Sulfate (${\mathrm{S}\mathrm{O}}_4^{2-}$) and nitrate (${\mathrm{N}\mathrm{O}}_3^{-}$) account for half of the fine particulate matter mass over the eastern United States. Their wintertime concentrations have changed little in the past decade despite considerable precursor emissions reductions. The reasons for this have remained unclear because detailed observations to constrain the wintertime gas–particle chemical system have been lacking. We use extensive airborne observations over the eastern United States from the 2015 Wintertime Investigation of Transport, Emissions, and Reactivity (WINTER) campaign; ground-based observations; and the GEOS-Chem chemical transport model to determine the controls on winter ${\mathrm{S}\mathrm{O}}_4^{2-}$ and ${\mathrm{N}\mathrm{O}}_3^{-}$. GEOS-Chem reproduces observed ${\mathrm{S}\mathrm{O}}_4^{2-}$–${\mathrm{N}\mathrm{O}}_3^{-}$–${\mathrm{N}\mathrm{H}}_4^{+}$ particulate concentrations (2.45 μg ${\mathrm{s}\mathrm{m}}^{-3}$) and composition (${\mathrm{S}\mathrm{O}}_4^{2-}$: 47%; ${\mathrm{N}\mathrm{O}}_3^{-}$: 32%; ${\mathrm{N}\mathrm{H}}_4^{+}$: 21%) during WINTER. Only 18% of ${\mathrm{S}\mathrm{O}}_2$ emissions were regionally oxidized to ${\mathrm{S}\mathrm{O}}_4^{2-}$ during WINTER, limited by low [H₂O₂] and [OH]. Relatively acidic fine particulates (pH∼1.3) allow 45% of nitrate to partition to the particle phase. Using GEOS-Chem, we examine the impact of the 58% decrease in winter ${\mathrm{S}\mathrm{O}}_2$ emissions from 2007 to 2015 and find that the H₂O₂ limitation on ${\mathrm{S}\mathrm{O}}_2$ oxidation weakened, which increased the fraction of ${\mathrm{S}\mathrm{O}}_2$ emissions oxidizing to ${\mathrm{S}\mathrm{O}}_4^{2-}$. Simultaneously, NOx emissions decreased by 35%, but the modeled ${\mathrm{N}\mathrm{O}}_3^{-}$ particle fraction increased as fine particle acidity decreased. These feedbacks resulted in a 40% decrease of modeled [${\mathrm{S}\mathrm{O}}_4^{2-}$] and no change in [${\mathrm{N}\mathrm{O}}_3^{-}$], as observed. Wintertime [${\mathrm{S}\mathrm{O}}_4^{2-}$] and [${\mathrm{N}\mathrm{O}}_3^{-}$] are expected to change slowly between 2015 and 2023, unless ${\mathrm{S}\mathrm{O}}_2$ and NOx emissions decrease faster in the future than in the recent past. Significance Exposure to fine particulate matter is a leading cause of premature deaths and illnesses globally. In the eastern United States, substantial cuts in sulfur dioxide and nitrogen oxides emissions have considerably lowered particulate sulfate and nitrate concentrations for all seasons except winter. Simulations that reproduce detailed airborne observations of wintertime atmospheric chemistry over the eastern United States indicate that particulate sulfate and nitrate formation is limited by the availability of oxidants and by the acidity of fine particles, respectively. These limitations relax at lower ambient concentrations, forming particulate matter more efficiently, and weaken the effect of emission reductions. These results imply that larger emission reductions, especially during winter, are necessary for substantial improvements in wintertime air quality in the eastern United States.

Lasers and laser spectroscopic techniques have been extensively used in several applications since their advent, and the subject has been reviewed extensively in the last several decades. This review is focused on three areas of laser spectroscopic applications in atmospheric and environmental sensing; namely laser-induced fluorescence (LIF), cavity ring-down spectroscopy (CRDS), and photoluminescence (PL) techniques used in the detection of solids, liquids, aerosols, trace gases, and volatile organic compounds (VOCs).

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).

Isoprene is an important atmospheric volatile organic compound involved in ozone production and NO_x (NO+NO₂) sequestration and transport. Isoprene reaction with OH in the presence of NO can form either isoprene hydroxy nitrates ("isoprene nitrates") or convert NO to NO₂ which can photolyze to form ozone. While it has been shown that isoprene nitrate production can represent an important sink for NO_x in forest impacted environments, there is little experimental knowledge of the relative importance of the individual isoprene nitrate isomers, each of which has a different fate and reactivity. In this work, we have identified the 8 individual isomers and determined their total and individual production yields. The overall yield of isoprene nitrates at atmospheric pressure and 295 K was found to be 0.070(+0.025/−0.015). Three isomers, representing nitrates resulting from OH addition to a terminal carbon, represent 90% of the total IN yield. We also determined the ozone rate constants for three of the isomers, and have calculated their atmospheric lifetimes, which range from ~1–2 h, making their oxidation products likely more important as atmospheric organic nitrates and sinks for nitrogen.

We present airborne observations of gaseous reactive halogen species (HCl, Cl₂, ClNO₂, Br₂, BrNO₂, and BrCl), sulfur dioxide (SO₂), and nonrefractory fine particulate chloride (pCl) and sulfate (pSO₄) in power plant exhaust. Measurements were conducted during the Wintertime INvestigation of Transport, Emissions, and Reactivity campaign in February–March of 2015 aboard the NCAR‐NSF C‐130 aircraft. Fifty air mass encounters were identified in which SO₂ levels were elevated ~5 ppb above ambient background levels and in proximity to operational power plants. Each encounter was attributed to one or more potential emission sources using a simple wind trajectory analysis. In case studies, we compare measured emission ratios to those reported in the 2011 National Emissions Inventory and present evidence of the conversion of HCl emitted from power plants to ClNO₂. Taking into account possible chemical conversion downwind, there was general agreement between the observed and reported HCl:SO₂ emission ratios. Reactive bromine species (Br₂, BrNO₂, and/or BrCl) were detected in the exhaust of some coal‐fired power plants, likely related to the absence of wet flue gas desulfurization emission control technology. Levels of bromine species enhanced in some encounters exceeded those expected assuming all of the native bromide in coal was released to the atmosphere, though there was no reported use of bromide salts (as a way to reduce mercury emissions) during Wintertime INvestigation of Transport, Emissions, and Reactivity observations. These measurements represent the first ever in‐flight observations of reactive gaseous chlorine and bromine containing compounds present in coal‐fired power plant exhaust.