Snowpack Chemistry Emerges as Hidden Engine of Winter Air Pollution in Northeast China

The pristine white blanket of snow that covers northeastern China each winter has long been seen as a passive backdrop to the region’s notorious smog episodes. A new modeling study published in The Cryosphere argues that it is anything but passive. A team led by Shengjin Xie and Xuelei Zhang of the Chinese Academy of Sciences’ Northeast Institute of Geography and Agroecology shows that chemical reactions taking place on snow-covered ground surfaces are a major, previously undercounted source of nitryl chloride, a reactive molecule that fuels the formation of fine particulate matter and ozone. According to the simulations, snow-surface chemistry alone accounts for roughly 28 percent of the nighttime buildup of nitryl chloride across Northeast China, a finding that could force air quality forecasters to rethink how they treat frozen landscapes in pollution models.

The chemistry at the heart of the study begins after dark. Dinitrogen pentoxide, or N2O5, is a key player in nocturnal tropospheric chemistry, forming when nitrogen oxides react with the nitrate radical under dark conditions. When N2O5 collides with chloride-laden surfaces, whether airborne aerosol particles, saline snowpack, vegetation, buildings, or soil, it can undergo heterogeneous hydrolysis to produce nitryl chloride, ClNO2. This molecule acts as a reservoir for reactive chlorine: after sunrise, sunlight photolyzes ClNO2, releasing highly reactive chlorine atoms. Those chlorine radicals are ferocious oxidants, attacking alkanes at rates roughly two orders of magnitude faster than the hydroxyl radical, the atmosphere’s usual cleaning agent. The result is a cascade of reactions that can enhance the atmospheric oxidizing capacity and drive the formation of secondary pollutants, including fine particles and ground-level ozone.

What has remained murky is the role of snow itself. Field studies in high-latitude regions have previously confirmed that ice and snow surfaces catalyze the heterogeneous hydrolysis of N2O5 and that saline snowpack can emit an upward net flux of ClNO2. One-dimensional models have suggested that urban winter snowpack could contribute up to 60 percent of near-surface ClNO2. But such models struggle to capture the full complexity of air-snow exchange. The new work takes a decisive step forward by embedding surface chemistry into a fully three-dimensional air quality model, the WRF-CAMx system, allowing bidirectional fluxes between the atmosphere and the ground to be represented across an entire region rather than a single vertical column.

The study region could hardly be better suited to the question. Northeast China endures months of continuous snow cover, and its heating season stretches for nearly half the year. Large-scale coal burning during that period releases substantial quantities of particulate chloride and hydrogen chloride into the atmosphere. Some of these chlorides adsorb onto particle surfaces or deposit onto the snowpack, where they become feedstock for N2O5 reactions. To anchor the modeling, the team conducted field observations from 23 February to 3 March 2024 at a station on experimental farmland in the northern suburbs of Changchun, using an iodide-adduct chemical ionization mass spectrometer to measure N2O5 and ClNO2 amid recurrent snowfall that maintained continuous snow cover.

The observations revealed textbook diurnal cycles: N2O5 concentrations averaged 58.73 pptv and ranged as high as 424.15 pptv, while ClNO2 averaged 102.64 pptv with a peak of 902.45 pptv on 27 February. Both species accumulated at night and collapsed after dawn as photolysis destroyed ClNO2 and released chlorine atoms. Crucially, the data showed that ClNO2 formation was limited by the availability of N2O5 rather than by chloride supply. During one episode, when N2O5 mixing ratios stayed below roughly 120 pptv, no ClNO2 peaks appeared despite abundant chloride, evidence that the region’s chloride-containing aerosols provide a non-limiting reservoir of reactive chlorine throughout the winter.

On the modeling side, the researchers confronted a long-standing discrepancy between laboratory and field estimates of the N2O5 uptake coefficient, the probability that a colliding N2O5 molecule is irreversibly taken up by a surface. The widely used Bertram and Thornton parameterization, derived from chamber experiments, has consistently overestimated uptake compared with ambient measurements. The team therefore compared it against the YU20 scheme, which was fitted to field observations from multiple Chinese sites. Under snow-covered conditions, YU20 clearly outperformed BT09: for N2O5, the mean bias dropped from 114.62 to 69.66 pptv and the index of agreement rose from 0.49 to 0.61, while for ClNO2 the mean bias improved from −29.28 to −16.08 pptv. The researchers attribute the difference to the altered humidity and elevated anthropogenic chlorine emissions that reshape aerosol composition under snowy winter conditions, and they applied YU20 to snow-covered grid cells and BT09 elsewhere.

