Theoretical evaluation of different factors affecting the HO 2 uptake coefficient driven by aqueous-phase first-order loss reaction

Abstract

The heterogeneous loss on aerosols is an important sink of HO 2 , affecting the radical chemistry and cycling, and thus it plays a key role in the atmospheric photochemistry. Gaining a reasonable HO 2 uptake coefficient (γ HO2 ) would be of great importance in evaluating the heterogeneous loss rate of HO 2 on aerosols. This work was motivated by the large variance of reported HO 2 mass accommodation coefficients (α HO2 ) in laboratory studies (0.1–1), which can cause consequent bias in the parameterized HO 2 uptake coefficient (γ HO2 ). We conducted a theoretical analysis of the roles of several key factors or parameters in determining γ HO2 on a sphere droplet with adjustable Cu 2+ ion concentration including α HO2 , aqueous-phase acidity, the first-order loss-rate constant K I value, and the aqueous phase production of HO 2 . The results intuitively demonstrate that utilizing a single γ HO2 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, γ HO2 decreased with increasing aerosol size, because of the increased gas phase diffusion resistance and dilution of aqueous-phase HO 2 consuming ions. Aerosol pH and metal abundance influence γ HO2 by determining the aqueous-phase loss-rate constants, and these two factors were found to be only predominant for large particles/droplets (R p > 1 μm). For small and middle size aerosols, the mass accommodation process plays the determining role in controlling HO 2 uptake. Considering ambient aerosols rarely grow to cloud droplet size on sunny days when photochemical budget of HO 2 radicals is of more concern, it is crucial to adopt appropriate α HO2 in models, as arbitrarily choosing the α HO2 value can lead to large bias when simulating HO 2 heterogeneous process on ambient aerosols.