Comments on 'Possible contribution of triboelectricity to snow–air interactions'

There have been numerous new discoveries of chemical activity at the snow–atmosphere interface over the last 20 years. These observations have stimulated an increasing body of research on snow chemical systems. The findings from this research have led to a general consensus that photochemical processes are the determining control of this snow chemistry. Traditional gasphase chemical reactions have fallen short of fully explaining observed behaviour. Heterogeneous and quasi-liquid layer chemistry has been postulated to play a role in accounting for the discrepancies. The Tkachenko and Kozachkov article is therefore a timely presentation of new ideas for further elucidating snowpack chemical mechanisms. In the following discussion we present recent data relevant to testing hypothesis proposed by Tkachenko and Kozachkov. Some of the hypotheses presented in this paper build upon data from measurements of ozone (O3) in interstitial air of the deep, glacial snowpack at Summit, Greenland, that were presented in our previous work. These experiments reported on positive O3 atmosphere–snowpack gradients, i.e. depleted O3 within the snowpack, and their dynamical behaviour dependent on environmental conditions. We have since taken this research to other locations and further investigated the dependencies of O3 within snow on a variety of parameters. A particular interest of this new research was to further investigate snowpack chemistry in environments with different snowpack conditions (year-round dry, polar snow; seasonal mid-latitude snowpack; snow over permafrost; snow over frozen freshwater lakes). We have also conducted a further 2 years of experiments at Summit, where the measurement depth in the snowpack was increased to 2.5m and experiments were extended year round to include winter observations. Another environment with polar year-round snowpack over glacial ice was investigated at South Pole, Antarctica. From the comparison with these other sites it has become obvious that the Summit snowpack is somewhat unique in that (1) O3 levels within the snowpack are generally higher than observed at other locations, and (2) interstitial air O3 shows a muchmore dynamic dependence on time of day and season than at the other locations that we have investigated. This behaviour might be in agreement with some hypotheses presented by Tkachenko and Kozachkov as they propose that O3 production by the triboelectrical effect would be most pronounced at sites characterised by a thick, glacial snowpack, low humidity, periodic high wind speeds, and low temperatures; all conditions that are typical for Summit. Our series of experiments have shown that different processes dominate snowpack chemistry in non-glacial snowpack. For example, one definite influence on O3 reactions in seasonal, midlatitude snowpack is the role of NO emitted from microbial processes in the subniveal soil. These influences were evident at a high alpine site at Niwot Ridge, CO, in the RockyMountains; snowpack under a canopy at the University of Michigan Biological Station (B. Seok, D. Helmig, M. W. Williams, C. Vogel, P. Curtis, unpubl. data); and snowpack over permafrost at Toolik Field Station, AK (B. Van Dam, D. Helmig, R. Honrath, L. Kramer, C. Toro, unpubl. data). For the Summit snowpack, Helmig et al. noted that O3 atmosphere–snowpack concentration gradients show three