A Study of the Aqueous Phase Processing of Organic Aerosols through Carbon Stable Isotope Analysis

Secondary organic aerosols (SOAs) are formed in the atmosphere through oxidative and photochemical reactions of primary organic aerosols (POAs) or volatile organic compounds (VOCs). SOAs that form from gas-phase species (gasSOA) have been well studied and documented in literature. However, atmospheric models continue to underestimate the concentration of aerosol organic carbon. A likely cause of this modeling error is our developing understanding of SOAs that form in the aqueous phase (aqSOA): It remains unclear if the species that form aqSOA originate as VOCs or aerosol particulate matter (PM). Both an understanding of how aqSOA forms, as well as whether aqSOA precursors originate from POAs or VOCs, can be addressed through studying the stable carbon isotopic composition of organic atmospheric species. Aerosol PM and fog samples were collected in Davis, CA and Whistler, BC and analyzed for total carbon isotopic composition by isotope ratio mass spectrometry (IRMS). These samples clearly reveal processes that involve isotope effects when fog is present: In both locations, the isotopic composition of the fog was found to be consistently lighter than that of the interstitial aerosol by up to one per mil. We will present these observations and discuss hypotheses on the origin of the observed isotopic differences and discuss the possibility of using bulk carbon isotope ratios as well as compound specific isotope ratios to investigate SOA formation. Preliminary data on compound specific isotope measurement sin atmospheric samples will also be presented.