Springtime Nitrogen Oxide-Influenced Chlorine Chemistry in the Coastal Arctic

Atomic chlorine (Cl) is a strong atmospheric oxidant that shortens the lifetimes of pollutants and methane in the springtime Arctic, where the molecular halogens Cl₂ and BrCl are known Cl precursors. Here, we quantify the contributions of reactive chlorine trace gases and present the first observations, to our knowledge, of ClNO₂ (another Cl precursor), N₂O₅, and HO₂NO₂ in the Arctic. During March – May 2016 near Utqiaġvik, Alaska, up to 21 ppt of ClNO₂, 154 ppt of Cl₂, 27 ppt of ClO, 71 ppt of N₂O₅, 21 ppt of BrCl, and 153 ppt of HO₂NO₂ were measured using chemical ionization mass spectrometry. The main Cl precursor was calculated to be Cl₂ (up to 73%) in March, while BrCl was a greater contributor (63%) in May, when total Cl production was lower. Elevated levels of ClNO₂, N₂O₅, Cl₂, and HO₂NO₂ coincided with pollution influence from the nearby town of Utqiaġvik and the North Slope of Alaska (Prudhoe Bay) Oilfields. We propose a coupled mechanism linking NO_x with Arctic chlorine chemistry. Enhanced Cl₂ was likely the result of the multiphase reaction of Cl^–_(aq) with ClONO₂, formed from the reaction of ClO and NO₂. In addition to this NO_x-enhanced chlorine chemistry, Cl₂ and BrCl were observed under clean Arctic conditions from snowpack photochemical production. These connections between NO_x and chlorine chemistry, and the role of snowpack recycling, are important given increasing shipping and fossil fuel extraction predicted to accompany Arctic sea ice loss.