Production of Aqueous Secondary Organic Aerosol during Cloud Processing Cycles

Aqueous secondary organic aerosol (aqSOA) production in clouds is believed to have significant contributions to the atmospheric aerosol burden, but large uncertainties in the production rate, pathways, and products still exist. The Multiphase Aging and Production of Particles (MAPP) chamber was used to investigate aqueous secondary organic aerosol (aqSOA) production within cloud droplets. MAPP consists of three layers, an outer vacuum-tight stainless steel casing, a middle enclosure, and an inner 1 m³ FEP Teflon cylindrical chamber. The formation of clouds occurs through adiabatic expansion within the inner chamber. A rising air parcel at a set updraft velocity is simulated through controlled changes in pressure and temperature. Known concentrations of isoprene and, for certain experiments, NOx were injected from compressed cylinders directly into the inner chamber before the cloud cycle. For the majority of the experiments, ozone was injected as the principal source of hydroxyl radicals. Seed particles were injected to serve as cloud condensation nuclei (CCN) and as a surface to which aqSOA was added. A narrow, monodisperse mode of the seed particles was injected so that small changes in size could be detected. The injected components were exposed to simulated daytime conditions, created by two 300 W xenon-arc lamps, and allotted time for gas phase chemistry to produce the soluble SOA precursors before cloud formation. Using a scanning mobility particle sizer (SMPS) and a tandem differential mobility analyzer (TDMA), pre-cloud and post-cloud measurements were used to quantify cloud aqSOA yield and hygroscopicity.