<p><strong>Abstract.</strong> Little attention has so far been paid to the nighttime atmospheric chemistry of iodine species. Current atmospheric models predict a buildup of HOI and I₂ during the night that leads to a spike of IO at sunrise, which is not observed by measurements. In this work, electronic structure calculations are used to survey possible reactions that HOI and I₂ could undergo at night in the lower troposphere, and hence reduce their nighttime

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... The new reaction NO₃+<span class="thinspace"></span>HOI<span class="thinspace"></span> → <span class="thinspace"></span>IO<span class="thinspace"></span>+<span class="thinspace"></span>HNO₃ is proposed, with a rate coefficient calculated from statistical rate theory over the temperature range 260–300<span class="thinspace"></span>K and at a pressure of 1000<span class="thinspace"></span>hPa to be <i>k</i>(<i>T</i>) = <span class="thinspace"></span>2.7<span class="thinspace"></span> × <span class="thinspace"></span>10⁻¹² (300<span class="thinspace"></span>K/<i>T</i>)^2.66<span class="thinspace"></span>cm³<span class="thinspace"></span>molecule⁻¹<span class="thinspace"></span>s⁻¹. This reaction is included in two atmospheric models, along with the known reaction between I₂ and NO₃, to explore a new nocturnal iodine radical activation mechanism. The results show that this iodine scheme leads to a considerable reduction of nighttime HOI and I₂, which results in the enhancement of more than 25<span class="thinspace"></span>% of nighttime ocean emissions of HOI<span class="thinspace"></span>+<span class="thinspace"></span>I₂ and the removal of the anomalous spike of IO at sunrise. We suggest that active nighttime iodine can also have a considerable, so far unrecognized, impact on the reduction of the NO₃ radical levels in the marine boundary layer (MBL) and hence upon the nocturnal oxidizing capacity of the marine atmosphere. The effect of this is exemplified by the indirect effect on dimethyl sulfide (DMS) oxidation.</p>