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  1. Abstract

    We present a comparison of instruments measuring nitrogen oxide species from an aircraft during the 2015 Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign over the northeast United States. Instrument techniques compared here include chemiluminescence (CL), thermal dissociation laser‐induced fluorescence (TD‐LIF), cavity ring‐down spectroscopy (CRDS), high‐resolution time of flight, iodide‐adduct chemical ionization mass spectrometry (ICIMS), and aerosol mass spectrometry. Species investigated include NO2, NO, total nitrogen oxides (NOy), N2O5, ClNO2, and HNO3. Particulate‐phase nitrate is also included for comparisons of HNO3and NOy. Instruments generally agreed within reported uncertainties, with individual flights sometimes showing much better agreement than the data set taken as a whole, due to flight‐to‐flight slope changes. NO measured by CRDS and CL showed an average relative slope of 1.16 ± 0.01 across all flights, which is outside of combined uncertainties. The source of the error was not identified. For NO2measured by CRDS and TD‐LIF the average was 1.02 ± 0.00; for NOymeasured by CRDS and CL the average was 1.01 ± 0.00; and for N2O5measured by CRDS and ICIMS the average was 0.89 ± 0.01. NOybudget closure to within 20% is demonstrated. We observe nonlinearity in NO2and NOycorrelations at concentrations above ~30 ppbv that may be related to the NO discrepancy noted above. For ClNO2there were significant differences between ICIMS and TD‐LIF, potentially due in part to the temperature used for thermal dissociation. Although the fraction of particulate nitrate measured by the TD‐LIF is not well characterized, it improves comparisons to include particulate measurements.

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  2. Abstract

    We use observations from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) aircraft campaign to constrain the proposed mechanism of Cl2production from ClNO2reaction in acidic particles. To reproduce Cl2concentrations observed during WINTER with a chemical box model that includes ClNO2reactive uptake to form Cl2, the model required the ClNO2reaction probability, γ (ClNO2), to range from 6 × 10−6to 7 × 10−5, with a mean value of 2.3 × 10−5(±1.8 × 10−5). These field‐determined γ (ClNO2) are more than an order of magnitude lower than those determined in previous laboratory experiments on acidic surfaces, even when calculated particle pH is ≤2. We hypothesize this is because thick salt films in the laboratory enhanced the reactive uptake ClNO2compared to that which would occur in submicron aerosol particles. Using the reacto‐diffusive length‐scale framework, we show that the field and laboratory observations can be reconciled if the net aqueous‐phase reaction rate constant for ClNO2(aq) + Cl(aq) in acidic particles is on the order of 104s−1. We show that wet particle diameter and particulate chloride mass together explain 90% of the observed variance in the box model‐derived γ (ClNO2), implying that the availability of chloride and particle volume limit the efficiency of the reaction. Despite a much lower conversion of ClNO2into Cl2, this mechanism can still be responsible for the nocturnal formation of 10–20 pptv of Cl2in polluted regions, yielding an atmospherically relevant concentration of Cl atoms the following morning.

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  3. Abstract

    Sulfur dioxide (SO2) is emitted in large quantities from coal‐burning power plants and leads to various harmful health and environmental effects. In this study, we use plume intercepts from the Wintertime INvestigation of Transport, Emission and Reactivity (WINTER) campaign to estimate the oxidation rates of SO2under wintertime conditions and the factors that determine SO2removal. Observations suggest that OH governs the rate SO2oxidation in the eastern United States during winter. The range of mean oxidation rates during the day from power plants were 0.22–0.71%/hr, producing SO2lifetimes of 13–43 days, if SO2consumption is assumed to occur during 10.5 hr of daylight in cloudless conditions. Though most nighttime rate measurements were zero within uncertainty, there is some evidence of nighttime removal, which suggests alternate oxidation mechanisms. The fastest nighttime observed SO2oxidation rate was 0.25±0.07%/hr, producing a combined day/night SO2lifetime of 8.5–21 days. The upper limit of the oxidation rate (the mean+1σof the fastest day and night observations) is 16.5%/day, corresponding to a lifetime of 6.1 days. The analysis also quantifies the primary emission of sulfate from power plants. The median mole percentage of SO4‐2from observed plumes was 1.7% and the mean percentage sulfate was 2.8% for intercepts within 1 hr of transit to power plants. The largest value observed from close intercepts was over 7% sulfate, and the largest extrapolated value was 18%, based on intercepts further from their source and fastest observed oxidation rate.

