Real-time field measurements of HO2 uptake coefficients onto ambient aerosols using laser flash photolysis coupled with laser induced fluorescence detection

Hydroperoxy radicals (HO₂) play a key role in atmospheric oxidative chemistry, participating in the transformation of primary emissions into secondary pollutants such as NO₂ and O₃. The uptake of HO₂ onto atmospheric aerosols can significantly impact ozone production in certain regions,[1] and may partly explain in some environments the overestimation of HO₂ concentrations by atmospheric chemistry models compared to field measurements. However, currently there have been limited real time field measurements of the HO₂ uptake coefficient (γ_HO2) for ambient air aerosol particles, conducted in Japan.[2-4]

In this study, a new instrument has been developed to directly measure γ_HO2 for the first time in the UK. A mixture of O₃ and H₂O vapour in air is introduced into a flow tube, where OH radicals are generated via photolysis of O₃ at 266 nm using a Nd:YAG laser. The OH radicals are then converted to HO₂ using excess CO. The temporal decay of HO₂ is measured by sampling the flow tube into a low pressure cell to sensitively monitor HO₂ following its conversion to OH and detection at 308 nm using the laser induced fluorescence technique. To quantify the removal of HO₂ that is due to uptake onto ambient aerosols, the ambient aerosol surface area is first enhanced using a Versatile Aerosol Concentration Enrichment System (VACES).[2] Ambient air is passed through VACES and then into the HO₂ reactivity instrument. Alternating between sampling lines—one containing an aerosol filter and one without—allows separation of gas-phase HO₂ reactivity from the total reactivity. The difference in HO₂ reactivity between the measurements with and without the filter is used to quantify the HO₂ uptake onto aerosols, and hence a real time observation of γ_HO2.

The instrument measured γ_HO2(ambient aerosols) vs. time at the Manchester Air Quality Supersite, UK in August 2025, alongside supporting measurements of aerosol composition and gas-phase species, including OH, HO₂ and RO₂ radicals and OH reactivity. This fieldwork enables the correlation of the measured γ_HO2vs. time with factors such as aerosol composition (e.g. transition metals, inorganic salts and organic species), temperature and humidity. The combined measurements will be used to improve the agreement of HO₂ concentrations in atmospheric chemistry models with [HO₂] in field measurements and understand the impact of the studied aerosol uptake on O₃ production.

[1]. Ivatt et al., Nat. Geosci., 15, 536-540, 2022

[2]. Zhou et al., Atmos. Environ., 223, 117189, 2020

[3]. Zhou et al., Atmos. Chem. Phys., 21, 12243–12260, 2021

[4]. Li et al., Environ. Sci. Technol., 56, 12926−12936, 2022