Abstract
Abstract. Four summer seasons of nitrogen oxide (NO) concentrations were obtained at the South Pole (SP) during the Sulfur Chemistry in the Antarctic Troposphere (ISCAT) program (1998 and 2000) and the Antarctic Tropospheric Chemistry Investigation (ANTCI) in (2003, 2005, 2006–2007). Together, analyses of the data collected from these studies provide insight into the large- to small-scale meteorology that sets the stage for extremes in NO and the significant variability that occurs day to day, within seasons, and year to year. In addition, these observations reveal the interplay between physical and chemical processes at work in the stable boundary layer of the high Antarctic plateau. We found a systematic evolution of the large-scale wind system over the ice sheet from winter to summer that controls the surface boundary layer and its effect on NO: initially in early spring (Days 280–310) the transport of warm air and clouds over West Antarctica dominates the environment over the SP; in late spring (Days 310–340), the winds at 300 hPa exhibit a bimodal behavior alternating between northwest and southeast quadrants, which is of significance to NO; in early summer (Days 340–375), the flow aloft is dominated by winds from the Weddell Sea; and finally, during late spring, winds aloft from the southeast are strongly associated with clear skies, shallow stable boundary layers, and light surface winds from the east – it is under these conditions that the highest NO occurs. Examination of the winds at 300 hPa from 1961 to 2013 shows that this seasonal pattern has not changed significantly, although the last twenty years have seen an increasing trend in easterly surface winds at the SP. What has also changed is the persistence of the ozone hole, often into early summer. With lower total ozone column density and higher sun elevation, the highest actinic flux responsible for the photolysis of snow nitrate now occurs in late spring under the shallow boundary layer conditions optimum for high accumulation of NO. This may occur via the non-linear HOX–NOx chemistry proposed after the first ISCAT field programs and NOx recycling to the surface where quantum yields may be large under the low-snow-accumulation regime of the Antarctic plateau. During the 2003 field program a sodar made direct measurements of the stable boundary layer depth (BLD), a key factor in explaining the chemistry of the high NO concentrations. Because direct measurements were not available in the other years, we developed an estimator for BLD using direct observations obtained in 2003 and step-wise linear regression with meteorological data from a 22 m tower (that was tested against independent data obtained in 1993). These data were then used with assumptions about the column abundance of NO to estimate surface fluxes of NOx. These results agreed in magnitude with results at Concordia Station and confirmed significant daily, intraseasonal and interannual variability in NO and its flux from the snow surface. Finally, we found that synoptic to mesoscale eddies governed the boundary layer circulation and accumulation pathways for NO at the SP rather than katabatic forcing. It was the small-scale features of the circulation including the transition from cloudy to clear conditions that set the stage for short-term extremes in NO, whereas larger-scale features were associated with more moderate concentrations.
Highlights
The Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) field programs, in the austral summers of 1998 and 2000 (Davis et al, 2004a, 2001), discovered unexpectedly high atmospheric nitrogen oxide (NO) concentrations at the South Pole (SP)
Using four spring-to-summer seasons of observations, we have described the influence of the synoptic-scale to mesoscale weather patterns and their seasonal cycle on stable boundary layer characteristics at the South Pole (SP) in the spring–summer period that set the stage for high NO episodes
Visualizations of near-surface airflow using ERA-I (Dee et al, 2011) revealed complex mesoscale circulations that belied any simple explanation of accumulation pathways for NO
Summary
The Investigation of Sulfur Chemistry in the Antarctic Troposphere (ISCAT) field programs, in the austral summers of 1998 and 2000 (Davis et al, 2004a, 2001), discovered unexpectedly high atmospheric nitrogen oxide (NO) concentrations at the South Pole (SP). The weather cycle typically includes a period of advection of warm air, higher wind speeds, and clouds with a deep boundary layer followed by clearing skies, strong radiative surface cooling, and formation of shallow statically stable boundary layers (Neff, 1999). Because of this combination the surface typically suffers a net loss of energy under clear sky conditions (Stone and Kahl, 1991) even at the summer solstice This effect is in contrast with summer observations at Concordia Station (∼ 75◦ S, 3233 m a.s.l.: diurnal solar elevation at solstice 8.5◦ to 38.5◦) and Dome Argus (∼ 80◦ S, 4093 m a.s.l.: diurnal solar elevation at solstice 13.5◦ to 33.8), which show a diurnal cycle of thermal convection followed by the formation of a stable boundary layer during the “evening (low sun elevation)” period (Frey et al, 2015) (Bonner, 2015; Bonner et al, 2010). For these reasons air masses arriving over long distances at the SP are likely to have encountered some convective mixing enroute and may need to be considered in future model simulations of surface chemistry
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