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  4. Abstract. Organic nitrate (RONO2) formation in the atmosphere represents a sink of NOx(NOx = NO + NO2) and termination of the NOx/HOx(HOx = HO2 + OH) ozone formation and radical propagation cycles, can act as a NOx reservoirtransporting reactive nitrogen, and contributes to secondary organic aerosol formation. While some fraction of RONO2 is thought to reside in the particle phase, particle-phase organic nitrates (pRONO2) are infrequently measured and thus poorly understood. There is anincreasing prevalence of aerosol mass spectrometer (AMS) instruments, which have shown promise for determining the quantitative total organic nitratefunctional group contribution to aerosols. A simple approach that relies on the relative intensities of NO+ and NO2+ ions inthe AMS spectrum, the calibrated NOx+ ratio for NH4NO3, and the inferred ratio for pRONO2 hasbeen proposed as a way to apportion the total nitrate signal to NH4NO3 and pRONO2. This method is increasingly beingapplied to field and laboratory data. However, the methods applied have been largely inconsistent and poorly characterized, and, therefore, adetailed evaluation is timely. Here, we compile an extensive survey of NOx+ ratios measured for variouspRONO2 compounds and mixtures from multiple AMS instruments, groups, and laboratory and field measurements. All data and analysispresented here are for use with the standard AMS vaporizer. We show that, in the absence of pRONO2 standards, thepRONO2 NOx+ ratio can be estimated using a ratio referenced to the calibrated NH4NO3 ratio, aso-called “Ratio-of-Ratios” method (RoR = 2.75 ± 0.41). We systematically explore the basis for quantifyingpRONO2 (and NH4NO3) with the RoR method using ground and aircraft field measurements conducted over a largerange of conditions. The method is compared to another AMS method (positive matrix factorization, PMF) and other pRONO2 andrelated (e.g., total gas + particle RONO2) measurements, generally showing good agreement/correlation. A broad survey of ground andaircraft AMS measurements shows a pervasive trend of higher fractional contribution of pRONO2 to total nitrate with lower totalnitrate concentrations, which generally corresponds to shifts from urban-influenced to rural/remote regions. Compared to ground campaigns,observations from all aircraft campaigns showed substantially lower pRONO2 contributions at midranges of total nitrate(0.01–0.1 up to 2–5 µg m−3), suggesting that the balance of effects controlling NH4NO3 and pRONO2formation and lifetimes – such as higher humidity, lower temperatures, greater dilution, different sources, higher particle acidity, andpRONO2 hydrolysis (possibly accelerated by particle acidity) – favors lower pRONO2 contributions for thoseenvironments and altitudes sampled. 
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  5. Abstract. Fires emit sufficient sulfur to affect local and regional airquality and climate. This study analyzes SO2 emission factors andvariability in smoke plumes from US wildfires and agricultural fires, as well as theirrelationship to sulfate and hydroxymethanesulfonate (HMS) formation.Observed SO2 emission factors for various fuel types show goodagreement with the latest reviews of biomass burning emission factors,producing an emission factor range of 0.47–1.2 g SO2 kg−1 C.These emission factors vary with geographic location in a way that suggeststhat deposition of coal burning emissions and application ofsulfur-containing fertilizers likely play a role in the larger observedvalues, which are primarily associated with agricultural burning. A 0-D boxmodel generally reproduces the observed trends of SO2 and total sulfate(inorganic + organic) in aging wildfire plumes. In many cases, modeled HMSis consistent with the observed organosulfur concentrations. However, acomparison of observed organosulfur and modeled HMS suggests that multipleorganosulfur compounds are likely responsible for the observations but thatthe chemistry of these compounds yields similar production and loss rates asthat of HMS, resulting in good agreement with the modeled results. Weprovide suggestions for constraining the organosulfur compounds observedduring these flights, and we show that the chemistry of HMS can alloworganosulfur to act as an S(IV) reservoir under conditions of pH > 6 and liquid water content>10−7 g sm−3. This canfacilitate long-range transport of sulfur emissions, resulting in increasedSO2 and eventually sulfate in transported smoke. 
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