Snow-pile and chamber experiments during the Polar Sunrise Experiment ‘Alert 2000’: exploration of nitrogen chemistry
Introduction
In addition to the measurements of many ambient trace species and parameters during the Polar Sunrise Experiment 2000 at Alert, Nunavut, several different snow experiments were carried out. These were mainly of two kinds; snow-piles and chambers. The experiments were made during both the dark and the light intensive and used either lamps or shading to effect rapid changes in the light conditions. This paper concerns specifically the nitrogen chemistry in snow and exchange processes in and above dark and sunlit snow surfaces. Snow chamber studies were developed recently to study in a defined ‘reactor’ the recycling of nitrate, and emissions of NOx and HONO (Honrath et al., 2000; Zhou et al., 2001; Dibb et al., 2002) and organic species (Sumner et al., 2002). The snow-pile studies described here are a new approach to investigate a confined amount of snow without the complication of wall effects inside a chamber. Also for the first time, the snow used in all experiments was characterized both for physical properties and chemical ion content. The aim of this study is (a) to give an overview of a number of snow experiments to which many researchers contributed with their measurements, and (b) to specifically explore nitrogen chemistry. We are interested in which species are released, which are the factors that lead to the release, and what is the relationship between the intake, reservoir and release of nitrogen in the snow.
As discussed by Honrath et al. (2000) NO3− photolysis in an aqueous surface phase of the snow layer may produce both NO2 and NO2 (aq)−. The latter would continue to react towards NO and HONO. A first aim of the snow experiments was to quantify release rates of these species, and their timing, to be able to test the mechanism. Recent laboratory experiments on sub-mm spray frozen aqueous nitrate solutions confirmed the production of both NO2 and NO2 (aq)− (Dubowski et al., 2001). In these experiments NO2 was directly released only from a <50 μm surface layer, while in deeper layers secondary reactions occurred.
The photochemical reactions of NO3− in snow so far have been discussed in terms of chemistry. We present evidence in this work for transport processes that occur in the snow, and identify gas-phase HNO3 as important link for re-distributing NO3− in the snowpack.
Although the species that contribute to the NO3− signal in the snow are not known with certainty, there are clear indications what these may (Bergin et al., 1995) or may not (Dibb et al., 1998) be. In order to identify the importance of a possible in situ source of NO3− onto the snowpack (Ianniello et al., manuscript in preparation), we attempt a mass balance and nitrogen balance for the snow used in our experiments.
Finally, there are several reasons why manipulated snow in such experiments may give different results than natural snow in a pristine surrounding. In particular, the experimental conditions in the snow piles turned out to be rather extreme. The discussion of the mechanisms in this study is an attempt to extract information about the natural behavior without over interpreting our individual results.
Section snippets
Experimental
The experimental methods and ambient data from the various measurements used here are described in detail in companion papers (Beine et al., 2002; Dominé et al., 2002; Simpson et al., 2002). Some of the results pertaining to HONO were recently published separately (Zhou et al., 2001). Briefly, NO was measured with the Michigan Tech. University (MTU) chemiluminescence instrument. NO2 was detected as NO following UV-broadband photolysis (Peterson and Honrath, 1999). For the NOx measurements in
Results
Results were obtained for the physical and chemical characteristics of the snow used in the experiments and for the air pulled through the chambers/piles from a number of instruments. In this work we describe the results from the snow characterization and for active nitrogen in the gas phase.
The ‘production’ of species in the chamber/piles refers to the difference between the measured mixing ratio from the chamber/pile and the ambient mixing ratio measured at the same time through the inlet at
Nitrogen chemistry in the snow
Neither the pile nor the chamber experiments produced the NOx:HONO flux ratios of ≈1:1 that were observed in ambient air at Alert (Beine et al., 2002). Since the ambient snow/air system operates on different time scales, and secondary reactions may change the original ratios, this was not expected. While it is still unclear what the sources or possible forms of NO3− in the snow are, there seems to be agreement that this NO3− is the origin of the release of the various active nitrogen species (
Conclusion
In this work we have presented evidence that NO3− in acidic snow is mobilized and re-distributed in the form of HNO3 by diffusion above a certain threshold temperature. In the snow-pile experiments a direct photolysis of gas-phase HNO3 was observable; this process may contribute to nitrate photolysis in natural snow as well. Transport in the form of HNO3 will re-distribute NO3− to the snow surface, where it can be more efficiently photolyzed. Because of shortened residence times in the snow,
Acknowledgements
We would like to thank the participants of the ‘Alert 2000’ experiment for initial discussions on the idea of snow-piles, and their support; especially P. ‘SnowBlaster’ Shepson, for carrying out the irradiation experiments at the piles using his lamps. Funding for this project was received from the National Science Foundation, Office of Polar Programs and the European Commission (EVK2-1999-00029 ‘NICE’).
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