The treatment of HO2 uptake onto aerosols in atmospheric models remains a major source of uncertainty due to the chemical complexity of atmospheric aerosols and the need for additional laboratory measurements to develop robust parameterisations of the uptake coefficient, γ(HO2). This work presents the first measurements of γ(HO2) onto secondary organic aerosol (SOA) across a range of relative humidities (RHs) (14–88 %). Atmospherically relevant SOA was generated in a Potential Aerosol Mass (PAM) chamber through OH and ozone oxidation of the volatile organic compounds α-pinene, d-limonene, 1,3,5-trimethylbenzene (TMB), and toluene, both in the absence and presence of inorganic seed aerosols (ammonium sulfate, ammonium nitrate, and sodium chloride). Gas-phase HO2 concentrations were measured using a flow tube coupled to a Fluorescence Assay by Gas Expansion (FAGE) detection cell employing laser-induced fluorescence (LIF) spectroscopy. The measured values of γ(HO2) show that aerosol liquid water (ALW) plays an important role in heterogeneous chemistry by enhancing HO2 uptake onto aerosols. The hygroscopicity of the SOA was investigated using the hygroscopic growth factor (HGF), defined as the ratio of particle diameter at a given RH to the dry particle diameter. SOA formed in the absence of inorganic seed aerosols exhibited low hygroscopicity, with HGF values at 90 % RH between 1.02 and 1.05, and no clear deliquescence point was observed. In contrast, SOA formed on inorganic seed aerosols displayed hygroscopic behaviour similar to that of the seed aerosol itself. Measured HGF values at 90 % RH for toluene, α-pinene, and TMB SOA formed on ammonium sulfate aerosols ranged from 1.40 to 1.51, comparable to that of pure ammonium sulfate (∼ 1.40 at 90 % RH). Measured γ(HO2) values were low (≤ 0.004) for purely organic TMB and toluene SOA and below the detection limit for purely organic α pinene and d-limonene SOA, with no clear relationship between RH and γ(HO2) due to the low ALW content. In contrast, SOA formed on ammonium sulfate and sodium chloride seed aerosols showed low HO2 uptake (≤ 0.004) below the deliquescence RH, followed by a linear increase in γ(HO2) with increasing RH, with a maximum value at RH 87 % of 0.029 ± 0.003. The measured γ(HO2) values for SOA formed on inorganic seed aerosols were similar to those of the corresponding seed aerosols. Owing to the strong influence of seed type on HO2 uptake, a parameterisation based on SOA formed on ammonium sulfate seeds was derived: γ(HO2) = 0.078(±0.011) × HGF − 0.077(±0.012), with an R2 value of 0.90 for the correlation of γ(HO2) with RH. A parameterisation for γ(HO2) was subsequently developed using both laboratory measurements and literature data and incorporated into the UK Chemistry and Aerosol (UKCA) scheme for inclusion in the UK Earth System Model (UKESM). The global surface monthly mean γ(HO2) was calculated to be 0.092 ± 0.006 for January and June, with regional values ranging from 0.078 to 0.119. Including the parameterisation reduced the global mean concentrations of [HO2] and [OH] by 10.5 % and 10.9 %, respectively. The impact of HO2 uptake on radical concentrations depended strongly on NOx levels, with larger decreases in HO2 over oceans (-20 to -70 %) than over continental regions (-10 to -40 %) because of lower NOx concentrations. Incorporating HO2 uptake onto aerosols decreased the global mean monthly [O3] by 8.2 % in January. These findings highlight the importance of accurately representing HO2 uptake onto aerosols in Earth system models.
