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
Authors: Majdzadeh Mahtab , Stroud Craig A. , Sioris Christopher , Makar Paul A. , Akingunola Ayodeji , McLinden Chris , Zhao Xiaoyi , Moran Michael D. , Abboud Ihab , Chen Jack 
Published: January 2022

Abstract. The photolysis module in Environment and Climate Change Canada's
online chemical transport model GEM-MACH (GEM: Global Environmental
Multi-scale – MACH: Modelling Air quality and Chemistry) was improved to
make use of the online 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 Submodel
System) photolysis module, through the use of the online 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 2 months, compared the model
aerosol optical depth (AOD) output to the previous version of MESSy-JVAL,
satellite data, ground-based measurements and reanalysis 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 1-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 reanalysis (Modern-Era Retrospective
analysis for Research and Applications – version 2), with an overall
underprediction of AOD over the common domain for both seasons. Our study
also suggests that the domain-wide impacts 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.