The polar springtime is a season of change, most notably in temperature, sea ice coverage, and weather, but it also signals the onset of many photochemically driven atmospheric chemical reactions, such as tropospheric ozone depletion events (ODEs) e.g.,. Boundary layer ozone in polar regions has been observed to fall dramatically periodically in the spring in both the Arctic and Antarctic . These tropospheric ODEs are initiated by increased concentrations of reactive bromine gases (BrO and Br) resulting in a catalytic removal of ozone ; however the mechanism by which Br enters the atmosphere is not well understood . The heterogeneous reaction producing bromine gas and hypothesized to explain the “bromine explosion” is given by : HOBr+Br-+H+→Br2+H2O. Proposed surfaces on which this reaction may occur have included sea spray , frost flowers , surface snow , aerosols , and blowing snow .

Although some bromide may arrive via sea spray produced aerosol , this mechanism is relatively less efficient and does not produce aerosol with the observed sulfate depletion . As seawater freezes, salt rejection leads to an increasingly saline brine . Specific salts precipitate out of solution when solubility limits are reached below -2 ∘C , producing brine with different ion ratios than those found in seawater. The brine is expelled to the ice/snow or ice/atmosphere surface, where it forms a fractionated surface skim and slush . This reservoir of bromide and other sea salts can be incorporated into frost flowers by wicking, or into the snow lying on sea ice by upward migration through capillary forces . It may be transported by the lofting of snow from sea ice with subsequent sublimation and return of the aerosol to the snowpack . However, the physical surface at which bromide activation occurs remains a subject of some debate.

It has been shown that brine is wicked upward by frost flowers that form on fresh sea ice, yielding a highly saline surface with the aforementioned sulfate depletion, and that the dendritic nature of frost flowers magnifies the surface area available for atmospheric interactions . However, studies have shown that frost flowers do not significantly increase the surface area available for heterogeneous reactions and that frost flowers are not easily broken and lofted . Instead, they accumulate snow on their windward side in strong winds and when broken, fall where they are . Laboratory studies of frost flowers have also shown that frost flowers are quite stable in winds up to 12 ms-1, and aerosol production is not observed under these conditions . Further, and found that frost flowers are not depleted in bromide relative to sodium and chloride, indicating they are not the immediate source of bromine-depleted snow nor the location of the heterogeneous reactions activating bromine. It has also been shown that first-year sea ice contact better correlates to increased BrO concentrations than does contact with potential frost flower regions .

In however, the snowpack did have Br-/Na+ ratio fluctuations by 2 orders of magnitude with bromide depletions of up to 90%, suggesting bromine was released in the aerosol phase or in the snowpack itself. Sea salts can migrate upwards through the snowpack as high as 17 cm, even in cold conditions where slush is not present . However, in thicker snowpacks, the surface snow is more readily acidified and influenced by atmospheric processes, and thus, more likely to play a role in halogen activation. Bromine activation is more efficient in acidic conditions and in environments with a higher Br-/Cl- ratio . further hypothesized that the snowpack interstitial air is the primary location for the heterogeneous reactions, with wind pumping providing a vehicle to release the bromine into the boundary layer.

proposed a fourth mechanism for bromine activation that involves the lofting of snow from sea ice with subsequent sublimation and either direct bromine release or return of the aerosol to the snowpack. In the model, lofted snow particles were allowed to sublimate to a particle size of <10 µm, whereby the particles instantaneously released bromine or were potentially scoured by more blowing snow particles. Snow salinity was found to have a nonlinear relationship with bromine production, and the reaction rate varied with dry aerosol particle size. It was shown that 1.3 µm particles yielded the most efficient release of bromine since the smaller aerosol could be lofted further and longer . Further work showed that snow in the sea ice zone may contain enough salt when lofted by wind to produce the necessary aerosol to explain the observed Antarctic ODEs . While it may be difficult for saturated snow lying on surface skim to become lofted, snow in the upper layers could wick up salts yet remain mobile, providing enormous capacity for transporting bromide into the boundary layer. provides the theoretical framework and model-based data for the physical measurements observed and presented in this manuscript.

