Theoretical evaluation of different factors affecting the HO2 uptake coefficient driven by aqueous-phase first-order loss reaction

Abstract

The heterogeneous loss on aerosols is an important sink of HO2, affecting the radical chemistry and cycling, and thus it plays a key role in the atmospheric photochemistry. Gaining a reasonable HO2 uptake coefficient (γHO2) would be of great importance in evaluating the heterogeneous loss rate of HO2 on aerosols. This work was motivated by the large variance of reported HO2 mass accommodation coefficients (αHO2) in laboratory studies (0.1–1), which can cause consequent bias in the parameterized HO2 uptake coefficient (γHO2). We conducted a theoretical analysis of the roles of several key factors or parameters in determining γHO2 on a sphere droplet with adjustable Cu2+ ion concentration including αHO2, aqueous-phase acidity, the first-order loss-rate constant KI value, and the aqueous phase production of HO2. The results intuitively demonstrate that utilizing a single γHO2 value for aerosols of different sizes, compositions or hygroscopic states is unsafe in atmospheric models. The theoretical analysis indicated that for a single aerosol experiencing hygroscopic growth, γHO2 decreased with increasing aerosol size, because of the increased gas phase diffusion resistance and dilution of aqueous-phase HO2 consuming ions. Aerosol pH and metal abundance influence γHO2 by determining the aqueous-phase loss-rate constants, and these two factors were found to be only predominant for large particles/droplets (Rp > 1 μm). For small and middle size aerosols, the mass accommodation process plays the determining role in controlling HO2 uptake. Considering ambient aerosols rarely grow to cloud droplet size on sunny days when photochemical budget of HO2 radicals is of more concern, it is crucial to adopt appropriate αHO2 in models, as arbitrarily choosing the αHO2 value can lead to large bias when simulating HO2 heterogeneous process on ambient aerosols.