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.

Abstract. We implement a detailed representation of aerosol mixing state in the Global Environmental Multiscale – Modelling Air quality and CHemistry (GEM-MACH) air quality and weather forecast model. Our mixing-state representation includes three categories: one for more hygroscopic aerosol, one for less hygroscopic aerosol with a high black carbon (BC) mass fraction, and one for less hygroscopic aerosol with a low BC mass fraction. The more detailed representation allows us to better resolve two different aspects of aerosol mixing state: differences in hygroscopicity due to aerosol composition and the amount of absorption enhancement of BC due to non-absorbing coatings. Notably, this three-category representation allows us to account for BC thickly coated with primary organic matter, which enhances the absorption of the BC but has a low hygroscopicity. We compare the results of the three-category representation (1L2B, (one hydrophilic, two hydrophobic)) with a simulation that uses two categories, split by hygroscopicity (HYGRO), and a simulation using the original size-resolved internally mixed assumption (SRIM). We perform a case study that is focused on North America during July 2016, when there were intense wildfires over northwestern North America. We find that the more detailed representation of the aerosol hygroscopicity in both 1L2B and HYGRO decreases wet deposition, which increases aerosol concentrations, particularly of less hygroscopic species. The concentration of PM2.5 increases by 23 % on average. We show that these increased aerosol concentrations increase cloud droplet number concentrations and cloud reflectivity in the model, decreasing surface temperatures. Using two categories based on hygroscopicity yields only a modest benefit in resolving the coating thickness on black carbon, however. The 1L2B representation resolves BC with thinner coatings than the HYGRO simulation, resulting in absorption aerosol optical depths that are 3 % less on average, with greater differences over strong anthropogenic source regions. We did not find strong subsequent effects of this decreased absorption on meteorology.

A column model based on CIRA wind and temperature profiles is employed to assess the characteristics of the Hines Doppler‐spread and Warner–McIntyre–Scinocca (WMS) non‐orographic drag parametrizations for internal gravity waves. The “Alexander–Dunkerton” variant of the WMS scheme is also briefly considered. This study goes into more detail than previous comparisons by performing a spectral analysis of the momentum deposition and drag, and by examining the ability of each scheme to reproduce a high vertical wavenumber tail consistent with atmospheric measurements. We find several undesirable characteristics in the drag produced by the Hines scheme. For typical midlatitude wind profiles, it produces an abrupt onset of large accelerations that vary strongly from one layer to the next. It is also unable to reproduce spectra consistent with observed wave saturation at high vertical vertical wavenumbers, even for the windless case. The WMS scheme has the ability to reproduce the observed spectral tail for the case of no background wind. In the presence of typical CIRA midlatitude background winds, however, it is demonstrated that the WMS saturation threshold generally does not follow the observed spectral behaviour at high vertical wavenumbers, except for one specific frequency dependence not considered in previous work. Doppler shifting is also found to interfere with the production of high‐wavenumber spectral tails consistent with observations.

<p>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 by using the on-line chemical composition and size-resolved representation of atmospheric aerosols in GEM-MACH to calculate the attenuation of radiation in the photolysis module.</p><p>We coupled both the GEM-MACH aerosol module and the MESSy-JVAL (Modular Earth Sub-Model System) photolysis routine 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. The new algorithm applies a lensing correction factor to the black carbon absorption efficiency (core-shell parametrization) and calculates the scattering and absorption optical depth and asymmetry factor of black carbon, sea-salt, dust and other internally mixed components.</p><p>In order to evaluate the effects of these modifications on the performance of the GEM-MACH model, a series of simulations with the updated version of MESSy-JVAL and wildfire emission inputs from the Canadian Forest Fire Emissions Prediction System (CFFEPS) were carried out, and the model aerosol optical depth (AOD) output was compared to the previous version of MESSy-JVAL, satellite data, ground-based measurements, and re-analysis products. The comparison of the updated version of MESSy-JVAL with the previous version showed significant improvements in the model performance with the implementation of the new photolysis module and adopting the online interactive aerosol concentrations in GEM-MACH.</p>

No abstract