Cloud processing of gas phase species and their transformation into non-volatile species is a potential mechanism for the formation of secondary organic aerosol (SOA) material. Recent research efforts focused largely on heterogeneous chemistry as well as aqueous phase chemistry of species from predominately biogenic precursors. Few studies have focused on the volatile, anthropogenic species including benzene, toluene, ethylbenzene, and xylenes (BTEX) and their processing by clouds.

We present data on field measurements of the occurrence of these VOCs in clouds collected at Mt. Elden in Flagstaff, Arizona. While the species are only a small fraction of the Total Organic Carbon in cloud droplets (<1%), their concentrations are orders of magnitude higher than expected from equilibrium partitioning using standard Henry's law. The ambient vapor phase concentrations of TEX (0.02-1.0 ƒÝgm-3) are lower than typical concentrations found at many other sites. Based on the measured gas phase concentrations at Mt. Elden, the theoretical Henry's law concentration of toluene would be ~2ngL-1 compared to measured concentrations of ~1ƒÝgL-1.

In a second step, laboratory experiments have been performed to investigate the aqueous phase reactivity of BTEX species in simulated and authentic cloudwater under controlled conditions. Results of these experiments in terms of products formed and reaction kinetics will be presented. Identified reaction products include aldehydes and acids that are less volatile than the BTEX precursors, hence evidence for the cloud processing pathway to SOA generation.