Abstract. Heterogeneous photochemistry converts bromide (Br − ) to reactive bromine species (Br atoms and bromine monoxide, BrO) that dominate Arctic springtime chemistry. This phenomenon has many impacts such as boundarylayer ozone depletion, mercury oxidation and deposition, and modification of the fate of hydrocarbon species. To study environmental controls on reactive bromine events, the BRomine, Ozone, and Mercury EXperiment (BROMEX) was carried out from early March to mid-April 2012 near Barrow (Utqiaġvik), Alaska. We measured horizontal and vertical gradients in BrO with multiple-axis differential optical absorption spectroscopy (MAX-DOAS) instrumentation at three sites, two mobile and one fixed. During the campaign, a large crack in the sea ice (an open lead) formed pushing one instrument package ∼ 250 km downwind from Barrow (Utqiaġvik). Convection associated with the open lead converted the BrO vertical structure from a surfacebased event to a lofted event downwind of the lead influence. The column abundance of BrO downwind of the refreezing lead was comparable to upwind amounts, indicating direct reactions on frost flowers or open seawater was not a major reactive bromine source. When these three sites were separated by ∼ 30 km length scales of unbroken sea ice, the BrO amount and vertical distributions were highly correlated for most of the time, indicating the horizontal length scales of BrO events were typically larger than ∼ 30 km in the absence of sea ice features. Although BrO amount and vertical distribution were similar between sites most of the time, rapid changes in BrO with edges significantly smaller than this ∼ 30 km length scale episodically transported between the sites, indicating BrO events were large but with sharp edge contrasts. BrO was often found in shallow layers that recycled reactive bromine via heterogeneous reactions on snowpack. Episodically, these surface-based events propagated aloft when aerosol extinction was higher (> 0.1 km −1 ); however, the presence of aerosol particles aloft was not sufficient to produce BrO aloft. Highly depleted ozone (< 1 nmol mol −1 ) repartitioned reactive bromine away from BrO and drove BrO events aloft in cases. This work demonstrates the interplay between atmospheric mixing and heterogeneous chemistry that affects the vertical structure and horizontal extent of reactive bromine events.
Abstract. Following polar sunrise in the Arctic springtime, tropospheric ozone episodically decreases rapidly to near-zero levels during ozone depletion events (ODEs). Many uncertainties remain in our understanding of ODE characteristics, including the temporal and spatial scales, as well as environmental drivers. Measurements of ozone, bromine monoxide (BrO), and meteorology were obtained during several deployments of autonomous, ice-tethered buoys (O-Buoys) from both coastal sites and over the Arctic Ocean; these data were used to characterize observed ODEs. Detected decreases in surface ozone levels during the onset of ODEs corresponded to a median estimated apparent ozone depletion timescale (based on both chemistry and the advection of O3-depleted air) of 11 h. If assumed to be dominated by chemical mechanisms, these timescales would correspond to larger-than-observed BrO mole fractions based on known chemistry and assumed other radical levels. Using backward air mass trajectories and an assumption that transport mechanisms dominate observations, the spatial scales for ODEs (defined by time periods in which ozone levels ≤15 nmol mol−1) were estimated to be 877 km (median), while areas estimated to represent major ozone depletions (<10 nmol mol−1) had dimensions of 282 km (median). These observations point to a heterogeneous boundary layer with localized regions of active, ozone-destroying halogen chemistry, interspersed among larger regions of previously depleted air that retain reduced ozone levels through hindered atmospheric mixing. Based on the estimated size distribution, Monte Carlo simulations showed it was statistically possible that all ODEs observed could have originated upwind, followed by transport to the measurement site. Local wind speed averages were low during most ODEs (median of ~3.6 m s−1), and there was no apparent dependence on local temperature.
Abstract. Following polar sunrise in the Arctic springtime, tropospheric ozone episodically decreases rapidly to near zero levels during ozone depletion events (ODEs). Many uncertainties remain in our understanding of ODE characteristics, including the temporal and spatial scales, as well as environmental drivers. Measurements of ozone, bromine monoxide (BrO), and meteorology were obtained during several deployments of autonomous, ice-tethered buoys (O-Buoys) from both coastal sites and over the Arctic Ocean; these data were used to characterize observed ODEs. Detected decreases in surface ozone levels during the onset of ODEs corresponded to a median estimated apparent ozone depletion timescale (based on chemistry and the advection of O3-depleted air) of 11 h. If assumed to be dominated by chemistry, these timescales would correspond to larger-than-observed BrO mole fractions based on known chemical mechanisms and assumed other radical levels. Using backward air mass trajectories, the spatial scales for ODEs (defined by time periods in which ozone mole fraction ≤15 nmol mol−1) were estimated to be ~900 km (median), while areas estimated to represent major ozone depletions (<10 nmol mol−1) had dimensions of ~280 km (median). These observations point to a heterogeneous boundary layer with localized regions of active, ozone-destroying halogen chemistry, interspersed among larger regions of previously depleted air that retain reduced ozone levels through hindered atmospheric mixing. Based on the estimated size distribution, Monte Carlo simulations showed it was statistically possible that all ODEs observed could have originated upwind, followed by transport to the measurement site. Local wind speed averages were low during most ODEs (median of ~3.6 m s−1), and there was no apparent dependence on local temperature.
<p><strong>Abstract.</strong> The mechanisms of molecular halogen productions from frozen saline surfaces remain incompletely understood, limiting our ability to predict atmospheric oxidation and composition in the polar regions. In this laboratory study, condensed-phase hydroxyl radicals (OH) were photochemically generated in frozen saltwater solutions that mimicked the ionic composition of ocean water. These hydroxyl radicals were found to oxidize Cl<sup>&#8722;</sup>, Br<sup>&#8722;</sup>, and I<sup>&#8722;</sup>, leading to the release of Cl<sub>2</sub>, Br<sub>2</sub>, and I<sub>2</sub>. This finding is consistent with mechanisms proposed from recent Arctic field studies for observed snowpack molecular halogen production. At moderately acidic pre-freezing pH (buffered between 4.5&#8211;4.8), irradiation of ice surfaces containing OH-precursors produced elevated amounts of I<sub>2</sub>. Subsequent addition of O<sub>3</sub> produced additional I<sub>2</sub>, as well as low amounts of Br<sub>2</sub>. At lower pH (1.7&#8211;2.2), substantial photochemical production of Br<sub>2</sub> was observed, following rapid dark conversion of I<sup>&#8722;</sup> to I<sub>2</sub> via reactions with hydrogen peroxide or nitrite. Exposure to O<sub>3</sub> under these low pH conditions also increased production of Br<sub>2</sub> and I<sub>2</sub>, possibly through surfaced-based reactions with O<sub>3</sub>, or the production and heterogeneous recycling of gas-phase HOBr and HOI. Our results suggest the observed products are dependent on the relative concentrations of halides at the ice surface. Finally, photochemical production of Cl<sub>2</sub> was only observed when the irradiated sample was composed of high-purity NaCl and hydrogen peroxide (acting as the OH precursor) at low pH (~&#8201;1.8). While OH was shown to produce Cl<sub>2</sub> in this study, kinetics calculations suggest that heterogeneous recycling chemistry may be equally or more important for Cl<sub>2</sub> production in the Arctic atmosphere.</p>
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