Seasonal comparisons of GEOS-Chem-TOMAS (GCT) simulations with AERONET-inversion retrievals over sites in the North American and European Arctic

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GEOS-Chem TOMAS (GCT) simulations of AERONET-inversion products during 2015 were compared with AERONET-inversion products from the multi-year climatology of AboEl-Fetouh et al. (2020) (AeF) and for 2015 over 5 stations in the North American and European Arctic. The GCT simulations of particle size distributions (PSD) did not capture a spring to summer radius increase of the fine mode (FM) peak observed by AeF but did capture AeF's springtime coarse mode (CM) peak (small-sized CM peak with a radius ∼ 1.3 μm) and a weak late summer/fall increase in the amplitude of that peak. The lack of a spring to summer FM radius increase was likely due to the large GCT cell size (4° × 5°) and associated difficulties in the modelling of coagulation-induced smoke particle size. Conversely, the GCT simulation of the small-sized CM peak indicated a successful capture of the springtime influx of Asian dust. The fall increase of that GCT peak was associated with an increase of a larger (4–7 μm) PSD mode that AeF suggested was due to local dust. GCT captured the general (climatological-scale) FM AOD amplitude, the decreasing CM AOD trend, and the increasing trend of the FM fraction. The GCT CM AOD also showed a fall increase that was coherent with the increase of the simulated small-sized CM peak and with a lesser rate of decrease of the AeF CM AOD. Large GCT deviations from the AERONET retrievals were attributed to an extreme July 2015 forest fire event.