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

In the most aged air masses (NOx / NOy  <  0.5), background organic aerosols over the Denver metropolitan area increased by a factor of ∼  2.5 due to transport from Northern Front Range. Furthermore, enhanced partitioning of nitric acid to the aerosol phase was observed during the cyclone episodes, mainly due to increased abundance of gas phase ammonia. During the non-cyclone events, βext displayed strong correlations (r  =  0.71) with organic and nitrate in the Northern Front Range and only with organics (r  =  0.70) in the Denver metropolitan area, while correlation of βext during the cyclone was strongest (r  =  0.86) with nitrate over Denver.

Mass extinction efficiency (MEE) values in the Denver metropolitan area were similar on cyclone and non-cyclone days despite the dominant influence of different aerosol species on βext. Our analysis showed that the meteorological patterns associated with the Denver Cyclone increased aerosol mass loadings in the Denver metropolitan area mainly by transporting aerosols and/or aerosol precursors from the northern regions, leading to impaired visibility and air quality deterioration.