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Following the springtime polar sunrise, ozone concentrations in the lower troposphere episodically decline to near-zero levels 1 . These ozone depletion events are initiated by an increase in reactive bromine levels in the atmosphere 2-5 . Under these conditions, the oxidative capacity of the Arctic troposphere is altered, leading to the removal of numerous transported trace gas pollutants, including mercury 6 . However, the sources and mechanisms leading to increased atmospheric reactive bromine levels have remained uncertain, limiting simulations of Arctic atmospheric chemistry with the rapidly transforming sea-ice landscape 7,8 . Here, we examine the potential for molecular bromine production in various samples of saline snow and sea ice, in the presence and absence of sunlight and ozone, in an outdoor snow chamber in Alaska. Molecular bromine was detected only on exposure of surface snow (collected above tundra and first-year sea ice) to sunlight. This suggests that the oxidation of bromide is facilitated by a photochemical mechanism, which was most efficient for more acidic samples characterized by enhanced bromide to chloride ratios. Molecular bromine concentrations increased significantly when the snow was exposed to ozone, consistent with an interstitial air amplification mechanism. Aircraft-based observations confirm that bromine oxide levels were enhanced near the snow surface. We suggest that the photochemical production of molecular bromine in surface snow serves as a major source of reactive bromine, which leads to the episodic depletion of tropospheric ozone in the Arctic springtime.Proposed substrates for Arctic halogen activation include open water, frost flowers, sea ice, surface snow, blowing snow and aerosols 7 . To test the effectiveness of various snow and ice surfaces for bromine activation, ten outdoor snow chamber experiments were conducted during the March-April 2012 Bromine, Ozone and Mercury Experiment (BROMEX) in Barrow, Alaska. As listed in Table 1, locally obtained samples included first-year sea ice, brine icicles that drained through the base of uplifted sea-ice blocks, several different layers of snow located on first-year sea ice, and surface snow on the tundra. Real-time chemical ionization mass spectrometry was used to monitor Br 2 production 9 from the snow/ice samples in a perfluoroalkoxy-coated chamber, through which clean air, with and without ozone, was allowed to flow. Br 2 was observed only when snow samples were exposed to ambient sunlight, as shown in Fig. 1 and in the Supplementary Information. This indicates active snowpack photochemistry. On O 3 addition, chamber Br 2 concentrations increased, consistent with the autocatalytic bromine explosion mechanism, described below.Photochemical production of the hydroxyl radical (OH) in the snowpack condensed phase and the subsequent oxidation of bromide explains the initially observed Br 2 production. Photoactivated release of Br 2 into the atmosphere was previously proposed to explain boundary-layer ozone destruction beginn...

A framework for an empirical parameterization (EP) of heterogeneous nucleation of ice crystals by multiple species of aerosol material in clouds was proposed in a 2008 paper by the authors. The present paper reports improvements to specification of a few of its empirical parameters. These include temperatures for onset of freezing, baseline surface areas of aerosol observed in field campaigns over Colorado, and new parameters for properties of black carbon, such as surface hydrophilicity and organic coatings. The EP's third group of ice nucleus (IN) aerosols is redefined as that of primary biological aerosol particles (PBAPs), replacing insoluble organic aerosols. A fourth group of IN is introduced-namely, soluble organic aerosols.The new EP predicts IN concentrations that agree well with aircraft data from selected traverses of shallow wave clouds observed in five flights (1, 3, 4, 6, and 12) of the 2007 Ice in Clouds Experiment-Layer Clouds (ICE-L). Selected traverses were confined to temperatures between about 2258 and 2298C in layer cloud without homogeneously nucleated ice from aloft. Some of the wave clouds were affected by carbonaceous aerosols from biomass burning and by dust from dry lakebeds and elsewhere. The EP predicts a trend between number concentrations of heterogeneously nucleated ice crystals and apparent black carbon among the five wave clouds, observed by aircraft in ICE-L. It is predicted in terms of IN activity of black carbon.The EP's predictions are consistent with laboratory and field observations not used in its construction, for black carbon, dust, primary biological aerosols, and soluble organics. The EP's prediction of biological ice nucleation is validated using coincident field observations of PBAP IN and PBAPs in Colorado.

Many of the significant advances in our understanding of atmospheric particles can be attributed to the application of mass spectrometry. Mass spectrometry provides high sensitivity with fast response time to probe chemically complex particles. This review focuses on recent developments and applications in the field of mass spectrometry of atmospheric aerosols. In Part II of this two-part review, we concentrate on real-time mass spectrometry techniques, which provide high time resolution for insight into brief events and diurnal changes while eliminating the potential artifacts acquired during long-term filter sampling. In particular, real-time mass spectrometry has been shown recently to provide the ability to probe the chemical composition of ambient individual particles <30 nm in diameter to further our understanding of how particles are formed through nucleation in the atmosphere. Further, transportable real-time mass spectrometry techniques are now used frequently on ground-, ship-, and aircraft-based studies around the globe to further our understanding of the spatial distribution of atmospheric aerosols. In addition, coupling aerosol mass spectrometry techniques with other measurements in series has allowed the in situ determination of chemically resolved particle effective density, refractive index, volatility, and cloud activation properties.

