Diurnal and Seasonal V ariations of Nitrogen O xides Within S nowpack A ir

1. Diurnal and Seasonal Variations of Nitrogen Oxides Within Snowpack Air and the Overlying Atmosphere at Summit, Greenland C. Toro1, R.E. Honrath1†, L.J. Kramer1, D.Helmig2, B.A. Van Dam2, B. Seok2, L. Ganzeveld3, P.V. Doskey1 Overview Results Diurnal cycle of NOx : seasonal and vertical variations through the snowpack, and emissions to the overlying atmosphere. Response of NO2 and NO to Wind Speed, Snowpack Temperature and Ozone. Seasonal behavior of NOx enhancements (snowpack concentrations minus ambient mixing ratios measured at 2 m above snow ). The Arctic is one of the Earth’s most sensitive regions to climate change. Reductions in snow cover concomitant with a warming atmosphere will affect cryosphere-atmosphere exchange of trace gases; thus, improved understanding of atmospheric and surface processes is required to predict future impacts of climate change on the atmospheric chemistry of the Arctic. Our research focuses on snowpack chemistry and air-snow exchange of nitrogen oxides and ozone. Specific goals of the research are to improve representations of surface and atmospheric processes that influence the oxidative capacity of the Arctic atmosphere and to develop and evaluate global chemistry-climate models that simulate effects of snowpack processes on the overlying atmosphere. Here, we present the first set of continuous measurements of nitrogen oxides within and above snowpack that detail the annual cycle of the trace gases at Summit, Greenland. Seasonal and diurnal variations of the reactive species are discussed. GeoSummit Flux Facility Late Summer Aug 02, 2009-Aug 07, 2009 Early Spring April 27, 2009-May 05, 2009 a. a. a. NO2 NO NO2 NO b. The GEOSummit Flux Facility was designed for air-snow exchange studies. Conduits for sampling tubes and cables run from the 10 m tower to a heated, insulated facility that is located under the snow (downwind from the tower during clean sector flow), and which is accessed through the entryway visible on the right of the photo. b. c. b. Figure 2: a.Time series of daily NO2 and NO enhancements (left and right, respectively) at the major activity levels for each specie, where level 4 is at 110 cm, 5 at 80 cm, 6 at 50 cm and 7 at 20 cm from the snowpack surface. b. Contour plot of daily NOx enhancements inside and above the snowpack. The depth of the lowest inlet was used as reference and expressed as 0 m. The surface of the snowpack is over plotted with black dots. From August 21, 2009 the plot shows additional data from 6 new inlets that were installed at that time. Experimental Site Description Figure 4: a. Diurnal cycles for NO2 and NO enhancements at different depths, during two periods: early spring and late summer. Data is 4 hr averaged and all times are expressed as local solar time. b. Two-hourly averaged data for NOx at three different heights above the snowpack. Values are expressed as NOx with daily median subtracted. d. Conclusions: Sections I & II of Results • NO2 represents most of NOx in the snowpack at Summit, with a maximum approximately 4-5 times higher compared to the maximum period of NO production. NO2 profile is clearly shifted to early spring, while NO follows the radiation profile with a peak around June-July (see Figure 2a). • NO is produced at the upper levels of the snowpack where NO2 photolysis is possible. At these levels, ventilation and NO emission to the atmosphere can occur, explaining why negative values of NO enhancements are observed at deep levels during the peak period of NO production. Low values of NO2 are observed in the upper layers, most likely to ventilation and photolysis (see Figures 2a, 3a and 3b). • High wind speed events increase the transport and mixing of gases through wind pumping mechanism. This is clearly observed in NO2 profiles, mainly due to the high concentrations of this specie in the snowpack (see Figure 3e). • No evidence of a strong relationship between NOx and ozone. Minimum values of O3 are observed during maximum of NO production, but the magnitude of this last specie does not account for the decrease in ozone concentrations. Conclusions: section III of Results • Diurnal cycles for NO and NO2 are clear during both periods, with profiles varying with availability of radiation and with depth. • NO enhancements observed right above the snow (see Figure 4a, first row) follows perfectly the radiation profile in both periods. On the other hand, NO2 diurnal cycle at the same level is slightly offset during early spring, but in late summer the peak is observed at times of maximum isolation. This same behavior is also observed in the first 50 cm of the snowpack (see Figure 4a, second row). • At deeper levels (see Figure 4a, third row), NO2 maximums are observed at night during early spring, while in late summer all levels reach their maximum values earlier in the day, though not coincident with maximum isolation hours. NO at 80 cm in the snowpack peaks with radiation in both periods analyzed, with NO at 110 cm also peaking with radiation in late summer. • Snowpack emissions can eventually be transported downwind at Summit. • Figure 4b shows NOx mixing ratios at three levels above the snowpack, and at all these heights is possible to observe a diurnal cycle, indicative that emissions from the snowpack are possible to observe as high as 9 m, especially in early spring, and thus possibly transported downwind. To the right is a picture of the snowtower with two, air inlets at levels separated by 30 cm between them. The total flow (~2 lpm) is divided between the inlets to minimize disturbance of interstitial air. The experiment started in June 2008 with 8 levels on the snowtower. Six new levels were added in August 2009, as the inlets were buried by accumulating snow. As of February 2010, the snowtower has a vertical profile of 4.2 meters with 2.5 meters into the snowpack. e. Figure 3:a. Contour plot of daily NO2 enhancements. b. Contour plot of daily NO enhancements. c. Contour plot for daily O3mixing ratios. d. Snowpack temperature (°C, daily averages). e. Major daily enhancements events for NO2 compared with wind speed data, averaged over 24 hr. Figure 1: Snow Tower Acknowledgements: This work is funded by National Science Foundation grants ARC0713992 and ARC0713943. Michael Dziobak provided essential instrumentation and field support. We especially thank the Summit Science Technicians and support staff and the Flux Facility design and construction team for their assistance.