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We present airborne measurements made during the\ud2014 Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ)\udproject to investigate the impacts of the Denver Cyclone on regional air\udquality in the greater Denver area. Data on trace gases, non-refractory\udsubmicron aerosol chemical constituents, and aerosol optical extinction (βext)\udat λ  =  632 nm were evaluated in the presence\udand absence of the surface mesoscale circulation in three distinct study\udregions of the Front Range: In-Flow, Northern Front Range, and the Denver\udmetropolitan area. Pronounced increases in mass concentrations of organics,\udnitrate, and sulfate in the Northern Front Range and the Denver metropolitan area were observed during the cyclone episodes (27–28 July) compared to the non-cyclonic days (26 July, 2–3 August). Organic aerosols dominated the mass concentrations on all evaluated days, with a 45 % increase in\udorganics on cyclone days across all three regions, while the increase during\udthe cyclone episode was up to  ∼  80 % over the Denver metropolitan area. In the most aged air masses (NOx / NOy  <  0.5),\udbackground organic aerosols over the Denver metropolitan area increased by\uda factor of ∼  2.5 due to transport from Northern Front Range.\udFurthermore, enhanced partitioning of nitric acid to the aerosol phase was\udobserved during the cyclone episodes, mainly due to increased abundance of\udgas phase ammonia. During the non-cyclone events, βext displayed strong correlations (r  =  0.71) with organic and nitrate in the\udNorthern Front Range and only with organics (r  =  0.70) in the\udDenver metropolitan area, while correlation of βext during the\udcyclone was strongest (r  =  0.86) with nitrate over Denver. Mass\udextinction efficiency (MEE) values in the Denver metropolitan area were similar\udon cyclone and non-cyclone days despite the dominant influence of different\udaerosol species on βext. Our analysis showed that the\udmeteorological patterns associated with the Denver Cyclone increased aerosol\udmass loadings in the Denver metropolitan area mainly by transporting\udaerosols and/or aerosol precursors from the northern regions, leading to\udimpaired visibility and air quality deterioration

Impacts of the Denver Cyclone on regional air quality and aerosol formation in the Colorado Front Range during FRAPPÉ 2014