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.

We examine the distribution and fate of nitrogen oxides (NO_x) in the lower troposphere over the Northeast United States (NE US) using aircraft observations from the Wintertime INvestigation of Transport, Emissions, and Reactivity (WINTER) campaign in February–March 2015, as well as the GEOS‐Chem chemical transport model and concurrent ground‐based observations. We find that the National Emission Inventory from the U.S. Environmental Protection Agency is consistent with WINTER observations of total reactive nitrogen (^TNO_y) to within 10% on average, in contrast to the significant overestimate reported in past studies under warmer conditions. Updates to the dry deposition scheme and dinitrogen pentoxide (N₂O₅) reactive uptake probability, ɣ(N₂O₅), result in an improved simulation of gas‐phase nitric acid (HNO₃) and submicron particulate nitrate (pNO₃⁻), reducing the longstanding factor of 2–3 overestimate in wintertime HNO₃ + pNO₃⁻ to a 50% positive bias. We find a NO_x lifetime against chemical loss and deposition of 22 hr in the lower troposphere over the NE US. Chemical loss of NO_x is dominated by N₂O₅ hydrolysis (58% of loss) and reaction with OH (33%), while 7% of NO_x leads to the production of organic nitrates. Wet and dry deposition account for 55% and 45% of ^TNO_y deposition over land, respectively. We estimate that 42% of the NO_x emitted is exported from the NE US boundary layer during winter, mostly in the form of HNO₃ + pNO₃⁻ (40%) and NO_x (38%).

Although urban NO_x lifetimes have been examined extensively during summertime conditions, wintertime NO_x chemistry has been comparatively less studied. We use measurements of NO_x and its oxidation products from the aircraft‐based WINTER (Wintertime INvestigation of Transport, Emissions, and Reactivity) experiment over the northeastern United States during February–March 2015 to describe the NO_x lifetime during conditions when days are shorter, actinic flux is reduced, and temperatures are colder. By analyzing regional outflow from the East Coast, we show that NO_x is long lived during the winter, with a longer daytime lifetime (29 hr) than nighttime lifetime (6.3 hr). We demonstrate that wintertime NO_x emissions have an overall lifetime controlled by the nighttime conversion of NO_x to nitric acid (HNO₃) via N₂O₅ heterogeneous chemistry, and we discuss constraints on the rates of NO_x conversion to HNO₃. Additionally, analysis of the nighttime O_x budget suggests that approximately 15% of O₃ is lost overnight through N₂O₅ production and subsequent reaction with aerosol to form HNO₃.

We describe the University of Washington airborne high‐resolution time‐of‐flight chemical ionization mass spectrometer (HRToF‐CIMS) and evaluate its performance aboard the NCAR‐NSF C‐130 aircraft during the recent Wintertime INvestigation of Transport, Emissions and Reactivity (WINTER) experiment in February–March of 2015. New features include (i) a computer‐controlled dynamic pinhole that maintains constant mass flow‐rate into the instrument independent of altitude changes to minimize variations in instrument response times; (ii) continuous addition of low flow‐rate humidified ultrahigh purity nitrogen to minimize the difference in water vapor pressure, hence instrument sensitivity, between ambient and background determinations; (iii) deployment of a calibration source continuously generating isotopically labeled dinitrogen pentoxide (¹⁵N₂O₅) for in‐flight delivery; and (iv) frequent instrument background determinations to account for memory effects resulting from the interaction between sticky compounds and instrument surface following encounters with concentrated air parcels. The resulting improvements to precision and accuracy, along with the simultaneous acquisition of these species and the full set of their isotopologues, allow for more reliable identification, source attribution, and budget accounting, for example, by speciating the individual constituents of nocturnal reactive nitrogen oxides (NO_z = ClNO₂ + 2 × N₂O₅ + HNO₃ + etc.). We report on an expanded set of species quantified using iodide‐adduct ionization such as sulfur dioxide (SO₂), hydrogen chloride (HCl), and other inorganic reactive halogen species including hypochlorous acid, nitryl chloride, chlorine, nitryl bromide, bromine, and bromine chloride (HOCl, ClNO₂, Cl₂, BrNO₂, Br₂, and BrCl, respectively).

The composition of wildfire smoke particulate matter (PM) was investigated during a 2018 smoke episode in the San Francisco Bay Area and compared to non-wildfire periods. Scanning electron microscopy (SEM) of passive PM samples deployed at four sites exhibited a higher concentration of submicron, spherical, carbonaceous PM (“tar balls”) during the wildfire period compared to the non-wildfire period. Coarse ash aluminosilicates and potassium bearing particles were enhanced as well. Disproportionately more UV light was absorbed by PM during the wildfire period than during the non-wildfire period, indicating a higher organic content consistent with the observed tar balls. An indoor PM sample from a residence operating a medium-efficiency furnace filter during the smoke episode exhibited fine, carbonaceous PM comparable to outdoors, but reduced coarse ash PM. Indoor/outdoor ratios for PM_2.5, PM_10-2.5, and PM₁₀ were 0.6, 0.1, and 0.5 respectively. SEM of the wildfire furnace filter revealed a greater proportion of tar balls compared to a non-wildfire filter. Relatively rare particles enriched in Cu, Zn, Sn and Pb were observed in wildfire samples, but not in non-wildfire samples. Higher concentrations of Cu, Zn, and Pb during the wildfire were confirmed by inductively coupled plasma mass spectrometry (ICP-MS) of the furnace filter. The use of low-cost passive samplers enabled rapid deployment at multiple locations and indoors and outdoors during the wildfire episode. These measurements suggest smoke impacts were regional in scale and similar across sites, though local variations in PM_2.5 magnitudes, particle types, and confounding by local sources were observed. Copyright © 2021 American Association for Aerosol Research

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.