ODEs have been found to occur at a range of wind speeds, something that has made agreement upon a mechanism difficult. Some have observed that ODEs require low winds, a stable boundary layer, and a relatively clear sky, or the movement of air masses from such regions . Using measurements and modeling, found rapid ozone depletion in the top 10 cm of the snowpack due to diffusion and ventilation in moderate winds. More recently, ODEs have been recorded during high winds (>12 ms-1), which increase the snow surface area exposed via blowing snow, and thus, the probability of contact between the gaseous and condensed phases required to initiate bromine release . During the 2009 OASIS campaign in Barrow, AK, high winds were observed to correspond with increased BrO concentrations . This was accompanied by decreased HOBr, suggesting that blowing snow likely provided additional surfaces for heterogeneous HOBr loss . These high wind halogen activation events may have a more widespread role when considering the impact of ODEs on regional tropospheric chemistry . Additionally, higher winds may enhance the availability of radical bromine precursors in the snowpack, accelerating bromine activation . On the contrary, others e.g., have found no correlation between ODEs and wind speed. However, the lack of low ozone observations at high wind speed may be a result of enhanced mixing . Although the chemistry of snow has been well studied , there has been no attempt until now at segregating snow capable of wind transport or snow actually blown by wind.

It is noted that both Cl2 and BrCl play a role in halide boundary layer chemistry and ODEs , but the focus of this work is on bromine because it is a far more efficient catalyst of ozone loss. As the main constituents of seawater, both chloride and sodium have been used as sea salt tracers, and thus chloride was used here to both determine snow and ice salinity and relative activity of other anions. Previous work has shown that sulfate is influenced both by mirabilite precipitation and atmospheric interactions, while nitrate is uncorrelated with sea salt and is deposited as a byproduct of other reactions .

Two field sites on first-year sea ice in the Ross Sea, Antarctica, were chosen, based upon prevailing wind direction, to maximize contact of the blowing snow with the first-year sea ice. Their locations are shown in Fig. . Butter Point Site was located 6.5 km from the open ocean and 35 km north of the Ross Ice Shelf. Iceberg Site was located 55 km northwest of Butter Point, and since the ice edge located to the east follows the shore, the distance to open water here was also 5 km. Iceberg Site was chosen based upon the presence of a 740 m long and 35 m high iceberg located 250 m to the west that would block winds sourced over land, maximizing blowing snow collection from over sea ice. At each site, a 5.5 m blowing snow collection tower was erected for the time period of 11 October–08 November 2012. Collection baskets made out of 150 µm nylon monofilament mesh and 7′′ diameter by 14′′ long (Midwest Filter Corporation, Lake Forest, IL) were placed at four different heights: 0.3, 2.0, 3.0, and 5.5 m above the snow surface.

The sites were visited approximately every other day, and collection baskets were switched even if no snow was collected. Baskets were triple rinsed with DI water between uses. Surface snow samples were collected each of the 12 days the sites were visited. In addition, 1.78 and 1.89 m ice cores were extracted from Butter Point and Iceberg sites, respectively. Snow samples and sea ice cores were transported at a constant temperature of -20 ∘C back to Thayer School of Engineering's Ice Research Laboratory at Dartmouth College and stored in a -33 ∘C cold room.

Winds during the field campaign were generally mild, only surpassing 10 ms-1 on a few occasions. Site visits were not feasible during stormy weather, and much of the blowing snow captured during those periods sublimated in the baskets prior to collection as observed by a webcam. Thus, only on 25 October were blowing snow samples collected at both sites and all four heights. The collected snow was the result of approximately 10 ms-1 southerly winds on 24 October. Visibility at McMurdo station during this blowing snow event dropped below 0.5 miles for about 4 h. Temperatures ranged from -20.5 to -11.5 ∘C with an average relative humidity of 69% on 24 October and -17.5 to -13.5 ∘C with an average relative humidity of 61% on 25 October. These temperatures were a couple of degrees warmer than the average for the rest of the field campaign.

