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

Abstract. Triple oxygen isotopic compositions (Δ 17 O  =  δ 17 O − 0.52  ×   δ 18 O) of atmospheric sulfate (SO 4 2− ) and nitrate (NO 3 − ) 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 Δ 17 O(SO 4 2− ) and Δ 17 O(NO 3 − ) values in those model estimations. To test one of those assumption that Δ 17 O values of ozone (O 3 ) have a flat value and do not influence the seasonality of Δ 17 O(SO 4 2− ) and Δ 17 O(NO 3 − ) values, we performed the first simultaneous measurement of Δ 17 O values of atmospheric sulfate, nitrate, and ozone collected at Dumont d'Urville (DDU) Station (66°40′ S, 140°01′ E) throughout 2011. Δ 17 O 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, Δ 17 O values of ozone showed no significant seasonal variation, with values of 26 ± 1 ‰ throughout the year. These contrasting seasonal trends suggest that seasonality in Δ 17 O(SO 4 2− ) and Δ 17 O(NO 3 − ) values is not the result of changes in Δ 17 O(O 3 ), but of the changes in oxidation chemistry. The trends with summer minima and winter maxima for Δ 17 O(SO 4 2− ) and Δ 17 O(NO 3 − ) values are caused by sunlight-driven changes in the relative contribution of O 3 oxidation to the oxidation by HO x , RO x , and H 2 O 2 . In addition to that general trend, by comparing Δ 17 O(SO 4 2− ) and Δ 17 O(NO 3 − ) values to ozone mixing ratios, we found that Δ 17 O(SO 4 2− ) 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 Δ 17 O(NO 3 − ) values. The relatively lower sensitivity of Δ 17 O(SO 4 2− ) values to the ozone mixing ratio in spring compared to fall is possibly explained by (i) the increased contribution of SO 2 oxidations by OH and H 2 O 2 caused by NO x emission from snowpack and/or (ii) SO 2 oxidation by hypohalous acids (HOX  =  HOCl + HOBr) in the aqueous phase.