A reassessment of Antarctic plateau reactive nitrogen based on ANTCI 2003 airborne and ground based measurements
Introduction
During the last decade there has been a growing interest in the atmospheric chemistry of polar regions (Domine’ and Shepson, 2002 and references therein). Among the reasons for this increased interest is the relevance of this chemistry to the interpretation of chemical signatures in polar ice cores. The latter can provide insights about major geophysical events in the earth's past history as well as point to changes that have occurred in the planet's climate (Legrand and Delmas,1987; Legrand and Feniet-Saigne, 1991). An understanding of atmospheric chemistry is relevant because it provides one of the critical inputs for evaluating the air-to-snow “transfer function” for a chemical proxy species. An evaluation of this transfer function requires that all of the processes that modulate the concentration of a chemical species be known from its point of origin to the point of deposition. As applied to the generic species “reactive nitrogen,” earlier efforts to interpret ice cores have resulted in minimal success (Legrand and Kirchner, 1990; Legrand and Mayewski, 1997; Wolff, 1995). Having the ability to interpret the levels of nitrogen in ice cores potentially could provide important insights about the planet's past atmospheric chemical fluctuations, particularly as related to its oxidizing capacity. The significance of “reactive nitrogen” lies in the fact that in the chemical form of NO it has a major impact on the levels of both OH and O3, the two most important oxidizing agents in the earth's atmosphere (Finlayson-Pitts and Pitts, 2000 and references therein).
Reflecting the uncertainties that still exist in interpreting ice-core nitrate levels are several recent studies showing that polar snow fields can release significant emissions of NOx (Honrath et al., 1999, Honrath et al., 2000; Jones et al., 2000; Ridley et al., 2000; Davis et al., 2001; Beine, 2002a, Beine et al., 2002b). One of the major mechanisms identified as being responsible for these emissions is the UV photolysis of nitrate ions in firn (Honrath et al., 1999, Honrath et al., 2000). Possibly even more complex processes may generate still other nitrogen species such as HONO (Zhou et al., 2001; Beine, 2002a, Beine et al., 2002b). Collectively, these field observations have led to a flurry of laboratory studies designed to better understand the detailed photochemical and physical processes operating within polar firn (Dubowski et al., 2001, Dubowski et al., 2002; Boxe et al., 2003; Cotter et al., 2003).
Nowhere has this newly identified polar-atmospheric source of NOx resulted in larger perturbations to the background levels of this species than observed at South Pole (SP), Antarctica (Davis et al., 2001, Davis et al., 2004a, Davis et al., 2004b). In two of the earliest field studies carried out in 1998 and 2000 (e.g., Investigation of Sulfur Chemistry in Antarctica (ISCAT), observed NO levels were found to range from a low of 10 pptv to a high of ∼600 pptv, with median levels estimated at 223 and 86 pptv, respectively. As previously found in the Arctic (Honrath et al., 1999, Honrath et al., 2000), shading experiments at SP, along with measurements of snow surface nitrate levels (Dibb et al., 2004), have all pointed to the importance of nitrate photolysis as the dominant source of NO. Reflecting these enhancements in NO, 24h averaged atmospheric hydroxyl radical (OH) concentrations have been measured at 2×106 molecules cm−3, rivaling in magnitude time averaged values found in the tropical marine boundary layer (Mauldin et al., 2001, Mauldin et al., 2004). Not surprisingly, the near surface atmosphere at SP has also been found to be a net-photochemical source of ozone (Crawford et al., 2001; Chen et al., 2004; Helmig et al., 2007a, Helmig and et al., 2007b).
Davis et al., in their 2004 paper, undertook a detailed examination of the possible causes for the highly elevated levels of NO at SP. Specifically, they explored the possible reasons why the SP environment might support higher NO levels than observed at other polar sites (e.g., Summit, Alert, Neumayer, and Halley Bay) where median values are typically an order of magnitude lower. Using data from the 1998 and 2000 ISCAT studies, Davis et al., 2004a, Davis et al., 2004b concluded that among the important factors favoring SP were: (1) 24 h of continuous sunlight during summer; (2) a strong tendency for shallow planetary boundary layers (PBL) to occur during the summer season; (3) a geographical location that places SP near the base of a large air drainage field; and (4) extremely low temperatures throughout the summer season resulting in very low primary production rates for HOx radicals. The latter two factors are of central importance in that the presence of a drainage basin provides an excellent opportunity for the accumulation of NOx. Very low rates for primary production, of HOx, on the other hand, can lead to the creation of a HOx–NOx chemical system that is highly non-linear. In this system, because NOx can act as a sink for HOx radicals, the NOx lifetime can increase with increasing concentrations of NOx. This, in turn, can lead to rapid increases in NOx when the latter reaches levels of ∼ 250 pptv or higher.
