Atmospheric Environment (v.41, #24)

An overview of air-snow exchange at Summit, Greenland: Recent experiments and findings by Jack E. Dibb; Mary Albert; Cort Anastasio; Elliot Atlas; Andreas J. Beyersdorf; Nicola J. Blake; Donald R. Blake; Florence Bocquet; John F. Burkhart; Gao Chen; Lana Cohen; Thomas J. Conway; Zoe Courville; Markus M. Frey; Donna K. Friel; Edward S. Galbavy; Samuel Hall; Meredith G. Hastings; Detlev Helmig; L. Greg Huey; Manuel A. Hutterli; Julia C. Jarvis; Barry L. Lefer; Simone Meinardi; William Neff; Samuel J. Oltmans; F. Sherwood Rowland; Steve J. Sjostedt; Eric J. Steig; Aaron L. Swanson; David J. Tanner (4995-5006).

Daily samples of the surface snow at Summit, Greenland were collected from June 1997 to April 1998 and then from August 2000 to August 2002. Concentrations of nine soluble ions (only eight in the first year) were determined in order to assess the validity of seasonal variations in snow composition at this site inferred from earlier snowpit and core studies. Strong and consistently sharp spring (April) peaks in dust, and broader summer (June–August) enhancements of NH 4 + and excess Cl - in the surface snow fully support the timing of these signals inferred from the pit profiles. Sea-salt reached maximum concentrations in the surface snow in late winter (February–March), based on averaging all three years of monthly means, but showed different patterns in winter each of the years. This is also consistent with the variable late-winter to spring timing inferred from pits. Simulated snowpit profiles constructed from the surface snow samples compared well with the ion profiles recovered from well dated snowpits sampled in this investigation, suggesting that early postdepositional changes do not greatly impact the glaciochemical records preserved at Summit. Nitrate in the real snowpits was approximately 25% lower than in simulated pits, this was the worst agreement for any ion but is consistent with several processes being known to deplete NO 3 - from near-surface snow.
Keywords: Soluble ions in snow; Seasonality; Summit;

The activity of the natural radionuclide tracers 7Be and 210Pb has been determined in bulk aerosol samples collected over 2-day intervals for nearly five full years at Summit, Greenland. Year-round sampling was conducted in three campaigns; summer 1997 to summer 1998, summer 2000 to summer 2002, and summer 2003 to present. As in previous summer campaigns at Summit, and a year-round investigation at Dye 3, variations in the activities of the tracers on short time scales were strongly correlated despite the upper troposphere/lower stratosphere source of 7Be and the continental surface source of 222Rn (precursor of 210Pb). This behavior is attributed to boundary layer dynamics exerting the dominant control on activities in air just above the ice sheet. Aerosols and associated species are depleted from the boundary layer above the snow when a strong inversion limits exchange with the free troposphere. Episodic weakening of the inversion allows ventilation of the boundary layer. This cycle drives simultaneous decreases and increases in the radionuclide tracers. The correlation between 7Be and 210Pb on seasonal and annual bases was found to be stronger than at Dye 3, and the average activity of 7Be was lower at Summit despite the higher elevation (3.0 versus 2.5 km). These observations indicate that the boundary layer at Summit is more effectively isolated than at Dye 3. The activity of 7Be at Summit peaked in June or July all 5 years, closely following the seasonality of stratospheric injection of 7Be into the Arctic troposphere (based on seasonality of the 10Be/7Be ratio previously measured at Alert, NWT). This suggests that when the boundary layer at Summit is replenished by ventilation, it receives air reflecting the composition of the mid and upper troposphere.
Keywords: Arctic aerosol; Beryllium-7; Lead-210; Boundary layer/free troposphere exchange; Greenland ice sheet;

