Cross-Reactions of Organic Trace Compounds in Cold, Acidic Sulfate Particles

Despite the detection of organic trace compounds in atmospheric particles, the catalytic role of sulfuric acid in atmospheric aerosols has not been fully explored. Several studies have demonstrated possible acid-catalyzed reactions on particles, generally involving carbonyl groups (C=O) and leading to the formation of larger, less volatile molecules. Reactions of oxygenated organic compounds in acidic solution are most often studied near room temperature, but sulfate particles in the upper troposphere and lower stratosphere (UT/LS) are significantly colder. No reactions have yet proven sufficiently fast to lead to any significant accumulation of organic carbon in particles under conditions of the UT/LS. We report here the first observation of reaction between organic trace species in cold sulfate solutions, where one of the compounds is taken up from the gas phase. Cold, acidic solutions doped with a small amount of an organic compound were exposed to different gaseous oxygenated compounds (formaldehyde, acetaldehyde, propanal, acetone and ethanol). The uptake of the gaseous species was monitored in a Knudsen cell reactor. For some organic pairs under some conditions, enhanced uptake due to reaction was observed. Where possible, we comment on solubility and reaction rate. Moreover, we present evidence that low-temperature (218 - 240 K) reactions are faster in 39 wt% H2SO4 solution than in 72 wt% H2SO4. This observation is in direct opposition to studies at 298 K, in which reactivity increases with increasing sulfuric acid content. These results indicate that rates of acid-catalyzed organic reactions must be measured under atmospheric temperature and acidity conditions. Furthermore, reactions between different organic species may provide a sufficiently fast pathway for the accumulation of organic material in sulfate particles in the UT/LS.