Evaporation-induced Brown Carbon Formation in Secondary Organic Aerosols: Effects of Drying Conditions and Gas-phase Precursors
Nethmi Kasthuriarachchi, Laura-Helena Rivellini, ALEX K.Y. LEE, Environment and Climate Change Canada
Abstract Number: 234
Working Group: Aerosol Chemistry
Abstract
Aqueous-phase processing of secondary organic aerosols produced from dark ozonolysis of limonene (limonene-SOA) can lead to brown carbon (BrC) via reactions with reduced nitrogen species. While many previous studies have investigated BrC formation from limonene-SOA in bulk solutions, little is known about their formation behaviour in aerosol/cloud droplets upon evaporation and in the presence of other SOA precursors. This work illustrates that co-oxidation of limonene with other biogenic (α-pinene) or anthropogenic (toluene) VOCs by ozone produce SOA that are more effective in BrC formation compared to those observed in limonene-SOA alone when the SOA extracts are dried in bulk solutions with ammonium sulfate/glycine. Our results also show that drying of SOA droplets produced by atomization of limonene-SOA and limonene-toluene-SOA extracts with ammonium sulfate/glycine can result in significant BrC formation in the time scale of seconds, and the BrC absorbance and product distribution vary as a function of relative humidity (RH) for both chemical systems. Starting from low values in the high RH condition (e.g., > 90%), the mass absorption efficiency (MAE) of evaporated SOA droplets increase with decreasing RH and reach a maximum at intermediate RH, and then reduce at lower RH conditions (e.g., < 65%). We hypothesise that rapid re-partitioning of semi-volatile BrC precursors in limonene-SOA may occur upon drying, limiting the multi-step oligomerization mechanism for producing BrC with longer wavelength absorption. Overall, this study highlights the potential effect of VOC mixtures on aqueous-phase BrC formation, and the need of more studies that mimic atmospherically relevant drying conditions to better understand BrC chemistry via aqueous aerosol and in-clouding processing.