Abstract
Abstract. We measured organic and inorganic gas-phase acids in the Front Range of Colorado to better understand their tropospheric sources and sinks using a high-resolution time-of-flight chemical ionization mass spectrometer. Measurements were conducted from 4 to 13 August 2014 at the Boulder Atmospheric Observatory during the Front Range Air Pollution and Photochemistry Éxperiment. Diurnal increases in mixing ratios are consistent with photochemical sources of HNO3, HNCO, formic, propionic, butyric, valeric, and pyruvic acid. Vertical profiles taken on the 300 m tower demonstrate net surface-level emissions of alkanoic acids, but net surface deposition of HNO3 and pyruvic acid. The surface-level alkanoic acid source persists through both day and night, and is thus not solely photochemical. Reactions between O3 and organic surfaces may contribute to the surface-level alkanoic acid source. Nearby traffic emissions and agricultural activity are a primary source of propionic, butyric, and valeric acids, and likely contribute photochemical precursors to HNO3 and HNCO. The combined diel and vertical profiles of the alkanoic acids and HNCO are inconsistent with dry deposition and photochemical losses being the only sinks, suggesting additional loss mechanisms.
Highlights
Organic acids comprise a major fraction of gas-phase acids in the troposphere
Organic acids are involved in the formation of secondary organic aerosol (SOA; Vogel et al, 2013; Yatavelli et al, 2014, 2015), which affects human health, visibility, and climate
We speculate that reactions between O3 and organic surfaces near the site could be driving this persistent upward alkanoic acid flux
Summary
Organic acids comprise a major fraction of gas-phase acids in the troposphere. They influence the acidity of precipitation, fog, and cloud droplets, in rural areas (Keene and Galloway, 1984; Andreae et al, 1988), and can impact ecosystem health (Sverdrup et al, 2001; Himanen et al., 2012). Sources and sinks determine tropospheric concentrations of gas-phase organic acids, and their impacts on biological health and air quality. Model–measurement comparisons for other tropospheric organic acids are lacking. Field and laboratory measurements investigating the sources and sinks of these compounds are necessary to reduce model uncertainties and improve our understanding of organic acids in the troposphere
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