What is causing high ozone at Summit, Greenland? by Detlev Helmig; Samuel J. Oltmans; Thomas O. Morse; Jack E. Dibb (5031-5043).
Causes for the unusually high and seasonally anomalous ozone concentrations at Summit, Greenland were investigated. Surface data from continuous monitoring, ozone sonde data, tethered balloon vertical profiling data, correlation of ozone with the radionuclide tracers 7Be and 210Pb, and synoptic transport analysis were used to identify processes that contribute to sources and sinks of ozone at Summit. Northern Hemisphere (NH) lower free troposphere ozone mixing ratios in the polar regions are ∼20 ppbv higher than in Antarctica. Ozone at Summit, which is at 3212 m above sea level, reflects its altitude location in the lower free troposphere. Transport events that bring high ozone and dry air, likely from lower stratospheric/higher tropospheric origin, were observed ∼40% of time during June 2000. Comparison of ozone enhancements with radionuclide tracer records shows a year-round correlation of ozone with the stratospheric tracer 7Be. Summit lacks the episodic, sunrise ozone depletion events, which were found to reduce the annual, median ozone at NH coastal sites by up to ∼3 ppbv. Synoptic trajectory analyses indicated that, under selected conditions, Summit encounters polluted continental air with increased ozone from central and western Europe. Low ozone surface deposition fluxes over long distances upwind of Summit reduce ozone deposition losses in comparison to other NH sites, particularly during the summer months. Surface-layer photochemical ozone production does not appear to have a noticeable influence on Summit's ozone levels.
Keywords: Tropospheric ozone; Snowpack-atmosphere gas exchange; Snow photochemistry; Synoptic transport;

Boundary-layer dynamics and its influence on atmospheric chemistry at Summit, Greenland by Lana Cohen; Detlev Helmig; William D. Neff; Andrey A. Grachev; Christopher W. Fairall (5044-5060).
Sonic anemometer turbulence measurements were made at Summit, Greenland during summer 2004 and spring 2005. These measurements allow for the characterization of the variability of the atmospheric boundary layer at this site by describing seasonal and diurnal changes in sensible heat flux and boundary layer stability as well as providing estimates of mixing layer height. Diurnal sensible heat fluxes at Summit ranged from −18 to −2 W m−2 in the spring and from −7 to +10 W m−2 in the summer. Sustained stable surface layer conditions and low wind speeds occured during the spring but not during the summer months. Unstable conditions were not observed at Summit until late April. Diurnal cycles of convective conditions during the daytime (0700–1700 h local time) were observed throughout July and August. Boundary layer heights, which were estimated for neutral to stable conditions, averaged 156 m for the spring 2005 observations. Comparisons of the boundary layer characteristics of Summit with those from South Pole, Antarctica, provide possible explanations for the significant differences in snowpack and surface-layer chemistry between the two sites.
Keywords: Polar boundary layer; Seasonal changes; Surface-atmosphere fluxes; Surface layer chemistry;

Ozone uptake to the polar snowpack at Summit, Greenland by Detlev Helmig; Florence Bocquet; Lana Cohen; Samuel J. Oltmans (5061-5076).
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.
Keywords: Ozone; Interstitial air; Polar snowpack; Atmosphere–snowpack gas exchange; Snow-photochemistry; Wind pumping;

Light penetration in the snowpack at Summit, Greenland: Part 1 by Edward S. Galbavy; Cort Anastasio; Barry L. Lefer; Samuel R. Hall (5077-5090).
Photochemical rate constants (j values) are crucial indicators for evaluating the importance of photochemical reactions in environmental systems. While measurement of aqueous j values via chemical actinometry is relatively straightforward under most conditions, problems arise with ambient conditions below freezing, such as at very high latitudes or altitudes. To address this problem, we have developed a new method for low temperature actinometry using solutions of acetonitrile (ACN) and water, which have freezing points down to - 44 ∘ C . In this method we measure the formation of phenol from the photolysis of an OH • -generating chromophore in the presence of benzene. Using results from laboratory tests we correct our phenol field results in ACN / H 2 O to rate constants for chromophore photolysis expected for water–ice (i.e., in the quasi-liquid layer of snow grains) under the same conditions.In part 1 of this study, we use this method at Summit, Greenland on the surface snow and to depths of ∼ 30 cm using hydrogen peroxide (HOOH) and nitrite ( NO 2 - ) as the chromophores. While the method works well for determining the rate constant for HOOH photolysis (j(HOOH)), we encountered problems using the technique with nitrite. However, measured PhOH formation rate constants for nitrite in acetonitrile, j NO 2 - → PhOH ACN , still provide an excellent means for calculating snowpack e -folding depths for NO 2 - photolysis (i.e., the depth over which the rate constant decreases by a factor of e ). Values of j(HOOH) and j ( NO 2 - ) determined from measurements of actinic flux (above the snow) and irradiance (in snow) suggest that the value of j(HOOH) on the surface snow at midday was 8.6 × 10 - 7 s - 1 in mid-March and increased by 300% by the start of May, while j NO 2 - → PhOH ACN midday surface values were consistently ( 1 – 3 ) × 10 - 7 s - 1 throughout the season. Within the snowpack, average e -folding depths determined from chemical actinometry were 13.3 ( ± 0.88 ) cm for j(HOOH) and 16.3 ( ± 4.2 ) cm for j NO 2 - → PhOH ACN ; e -folding depths determined from in-snow spectral radiometer measurements of irradiance were similar. The larger e -folding depth for nitrite is because this chromophore absorbs at longer wavelengths where there is less light extinction in the snow.
Keywords: Actinometry; Snowpack photolysis; Polar chemistry; Actinic flux;

