Understanding HONO concentrations, its role as a hydroxyl radical source and the impact on summertime ozone production in London

Abstract:

Understanding the chemistry of free-radicals in the atmosphere is necessary to understand the lifetime of primary pollutants and production of secondary pollutants, such as ozone. In the urban environment, field observations of HONO have revealed elevated concentrations persisting throughout the day and subsequent modelling studies have identified HONO as the major OH precursor (e.g. Elshorbany et al. 2009). Attempts to reproduce the strong daytime HONO signature in models, however, have revealed that the currently known chemistry is unable to account for the levels observed.

Here we present simultaneous measurements of OH, HO₂, RO₂ and HONO made during the Clean air for London project in the summer of 2012. HONO concentrations were observed to build up throughout the night, with concentrations exceeding 2ppbV on several nights. Daytime concentrations were lower, but ~ 300 pptv was observed to persist throughout the afternoon. Zero dimensional box modelling studies, constrained with the detailed MCM chemistry and to the measured HONO, suggest that HONO makes up ~85% of the primary OH budget and just over 50% of the total primary radical budget at noon. The model, however, greatly over-predicts the OH concentrations (and HO₂ and RO₂concentrations) observed.

Unconstrained to HONO, the basic model is unable to reproduce the measured HONO concentrations. A source of HONO from the reaction of NO₂ with HO₂.H₂O, as postulated by Li et al. (2014), can enhance HONO concentrations considerably and also reduces the discrepancy between modelled and measured radicals by reducing the fraction of HONO acting as a net radical source. The model still underestimates the observed HONO by ~ 80% at noon, suggesting that this portion of HONO should still be considered as a primary radical source.

The net in-situ ozone production estimated from the measured peroxy radical concentrations and their reaction with NO is sufficient to account for the daily increases in ozone that were observed. Enhancements in radical concentrations on warm, sunny days, driven by two-fold enhancements in production from HONO and other photolytic sources, have been found to elevate ozone levels beyond the EU air quality recommendations.

Elshorbany et al. Atmospheric Chemistry and Physics, 2009, 9, 2257-2273

Li et al. Science, 2014, 344, 292