Snowpack Air Research Articles

Impact of exhaust emissions on chemical snowpack composition at Concordia Station, Antarctica

Abstract. The chemistry of reactive gases inside the snowpack and in the lower atmosphere was investigated at Concordia Station (Dome C), Antarctica, from December 2012 to January 2014. Measured species included ozone, nitrogen oxides, gaseous elemental mercury (GEM), and formaldehyde, for study of photochemical reactions, surface exchange, and the seasonal cycles and atmospheric chemistry of these gases. The experiment was installed ≈1 km from the station main infrastructure inside the station clean air sector and within the station electrical power grid boundary. Ambient air was sampled continuously from inlets mounted above the surface on a 10 m meteorological tower. In addition, snowpack air was collected at 30 cm intervals to 1.2 m depth from two manifolds that had both above- and below-surface sampling inlets. Despite being in the clean air sector, over the course of the 1.2-year study, we observed on the order of 50 occasions when exhaust plumes from the camp, most notably from the power generation system, were transported to the study site. Continuous monitoring of nitrogen oxides (NOx) provided a measurement of a chemical tracer for exhaust plumes. Highly elevated levels of NOx (up to 1000 × background) and lowered ozone (down to ≈50 %), most likely from reaction of ozone with nitric oxide, were measured in air from above and within the snowpack. Within 5–15 min from observing elevated pollutant levels above the snow, rapidly increasing and long-lasting concentration enhancements were measured in snowpack air. While pollution events typically lasted only a few minutes to an hour above the snow surface, elevated NOx levels were observed in the snowpack lasting from a few days to ≈ 1 week. GEM and formaldehyde measurements were less sensitive and covered a shorter measurement period; neither of these species' data showed noticeable concentration changes during these events that were above the normal variability seen in the data. Nonetheless, the clarity of the NOx and ozone observations adds important new insight into the discussion of if and how snow photochemical experiments within reach of the power grid of polar research sites are possibly compromised by the snowpack being chemically influenced (contaminated) by gaseous and particulate emissions from the research camp activities. This question is critical for evaluating if snowpack trace chemical measurements from within the camp boundaries are representative for the vast polar ice sheets.

Release and uptake of volatile inorganic and organic gases through the snowpack at Niwot Ridge, Colorado

Whole air drawn from four heights within the high elevation (3,340 m asl), deep, winter snowpack at Niwot Ridge, Colorado, were sampled into stainless steel canisters, and subsequently analyzed by gas chromatography for 51 volatile inorganic and organic gases. Two adjacent plots with similar snow cover were sampled, one over bare soil and a second one from within a snow-filled chamber where Tedlar/Teflon-film covered the ground and isolated it from the soil. This comparison allowed for studying effects from processes in the snowpack itself versus soil influences on the gas concentrations and fluxes within and through the snowpack. Samples were also collected from ambient air above the snow surface for comparison with the snowpack air. Analyzed gas species were found to exhibit three different kinds of behavior: (1) One group of gases, i.e., carbon dioxide (CO2), chloroform (CHCl3), dimethylsulfide (CH3)2S, carbondisulfide (CS2), and dichlorobromomethane (CHBrCl2), displayed higher concentrations inside the snow, indicating a formation of these species and release into the atmosphere. (2) A second group of compounds, including carbon monoxide (CO), carbonyl sulfide (COS), the hydrocarbons methane, ethane, ethyne, benzene, and the halogenated compounds methylchloride (CH3Cl), methylbromide (CH3Br), dibromomethane (CH2Br2), bromoform (CHBr3), tetrachloromethane (CCl4), CFC-11, CFC-12, HCFC-22, CFC-113, 1,2-dichloroethane, methylchloroform, HCFC-141b, and HCFC-142b, were found at lower concentrations in the snow, indicating that the snow and/or soil constitute a sink for these gases. (3) For 21 other gases absolute concentrations, respectively concentration gradients, were too low to unequivocally identify their uptake or release behavior. For gases listed in the first two groups, concentration gradients were incorporated into a snowpack gas diffusion model to derive preliminary estimates of fluxes at the snow-atmosphere interface. The snowpack gradient flux technique was found to offer a highly sensitive method for the study of these surface gas exchanges. Microbial activities below this deep, winter snowpack appear to be the driving mechanism behind these gas sources and sinks. Flux results were applied to a simple box model to assess the potential contribution of the snowpack uptake rates to atmospheric lifetimes of these species.