Abstract. Multiple axis differential absorption spectroscopy (MAX-DOAS) measurements of bromine monoxide (BrO) probed the vertical structure of halogen activation events during March–May 2012 at Barrow, Alaska. An analysis of the BrO averaging kernels and degrees of freedom obtained by optimal-estimation-based inversions from raw MAX-DOAS measurements reveals the information is best represented by reducing the retrieved BrO profile to two quantities: the integrated column from the surface through 200 m (VCD200 m), and the lower tropospheric vertical column density (LT-VCD), which represents the integrated column of BrO from the surface through 2 km. The percentage of lower tropospheric BrO in the lowest 200 m was found to be highly variable ranging from shallow layer events, where BrO is present primarily in the lowest 200 m, to distributed column events where BrO is observed at higher altitudes. The highest observed LT-VCD events occurred when BrO was distributed throughout the lower troposphere, rather than concentrated near the surface. Atmospheric stability in the lowest 200 m influenced the percentage of LT-VCD that is in the lowest 200 m, with inverted temperature structures having a first-to-third quartile range (Q1–Q3) of VCD200 m/LT-VCD from 15–39%, while near-neutral-temperature structures had a Q1–Q3 range of 7–13%. Data from this campaign show no clear influence of wind speed on either lower tropospheric bromine activation (LT-VCD) or the vertical distribution of BrO, while examination of seasonal trends and the temperature dependence of the vertical distribution supported the conclusion that the atmospheric stability affects the vertical distribution of BrO.

During the summer and fall of 2005 in Riverside, California, the seasonal volatility behavior of submicrometer aerosol particles was investigated by coupling an automated thermodenuder system to an online single-particle mass spectrometer. A strong seasonal dependence was observed for the gas/particle partitioning of alkylamines within individual ambient submicrometer aged organic carbon particles internally mixed with ammonium, nitrate, and sulfate. In the summer, the amines were strongly correlated with nitrate and sulfate, suggesting the presence of aminium nitrate and sulfate salts which were nonvolatile and comprised approximately 6-9% of the average particle mass at 230 degrees C. In the fall, 86 +/- 1% of the amines volatilized below 113 degrees C with aminium nitrate and sulfate salts representing less than 1% of the particle mass at 230 degrees C. In the summer, a more acidic particle core led to protonation of the amines and subsequent formation of aminium sulfate and nitrate salts; whereas, in the fall, the particles contained more ammonium and thus were less acidic, causing fewer aminium salts to form. Therefore, the acidity of individual particles can greatly affect gas/particle partitioning of organic species in the atmosphere, and the concentrations of amines, as strong bases, should be included in estimations of aerosol pH.

Atmospheric bromine and chlorine atoms have a significant influence on the pathways of atmospheric chemical species processing. The photolysis of molecular halogens and subsequent reactions with ozone, mercury, and hydrocarbons are common occurrences in the Arctic boundary layer during spring, following polar sunrise. While it was recently determined that Br 2 is released from the sunlit surface snowpack, the source(s) and mechanisms of Cl 2 and BrCl production have remained unknown. Current efforts to model Arctic atmospheric composition are limited by the lack of knowledge of the sources and emission rates of these species. Here, we present the first simultaneous direct measurements of Br 2 , Cl 2 , and BrCl in snowpack interstitial air, as well as the first measured emission rates of Br 2 and Cl 2 out of the snowpack into the atmosphere. Using chemical ionization mass spectrometry, Br 2 , Cl 2 , and BrCl were observed to be produced within the tundra surface snowpack near Utqiagvik, AK, during Feb 2014, following both artificial and natural irradiation, consistent with a photolytic production mechanism. Maximum Cl 2 and Br 2 fluxes from the snowpack to the overlying atmosphere were quantified and reached maxima at mid-day during peak radiation. In-snowpack Br 2 and BrCl production was enhanced, with Cl 2 production reduced, at air temperatures below the eutectic point for the formation of NaCl• 2H 2 O, suggesting limited chloride availability, as compared to production at air temperatures above this eutectic point. These new observations improves the ability of the community to simulate Arctic boundary layer composition and pollutant fate.

Inland sources of particulate chloride for atmospheric nitryl chloride (ClNO 2 ) formation remain unknown and unquantified, hindering air quality assessments. Globally each winter, tens of millions of tons of road salt are spread on roadways for deicing. Here, we identify road salt aerosol as the primary chloride aerosol source, accounting for 80−100% of ClNO 2 formation, at an inland urban area in the wintertime. This study provides experimental evidence of the connection between road salt and air quality through the production of this important reservoir for nitrogen oxides and chlorine radicals, which significantly impact atmospheric composition and pollutant fates. A numerical model was employed to quantify the contributions of chloride sources to ClNO 2 production. The traditional method for simulating ClNO 2 considers chloride to be homogeneously distributed across the atmospheric particle population; yet, we show that only a fraction of the particulate surface area contains chloride. Our new single-particle parametrization considers this heterogeneity, dramatically lowering overestimations of ClNO 2 levels that have been routinely reported using the prevailing methods. The identification of road salt as a ClNO 2 source links this common deicing practice to atmospheric composition and air quality in the urban wintertime environment.