Radical chemistry at a UK coastal receptor site -- Part 1: observations of OH, HO2, RO2, and OH reactivity and comparison to MCM model predictions.

OH, HO₂, total and partially-speciated RO₂, and OH reactivity (k'OH) were measured during the July 2015 ICOZA (Integrated Chemistry of OZone in the Atmosphere) project that took place at a coastal site in North Norfolk, UK. Maximum measured daily OH, HO₂, and total RO₂ radical concentrations were in the range 2.6-17 × 106, 0.75-4.2 × 108, and 2.3-8.0 × 108 molecule cm^-3, respectively. k'OH ranged from 1.7 to 17.6 s^-1 with a median value of 4.7 s^-1. ICOZA data were split by wind direction to assess differences in the radical chemistry between air that had passed over the North Sea (NW-SE sectors) or major urban conurbations such as London (SW sector). A photostationary steady-state (PSS) calculation underpredicted daytime OH in NW-SE air by ~35%, whereas agreement (~15%) was found within instrumental uncertainty (~26% at 2σ) in SW air. A box model using MCMv3.3.1 chemistry was in better agreement with the OH measurements, but it overpredicted HO₂ observations in NW-SE air in the afternoon by a factor of ~2-3, although slightly be_tter agreement was found for HO₂ in SW air (factor of ~1.4-2.0 underprediction). The box model severely underpredicted total RO₂ observations in both NW-SE and SW air by factors of ~8-9 on average. Measured radical and k'OH levels and measurement-to-model ratios displayed strong dependences on NO mixing ratios. The PSS calculation could capture OH observations at high NO but underpredicted the observations at low NO. The box model overpredicted HO₂ concentrations at low NO in NW-SE air, whereas in SW air, the measurements and model results were in agreement across the full NO range. The box model underpredicted total RO₂ at all NO levels, where the measurement-to-model ratio scaled with NO. This trend has been found in all previous field campaigns in which total RO₂ was measured using the RO_xLIF technique and suggests that peroxy radical chemistry is not well understood under high NO_x conditions. [ABSTRACT FROM AUTHOR]