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 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-enhanced chlorine chemistry, Cl₂ and BrCl were observed under clean Arctic conditions from snowpack photochemical production. These connections between NO 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.