The most striking gains came from adding ground-surface chemistry. The team modified CAMx’s surface chemistry module to dynamically calculate surface-to-volume ratios from three-dimensional surfaces within each grid cell, drawing on field surveys and satellite retrievals of building areas across Northeast China, and to include N2O5 reactions on snow, soil, vegetation, and buildings. Before the revision, the model badly underestimated ClNO2, with a mean bias of −105.78 pptv and an index of agreement of just 0.39. After incorporating anthropogenic chlorine emissions and surface reactions, the mean bias shrank to 2.66 pptv and the index of agreement climbed to 0.86, while the root mean square error fell from 170.26 to 88.28 pptv. Simulated ClNO2 averages rose from 7.73 to 110.30 pptv, closely matching observations. Improvements rippled through secondary pollutants as well: for PM2.5 the mean bias improved from −10.56 to −1.39 micrograms per cubic meter, and for ozone from 19.42 to 13.91.

Quantifying the individual contributions, the researchers found that anthropogenic chlorine emissions alone raised ClNO2 by up to 151.67 pptv, with contributions exceeding 50 percent across most of the region, a larger share than the 30 to 50 percent reported in earlier snow-free modeling studies. The team links this amplification to snow’s high albedo, which boosts the photochemistry involved in ClNO2 generation. Ground-surface chemistry added a further 0 to 38.96 pptv, with contribution ratios reaching 48.20 percent in Heilongjiang, Jilin, and northeastern Liaoning. The day-night contrast was dramatic: nighttime surface-chemistry contributions peaked at 72.14 pptv, roughly ten times the daytime maximum, with ratios near 92 percent in parts of Heilongjiang and Jilin. Low temperatures favor the equilibrium shift of the N2O5–NO3 reaction toward N2O5, and long winter nights allow dark accumulation, while snow’s large reactive interface and moisture favor gas deposition and heterogeneous reaction.

These chemical changes translated into measurable air quality impacts. Chlorine chemistry increased hourly PM2.5 concentrations by up to 3.65 micrograms per cubic meter, a relative contribution of 15.34 percent, concentrated around cities such as Harbin and Changchun, while promoting ammonium and sulfate particle formation and suppressing particulate nitrate by as much as 23.77 percent. Maximum daily 8-hour average ozone rose by up to 3.41 ppbv, or 5.68 percent. The simulations also showed chlorine chemistry reshaping the atmospheric oxidizing capacity, promoting OH generation with relative contributions up to 7.59 percent, consistent with earlier estimates of chlorine’s role in regional oxidation.

The authors caution that their surface chemistry module remains simplified, focusing on re-emission from predefined reactions, and that the behavior of snowmelt, where shifting liquid water content in snow and ice pores alters reaction rate constants, deserves particular attention. They propose establishing quantitative relationships among snow depth, total ion concentration, and liquid brine fraction through field sampling and laboratory experiments, which would allow uptake coefficients and ClNO2 yields to be pinned down for different snow depths. Even so, the study delivers a clear message: frozen landscapes are chemically active players in winter air pollution, and accurately representing the flux exchange between the atmosphere and the cryosphere is essential for forecasting, and ultimately mitigating, the smog that blankets cold regions each winter.

Subject of Research: Heterogeneous chlorine chemistry on snowpack surfaces and its role in secondary atmospheric pollution in Northeast China

Article Title: Numerical modeling on the mechanisms of chlorine chemistry in snowpack and their impact on secondary atmospheric pollution

Article References: Xie, S., Zhang, X., Xiu, A., Qi, H., Tong, S., Chen, Q., Gao, C., Zhao, H., Zhang, S., Yabo, S. D., Liu, Y., Li, S., & Zhang, M. (2026). Numerical modeling on the mechanisms of chlorine chemistry in snowpack and their impact on secondary atmospheric pollution. The Cryosphere, 20(9), 5435-5451. https://doi.org/10.5194/tc-20-5435-2026

Image Credits: AI Generated

DOI: 10.5194/tc-20-5435-2026

Keywords: snowpack chemistry, nitryl chloride, dinitrogen pentoxide, chlorine radicals, WRF-CAMx, air quality modeling, PM2.5, ozone, Northeast China, heterogeneous reactions, atmospheric oxidizing capacity, The Cryosphere