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    N2O5 uptake coefficients and nocturnal NO2 removal rates determined from ambient wintertime measurements
    (Amer Geophysical Union, 2013) Wagner, N. L.; Riedel, T. P.; Young, Cora J.; Bahreini, R.; Brock, Charles A.; Öztürk, Fatma
    Heterogeneous N2O5 uptake onto aerosol is the primary nocturnal path for removal of NOx (= NO+NO2) from the atmosphere and can also result in halogen activation through production of ClNO2. The N2O5 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 NOx removed by N2O5 uptake is the largest. In this work, N2O5 uptake coefficients are determined from ambient wintertime measurements of N2O5 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 N2O5 concentration. The uptake coefficient in the model is iterated until the predicted N2O5 concentration matches the measured concentration. The results suggest that during winter, the most important influence that might suppress N2O5 uptake is aerosol nitrate but that this effect does not suppress uptake coefficients enough to limit the rate of NOx loss through N2O5 hydrolysis. N2O5 hydrolysis was found to dominate the nocturnal chemistry during this study consuming similar to 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 NO2.
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    Vertically resolved chemical characteristics and sources of submicron aerosols measured on a Tall Tower in a suburban area near Denver, Colorado in winter
    (Amer Geophysical Union, 2013) Öztürk, Fatma; Bahreini, R.; Wagner, Nicholas L.; Dube, W. P.; Young, Cora J.; Brown, S. S.; Brock, Charles A.
    The Nitrogen, Aerosol Composition, and Halogens on a Tall Tower study was conductedat the Boulder Atmospheric Observatory in Colorado during February–March 2011. Acompact time-of-flight aerosol mass spectrometer was installed in a moving carriage on thetower, obtaining vertical profiles of submicron nonrefractory aerosol mass concentrations(PMnr1)from0–265 m above ground level. The average PMnr1was 4.6 ± 5.7 μg/m3, withaverage contributions of nitrate, organics, sulfate, ammonium, and chloride of 35%, 26%,20%, 17%, and 1%, respectively. Positive Matrix Factorization analysis of the organic aerosol(OA) mass spectra indicated that average contributions of oxygenated organic aerosol(OOA)-I, OOA-II, and hydrocarbon-like organic aerosol (surrogates for aged and freshsecondary OA and primary OA, respectively) to OA mass were 52%, 32%, and 16%,respectively. There was considerable variability in the vertical profiles of aerosol mass loadingand composition, especially at the lowest heights. Below 40 m, the highest PMnr1concentrationswere composed of mostly nitrate (30–46%) and were associated with winds from the northeastwhere there are large agricultural facilities. When winds were southerly, PMnr1massdistributions near the surface had small, fresh OA, indicating the influence of nearby Denverurban emissions at the site. The largest contribution to OA mass at these heights was OOA-II(~43%). Between 40 and 120 m, trajectory cluster analysis indicated that during high-altitudelong-range transport events, daytime aerosol composition was dominated by sulfate, whereasduring low-altitude transport events, the contributions of sulfate, nitrate, and OA werecomparable. OOA-I contributed the most (53–68%) to OA mass at these tower heights.