Abstract
Abstract. We present airborne measurements made during the 2014 Front Range Air Pollution and Photochemistry Experiment (FRAPPÉ) project to investigate the impacts of the Denver Cyclone on regional air quality in the greater Denver area. Data on trace gases, non-refractory submicron aerosol chemical constituents, and aerosol optical extinction (βext) at λ = 632 nm were evaluated in the presence and absence of the surface mesoscale circulation in three distinct study regions of the Front Range: In-Flow, Northern Front Range, and the Denver metropolitan area. Pronounced increases in mass concentrations of organics, nitrate, 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 organics on cyclone days across all three regions, while the increase during the cyclone episode was up to ∼ 80 % over the Denver metropolitan area. 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.
Highlights
Atmospheric aerosols are of interest due to their impacts on human health, visibility, and climate radiative forcing through scattering and absorption of solar radiation (Monks et al, 2009; Stocker et al, 2013)
Data from the FRAPPÉ 2014 project in the Colorado Front Range were presented to understand the influence of the Denver Cyclone on source distribution and processes that impact regional air quality and visibility in the summer
The analysis demonstrated that mesoscale recirculation patterns changed the spatial distribution of pollutants emitted in the northern latitudes of the study area, transporting pollutants over Denver metropolitan area (DM), leading to enhanced concentration of secondary aerosol species
Summary
Atmospheric aerosols are of interest due to their impacts on human health, visibility, and climate radiative forcing through scattering and absorption of solar radiation (Monks et al, 2009; Stocker et al, 2013). The complex topography of the Colorado Front Range leads to terrain-induced flows and mesoscale circulations that have a significant impact on air quality These include cycles of daytime thermally driven upslope from the plains into the mountains and decoupled, downslope nighttime drainage and slope flows which can transport and pool particulates and precursors of secondary aerosols into the wider Platte River valley between Denver and Greeley, Colorado. During 1996–1997, measurements of aerosol composition and inorganic aerosol precursors were carried out in winter and summer months at several urban and rural sites during the Northern Front Range Air Quality Study (NFRAQS). We focus our analysis on the data obtained during FRAPPÉ to assess the impact of the Denver Cyclone on the region’s air quality (Flocke, 2015)
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