Atmospheric near‐surface nitrate at coastal Antarctic sites

Abstract

Sampling Sites and MethodsThe 2. The seasonal change in the occurrence and strength of the surface inversion may contribute to the variability of all short-lived species in the boundary layer except locally produced sea-salt aerosol. As summarized by K6nig-Langlo et al.[this issue], the period from November to February shows a relatively weak or absent surface inversion at NM and DDU but generally stronger temperature gradients at NM (due to radiative cooling). Cyclonic activity, however, may regularly interrupt the isolation from higher atmospheric levels •' at both sites throughout all seasons.3. The surface'wind system is dominated by zonal flow at NM but by a meridional one at DDU and Mawson [Parish, 1988]. The latter two sites are therefore more directly influenced by air masses flowing down from the Antarctic plateau and by upper tropospheric air from midlatitudes eventually entrained in the downslope winds.In summary, the climatologies at DDU and Mawson are similar, whereas NM is characterized by its ice shelf position (leading to zonal winds) and by its relatively more frequent location inside the polar vortex. Palmer station stands out by its position on the Antarctic Peninsula and reflects mainly the subpolar maritime regime. Sampling and AnalysesDetails on the sampling procedure and analytical methods used for aerosol chemical analyses are summarized in Table 1 2.2.1. Assessing local contamination at DDU. The very small size of the island (Ile des P6trels) on which DDU is located makes it impossible to define a "clean air sector" there. In order to estimate the effects both of human activities (which confined to the summer period and to individual filters with very low sea-salt loads. On the other hand, the HNO 3 collection efficiency for the backup aerosol filter was found to be between 60 and 90%, allowing resampling of most of the evaporated nitrate. Again, inspection of the summer nitrate variability at NM did not reveal a systematic relationship to the sea-salt or acidity loads of the filters. In particular, the general nitrate trend during the summer season is not substantially influenced by the dramatic change in the aerosol acidity during that season. We cannot exclude, however, the possibility that some filters collected during the acidity peak in January under low sea-salt loads may underestimate the total atmospheric nitrate concentration. We expect that the same holds true for the total nitrate data reported by Savoie et al. [1992] from Mawson station since sea salt is lower there by a factor of-4 compared to NM and since nonhygroscopic polycarbonate fibre filters are used.In conclusion, total atmospheric nitrate records reported here are affected to a minor extent by year-round contamination events at DDU and by HNO 3 remobilization from summer filters at NM, respectively. None of these shortcomings occurring on individual filter samples could, however, substantially perturb the observed mean nitrate levels and their seasonal patterns. ResultsBecause of the large difference in the sampl...

Seasonally resolved ice core records from West Antarctica indicate a sea ice source of sea‐salt aerosol and a biomass burning source of ammonium

“…Moderate and low snow accumulation sites including those on the East Antarctic Plateau (EAP) do not preserve the annual cycle well because NO 3 À is reemitted to the atmosphere before it is buried by subsequent snowfall [Goktas et al, 2002;Röthlisberger et al, 2002a]. The annual cycle of nitrate in this study is also similar to that observed in boundary layer air studies at Neumayer, Mawson, Halley, and Dumont d'Urville, and Kohnen Stations [Wagenbach et al, 1998b;Weller and Wagenbach, 2007], although at Neumayer and Mawson the timing of the primary atmospheric peak occurs in November and an additional secondary peak in the fall is typically observed [Wagenbach et al, 1998b].…”

“…The depth of these samples did not exceed 15 m. Figure S4 shows essentially the same pattern at each site in the well-resolved, high-accumulation years of any age. A similar pattern in NO 3 À concentration, with a primary peak in summer and a secondary peak in late winter, has been observed in freshly fallen snow samples from coastal sites at Halley station and Neumayer station Wagenbach et al, 1998b], and from firn and ice core records from Law Dome [Curran et al, 1998;McMorrow et al, 2004]. Moderate and low snow accumulation sites including those on the East Antarctic Plateau (EAP) do not preserve the annual cycle well because NO 3 À is reemitted to the atmosphere before it is buried by subsequent snowfall [Goktas et al, 2002;Röthlisberger et al, 2002a].…”

“…The subannual nitrate signal is well resolved within the top~15 m of the snowpack at the sites it this study, which contain~20 years of data and are easily accessible by hand auger or mechanical drill. It may be possible to gain new insights into the sources of NO 3 À in Antarctica using ice cores from the WAIS by analyzing the isotope composition of NO 3 À , stratospheric tracers such as 10 Be and 210 Pb, or other analytes from the many years of seasonally resolved samples, similar to what has been done in bulk aerosol studies [e.g., Wagenbach et al, 1998b].…”

“…Ammonium (NH 4 + ) previously has been observed to co-occur with biogenic sulfur in the summertime [Legrand et al, 1998;Savoie et al, 1992], and nitrate (NO 3 À ) has shown a late-polar stratospheric clouds (PSCs) and snowpack remission during late winter/ spring and summer, respectively [Jones et al, 2011;Savarino et al, 2007;Wagenbach et al, 1998b;Weller et al, 2002]; however, debate still exists regarding the origin of the summertime peak in high-accumulation coastal locations and the relative magnitude of tropospheric versus stratospheric sources [Frey et al, 2009;Lee et al, 2014]. The chloride to sodium ratio (Cl À /Na + ) of aerosols at coastal Antarctic sites has shown a minimum in summer attributed to displacement of volatile chlorine (primarily hydrochloric acid, HCl) from sea-salt aerosols and a maximum in winter that is interpreted as fractionation from mirabilite (Na 2 SO 4 ·10H 2 O) mineral precipitation in sea ice brine prior to aerosol formation [Hara et al, 2004[Hara et al, , 2012[Hara et al, , 2013Jourdain and Legrand, 2002].…”

Iodine monoxide in the Antarctic snowpack

“…Nitrate and MSA formation are both dependent on photolytic processes, with MSA being a product of dimethyl sulphide oxidation by OH radicals during abstraction and addition oxidation pathways (von Glasow and Crutzen, 2004). It is thought that most of the nitrate excess (compared to background winter levels) observed during spring and summer originates from sedimentation from the stratosphere, and to a lesser degree (and perhaps latter in the summer season) from photolytic release of nitrate from the snow pack (Wagenbach et al, 1998;Jones et al, 2001;Weller et al, 2004;Savarino et al, 2007). Thus these three independent indicators should give a reliable representation of summer/winter stratification in the snow pit, and can be clearly seen in Fig.…”