An issue not clearly resolved by the Davis et al. analysis was whether NOx surface emissions at SP might be substantially larger than those at other polar sites. Based on the limited flux studies at each site, it might be concluded that emissions at SP are as much as three times larger than those at Neumayer, Antarctica (Jones et al., 2001); whereas, for Summit (Honrath et al., 2002) it could be argued that they are perhaps a factor of 1.5 larger. Considering, however, that quite different techniques were used in each of these studies, it is still questionable whether the above comparison is meaningful. Among the concerns is the fact that very little effort appears to have been made to obtain high-resolution vertical distribution data on nitrate in firn at each site. As discussed later in the text, this could be a critical factor. Yet another concern is the strong possibility that there might be considerable variability in the NOx emission rate on the plateau and at other sites, both temporally and spatially. More specifically, flux measurements of NOx have only been recorded at the SP's Atmospheric Research Observatory (ARO) and then only over a two week period. Thus, questions have persisted about the station somehow influencing the surrounding environment.
This paper expands on what was learned from the two previous field studies at SP using the results from the ANTCI 2003 field study. In particular, it expands the geographical area over which NO and NOy observations will have been recorded. This was achieved using an airborne sampling platform. The results from this airborne study together with the SP observations have made possible an examination of the following topics: (1) contrasting plateau levels of reactive nitrogen with those measured at coastal sites; (2) examining new photochemical evidence to determine its role in post-depositional losses of nitrate; (3) examining possible species and processes that might provide a basis for reactive nitrogen undergoing multi-recycling events within a given season; and (4) exploring the large scale influence of plateau NOx emissions on the oxidizing properties of this unique environment.
Section snippets
Sampling platforms, measurement techniques and model description
Three independent sampling efforts took place during the ANTCI 2003 field study. The Twin Otter aircraft sampled from 27 November to 6 December 2003 (see Fig. 1 for flight tracks and area of investigation); at SP, ground-based sampling was carried out from 22 November to 31 December; and also at SP, tethered balloon sampling was implemented over the time period of 13 December to 31 December. Airborne chemical measurements included the trace gases NO, NOy, and dimethyl sulfide (DMS). The
Plateau
Although a major focus of this paper is on NO data generated from Twin Otter sampling, the SP ground-based NO observations have also provided an important nitrogen data base. For example, as shown in Fig. 1(b) some of the highest NO values ever measured at the ARO facility were those recorded in late November 2003 (i.e., consistently in excess of 600 pptv (parts per trillion by volume)). The maximum level of ∼1000 pptv (i.e., reached on 25 November) represents the single highest value of NO ever
The Nitrogen Cycle
As presented in the “Introduction” section, the photochemically driven release of reactive nitrogen in polar environments is currently an area of major scientific interest. Much of this interest is focused on the well known chemical impact that NOx has on the critical atmospheric oxidant, OH. Not as well documented but of great interest to glacio-chemists is the potential importance of this process in understanding post-depositional losses of nitrogen from ice fields (De Angelis and Legrand,
Summary and Conclusions
The findings reported here have provided major new insights on the chemical/physical processes controlling the composition of the Antarctic plateaus near surface atmosphere. The airborne measurements, in particular, have removed many of the reservations about the representativeness of the earlier NO observations recorded at SP. We now know that highly elevated NO concentrations can be found over significant areas of the plateau. Still larger regions need to be explored, but the current findings
Acknowledgments
The author Doug Davis would like to express his appreciation to NSF's Office of Polar Programs (Grant # OPP-0230246) for their partial support of this research. He would also like to thank NOAA's CMDL personnel for their support of this research at the ARO facility at SP. Similarly, he is grateful for the dedicated efforts of Biospherical Instrument Inc. personnel in their providing J-value compellations to us based on real-time actinic flux measurements. Finally, he would like to thank the
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