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. The photolysis module in Environment and Climate Change Canada'sonline chemical transport model GEM-MACH (GEM: Global EnvironmentalMulti-scale – MACH: Modelling Air quality and Chemistry) was improved tomake use of the online size and composition-resolved representation ofatmospheric aerosols and relative humidity in GEM-MACH, to account foraerosol attenuation of radiation in the photolysis calculation. We coupledboth the GEM-MACH aerosol module and the MESSy-JVAL (Modular Earth SubmodelSystem) photolysis module, through the use of the online aerosol modeleddata and a new Mie lookup table for the model-generated extinctionefficiency, absorption and scattering cross sections of each aerosol type.The new algorithm applies a lensing correction factor to the black carbonabsorption efficiency (core-shell parameterization) and calculates thescattering and absorption optical depth and asymmetry factor of blackcarbon, sea salt, dust and other internally mixed components. We carried out a series of simulations with the improved version ofMESSy-JVAL and wildfire emission inputs from the Canadian Forest FireEmissions Prediction System (CFFEPS) for 2 months, compared the modelaerosol optical depth (AOD) output to the previous version of MESSy-JVAL,satellite data, ground-based measurements and reanalysis products, andevaluated the effects of AOD calculations and the interactive aerosolfeedback on the performance of the GEM-MACH model. The comparison of theimproved version of MESSy-JVAL with the previous version showed significantimprovements in the model performance with the implementation of the newphotolysis module and with adopting the online interactive aerosolconcentrations in GEM-MACH. Incorporating these changes to the modelresulted in an increase in the correlation coefficient from 0.17 to 0.37between the GEM-MACH model AOD 1-month hourly output and AERONET (AerosolRobotic Network) measurements across all the North American sites.Comparisons of the updated model AOD with AERONET measurements for selectedCanadian urban and industrial sites, specifically, showed better correlationcoefficients for urban AERONET sites and for stations located further southin the domain for both simulation periods (June and January 2018). Thepredicted monthly averaged AOD using the improved photolysis module followedthe spatial patterns of MERRA-2 reanalysis (Modern-Era Retrospectiveanalysis for Research and Applications – version 2), with an overallunderprediction of AOD over the common domain for both seasons. Our studyalso suggests that the domain-wide impacts of direct and indirect effectaerosol feedbacks on the photolysis rates from meteorological changes areconsiderably greater (3 to 4 times) than the direct aerosol optical effecton the photolysis rate calculations.

geoscientific model development

Development of aerosol optical properties for improving the MESSy photolysis module in the GEM-MACH v2.4 air quality model and application for calculating photolysis rates in a biomass burning plume