Evaluation of local measurement-driven adjustments of modelled cloud-free atmospheric photolysis rate coefficients

Photolysis rate constants (j-values) play a crucial role in atmospheric chemistry modelling, but capturing the variability in local conditions needed for their accurate simulation is computationally challenging. One approach is to adjust modelled clear-sky estimates using ratios of measured-to-modelled j-values of a reference photolysis, typically j(NO₂) or j(O¹D). However, application of such adjustments to other photolysis reactions introduces uncertainty. Using spectral radiometer data from the UK, this study examines how hourly measurement driven adjustment factors (MDAF) across a set of 12 photolysis reactions group together using cluster analysis, and evaluates the uncertainties in using j(NO₂) and j(O¹D)-derived MDAF values to adjust modelled j-values of other photolysis reactions. The NO₂-MDAF reference is suitable for adjusting photolysis reactions that absorb at λ > 360 nm (HONO, methylglyoxal, ClNO₂, ClONO₂ → Cl), which are largely independent of solar zenith angle and total ozone column (<31% error). In particular, NO₂-MDAF is a good reference for j(HONO) and j(ClNO₂). The O¹D-MDAF performed better at adjusting modelled j-values for species that predominantly photodissociate at λ < 350 nm, such as HNO₃, H₂O₂, CH₃CHO, HCHO → H, HCHO → H₂ and ClONO₂ → ClO (errors ≤ 30%). However, j(O¹D) radiometers require more data processing to account for local conditions. The maximum error determined using NO₂-MDAF was within a factor of two (91% for j(H₂O₂)), which may still be acceptable in some instances. It is important that MDAFs are used to improve accuracy and uncertainty in simulated j-values caused by variation in local conditions.