Urban Snowpack ClNO₂Production and Fate: A One-Dimensional Modeling Study

Nitryl chloride (ClNO2) is formed in urban areas from the multiphase reaction of dinitrogen pentoxide (N2O5) on chloride-containing surfaces. ClNO2 undergoes photolysis to produce atomic chlorine (Cl•), a strong atmospheric oxidant. While previous ClNO2 studies have focused on atmospheric particulate chloride, the saline snowpack in locations impacted by sea spray and road salt usage represents an additional, potentially large, source of ClNO2. Here, we present the first modeling study to explore the production of ClNO2 from the inland urban snowpack. The coupled snowpack-atmospheric one-dimensional model is constrained to and evaluated by an array of ambient measurements in Ann Arbor, Michigan, during February 2016. The model predicts strong N2O5 deposition onto the snowpack, with ClNO2 formation and release to the atmosphere at low temperatures (<∼260 K). However, at higher temperatures (>∼270 K), the ClNO2 yield is low (e.g., 10%), with ClNO2 undergoing hydrolysis on the snow grains, making the snowpack a net sink for ClNO2. These results motivate measurements to quantify ClNO2 production from the urban snowpack because of potential broader impacts on atmospheric composition and air quality.