All surface snow and blowing snow samples were melted into acid-rinsed HDPE vials, and diluted by a factor of 5 with DI water. Those samples falling outside of the calibration standard concentrations were further diluted to fall within the desired range. Each ice core was sub-sampled every 10 cm, melted, and diluted by a factor of 50, except for the bottom-most sample that was diluted by a factor of 100. A 0.5 mL aliquot of each diluted snow and ice sample was then analyzed for anion concentrations.

Anion chromatography was completed using a Dionex-600 IC system, equipped with a LC 25 chromatogram oven set at 30 ∘C, GD 50 gradient pump, CD 25 conductivity detector, AS15 anion column with a AG15 guard column, and an auto-sampler. A 38 mM NaOH eluent was used at a flow rate of 1.2 mLmin-1 and the SRS current was set to 100 µA. Pressure in the column was 1600 psi and the signal noise was approximately 0.003 µS. Peak analysis was performed using Chromeleon 6.8 software with a 5 point standard calibration curve. IC detection limits were 0.6, 0.1, 0.04, and 0.03 gm-3, yielding limits of quantification of 2, 0.3, 0.1, and 0.1 gm-3 for chloride, sulfate, bromide, and nitrate, respectively.

Raw anion concentrations for sea ice, surface snow, and blowing snow samples are provided in the Supplement. Sea ice anion concentrations had nearly identical profiles for Butter Point and Iceberg cores, with both showing the expected C-shape profile (see Fig. ) . Note that the deepest sample for Iceberg Site was directly at the base of the core, while the deepest sample for Butter Point was 8 cm above the base, due to a mushy, poorly defined basal layer, and thus missed the saltiest portion. Chloride concentrations ranged from 2000–8000 gm-3, while bromide concentrations ranged from 5–35 gm-3. Sulfate concentrations ranged from 2–3000 gm-3, with increased variation observed over the top 40 cm of both cores. Nitrate concentrations were below detection limits for all sea ice core samples.

Surface snow anion concentrations varied greatly, but were of similar magnitude at the two sites and showed no noticeable trend with time over the field campaign. Concentrations ranged from 200–5000 gm-3 chloride, 1.0–16 gm-3 bromide, 20–2300 gm-3 sulfate, and 0–1.2 gm-3 nitrate, though the majority of samples were below detection limits in nitrate. If we assume Cl concentrations to be a good proxy to estimate salinity using sea water ion ratios, this corresponds to a mean surface snow salinity of 3 PSU, with a range of 0.4–9 PSU. There was little snow cover at Butter Point with the surface varying from completely wind scoured to several cm of snow. Iceberg Site had a more variable snowpack, with prevalent rafted ice and sastrugi, resulting in snow depths varying from tens of cm to over 1 m. A 21 cm snow pit was dug 15 km from Butter Point Site to study a more consistent snowpack that is representative of the greater first-year sea ice of the region. In this snow pit, both chloride and bromide concentrations increased steadily from the snow surface to the snow/ice interface, with concentrations ranging from 500–1600 gm-3 chloride and 1.5–6 gm-3 bromide. Sulfate concentrations ranged from 30–50 gm-3, but showed a C-shaped profile with highest concentrations found near the top and bottom of the snow pit. A second snow pit located slightly closer to the open ocean showed similar trends.

Anion concentrations from the blowing snow samples collected on 25 October in both towers varied greatly, including separate samples from the same collection baskets, indicating that blown snow can be quite varied in its salt concentration. Concentrations ranged from 50–6800 gm-3 chloride, 0.2–18.5 gm-3 bromide, 3–240 gm-3 sulfate, and 0.4–23 gm-3 nitrate. Using chloride concentrations to derive an estimate of salinity, blowing snow samples ranged from 0.1 to 12 PSU. Comparing the 0.3 and 5.5 m baskets at both sites suggests a 10-fold increase in salinity from 0.6 to 6 PSU. However, when all blowing snow samples are considered, there is not a statistically significant trend of increasing salt concentration with basket height, except for nitrate where concentrations increased with height above the snow surface.

