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[1] Heterogeneous N 2 O 5 uptake onto aerosol is the primary nocturnal path for removal of NO x (= NO + NO 2 ) from the atmosphere and can also result in halogen activation through production of ClNO 2 . The N 2 O 5 uptake coefficient has been the subject of numerous laboratory studies; however, only a few studies have determined the uptake coefficient from ambient measurements, and none has been focused on winter conditions, when the portion of NO x removed by N 2 O 5 uptake is the largest. In this work, N 2 O 5 uptake coefficients are determined from ambient wintertime measurements of N 2 O 5 and related species at the Boulder Atmospheric Observatory in Weld County, CO, a location that is highly impacted by urban pollution from Denver, as well as emissions from agricultural activities and oil and gas extraction. A box model is used to analyze the nocturnal nitrate radical chemistry and predict the N 2 O 5 concentration. The uptake coefficient in the model is iterated until the predicted N 2 O 5 concentration matches the measured concentration. The results suggest that during winter, the most important influence that might suppress N 2 O 5 uptake is aerosol nitrate but that this effect does not suppress uptake coefficients enough to limit the rate of NO x loss through N 2 O 5 hydrolysis. N 2 O 5 hydrolysis was found to dominate the nocturnal chemistry during this study consuming~80% of nocturnal gas phase nitrate radical production. Typically, less than 15% of the total nitrate radical production remained in the form of nocturnal species at sunrise when they are photolyzed and reform NO 2 . , et al. (2013), N 2 O 5 uptake coefficients and nocturnal NO 2 removal rates determined from ambient wintertime measurements, J. Geophys. Res. Atmos., 118,[9331][9332][9333][9334][9335][9336][9337][9338][9339][9340][9341][9342][9343][9344][9345][9346][9347][9348][9349][9350]

Abstract. Two proton-transfer-reaction mass spectrometry systems were deployed at the Bio-hydro-atmosphere interactions of Energy, Aerosols, Carbon, H 2 O, Organics and Nitrogen-Southern Rocky Mountain 2008 field campaign (BEACHON-SRM08; July to September, 2008) at the Manitou Forest Observatory in a ponderosa pine woodland near Woodland Park, Colorado USA. The two PTR-MS systems simultaneously measured BVOC emissions and ambient distributions of their oxidation products. Here, we present mass spectral analysis in a wide range of masses (m/z 40 + to 210 + ) to assess our understanding of BVOC emissions and their photochemical processing inside of the forest canopy. The biogenic terpenoids, 2-methyl-3-butene-2-ol (MBO, 50.2%) and several monoterpenes (MT, 33.5%) were identified as the dominant BVOC emissions from a transmission corrected mass spectrum (PTR-MS), averaged over the daytime (11 a.m. to 3 p.m., local time) of three days. To assess contributions of oxidation products of local BVOC, we calculate an oxidation product spectrum with the OH-and ozone-initiated oxidation product distribution mass spectra of two major BVOC emissions at the ecosystem (MBO and β-pinene) that were observed from laboratory oxidation experiments. The majority (∼ 76%) of the total signal in the transmission corrected PTR-MS spectra could be explained by identified compounds. The remainder are attributed to oxidation products of BVOC emitted from nearby ecosystems and transported to the site, and oxidation products of unidentified BVOC emitted from the ponderosa pine ecosystem.

[1] A negative-ion proton-transfer chemical ionization mass spectrometer was deployed on a mobile tower-mounted platform during Nitrogen, Aerosol Composition, and Halogens on a Tall Tower (NACHTT) to measure nitrous acid (HONO) in the winter of 2011. High resolution vertical profiles revealed (i) HONO gradients in nocturnal boundary layers, (ii) ground surface dominates HONO production by heterogeneous uptake of NO 2 , (iii) significant quantities of HONO may be deposited to the ground surface at night, (iv) daytime gradients indicative of ground HONO production or emission, and (v) an estimated surface HONO reservoir comparable or larger than integrated daytime HONO surface production. Nocturnal integrated column observations of HONO and NO 2 allowed direct evaluation of nocturnal ground surface uptake coefficients for these species (γ NO2, surf = 2 × 10 À6 to 1.6 × 10 À5 and γ HONO, surf = 2 × 10 À5 to 2 × 10 À4 ). The quantity of surface-deposited HONO was also modeled, showing that HONO deposited to the surface at night was at least 25%, and likely in excess of 100%, of the calculated unknown daytime HONO source. These results suggest that if nocturnally deposited HONO forms a conservative surface reservoir, which can be released the following day, a significant fraction of the daytime HONO source can be explained for the NACHTT observations. Citation: VandenBoer, T. C., et al. (2013), Understanding the role of the ground surface in HONO vertical structure: High resolution vertical profiles during