Urban Snowpack ClNO₂ Production and Fate: A One-Dimensional Modeling Study

Abstract

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.

Observation of N2O5 Deposition and ClNO2 Production on the Saline Snowpack

“…The observed N 2 O 5 deposition velocities in Kalamazoo are aligned with the recent numerical modeling by Wang et al . of N 2 O 5 and ClNO 2 during February–March 2016 in Ann Arbor, Michigan.…”

“…In that prior work, the N 2 O 5 deposition velocities over snow were calculated to increase from 0.23 cm s –1 at ∼258 K to 1.06 cm s –1 at 272–280 K. At the higher temperatures, 95% of N 2 O 5 was simulated to undergo hydrolysis following deposition . Our measurements in Kalamazoo generally followed the predictions of the Wang et al modeling; the average N 2 O 5 deposition velocity at 265 K and lower was 0.3 ± 0.2 cm s –1 , compared to the average of 0.5 ± 0.2 cm s –1 (significantly different at p = 0.08) for warmer (>265 K) conditions (Figure ). Increased atmospheric mixing is expected with higher temperatures, as was indeed the case for this study (Figure S7).…”

Coupled Air Quality and Boundary-Layer Meteorology in Western U.S. Basins during Winter: Design and Rationale for a Comprehensive Study

“…Snowpack photochemistry also contributes to near-surface OH through the production and subsequent photolysis of H2O2 and carbonyls, including formaldehyde, acetaldehyde, and acetone (Couch et al 2000). Dinitrogen pentoxide (N2O5) reactions on saline snow grains can result in ClNO2 formation, with the snowpack serving as a net source or sink of ClNO2 depending on temperature (Wang et al 2020).…”

Quantifying Nitrous Acid Formation Mechanisms Using Measured Vertical Profiles During the CalNex 2010 Campaign and 1D Column Modeling

“…One-dimensional chemistry and transport models are an ideal tool to study poorly constrained surface chemistry. A number of 1D models have provided valuable insight into similar atmospheric systems, such as the interaction of snow with the atmosphere (Cao et al, 2014(Cao et al, , 2016Thomas et al, 2011Thomas et al, , 2012Toyota et al, 2014;Wang et al, 2020), forest canopies (Boy et al, 2011), and the marine boundary layer (von Glasow et al, 2002a(von Glasow et al, , 2002b). However, only few studies have addressed the surface chemistry of HONO (Karamchandani et al, 2014;Tsai et al, 2018;Wong et al, 2011Wong et al, , 2013.…”