continuous measurements of gas phase species SO2, H2O2, and O-3 were carried out at the summit and below-cloud sites. Concentrations of cloud water SO42-, NO3-, H2O2, and trace elements, as well as pH, were largely independent of droplet size. The component of cloud water SO42- produced from in situ oxidation (SO4in2-) was also largely independent of droplet size. The results are in agreement with calculated relative production rates in the small and large drop sizes based on known laboratory reaction rates.