Abstract
Abstract. Summertime aerosol optical extinction (βext) was measured in the Colorado Front Range and Denver metropolitan area as part of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) campaign during July–August 2014. An Aerodyne cavity attenuated phase shift particle light extinction monitor (CAPS-PMex) was deployed to measure βext (at average relative humidity of 20 ± 7 %) of submicron aerosols at λ = 632 nm at 1 Hz. Data from a suite of gas-phase instrumentation were used to interpret βext behavior in various categories of air masses and sources. Extinction enhancement ratios relative to CO (Δβext ∕ ΔCO) were higher in aged urban air masses compared to fresh air masses by ∼ 50 %. The resulting increase in Δβext ∕ ΔCO for highly aged air masses was accompanied by formation of secondary organic aerosols (SOAs). In addition, the impacts of aerosol composition on βext in air masses under the influence of urban, natural oil and gas operations (O&G), and agriculture and livestock operations were evaluated. Estimated non-refractory mass extinction efficiency (MEE) values for different air mass types ranged from 1.51 to 2.27 m2 g−1, with the minimum and maximum values observed in urban and agriculture-influenced air masses, respectively. The mass distribution for organic, nitrate, and sulfate aerosols presented distinct profiles in different air mass types. During 11–12 August, regional influence of a biomass burning event was observed, increasing the background βext and estimated MEE values in the Front Range.
Highlights
The aerosol optical extinction coefficient represents the attenuation of light due to aerosol absorption and scattering of solar radiation
Keeping in mind that in the presence of absorbing species, mass extinction efficiency (MEE) is higher than MSE, in the absence of estimates of MEE in other regions we present estimates of MSE from previous studies for comparison with the current MEE estimates in the Front Range
Airborne aerosol optical extinction (632 nm) and submicron non-refractory aerosol composition were measured during the summer in the Colorado Front Range to understand sources and processes that impact summertime visibility in the area
Summary
The aerosol optical extinction coefficient (βext) represents the attenuation of light due to aerosol absorption and scattering of solar radiation. The composition of the Denver brown cloud and contribution of different chemical species to the observed βext during the wintertime have been investigated in the 1970s to late 1980s (Groblicki et al, 1981; Wolff et al, 1981; Neff, 1997). These studies concluded that, among all the measured aerosol species, elemental carbon, ammonium sulfate, and ammonium nitrate contributed the most (37.7, 20.2, and 17.2 %, respectively) to wintertime optical extinction in the visible range. This paper will discuss the role of local aerosol sources in the Front Range and regional wildfires on aerosol optical extinction in the absence of the Denver cyclone by investigating chemical and optical properties of aerosols in different air masses
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