Light penetration in the snowpack at Summit, Greenland: Part 2 Nitrate photolysis by Edward S. Galbavy; Cort Anastasio; Barry Lefer; Samuel Hall (5091-5100).
Rates of photochemical reactions within the snowpack, both on snow grains and in the firn air, depend on how actinic flux is attenuated as a function of depth. This snowpack photon flux can either be measured directly (e.g., with spectral radiometers in the snow) or indirectly (e.g., by chemical actinometry where the rate of a photochemical reaction is measured). In this work we use both techniques to measure the rate constant for nitrate photolysis on water–ice, j ( NO 3 - ) , on the snowpack surface and beneath at Summit, Greenland during spring and summer. The surface measurements from these two methods are generally similar ((1−2)×10–7  s−1 during midday near the summer solstice) and follow expected diurnal and annual trends. In addition, both methods show a similar effect of snow albedo on photolysis at the surface: rate constants measured on the surface snow were approximately 45% higher than values predicted for the surface based on regressions of in-snow rate constants. Average (± σ ) j ( NO 3 - ) e-folding depths (the depth along which the rate constant decreases by a factor of e) during the 2 field seasons are 10.3(±2.8) cm from actinometry tubes and 8.4(±2.4) cm from in-snow spectral radiometers.
Keywords: Actinometry; Snowpack photochemistry; Polar atmospheric chemistry; Radiation;

Hydroxyl concentration estimates in the sunlit snowpack at Summit, Greenland by Andreas J. Beyersdorf; Nicola J. Blake; Aaron L. Swanson; Simone Meinardi; Jack E. Dibb; Steve Sjostedt; Greg Huey; Barry Lefer; F. Sherwood Rowland; Donald R. Blake (5101-5109).
Experiments were performed at Summit, Greenland (72°34′ N, 38°29′ W) to investigate hydroxyl mixing ratios in the sunlit surface snowpack (or firn). We added a carefully selected mixture of hydrocarbon gases (with a wide range of hydroxyl reactivities) to a UV and visible light transparent flow chamber containing undisturbed natural firn. The relative decrease in mixing ratios of these gases allowed estimation of the lower limit mixing ratio of hydroxyl radicals in the near-surface firn pore spaces. Hydroxyl mixing ratios in the firn air followed a diurnal cycle in summer 2003 (10–12 July), with peak values of more than 3.2×106 molecules cm−3 between 13:00 and 16:00 local time. The minimum value estimated was 1.1×106 molecules cm−3 at 20:00 local time. Results during spring of 2004 showed lower, but rapidly increasing, peak hydroxyl mixing ratios of 1.1×106 molecules cm−3 in the early afternoon on 15 April and 1.5×106 molecules cm−3 on 1 May. Our firn hydroxyl estimates were similar to directly measured above-snow ambient levels during the spring field season, but were only about 30% of ambient levels during summer.
Keywords: Snowpack photochemistry; Firn air; Hydroxyl; Summit;