Ozone uptake to the polar snowpack at Summit, Greenland

The uptake of atmospheric ozone to the polar, year-round snowpack on glacial ice was studied at Summit, Greenland during three experiments in 2003, 2004, and 2005. Ozone was measured at up to three depths in the snowpack, on the surface, and above the surface at three heights on a tower along with supporting meteorological parameters. Ozone in interstitial air decreased with depth, albeit ozone gradients showed a high variation depending on environmental conditions of solar radiation and wind speed. Under low irradiance levels, up to 90% of ozone was preserved up to 1 m depth in the snowpack. Ozone depletion rates increased significantly with the seasonal and diurnal cycle of solar irradiance, resulting in only 10% of ozone remaining in the snowpack following solar noon during summertime. Faster snowpack air exchange from wind pumping resulted in smaller above-surface-to-within snowpack ozone gradients. These data indicate that the uptake of ozone to polar snowpack is strongly dependent on solar irradiance and wind pumping. Ozone deposition fluxes to the polar snowpack are consequently expected to follow incoming solar radiation levels and to exhibit diurnal and seasonal cycles. The Summit observations are in stark contrast to recent findings in the seasonal, midlatitude snowpack [Bocquet, F., Helmig, D., Oltmans, S.J., 2007. Ozone in the mid-latitude snowpack at Niwot Ridge, Colorado. Arctic, Antarctic and Alpine Research, in press], where mostly light-independent ozone behavior was observed. These contrasting results imply different ozone chemistry and snowpack–atmosphere gas exchange in the snow-covered polar, glacial conditions compared to the temperate, mid-latitude environment.

Fast depletion of gaseous elemental mercury in the Kongsvegen Glacier snowpack in Svalbard

Gaseous elemental mercury (GEM, Hg°) was measured in the snowpack interstitial air on the Kongsvegen Glacier (Svalbard) between the surface and 190 cm depth. A rapid depletion of GEM from ∼5 to 0.4 ng.m−3 in the snowpack air was measured in less than 8 hours at a calculated depletion rate of ∼0.5–0.7 ng.m−3.h−1 while concentration of GEM above the snow stayed constant about 1.7 ng.m−3. This depletion could only be explained by chemical processes and this study suggests that Br. could be the most important reactant for the oxidation of GEM in the interstitial snow air. The lifetime of GEM was estimated to be ∼10 minutes with second order reaction rate constant between GEM and Br. of about ∼2 × 10−11 cm−3.molecule−1.s−1. These first experimental kinetic values for GEM oxidation in the snow air are in good agreement with both theoretical and modelling studies previously reported.

The fate of mercury species in a sub‐arctic snowpack during snowmelt

An extensive mercury study was conducted in April 2002 prior to and during the annual melting of a snowpack in a sub‐arctic site along the Hudson Bay (Canada). Gas‐phase measurements show that the snowmelt coincides with an elemental mercury (Hg°) pulse in the snowpack air far above ambient levels. Additional measurements of inorganic mercury (Hg2+) and methylmercury (MeHg+) in snow pits, in surface snow and in a meltwater sample clearly reveal that most of Hg is removed from the snow during the first days of snowmelt. We estimate that gas‐phase exchanges contribute poorly to remove Hg from the snowpack; consequently during a snowmelt day more than 90% of Hg present in the snow surface is likely released with the meltwater. In arctic areas, where Hg accumulates at an accelerated rate in the snow surfaces [Lu et al., 2001] during mercury depletion events (MDE), the discharge of this toxic and bio‐accumulating pollutant in water systems could be a threat to ecosystems and local indigenous populations.