Anion concentrations can provide a general understanding of the overall salinity of the snow and ice, but in order to understand chemical reaction activity, it is instructive to calculate anion concentration ratios. The Br-/Cl- mass ratio has long been used as a metric for analyzing both frozen and liquid saline water samples, and comparing measured values to the well established 1:290 ratio observed in seawater . This ratio is plotted in Fig.  for all samples at both sites. Br-/Cl- mass ratios were relatively constant for all sea ice samples (1:281±24) and all surface snow samples (1:296±22). However, there was a linear correlation between Br-/Cl- mass ratios for blowing snow relative to height at both Butter Point (r2=0.89) and Iceberg Site (r2=0.93). Sulfate mass ratios in sea ice, surface snow, and blowing snow did not show as clear a trend (Fig. ). In sea ice, the SO42-/Cl- mass ratio was roughly 1:7 in the bottom half of the cores, consistent with what is generally observed in seawater , but varied from approximately 1:2 to 1:12 in the upper halves of the cores. Surface snow mass ratios also were quite variable ranging from 1:1 to 1:19 with no clear correlations. Collected lofted snow was quite depleted in sulfate with mass ratios reaching 1:20 to 1:40, but did not show the linear trend with height observed with bromide.

Observation of bromide depletion relative to other salts is an indication of heterogeneous atmospheric chemical reactions and bromine activation . Although there was some variability in Br-/Cl- mass ratios for sea ice and surface snow, these ratios were remarkably constant, just over 1:300. As shown in Fig. , signs of bromide depletion occur above the snowpack, with greater depletion occurring at greater heights, reaching mass ratios of nearly 1:1000 at heights of 5.5 m. This is consistent with aerosol observations by that measured increased bromide depletion with height, albeit at tropical latitudes. They noted a strong particle size dependence with increased bromide depletion corresponding to mid-sized aerosol particles. Further studies have shown that particle size influences the mode of transport and it may not be realistic to differentiate snow saltation and suspension based solely on wind speed and height . Nevertheless, both processes greatly increase the snow surface area exposed for gas-phase reactions by as much as 3000 times that of surface snow . It is noted that snow blowing at the windspeed will have no relative motion compared to the gases and gas-phase diffusion becomes an important consideration, especially for large grain particles. Local turbulence however, will help to overcome some of these diffusion limitations . We also note that sublimation of blowing snow particles likely has a strong effect on anion concentrations. Consistent with the mechanism suggested by , lighter particles would remain airborne for longer periods of time, leading to suspension at greater heights and more time for photochemical reactions. However, since particle sizes were not measured in this study, we solely mention the potential sublimation may have on increasing bromine release through this pathway.

A depletion factor (DF) was defined such that DF =0 signifies no bromine depletion in particles, and DF=1 signifies complete depletion . We note that a negative value for DF indicates bromine enrichment. Averaging the Br-/Cl- mass ratios at each height for Butter Point yields DF =0.04,0.11,0.41, and 0.68 for heights of 0.3, 2.0, 3.0, and 5.5 m, respectively. At Iceberg Site, DF =-0.08, 0.20, 0.33, and 0.64 for the four heights, respectively. Depletion factors were relatively small for both surface snow and ice core samples, with values ranging from -0.20 to 0.13 and -0.13 to 0.17, respectively. These values are consistent with previously reported values and currently used in some models . Previously, observed up to 90% bromide depletion in surface snow, while observed up to 70% depletion in snow mobilized 5 cm above the surface. The latter study also noted that during a blowing snow event, there was first bromide enrichment in the snow during the initial part of the storm followed by later bromide depletion . An increased depletion factor can either signify bromide depletion due to bromine release or chloride enrichment from, for example, gaseous HCL uptake. If the latter hypothesis were true, one would expect to observe increased chloride concentrations in higher baskets. The lack of an observed chloride trend with height leads to our interpretation that the decreased mass ratio is a clear indicator of bromine activation. Interestingly, lower Br-/Cl- ratios have also been found to correspond with a decrease in chlorine release . This is due to the fact that BrCl plays a critical role in chlorine production. BrCl is initially formed from the reaction between HOBr and Cl-, and then upon later photolysis, can lead to chlorine release .

