The preservation of atmospheric nitrate in snow at Summit, Greenland

There is great interest in using nitrate NO3 isotopic composition in ice cores to track the history of precursor nitrogen oxides (NOx = NO + NO2) in the atmosphere. Nitrate NO3 however can be lost from the snow by surface processes, such as photolysis back to NOx upon exposure to sunlight, making it difficult to interpret records of NO3 as a tracer of atmospheric NOx loading. In a campaign consisting of two field seasons (May–June) at Summit, Greenland, high temporal frequency surface snow samples were collected and analyzed for the oxygen isotopic composition of NO3. The strong, linear relationship observed between the oxygen isotopes of NO3 in both 2010 and 2011, is difficult to explain in the presence of significant post depositional processing of NO3 unless several unrelated variables change in concert. Therefore, the isotopic signature of NO3 in the snow at Summit is most feasibly explained as preserved atmospheric NO3 deposition

Heterogeneous N2O5 Uptake During Winter: Aircraft Measurements During the 2015 WINTER Campaign and Critical Evaluation of Current Parameterizations

Nocturnal dinitrogen pentoxide (N2O5) heterogeneous chemistry impacts regional air quality and the distribution and lifetime of tropospheric oxidants. Formed from the oxidation of nitrogen oxides, N2O5 is heterogeneously lost to aerosol with a highly variable reaction probability, γ(N2O5), dependent on aerosol composition and ambient conditions. Reaction products include soluble nitrate (HNO3 or NO3−) and nitryl chloride (ClNO2). We report the first‐ever derivations of γ(N2O5) from ambient wintertime aircraft measurements in the critically important nocturnal residual boundary layer. Box modeling of the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the eastern United States derived 2,876 individual γ(N2O5) values with a median value of 0.0143 and range of 2 × 10−5 to 0.1751. WINTER γ(N2O5) values exhibited the strongest correlation with aerosol water content, but weak correlations with other variables, such as aerosol nitrate and organics, suggesting a complex, nonlinear dependence on multiple factors, or an additional dependence on a nonobserved factor. This factor may be related to aerosol phase, morphology (i.e., core shell), or mixing state, none of which are commonly measured during aircraft field studies. Despite general agreement with previous laboratory observations, comparison of WINTER data with 14 literature parameterizations (used to predict γ(N2O5) in chemical transport models) confirms that none of the current methods reproduce the full range of γ(N2O5) values. Nine reproduce the WINTER median within a factor of 2. Presented here is the first field‐based, empirical parameterization of γ(N2O5), fit to WINTER data, based on the functional form of previous parameterizations

Wintertime Spatial Distribution of Ammonia and its Emission Sources in the Great Salt Lake Region

Ammonium-containing aerosols are a major component of wintertime air pollution in many densely populated regions around the world. Especially in mountain basins, the formation of persistent cold-air pools (PCAPs) can enhance particulate matter with diameters less than 2.5 µm (PM2.5) to levels above air quality standards. Under these conditions, PM2.5 in the Great Salt Lake region of northern Utah has been shown to be primarily composed of ammonium nitrate; however, its formation processes and sources of its precursors are not fully understood. Hence, it is key to understanding the emission sources of its gas phase precursor, ammonia (NH3). To investigate the formation of ammonium nitrate, a suite of trace gases and aerosol composition were sampled from the NOAA Twin Otter aircraft during the Utah Winter Fine Particulate Study (UWFPS) in January and February 2017. NH3 was measured using a quantum cascade tunable infrared laser differential absorption spectrometer (QC-TILDAS), while aerosol composition, including particulate ammonium (pNH4), was measured with an aerosol mass spectrometer (AMS). The origin of the sampled air masses was investigated using the Stochastic Time-Inverted Lagrangian Transport (STILT) model and combined with an NH3 emission inventory to obtain model-predicted NHx (=NH3+pNH4) enhancements. Enhancements represent the increase in NH3 mixing ratios within the last 24 h due to emissions within the model footprint. Comparison of these NHx enhancements with measured NHx from the Twin Otter shows that modelled values are a factor of 1.6 to 4.4 lower for the three major valleys in the region. Among these, the underestimation is largest for Cache Valley, an area with intensive agricultural activities. We find that one explanation for the underestimation of wintertime emissions may be the seasonality factors applied to NH3 emissions from livestock. An investigation of inter-valley exchange revealed that transport of NH3 between major valleys was limited and PM2.5 in Salt Lake Valley (the most densely populated area in Utah) was not significantly impacted by NH3 from the agricultural areas in Cache Valley. We found that in Salt Lake Valley around two thirds of NHx originated within the valley, while about 30 % originated from mobile sources and 60 % from area source emissions in the region. For Cache Valley, a large fraction of NOx potentially leading to PM2.5 formation may not be locally emitted but mixed in from other counties

