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Remote and free-tropospheric aerosols represent a large fraction of the\udclimatic influence of aerosols; however, aerosol in these regions is less\udcharacterized than those polluted boundary layers. We evaluate aerosol size\uddistributions predicted by the GEOS-Chem-TOMAS global chemical transport\udmodel with online aerosol microphysics using measurements from the peak of\udWhistler Mountain, British Columbia, Canada\ud(2182 m a.s.l., hereafter\udreferred to as Whistler Peak). We evaluate the\udmodel for predictions of aerosol number, size, and composition during periods\udof free-tropospheric (FT) and boundary-layer (BL) influence at "coarse"\ud4°  ×  5° and "nested"\ud0.5°  ×  0.667° resolutions by developing simple FT/BL\udfiltering techniques. We find that using temperature as a proxy for upslope\udflow (BL influence) improved the model–measurement comparisons. The best\udthreshold temperature was around 2 °C for the coarse simulations and\udaround 6 °C for the nested simulations, with temperatures warmer\udthan the threshold indicating boundary-layer air. Additionally, the site was\udincreasingly likely to be in cloud when the measured relative humidity (RH)\udwas above 90  %, so we do not compare the modeled and measured size\uddistributions during these periods. With the inclusion of these temperature\udand RH filtering techniques, the model–measurement comparisons improved\udsignificantly. The slope of the regression for N80 (the total number of\udparticles with particle diameter, Dp,  >  80 nm) in the nested\udsimulations increased from 0.09 to 0.65, R2 increased from 0.04 to 0.46,\udand log-mean bias improved from 0.95 to 0.07. We also perform simulations at\udthe nested resolution without Asian anthropogenic emissions and without\udbiomass-burning emissions to quantify the contribution of these sources to\udaerosols at Whistler Peak (through\udcomparison with simulations with these emissions on). The long-range\udtransport of Asian anthropogenic aerosol was found to be significant\udthroughout all particle number concentrations, and increased N80 by more than\ud50 %, while decreasing the number of smaller particles because of\udsuppression of new-particle formation and enhanced coagulation sink.\udSimilarly, biomass burning influenced Whistler Peak during summer months,\udwith an increase in N80 exceeding 5000 cm−3. Occasionally, Whistler\udPeak experienced N80  >  1000 cm−3 without significant influence from\udAsian anthropogenic or biomass-burning aerosol. Air masses were advected at\udlow elevations through forested valleys during times when temperature and\uddownwelling insolation were high, ideal conditions for formation of large\udsources of low-volatility biogenic secondary organic aerosol (SOA). This\udcondensable material increased particle growth and hence N80. The low-cost\udfiltering techniques and source apportionment used in this study can be used\udin other global models to give insight into the sources and processes that\udshape the aerosol at mountain sites, leading to a better understanding of\udmountain meteorology and chemistry

Source attribution of aerosol size distributions and model evaluation using Whistler Mountain measurements and GEOS-Chem-TOMAS simulations