Substantial contribution of iodine to Arctic ozone destruction 

<p class="western" lang="en-US" align="justify">While the importance of bromine chemistry for the Arctic surface ozone budget is well documented, the effect of iodine chemistry has received less attention. Here we present observations performed as part of the ship-based Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, where halogen oxides were measured during the sunlit period from March to October 2020. Ozone shows drastic loss during the boreal spring (March, April and May), when near-complete depletion is regularly observed, highlighting that ozone depletion in the central Arctic is widespread. The drastic ozone depletion coincides with &#8216;bromine explosion&#8217; episodes, when bromine oxide (BrO) reaches values as high as 14.8&#177;0.8pptv. In contrast, observations of IO show the presence of active iodine chemistry on most days during the whole sunlit period (including boreal spring, summer and autumn), presenting elevated levels of IO (usually between 0.2 and 1 pptv) that peak during spring (2.9&#177;0.3 pptv). We find that chemical reactions between iodine and ozone are the second highest contributor to ozone loss over the study period, after ozone photolysis-initiated loss and ahead of bromine.</p>