Analysis of nitrate in the snow and atmosphere at Summit, Greenland: Chemistry and transport

International audienceAs a major sink of atmospheric nitrogen oxides (NOx = NO + NO2), nitrate (NO3-) in polar snow can reflect the long-range transport of NOx and related species (e.g., PAN). On the other hand, because NO3- in snow can be photolyzed, potentially producing gas-phase NOx locally, NO3- in snow (and thus, ice) may reflect local processes. Here we investigate the relationship between local atmospheric composition at Summit, Greenland (72°35’N, 38°25’W) and the isotopic composition of NO3- to determine the degree to which local processes influence atmospheric and snow NO3-. Based on snow and atmospheric observations during May-June 2010 and 2011, we find no connection between the local atmospheric concentrations of a suite of gases (BrO, NO, NOy, HNO3 and nitrite (NO2-)) and the NO3- isotopic composition or concentration in snow. This suggests that 1) the snow NO3- at Summit is primarily derived from long-range transport and 2) this NO3- is largely preserved in the snow. Additionally, three isotopically distinct NO3- sources were found to be contributing to the NO3- in the snow at Summit during both 2010 and 2011. Through the complete isotopic composition of NO3-, we suggest that these sources are local anthropogenic particulate NO3- from station activities (δ15N = 16‰, Δ17O = 4‰ and δ18O = 23‰), NO3- formed from mid-latitude NOx (δ15N = -10‰, Δ17O = 29‰, δ18O = 78‰) and a NO3- source that is possibly influenced or derived from stratospheric ozone NO3- (δ15N = 5‰, Δ17O = 39‰, δ18O = 100‰)

ClNO2 yields from aircraft measurements during the 2015 WINTER campaign and critical evaluation of the current parameterization

Nitryl chloride (ClNO2) plays an important role in the budget and distribution of tropospheric oxidants, halogens, and reactive nitrogen species. ClNO2 is formed from the heterogeneous uptake and reaction of dinitrogen pentoxide (N2O5) on chloride-containing aerosol, with a production yield, ϕ(ClNO2), defined as the moles of ClNO2 produced relative to N2O5 lost. The ϕ(ClNO2) has been increasingly incorporated into 3-D chemical models where it is parameterized based on laboratory-derived kinetics and currently accepted aqueous-phase formation mechanism. This parameterization models ϕ(ClNO2) as a function of the aerosol chloride to water molar ratio. Box model simulations of night flights during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign derived 3,425 individual ϕ(ClNO2) values with a median of 0.138 and range of 0.003 to 1. Comparison of the box model median to those predicted by two other field-based ϕ(ClNO2) derivation methods agreed within a factor of 1.3, within the uncertainties of each method. In contrast, the box model median was 75–84% lower than predictions from the laboratory-based parameterization (i.e., [parameterization − box model]/parameterization). An evaluation of factors influencing this difference reveals a positive dependence of ϕ(ClNO2) on aerosol water, opposite to the currently parameterized trend. Additional factors may include aqueous-phase competition reactions for the nitronium ion intermediate and/or direct ClNO2 loss mechanisms. Further laboratory studies of ClNO2 formation and the impacts of aerosol water, sulfate, organics, and ClNO2 aqueous-phase reactions are required to elucidate and quantify these processes on ambient aerosol, critical for the development of a robust ϕ(ClNO2) parameterization

Wintertime Overnight NOx Removal in a Southeastern United States Coal-Fired Power Plant Plume: A Model for Understanding Winter NOx Processing and Its Implications

Nitric oxide (NO) is emitted in large quantities from coal-burning power plants. During the day, the plumes from these sources are efficiently mixed into the boundary layer, while at night, they may remain concentrated due to limited vertical mixing during which they undergo horizontal fanning. At night, the degree to which NO is converted to HNO3 and therefore unable to participate in next-day ozone (O3) formation depends on the mixing rate of the plume, the composition of power plant emissions, and the composition of the background atmosphere. In this study, we use observed plume intercepts from the Wintertime INvestigation of Transport, Emissions and Reactivity (WINTER) campaign to test sensitivity of overnight NOx removal to the N2O5 loss rate constant, plume mixing rate, background O3, and background levels of volatile organic compounds using a 2-D box model of power plant plume transport and chemistry. The factor that exerted the greatest control over NOx removal was the loss rate constant of N2O5. At the lowest observed N2O5 loss rate constant, no other combination of conditions converts more than 10 percent of the initial NOx to HNO3. The other factors did not influence NOx removal to the same degree