Previously, bromine activation and ODEs were observed in both low (<5 ms-1) and high (>12 ms-1) winds . Here, the blowing snow event occurred during moderate winds of approximately 10 ms-1. Since the wind was from the south and there was no new precipitation, it is safe to assume that the snow originated from the surface of first-year sea ice. It is noted that the blowing snow event lasted approximately 4–6 h, and that snow samples from the baskets were not collected until 10–12 h later. During this time, wind speeds dropped to less than 5 ms-1, and it is possible that the collected snow was exposed to further wind scouring. However, all baskets would have experienced a similar degree of scouring in these minimal winds and the effect would be minor over the short time period prior to collection. Further, the collected snow was mostly windpacked, suggesting it was unlikely the wind penetrated both the 150 µm nylon monofilament mesh of the basket and the interior bulk of the snow. Future studies, however, may attempt to better seal off collected blown snow.

The surface snow Br-/Cl- mass ratios at both sites showed little variation and no observed trend with date or meteorology over the month-long campaign. If there was a significant amount of lofted snow being depleted in bromide and then falling back to the snow surface, one might expect to detect a signal in the surface snow chemistry. The lack of recorded bromide depletion in the surface snow may indicate that either surface snow bromide concentration is quickly replenished or that blowing snow represents only a small portion of the surface snowpack. Surface snow will be replenished in bromide through interaction with saline brine , deposition of HOBr as a byproduct of the reaction between BrO and HO2, deposition of HBr, and deposition of aerosol-phase bromine. In cold ambient environments, frost flowers, surface snow, and the sea ice surface have been shown to be bromide enriched sources, likely due to the precipitation of NaCl . Additionally, recycling of deposited bromine plays a critical role with as much as 75% of deposited bromine being re-emitted into the gas phase as Br2 or BrCl . In future work, it would be informative to collect both surface and blowing snow during stronger wind events.

The sulfate depletion observed in lofted snow is consistent with the observations of others, most likely a product of mirabilite precipitation occurring at temperatures below -8 ∘C . The range of SO42-/Cl- mass ratios measured in surface snow and the upper halves of the ice cores is also likely due to similar interactions. Sulfate depletion can result from a wide variety of processes involving brine and mirabilite formation as brine constantly moves through sea ice and is also wicked up by surface snow . In addition, non-sea salt sourced sulfate may provide additional sulfate to the surface snowpack .

Nitrate concentration in seawater is highly dependent upon phytoplankton blooms, and therefore varies greatly from single digit gm-3 to less than one gm-3 e.g.,. Thus, it was not surprising that sea ice samples were below the quantification limit for nitrate. Surface snow samples also were generally below nitrate detection limits, and those with quantifiable concentrations showed no noticeable trend. Nitrate snow photochemistry is quite complex, with many processes both adding and subtracting nitrate from the snowpack . Nitrate sources include biomass burning, lightning production, soil exhalation, and anthropogenic pollutants that can be carried long distances through atmospheric transport . The increase in nitrate concentration observed with basket height may be attributed to increased atmospheric interactions higher above the snowpack. Additionally, basket nitrate concentration may also be influenced by interactions occurring while the snow was sitting in the basket but prior to collection, as cycling of different nitrogen-containing compounds is relatively fast and can vary greatly during a day . Considering the number of competing reactions, more data points would be needed to make conclusive statements regarding the cycling of nitrate in the polar boundary layer.

In summary, this work examined the mass ratio of Br-/Cl- in blowing snow during mid-strength winds to better understand mechanisms leading to bromine activation. It was found that while Br-/Cl- mass ratios remained relatively constant for sea ice and surface snow, bromide becomes strongly depleted relative to chloride in lofted snow. Thus, we conclude the blowing snow particles provide a very viable surface for the initial heterogeneous reactions initializing ODEs to occur. The findings of this study are consistent with previous modeling results , suggesting that blowing snow has the capacity to explain the occurrence of ODEs in the polar springtime.