Interactive Aerosol Feedbacks on Photolysis Rates in the GEM-MACH v2.4 Air Quality Model in Canadian Urban and Industrial Areas

• Main Authors: Majdzadeh, Mahtab, Stroud, Craig A., Sioris, Christopher, Makar, Paul A., Akingunola, Ayodeji, McLinden, Chris, Zhao, Xiaoyi, Moran, Michael D., Abboud, Ihab, Chen, Jack
• Format: Text
• Language: English
• Published: 2021
• Subjects: Merra; Aerosol Robotic Network
• Online Access: https://doi.org/10.5194/gmd-2021-172; https://gmd.copernicus.org/preprints/gmd-2021-172/

The photolysis module in Environment and Climate Change Canada’s on-line chemical transport model GEM-MACH (GEM: Global Environmental Multi-scale – MACH: Modelling Air quality and Chemistry) was improved, to make use of the on-line size and composition-resolved representation of atmospheric aerosols and relative humidity in GEM-MACH, to account for aerosol attenuation of radiation in the photolysis calculation. We coupled both the GEM-MACH aerosol module and the MESSy-JVAL (Modular Earth Sub-Model System) photolysis module, through the use of the on-line aerosol modeled data and a new Mie lookup table for the model-generated extinction efficiency, absorption and scattering cross sections of each aerosol type. The new algorithm applies a lensing correction factor to the black carbon absorption efficiency (core-shell parameterization) and calculates the scattering and absorption optical depth and asymmetry factor of black carbon, sea-salt, dust, and other internally mixed components. We carried out a series of simulations with the improved version of MESSy-JVAL and wildfire emission inputs from the Canadian Forest Fire Emissions Prediction System (CFFEPS) for two months, compared the model aerosol optical depth (AOD) output to the previous version of MESSy-JVAL, satellite data, ground-based measurements and re-analysis products, and evaluated the effects of AOD calculations and the interactive aerosol feedback on the performance of the GEM-MACH model. The comparison of the improved version of MESSy-JVAL with the previous version showed significant improvements in the model performance with the implementation of the new photolysis module, and with adopting the online interactive aerosol concentrations in GEM-MACH. Incorporating these changes to the model resulted in an increase in the correlation coefficient from 0.17 to 0.37 between the GEM-MACH model AOD one-month hourly output and AERONET (Aerosol Robotic Network) measurements across all the North American sites. Comparisons of the updated model AOD with AERONET measurements for selected Canadian urban and industrial sites specifically, showed better correlation coefficients for urban AERONET sites, and for stations located further south in the domain for both simulation periods (June and January 2018). The predicted monthly averaged AOD using the improved photolysis module followed the spatial patterns of MERRA-2 re-analysis (Modern-Era Retrospective analysis for Research and Applications – Version 2), with an overall under-prediction of AOD over the common domain for both seasons. Our study also suggests that the domain-wide impact of direct and indirect effect aerosol feedbacks on the photolysis rates from meteorological changes, are considerably greater (3 to 4 times) than the direct aerosol optical effect on the photolysis rate calculations.