Abstract. We use an ensemble of aircraft, satellite, sonde, and surface observations for April–May 2006 (NASA/INTEX-B aircraft campaign) to better understand the mechanisms for transpacific ozone pollution and its implications for North American air quality. The observations are interpreted with a global 3-D chemical transport model (GEOS-Chem). OMI NO2 satellite observations constrain Asian anthropogenic NOx emissions and indicate a factor of 2 increase from 2000 to 2006 in China. Satellite observations of CO from AIRS and TES indicate two major events of Asian transpacific pollution during INTEX-B. Correlation between TES CO and ozone observations shows evidence for transpacific ozone pollution. The semi-permanent Pacific High and Aleutian Low cause splitting of transpacific pollution plumes over the Northeast Pacific. The northern branch circulates around the Aleutian Low and has little impact on North America. The southern branch circulates around the Pacific High and some of that air impacts western North America. Both aircraft measurements and model results show sustained ozone production driven by peroxyacetylnitrate (PAN) decomposition in the southern branch, roughly doubling the transpacific influence from ozone produced in the Asian boundary layer. Model simulation of ozone observations at Mt. Bachelor Observatory in Oregon (2.7 km altitude) indicates a mean Asian ozone pollution contribution of 9±3 ppbv to the mean observed concentration of 54 ppbv, reflecting mostly an enhancement in background ozone rather than episodic Asian plumes. Asian pollution enhanced surface ozone concentrations by 5–7 ppbv over western North America in spring 2006. The 2000–2006 rise in Asian anthropogenic emissions increased this influence by 1–2 ppbv.

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...

We report Thermal Desorption Chemical Ionization Mass Spectrometer (TDCIMS) measurements of the composition of ambient 10–33 nm diameter particles formed from nucleation on 16 March 2006 in Tecamac, Mexico during the Megacity Initiative: Local and Global Research Observations (MILAGRO) field study. On this day, nucleated particles contained far more organics than sulfates: average ion molar ratios with measurement uncertainties for nitrate, organics and sulfur species were 6 ± 2%, 84 ± 5%, and 10 ± 1%, respectively. The measured organic species include nitrogen‐containing organic compounds, organic acids, and hydroxy organic acids. Independent calculations show that sulfuric acid condensation could have accounted for only 10 ± 2% of the growth that was observed on this day, which is consistent with the TDCIMS measurements of composition. It follows that organic compounds play a dominant role the high growth rates that were observed.

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DC3 brought together simultaneous measurements of storm kinematics, structure, electrical activity, and chemistry to improve our knowledge of how thunderstorms affect the chemical composition of the troposphere.