Seasonal variations of triple oxygen isotopic compositions of atmospheric sulfate, nitrate, and ozone at Dumont d'Urville, coastal Antarctica

Abstract. Triple oxygen isotopic compositions (Δ¹⁷O  = δ¹⁷O − 0.52  ×  δ¹⁸O) of atmospheric sulfate (SO₄²⁻) and nitrate (NO₃⁻) in the atmosphere reflect the relative contribution of oxidation pathways involved in their formation processes, which potentially provides information to reveal missing reactions in atmospheric chemistry models. However, there remain many theoretical assumptions for the controlling factors of Δ¹⁷O(SO₄²⁻) and Δ¹⁷O(NO₃⁻) values in those model estimations. To test one of those assumption that Δ¹⁷O values of ozone (O₃) have a flat value and do not influence the seasonality of Δ¹⁷O(SO₄²⁻) and Δ¹⁷O(NO₃⁻) values, we performed the first simultaneous measurement of Δ¹⁷O values of atmospheric sulfate, nitrate, and ozone collected at Dumont d'Urville (DDU) Station (66°40′ S, 140°01′ E) throughout 2011. Δ¹⁷O values of sulfate and nitrate exhibited seasonal variation characterized by minima in the austral summer and maxima in winter, within the ranges of 0.9–3.4 and 23.0–41.9 ‰, respectively. In contrast, Δ¹⁷O values of ozone showed no significant seasonal variation, with values of 26 ± 1 ‰ throughout the year. These contrasting seasonal trends suggest that seasonality in Δ¹⁷O(SO₄²⁻) and Δ¹⁷O(NO₃⁻) values is not the result of changes in Δ¹⁷O(O₃), but of the changes in oxidation chemistry. The trends with summer minima and winter maxima for Δ¹⁷O(SO₄²⁻) and Δ¹⁷O(NO₃⁻) values are caused by sunlight-driven changes in the relative contribution of O₃ oxidation to the oxidation by HO_x, RO_x, and H₂O₂. In addition to that general trend, by comparing Δ¹⁷O(SO₄²⁻) and Δ¹⁷O(NO₃⁻) values to ozone mixing ratios, we found that Δ¹⁷O(SO₄²⁻) values observed in spring (September to November) were lower than in fall (March to May), while there was no significant spring and fall difference in Δ¹⁷O(NO₃⁻) values. The relatively lower sensitivity of Δ¹⁷O(SO₄²⁻) values to the ozone mixing ratio in spring compared to fall is possibly explained by (i) the increased contribution of SO₂ oxidations by OH and H₂O₂ caused by NO_x emission from snowpack and/or (ii) SO₂ oxidation by hypohalous acids (HOX  =  HOCl + HOBr) in the aqueous phase.