Abstract. Production of hydroxyl (OH) radicals is frequently dominated by the photolysis of tropospheric ozone (O₃). However, photolysis of nocturnal radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClNO₂), also produces radicals (OH and Cl atoms) that contribute to the oxidising capacity of the local atmosphere, and initiate many radical-chain reactions that lead to the formation of harmful secondary pollutants. Photolysis of nitric acid (HNO₃) is also a minor radical production mechanism. In this paper, locally representative photolysis rate constants (j-values) for these molecules are shown to be critical for quantifying and understanding the rate of radical production in a local atmosphere.

The first long-term 4-π filter radiometer dataset in the UK (21 November 2018–20 November 2019) available for direct atmospheric model validation is reported. Measurements were made at Auchencorth Moss, a Scottish rural background site, and j(NO₂) is used to generate a measurement-driven adjustment factor (MDAF) for calculated j-values that accounts for local changes in meteorological variables without significantly increasing computational cost.

Modelled clear-sky j-values and actinic flux for Auchencorth Moss were generated using the Tropospheric Ultraviolet and Visible radiation model (TUV; v.5.3.1). Applying the MDAF metric resulted in the calculated photolytic production rate of OH radicals, from all sources considered, being ~40 % lower over the year. Photolysis of HONO resulted in an increased rate of OH production compared to that from O₃ in low-light conditions, such as sunrise and sunset (Solar Zenith Angle > 80°). Hydroxyl radical production from HONO photolysis exceeded that from O₃ consistently throughout the day during the winter and autumn (by a factor of 5 and 2.1, respectively). Radical production rates from HONO and ClNO₂ reached maximum values during the early morning hours of summer (06:00–09:00 UTC), with OH produced at a rate of 1.06 × 10⁶ OH radicals cm⁻³ s⁻¹, and Cl radicals at 3.20 × 10⁴ Cl radicals cm⁻³ s⁻¹, with the MDAF metric applied.

This first application of the MDAF j-values demonstrates an efficient measurement and computational approach to improve modelling of the local atmospheric photochemistry that drives NO₂, O₃ and PM pollution levels. The incorporation of local radiation measurements in measurement networks, and the consequent greater spatial resolution of locally-relevant photolysis coefficients in model photolysis parameterisations, will improve the accuracy of assessment of air pollution and policy-intervention impacts.