Implementation and evaluation of updated photolysis rates in the EMEP model using Cloud-J v7.3e

Photolysis reactions are an essential component of atmospheric chemistry, with the accurate representation of solar radiation and its interactions with clouds and aerosols being a fundamental part of atmospheric chemistry-transport models (CTMs). The photolysis rate calculation scheme in the EMEP MSC-W CTM has recently been updated to the interactive Cloud-J v7.3e scheme, replacing the old system based on tabulated values (van Caspel, et. al., 2023). The current work highlights the comparison of the photolysis rate systems against aerial observations from the ATom-1 campaign over the Pacific Ocean (Hall, et. al., 2018). This comparison includes sensitivity analysis investigating the impact of model resolution, meteorological input model, and cloud averaging scheme. For air quality simulations, the Cloud-J photolysis rates lead to a clear shift in the partitioning of reactive oxygen into the ozone component, with simulations of surface ozone and carbon monoxide showing a general increase in performance. Annual mass-weighted tropospheric hydroxyl concentrations are increased by 26%, while the photolytic impact of aerosols is mostly limited to tropical biomass burning regions. The model run-time penalty of the interactive photolysis rate calculations is 15% at most, with the run-time of regional (forecasting) simulations being increased by no more than 3%. van Caspel, W. E., Simpson, D., Jonson, J. E., Benedictow, A. M., Ge, Y., di Sarra, A., Pace, G, Vieno, M, Walker, H.L., & Heal, M. R. (2023). Implementation and evaluation of updated photolysis rates in the EMEP MSC-W chemistry-transport model using Cloud-J v7. 3e. Geoscientific Model Development, 16(24), 7433-7459. https://doi.org/10.5194/gmd-16-7433-2023 Hall, S. R., Ullmann, K., Prather, M. J., Flynn, C. M., Murray, L. T., Fiore, A. M., Correa, G., Strode, S. A., Steenrod, S. D., Lamarque, J.-F., Guth, J., Josse, B., Flemming, J., Huijnen, V., Abraham, N. L., and Archibald, A. T. (2018): Cloud impacts on photochemistry: building a climatology of photolysis rates from the Atmospheric Tomography mission, Atmos. Chem. Phys., 18, 16809–16828, https://doi.org/10.5194/acp-18-16809-2018