Photoformation of hydroxyl radical on snow grains at Summit, Greenland by Cort Anastasio; Edward S. Galbavy; Manuel A. Hutterli; John F. Burkhart; Donna K. Friel (5110-5121).
We measured the photoformation of hydroxyl radical ( ⋅ OH ) on snow grains at Summit, Greenland during the spring and summer. Midday rates of ⋅ OH formation in the snow phase in the summer range from 130 to 610 nmol L - 1 h - 1 , expressed relative to the liquid equivalent volume of snow. Calculated formation rates of snow-grain ⋅ OH based on the photolysis of hydrogen peroxide and nitrate agree well with our measured rates during summer, indicating that there are probably not other major sources of ⋅ OH under these conditions. Throughout both the spring and summer, HOOH is by far the dominant source of snow-grain ⋅ OH ; on average, HOOH produces approximately 100 times more ⋅ OH than does NO 3 - . Rates of ⋅ OH photoformation have a strong seasonal dependence and increase by approximately a factor of 10 between early spring and summer at midday. The rate of ⋅ OH photoformation on snow grains decreases rapidly with depth in the snowpack, with approximately 90% of photoformation occurring within the top 10 cm, although ⋅ OH formation occurs to depths below 20 cm. The formation of ⋅ OH on snow grains likely initiates a suite of reactions in the snowpack, including the transformation of organic carbon (OC) and oxidation of halides. The reaction of ⋅ OH with OC probably forms a number of volatile organic compounds (VOCs) that are potentially emitted into the atmospheric boundary layer. Indeed, our measured rates of ⋅ OH photoformation on snow grains are large enough that they could account for previously reported fluxes of VOCs from the snowpack at Summit, although the relative importance of thermal desorption and photochemical production for most of these VOCs still needs to be determined.
Keywords: Snow chemistry; ⋅ OH ; Hydrogen peroxide; Nitrate; Organic carbon;

Observations of hydroxyl and the sum of peroxy radicals at Summit, Greenland during summer 2003 by S.J. Sjostedt; L.G. Huey; D.J. Tanner; J. Peischl; G. Chen; J.E. Dibb; B. Lefer; M.A. Hutterli; A.J. Beyersdorf; N.J. Blake; D.R. Blake; D. Sueper; T. Ryerson; J. Burkhart; A. Stohl (5122-5137).
The first measurements of peroxy (HO2+RO2) and hydroxyl (OH) radicals above the arctic snowpack were collected during the summer 2003 campaign at Summit, Greenland. The median measured number densities for peroxy and hydroxyl radicals were 2.2×108  mol cm−3 and 6.4×106  mol cm−3, respectively. The observed peroxy radical values are in excellent agreement ( R 2 = 0.83 , M / O = 1.06 ) with highly constrained model predictions. However, calculated hydroxyl number densities are consistently more than a factor of 2 lower than the observed values. These results indicate that our current understanding of radical sources and sinks is in accord with our observations in this environment but that there may be a mechanism that is perturbing the (HO2+RO2)/OH ratio. This observed ratio was also found to depend on meteorological conditions especially during periods of high winds accompanied by blowing snow. Backward transport model simulations indicate that these periods of high winds were characterized by rapid transport (1–2 days) of marine boundary layer air to Summit. These data suggest that the boundary layer photochemistry at Summit may be periodically impacted by halogens.
Keywords: HO x ; Greenland; Polar; Photochemistry; Hydroxyl radical; Summit;

