Aerosol Optical Extinction during the Front Range Air Pollution and Photochemistry Experiment (FRAPPE) 2014 Summertime Field Campaign, Colorado U.S.A.

Abstract:

Aerosol optical extinction (β_ext) was measured in the Colorado Front Range Denver Metropolitan Area as part of the summertime air quality airborne field campaign to characterize the influence of sources, photochemical processing, and transport of pollution on local air quality. An Aerodyne Cavity Attenuated Phase Shift particle light extinction monitor (CAPS-PMex) was deployed to measure dry β_ext at λ=632 nm at 1 Hz. Data from a suite of gas-phase instrumentation were used to interpret the β_ext under various categories of aged air masses and sources. Extinction enhancement ratios of Δβ_ext/ΔCO were evaluated under 3 differently aged air mass categories (fresh, intermediately aged, and aged) to investigate impacts of photochemistry on β_ext. Δβ_ext/ΔCO was significantly increased in heavily aged air masses compared to fresh air masses (0.17 Mm^-1/ppbv and 0.094 Mm^-1/ppbv respectively). The resulting increase in Δβ_ext/ΔCO under heavily aged air masses was represented by secondary organic aerosols (SOA) formation. Aerosol composition and sources from urban, natural oil and gas wells (OG), and agriculture and livestock operations were also evaluated for their impacts on β_ext. Linear regression fits to β_ext vs. organic aerosol mass showed higher correlation coefficients under the urban and OG plumes (r=0.55 and r=0.71 respectively) and weakest under agricultural and livestock plumes (r=0.28). The correlation between β_ext and nitrate aerosol mass however was best under the agriculture and livestock plumes (r=0.81), followed by OG plumes (r=0.74), suggesting co-location of aerosol nitrate precursor sources with OG emissions. Finally, non-refractory mass extinction efficiency (MEE) was analyzed. MEE was observed to be 1.37 g/m² and 1.30 g/m² in OG and urban+OG plumes, respectively.