Radical chemistry at a UK coastal receptor site – Part 2: experimental radical budgets and ozone production

Abstract. In our companion paper (Woodward-Massey et al., 2022), we presented measurements of radical species and OH reactivity (k ’_OH) made in summer 2015 during the ICOZA (Integrated Chemistry of OZone in the Atmosphere) field campaign at the Weybourne Atmospheric Observatory, a site on the east coast of the UK. In the present work, we used the simultaneous measurement of OH, HO₂, total RO₂, and k ’_OH to derive experimental (i.e., observationally determined) budgets for all radical species as well as total RO_x (= OH + HO₂ + RO₂). Data were separated according to wind direction: prevailing SW winds (with influence from London and other major conurbations), and all other winds (NW–SE; predominantly marine in origin). In NW–SE air, the RO_x budget could be closed during the daytime within experimental uncertainty but OH destruction exceeded OH production, and HO₂ production greatly exceeded HO₂ destruction while the opposite was true for RO₂. In SW air, the RO_x budget analysis indicated missing daytime RO_x sources but the OH budget was balanced, and the same imbalances were found with the HO₂ and RO₂ budgets as in NW–SE air. For HO₂ and RO₂, the budget imbalances were most severe at high NO mixing ratios.

We explored several mechanistic modifications to the experimental budgets to try to reconcile the HO₂ and RO₂ budget imbalances: (1) the addition of generic radical recycling processes, (2) reduction of the rate of RO₂ → HO₂ conversion, (3) inclusion of heterogeneous HO₂ uptake, and (4) addition of chlorine chemistry. The best agreement between HO₂ and RO₂ production and destruction rates was found for option (2), in which we reduced the RO₂ + NO rate constant by a factor of 5.

The rate of in situ ozone production (P(O_x)) was calculated from observations of RO_x, NO, and NO₂ and compared to that calculated from MCM-modelled radical concentrations. The MCM-calculated P(O_x) significantly underpredicted the measurement-calculated P(O_x) in the morning, and the degree of underprediction was found to scale with NO.