Interactive Aerosol Feedbacks on Photolysis Rates in the GEM-MACH Air Quality Model for an Athabasca Oil-Sands Modelling Study

The radiative transfer module in chemical transport models contains parameterizations for extinction efficiency, single scatter albedo and asymmetry factor for individual particles and calculates scattering and absorption coefficients, in order to predict the radiative state of the atmosphere. These aerosol optical properties are integrated over the vertical column to calculate aerosol optical depth (AOD). The current photolysis routine in the radiative transfer module, MESSY-JVAL (Modular Earth Sub-Model System) in ECCC's (Environment and Climate Change Canada) 3D air quality model, GEM-MACH (GEM: Global Environmental Multi-scale - MACH: Modelling Air quality and Chemistry), calculates photolysis rates using climatological aerosol vertical profiles. We calculated a new lookup table for the extinction efficiency, absorption and scattering cross sections of each aerosol type. The new version of MESSY-JVAL uses GEM-MACH predicted aerosol size distributions, chemical composition and relative humidity in each vertical column at each time step as input, reads aerosol absorption and scattering cross section data from the new lookup table and calculates aerosol optical properties, that are then used to attenuate the radiation stream.

In order to evaluate these modifications to the model, we performed a series of simulations with GEM-MACH with wildfire emissions inputs from the Canadian Forest Fire Emissions Prediction System (CFFEPS) and compared the model AOD output with satellite and AERONET (Aerosol Robotic Network) measurement data. Comparison of the hourly AERONET and monthly-averaged satellite AOD demonstrates major improvements in the revised model AOD predictions. One instrumented aircraft flight, which sampled a forest fire plume, will be used to compare to measurements. The impact of the updated photolysis rates on predictions of oxidant mixing ratios and rates of pollutant oxidation (nitrogen dioxide conversion to nitric acid) will be assessed both within and below the forest fire plume.