Gaseous mercury distribution in interstitial air of snow pack in Station Nord, Greenland. Evidence of permanent mercury depletion event in the air of snow during polar sunrise

Gaseous Elemental Mercury (Hg°) concentrations have been measured in Station Nord, Greenland in the air of the snowpack from February 25 th to march 15 th , 2002, during twilight and low solar irradiation periods. The concentration of Hg° decreases rapidly with depth in the snow pack air from ∼1.5 ng/m 3 outside to 0.1 ng/m 3 at 120 cm depth. Adsorption of mercury on snow could not explain such profile since temperature increases with depth (∼ -13 °C at 120 cm below the surface and -35 °C at the surface). Additionally calculations indicate that a poor fraction of Hg° could be adsorbed onto snow. A possible argument to explain such a decrease of Hg° with depth is to investigate homogeneous and/or heterogeneous chemistry occurring at the air/snow interface initiated by sun light. The Arctic snowpack is known to produce active bromine and chlorine species in the frame of interactions between sea-salts, ozone and acid species in the snow. The lifetime of Hg° is calculated on the basis of reactions with various reactive species in the gaseous phase existing in the interstitial air during twilight and under low solar irradiation periods, such as Brcl, Br 2 , HOBr, Br - , BrO and OH + . We assume that the decrease of Hg° concentration with depth is the result of the reaction between Hg° and Br and/or BrO, which give a lifetime for Hg°<∼0.8-8 hours based on known kinetic constants found in the recent literature. This decrease of mercury in the air of the snowpack indicates that during polar sunrise in the Arctic, mercury is probably accumulated in the snowpack on its oxidised form Hg(II), probably HgO. Therefore, the snowpack could be a sink of mercury in the Arctic in spring where a permanent Mercury Depletion Event (M.D.E.) could exist at polar sunrise.

Snowpack processing of acetaldehyde and acetone in the Arctic atmospheric boundary layer

Acetaldehyde (CH 3CHO) and acetone (CH 3C(O)CH 3) concentrations in ambient air, in snowpack air, and bulk snow were determined at Alert, Nunavut, Canada, as a part of the Polar Sunrise Experiment (PSE): ALERT 2000. During the period of continuous sunlight, vertical profiles of ambient and snowpack air exhibited large concentration gradients through the top ∼10 cm of the snowpack, implying a flux of carbonyl compounds from the surface to the atmosphere. From vertical profile and eddy diffusivity measurements made simultaneously on 22 April, acetaldehyde and acetone fluxes of 4.2(±2.1)×10 8 and 6.2(±4.2)×10 8 molecules cm −2 s −1 were derived, respectively. For this day, the sources and sinks of CH 3CHO from gas phase chemistry were estimated. The result showed that the snowpack flux of CH 3CHO to the atmosphere was as large as the calculated CH 3CHO loss rate from known atmospheric gas phase reactions, and at least 40 times larger (in the surface layer) than the volumetric rate of acetaldehyde produced from the assumed main atmospheric gas phase reaction, i.e. reaction of ethane with hydroxyl radicals. In addition, acetaldehyde bulk snow phase measurements showed that acetaldehyde was produced in or on the snow phase, likely from a photochemical origin. The time series for the observed CH 3C(O)CH 3, ozone (O 3), and propane during PSE 1995, PSE 1998, and ALERT 2000 showed a consistent anti-correlation between acetone and O 3 and between acetone and propane. However, our data and model simulations showed that the acetone increase during ozone depletion events cannot be explained by gas phase chemistry involving propane oxidation. These results suggest that the snowpack is a significant source of acetaldehyde and acetone to the Arctic boundary layer.

An investigation of the interaction of carbonyl compounds with the snowpack

Measurements of formaldehyde, acetaldehyde, and acetone in ambient and snowpack air were conducted as a part of the SNOW99 study in northern Michigan. Vertical profiles of ambient and snowpack air illustrate large concentration gradients through the top ∼10 cm of the snowpack, implying a positive flux of these species from the surface. Snow chamber experiments that involved flushing a snow‐filled 34L Teflon‐lined chamber with zero air at 20 slpm indicated that release from the snow followed first order kinetics, with decay constants of 0.19, 0.44, and 0.34 hr−1 for formaldehyde, acetaldehyde, and acetone, respectively. Although it is likely that temperature dependent adsorption/desorption processes play a role, the data are not inconsistent with loss from the snowpack via snow grain metamorphism. The data also imply that formaldehyde is not hydrated in the snow grain surface layer.