A review of surface ozone in the polar regions by Detlev Helmig; Samuel J. Oltmans; Daniel Carlson; Jean-Francois Lamarque; Anna Jones; Casper Labuschagne; Kurt Anlauf; Katherine Hayden (5138-5161).
Surface ozone records from ten polar research stations were investigated for the dependencies of ozone on radiative processes, snow-photochemisty, and synoptic and stratospheric transport. A total of 146 annual data records for the Arctic sites Barrow, Alaska; Summit, Greenland; Alert, Canada; Zeppelinfjellet, Norway; and the Antarctic stations Halley, McMurdo, Neumayer, Sanae, Syowa, and South Pole were analyzed. Mean ozone at the Northern Hemisphere (NH) stations (excluding Summit) is ∼5 ppbv higher than in Antarctica. Statistical analysis yielded best estimates for the projected year 2005 median annual ozone mixing ratios, which for the Arctic stations were 33.5 ppbv at Alert, 28.6 ppbv at Barrow, 46.3 ppbv ppb at Summit and 33.7 ppbv at Zeppelinfjellet. For the Antarctic stations the corresponding ozone mixing ratios were 21.6 ppbv at Halley, 27.0 ppbv at McMurdo, 24.9 ppbv at Neumayer, 27.2 ppbv at Sanae, 29.4 ppbv at South Pole, and 25.8 ppbv at Syowa. At both Summit (3212 m asl) and South Pole (2830 m asl), annual mean ozone is higher than at the lower elevation and coastal stations. A trend analysis revealed that all sites in recent years have experienced low to moderate increases in surface ozone ranging from 0.02 to 0.26 ppbv yr−1, albeit none of these changes were found to be statistically significant trends. A seasonal trend analysis showed above-average increases in ozone during the spring and early summer periods for both Arctic (Alert, Zeppelinfjellet) and Antarctic (McMurdo, Neumayer, South Pole) sites. In contrast, at Barrow, springtime ozone has been declining. All coastal stations experience springtime episodes with rapid depletion of ozone in the boundary layer, attributable to photochemically catalyzed ozone depletion from halogen chemistry. This effect is most obvious at Barrow, followed by Alert. Springtime depletion episodes are less pronounced at Antarctic stations. At South Pole, during the Antarctic spring and summer, photochemical ozone production yields frequent episodes with enhanced surface ozone. Other Antarctic stations show similar, though less frequent spring and summertime periods with enhanced ozone. The Antarctic data provide evidence that austral spring and summertime ozone production in Antarctica is widespread, respectively, affects all stations at least through transport events. This ozone production contributes to a several ppbv enhancement in the annual mean ozone over the Antarctic plateau; however, it is not the determining process in the Antarctic seasonal ozone cycle. Although Summit and South Pole have many similarities in their environmental conditions, this ozone production does not appear to be of equal importance at Summit. Amplitudes of diurnal, summertime ozone cycles at these polar sites are weaker than at lower latitude locations. Amplitudes of seasonal ozone changes are larger in the Southern Hemisphere (by ∼5 ppbv), most likely due to less summertime photochemical ozone loss and more transport of ozone-rich air to the Arctic during the NH spring and summer months.
Keywords: Ozone photochemistry; Diurnal ozone cycles; Seasonal ozone cycles; Long-term ozone trends; Snowpack–atmosphere interactions;

Are methyl halides produced on all ice surfaces? Observations from snow-laden field sites by Aaron L. Swanson; Nicola J. Blake; Donald R. Blake; F. Sherwood Rowland; Jack E. Dibb; Barry L. Lefer; Elliot Atlas (5162-5177).
We present data collected from a number of snow-covered environments including two polar locations (Summit, Greenland and the South Pole) and two mid-latitude regions (a remote site in northern Michigan, and Niwot Ridge, Colorado). At each site, concentrations of CH 3 I and C 2 H 5 I were enhanced within the interstitial air near the snow surface, compared to levels in boundary layer air. Fluxes of CH 3 Br from surface snow to the atmosphere were observed at each site except Niwot Ridge, where CH 3 Br appeared to have a sink. The mid-latitude sites showed significant emissions of CH 3 Cl , mostly originating at the ground surface and traveling up through the snow, while at the polar locations CH 3 Cl emissions from firn air were relatively small. In general, methyl halide mixing ratios in firn air were significantly greater at Summit than at the South Pole, with Summit showing a strong diurnal cycle in the production of alkyl halides that was well correlated with actinic radiation and firn temperature. We suggest that the most likely route to alkyl halide formation is through an acid catalyzed nucleophilic substitution of an alcohol type function by a halide, both of which should be preferentially segregated to the quasi-liquid layer at the surface of the snow grains. A series of experiments using a snow-filled quartz chamber irradiated by natural sunlight allowed estimation of emission trends that were hard to measure in the natural snowpack. These static chamber experiments confirmed significant production of the primary alkyl halides, following the order CH 3 Cl > CH 3 Br > C 2 H 5 Cl > CH 3 I > C 2 H 5 Br > C 2 H 5 I > 1 - C 3 H 7 Br > 1 - C 3 H 7 I . Our observations at all four locations, including polar and mid-latitude sites, imply that alkyl halide production may be associated with all surface snows.
Keywords: Snow chemistry; Methyl iodide; Methyl bromide; Organohalides; Snow composition;