Comment on “Constraining Chemical Transport PM2.5 Modeling Using Surface Monitor Measurements and Satellite Retrievals: Application over the San Joaquin Valley” by Mariel

Advances in satellite retrieval of aerosol type can improve the accuracy of near-surface air quality 10 characterization, by providing broad regional context and decreasing metric uncertainties and errors. The frequent, spatially extensive and radiometrically consistent instantaneous constraints can be especially useful in areas away from ground monitors and progressively downwind of emission sources. We present a physical approach to constraining regional-scale estimates of PM2.5, its major chemical component species estimates, and related uncertainty estimates of chemical transport model (CTM, e.g. the Community Multi-scale Air Quality Model) outputs. This approach uses ground-based monitors where 15 available, combined with aerosol optical depth and qualitative constraints on aerosol size, shape, and light-absorption properties from the Multi-angle Imaging SpectroRadiometer (MISR) on the NASA Earth Observing System’s Terra satellite. The CTM complements these data by providing complete spatial and temporal coverage. Unlike widely used approaches that train statistical regression models, the technique developed here leverages CTM physical constraints such as conservation of aerosol mass and meteorological consistency, independent of observations. The CTM also aids in identifying relationships 20 between observed species concentrations and emission sources. Aerosol airmass types over populated regions of Central California are characterized using satellite data acquired during the 2013 San Joaquin field deployment of the NASA DISCOVER-AQ project. We investigate the optimal application of incorporating 275 m horizontal-resolution aerosol airmass-type maps and total-column aerosol optical depth from the MISR 25 Research Aerosol retrieval algorithm (RA) into regional-scale CTM output. The impact on surface PM2.5 fields progressively downwind of large single sources is evaluated using contemporaneous surface observations. Spatiotemporal R and RMSE values for the model, constrained by both satellite and surface-monitor measurements based on 10-fold cross-validation, are 0.79 and 0.33 for PM2.5, 0.88 and 0.65 for NO3, 0.78 and 0.23 for SO4, 1.00 and 1.01 for NH4, 0.73 and 0.23 for OC, and 0.31 and 0.65 for EC, respectively. Regional cross-validation temporal and spatiotemporal R results for the satellite-based 30 PM2.5 improve by 30% and 13%, respectively, in comparison to unconstrained CTM simulations, and provide finer spatial Mariel Friberg 7/12/2018 5:13 PM Deleted: . In addition Mariel Friberg 7/12/2018 5:13 PM Deleted: , Mariel Friberg 7/12/2018 5:13 PM Deleted: single-scattering albedo provided by multi-angle instruments, such as 35 Mariel Friberg 7/12/2018 5:13 PM Deleted: , can provide frequent, spatially extensive, instantaneous constraints on chemical transport models (CTMs), which can be especially useful in areas away from ground monitors and progressively downwind of emission sources. CTMs 40 (e.g. the Community Multi-scale Air Quality Modeling System) complement such Mariel Friberg 7/12/2018 5:13 PM Deleted: , offering additional Mariel Friberg 7/12/2018 5:13 PM Deleted: (e.g., Mariel Friberg 7/12/2018 5:13 PM Deleted: , 45 Mariel Friberg 7/12/2018 5:13 PM Deleted: ) Mariel Friberg 7/12/2018 5:13 PM Deleted: , and aid Mariel Friberg 7/12/2018 5:13 PM Deleted: . Incorporating satellite aerosol information in the development of PM2.5 concentration metrics can lead to a decrease in 50 metric uncertainties and errors Mariel Friberg 7/12/2018 5:13 PM Deleted: This work focuses on the degree to which . [1] Mariel Friberg 7/12/2018 5:13 PM Deleted: Southern Mariel Friberg 7/12/2018 5:13 PM Deleted: Using the MISR Research Aerosol 70 . [2] Mariel Friberg 7/12/2018 5:13 PM Deleted: and evaluate 60 Mariel Friberg 7/12/2018 5:13 PM Deleted: a 2 km resolution, Mariel Friberg 7/12/2018 5:13 PM Deleted: , to obtain constrained fields of Mariel Friberg 7/12/2018 5:13 PM Deleted: . Contemporaneous surface observations . [3] Mariel Friberg 7/12/2018 5:13 PM Deleted: discussed. The spatiotemporal R Mariel Friberg 7/12/2018 5:13 PM Deleted: % withholding 2 resolution. SO4 cross-validation values showed the largest spatial and spatiotemporal R improvement with a 43% increase. Assessing this physical technique in a well-instrumented region opens the possibility of applying it globally, especially over areas where surface air quality measurements are scarce or entirely absent.