Photochemical production of molecular bromine in Arctic surface snowpacks

Abstract

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...

Temporally delineated sources of major chemical species in high Arctic snow

“…1; Table 2). These dominant analytes and their relative proportions are consistent with that of sea salt (Pytkowicz and Kester, 1971), suggesting a marine origin for Factor 1. The mass ratios of Cl − and K + to Na + in Factor 1 were similar to that expected for sea salt with enrichment ratios close to unity of 1.3 and 1.1, respectively (1.3-1.4 and 0.8-1.2 25th-75th percentiles per bootstrapping analysis).…”

Interactions of bromine, chlorine, and iodine photochemistry during ozone depletions in Barrow, Alaska

“…This suggests a surface activation mechanism controlled by O 3 , presumably in part through a snowpack halogen explosion mechanism, such as has been observed in laboratory studies by, e.g., Oum et al (1998a, b) and the recent Pratt et al (2013) study using natural Barrow snow. In our model, because the production mechanism of I 2 is undefined and we have no observations with which to compare, the I 2 flux is of the same magnitude each day, independent of [O 3 ].…”

“…The production of Br 2 can thus be sustained on saline snow, ice, and aerosol surfaces, as has been confirmed in laboratory studies that have observed production of Br 2 and BrCl from aqueous and frozen halide surfaces exposed to HOBr (Fickert et al, 1999;Adams et al, 2002;Huff and Abbatt, 2002), as well as in a recent field-based study that observed Br 2 production from sunlit snowpacks in Barrow, Alaska (Pratt et al, 2013).…”

“…Considering an e-folding photolytic lifetime of Br 2 at solar noon of 23 s (J max = 0.044 s −1 ), and using the method of Guimbaud et al (2002), the effective daytime mixing height (Z*) of Br 2 in the stable air typical of the Arctic (K c = 95 cm 2 s −1 ) is ∼ 0.5 m. Assuming simple first-order kinetics, the [Br 2 ] remaining after mixing up from the surface to the intake of the CIMS (∼ 1 m or 2 lifetimes) is 10 % that at the surface. A recent study examining Br 2 production from surface snow in Barrow demonstrates that enhanced Br 2 production is observed in the presence of solar radiation (Pratt et al, 2013). Given that the Br 2 concentrations in the snowpack interstitial air should be elevated due to heterogeneous production mechanisms (e.g., the bromine explosion), and that production should be greater during sunlit periods, it seems reasonable to conclude that Br 2 should sometimes be observable during the day.…”

Aqueous photochemical reactions of chloride, bromide, and iodide ions in a diode-array spectrophotometer. Autoinhibition in the photolysis of iodide ions

“…9 Similarly, the photochemical formation of bromine atoms from bromide ions on the surface of snow is understood to play a significant role in self-cleaning procedures in the atmosphere. 10,11 In addition, the photoinduced formation of carcinogenic bromate ions in seawater or simply in the presence of a lower amount of chloride ions is a major concern for the water industry. 12,13 Diode array spectrophotometers are becoming increasingly popular for a large number of different applications.…”