Investigating radical processes at the surface of secondary organic aerosols

Secondary Organic Aerosols (SOAs) have been estimated to be the highest proportion by mass of atmospheric aerosols averaged globally and a significant fraction of particulate matter below 2.5 μm (PM2.5). Previous studies have established its formation pathways, but fewer studies have focused on the processing of SOAs and how SOAs interact with trace gas species in the atmosphere. Concentrations of HO₂, a critical radical in many atmospheric processes, are often overestimated in atmospheric models. These discrepancies have sometimes been attributed to the heterogeneous uptake onto atmospheric aerosols. There is a significant lack of data with respect to the uptake of HO₂ onto secondary organic aerosols. The principal objective of this project is to explore the heterogeneous reactions of HO₂ occurring on the surface of atmospherically relevant secondary organic aerosols.

Atmospherically relevant SOAs have been produced in a Potential Aerosol Mass Chamber (PAM) from the oxidation with OH and ozone of volatile organic compounds, α-pinene, d-limonene and 1,3,5 – trimethyl benzene. A scanning mobility particle sizer (SMPS) characterised the aerosol's physical properties. Results from these chamber studies show that the size distribution of the SOA can be altered by changing the initial mixing ratio of the VOC or oxidant. A flow tube coupled to a Fluorescence Assay Gas Expansion (FAGE) detection cell, which utilises laser-induced fluorescence (LIF) spectroscopy, is used to measure radical species in the gas phase.

HO₂ uptake is observed by an increased loss of HO₂ with increasing aerosol surface area. There is competition between the uptake of HO₂ onto SOAs and the production of HO₂ from SOAs. Thus, both processes must be well understood to obtain an HO₂ uptake coefficient for SOAs and are investigated in this presentation.