Hydroxyl concentration estimates in the sunlit snowpack at Summit, Greenland

Abstract

Experiments were performed at Summit, Greenland (72134 0 N, 38129 0 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 nearsurface 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 Â 10 6 molecules cm À3 between 13:00 and 16:00 local time. The minimum value estimated was 1.1 Â 10 6 molecules cm À3 at 20:00 local time. Results during spring of 2004 showed lower, but rapidly increasing, peak hydroxyl mixing ratios of 1.1 Â 10 6 molecules cm À3 in the early afternoon on 15 April and 1.5 Â 10 6 molecules cm À3 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. r

How different would tropospheric oxidation be over an ice‐free Arctic?

“…We note that modelled mean OH concentrations over the Arctic in August are around 5 -7 Â 10 5 molecules/cm 3 . This figure agrees with previous studies [e.g., Spivakovsky et al, 2000] and underlines that the Arctic cannot be neglected as an environment for oxidation in the summer, as has also been indicated by measurement studies [e.g., Beyersdorf et al, 2007].…”

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

“…This behavior could imply that a chemical sink in the upper snow layers may contribute to the loss of these compounds. A series of recent investigations Beyersdorf et al 2007;France et al 2007) have pointed out the significant oxidation potential of sunlit snow. These studies demonstrated heterogeneous production of the hydroxyl radical from snow impurities, with hydrogen peroxide and nitrate being important substrates for this radical formation.…”

Snowpack concentrations and estimated fluxes of volatile organic compounds in a boreal forest

“…O 3 deposition to snow and concentrations inside the snowpack has been found to be low (Zeller and Hehn, 1995;Helmig et al, 2009b;Bocquet et al, 2011). OH concentrations may be higher under sunny conditions (Anastasio et al, 2007;Beyersdorf et al, 2007), however, light levels on boreal forest floor during winter are very low. Thus, for oxidative reactions the snow-air interface, including the top layers of the snowpack, seems to be more important than the deeper layers of the snowpack.…”

“…However, as discussed already in Sect. 4.2, the concentrations of the main oxidants, OH and O 3 , inside the snowpack are presumably low, except for the surface layer of the snowpack (Zeller and Hehn, 1995;Anastasio et al, 2007;Beyersdorf et al, 2007), as are the temperature and light levels, all of which reduce the reactivity of VOCs.…”