Assessing the Seasonal Dynamics of Nitrate and Sulfate Aerosols at the South Pole Utilizing Stable Isotopes

Abstract

Atmospheric nitrate (NO3− = particulate NO3− + gas‐phase nitric acid [HNO3]) and sulfate (SO42−) are key molecules that play important roles in numerous atmospheric processes. Here, the seasonal cycles of NO3− and total suspended particulate sulfate (SO42−(TSP)) were evaluated at the South Pole from aerosol samples collected weekly for approximately 10 months (26 January to 25 October) in 2002 and analyzed for their concentration and isotopic compositions. Aerosol NO3− was largely affected by snowpack emissions in which [NO3−] and δ15N(NO3−) were highest (49.3 ± 21.4 ng/m3, n = 8) and lowest (−47.0 ± 11.7‰, n = 5), respectively, during periods of sunlight in the interior of Antarctica. The seasonal cycle of Δ17O(NO3−) reflected tropospheric chemistry year‐round with lower values observed during sunlight periods and higher values observed during dark periods, reflecting shifts from HOx‐ to O3‐dominated oxidation chemistry. SO42−(TSP) concentrations were highest during austral summer and fall (86.7 ± 73.7 ng/m3, n = 18) and are indicated to be derived from dimethyl sulfide (DMS) emissions, as δ34S(SO42−)(TSP) values (18.5 ± 1.0‰, n = 10) were similar to literature δ34S(DMS) values. The seasonal cycle of Δ17O(SO42−)(TSP) exhibited minima during austral summer (0.9 ± 0.1‰, n = 5) and maxima during austral fall (1.3 ± 0.3‰, n = 6) and austral spring (1.6 ± 0.1‰, n = 5), indicating a shift from HOx‐ to O3‐dominated chemistry in the atmospheric derived SO42− component. Overall, the budgets of NO3− and SO42−(TSP) at the South Pole were complex functions of transport, localized chemistry, biological activity, and meteorological conditions, and these results will be important for interpretations of oxyanions in ice core records in the interior of Antarctica.

Regional Characteristics of Atmospheric Sulfate Formation in East Antarctica Imprinted on 17O‐Excess Signature

“…(2017) and Walters et al. (2019) suggested the possibility of an increased contribution of S(IV) + HOBr during austral spring at DDU and summer at South Pole, respectively, based on the relatively low Δ 17 O(SO 4 2− ) in those seasons. However, the importance of S(IV) + HOBr remains inconclusive, since both results in those two previous works can also be explained by the contribution of OH and H 2 O 2 oxidation.…”

“…There are only a few reports of Δ 17 O(SO 4 2− ) observations in the present Antarctic atmosphere, and little is known about the influence of characteristic oxidation processes in Antarctica on Δ 17 O(SO 4 2− ). Δ 17 O(SO 4 2− ) observations of aerosols at three different sites, Dome C (Hill‐Falkenthal et al., 2013) and South Pole (Walters et al., 2019) on the Antarctic Plateau and coastal Antarctic station Dumont d'Urville (DDU) (Ishino et al., 2017), show similar seasonality with minima in the austral summer and higher values in the autumn to spring, which likely reflects a seasonal shift from OH‐ and H 2 O 2 ‐ to O 3 ‐dominated chemistry. In addition, Ishino et al.…”

“…The contribution of this reaction is thus expected to be also significant in the Antarctic troposphere. ) observations of aerosols at three different sites, Dome C (Hill-Falkenthal et al, 2013) and South Pole (Walters et al, 2019) on the Antarctic Plateau and coastal Antarctic station Dumont d'Urville (DDU) (Ishino et al, 2017), show similar seasonality with minima in the austral summer and higher values in the autumn to spring, which likely reflects a seasonal shift from OH-and H 2 O 2 -to O 3 -dominated chemistry. In addition, Ishino et al (2017) and Walters et al (2019) suggested the possibility of an increased contribution of S(IV) + HOBr during austral spring at DDU and summer at South Pole, respectively, based on the relatively low Δ 17 O(SO 4 2−…”

“…Based on such results, they pointed out deficiencies in the understanding of sulfate formation other than the recognized SO 2 oxidation by OH, H 2 O 2 , and O 3 . Furthermore, there is increasing evidence for a significant difference between the Δ 17 O(SO 4 2− ) in aerosol samples (ca., 1.5‰;Hill-Falkenthal et al, 2013;Ishino et al, 2017;Walters et al, 2019) and those in ice cores corresponding to the present-day climate conditions (ca., 3‰;Alexander et al, 2002Alexander et al, , 2003Kunasek et al, 2010;Sofen et al, 2014). Despite the significance of this ∼1.5‰ shift in Δ 17 O(SO 4 2− ) compared to the observed variability in ice cores (1.3‰-4.8‰ for glacial-interglacial time scale), there has been no study pointing it out so far.…”

Comment on acp-2020-1279

“…The mass-independent oxygen-17 anomaly (D 17 O) is any deviation from the linear approximation of δ 17 O = 0.52 ´ δ 18 O, a relationship that describes the mass-dependent process, and can be quantified as D 17 O = δ 17 O -0.52 ´ δ 18 O (Thiemens, 1999), wherein δ 17,18 O = [( 17,18 O/ 16 O)sample/( 17,18 O/ 16 O)VSMOW -1]. Since it was discovered to be produced during the chemical formation of ozone (O3) for the first time in 1980s, (Thiemens and Heidenreich, 1983), the D 17 O has been studied extensively and proven to be a powerful tool in discerning the formation mechanisms of atmospheric sulfate (e.g., Ishino et al, 2017;Alexander et al, 2002Alexander et al, , 2005Alexander et al, , 2009Alexander et al, , 2012Li et al, 2013;Jenkins and Bao, 2006;He et al, 2018;McCabe et al, 2006;Lee and Thiemens, 2001;Dominguez et al, 2008;Walters et al, 2019;Lin et al, 2017;Lin et al, 2020). Because the oxidants transfer unique D 17 O signal to the produced sulfate (Savarino et al, 2000), D 17 O in sulfate (D 17 O(SO 2-4 )) reflects the relative importance of various oxidation pathways involved in its formation.…”