Investigating HO2 uptake onto the surface of secondary organic

The concentrations of HO₂, a critical radical in the atmosphere, are often overestimated in atmospheric models (1). These discrepancies have sometimes been attributed to the heterogeneous uptake to atmospheric aerosols.

However, the correct treatment of heterogeneous chemistry in models is a significant source of uncertainty, partly due to the complexity of atmospheric aerosols and the need for laboratory experiments to formulate a robust parameterisation of HO₂ uptake. There is a significant lack of experimental data for the uptake coefficient γ(HO₂) of HO₂ onto secondary organic aerosols (SOAs), even though they represent a high proportion of atmospheric aerosols and a significant fraction of particulate matter below 2.5 μm (PM_2.5).

We report the first γ(HO₂) measurements onto SOAs over a range of relative humidities (30 - 85 %). Atmospherically relevant SOA has been produced in a Potential Aerosol Mass Chamber (PAM) from the oxidation, by OH and ozone, of the volatile organic compounds α-pinene, ∆-limonene, 1,3,5 – trimethyl benzene (TMB) and toluene.  An aerosol flow tube coupled to a Fluorescence Assay Gas Expansion (FAGE) detection cell, which utilises laser-induced fluorescence (LIF) spectroscopy, is used to measure the uptake of gas-phase HO₂ onto the aerosols, with a limit of detection of γ(HO₂) = 0.003.

Results show that the aerosol liquid water (ALW) content plays an important role in heterogeneous reactions by enhancing HO₂ uptake onto aerosols. The measured γ(HO₂) was low (γ ≤ 0.004) for TMB and toluene SOA and undetectable for both α-pinene and ∆-limonene, and no correlation was observed between RH and γ(HO₂). The aerosol size distribution of the SOA remained constant over the range of relative humidities, suggesting the RH had little effect on the ALW of purely organic aerosols. Whereas, when ammonium sulfate seed aerosols are added to enhance SOA formation, the measured γ(HO₂) for toluene-derived SOA increases from 0.006 to 0.03 with an increase in RH from 38 – 84 %. The increase in the geometric mean of the toluene-derived SOA at higher RH suggests that the ALW increases. Thus, the presence of seed particles results in a more significant γ(HO₂), which increases with RH and potentially impacts the atmospheric abundance of HO_x.

 

 

1. Dyson, Joanna E., et al. "Impact of HO₂ aerosol uptake on radical levels and O₃ production during summertime in Beijing." Atmospheric Chemistry and Physics Discussions(2022): 1-43.