Cloud chamber studies of dark transformations of sulfur dioxide in cloud droplets

Abstract

A large (6.6m 3) expansion type cloud chamber, capable of simulating atmospheric clouds for periods up to twenty minutes has been used to investigate the chemical interactions that occur between SO 2 and cloud droplets under controlled dark conditions. Specialized apparatus has been developed and tested that will collect the cloud droplets in sufficient quantity for chemical analysis by means of ion chromatography. The cloud droplet collector has been designed specifically to discriminate between cloud droplets and any interstitial aerosols. Other specialized instrumentation includes a CO 2 laser transmissometer (10.6 μm wavelength) that permits real-time measurement of the liquid water content of the simulated clouds. The experimental work to date has focused upon the nighttime conversion of SO 2 to sulfate ions in cloud droplets that form on specific cloud condensation nuclei (CCN). Suitable variation of the size and chemical composition of the CCN is used to control the cloud droplet pH and the concentration of catalysts. Initial SO 2 concentrations used are approx. 500 ppb. Results to date indicate that when the CCN are small (~ 0.1 μm), the SO 2 conversion rates are very low and not dependent on CCN composition, including buffers, because of the extreme dilution that occurs during droplet growth. In these cases, the final cloud water pH is generally in the vicinity of 5, apparently due to the self-limiting action of the sulfuric acid production. On the other hand, when larger, buffered CCN (~ 0.5 μm) are used, SO 2 conversion rates rise dramatically when the droplet pH is held between 7 and 8, in contrast to those cases where it is held to near 4, as is expected from known reaction kinetics. The addition of catalysts such as manganese also shows markedly higher oxidation rates, especially at low pH values.