The fate of NO_x emissions due to nocturnal oxidation at high latitudes: 1-D simulations and sensitivity experiments

Abstract. The fate of nitrogen oxide pollution during high-latitude winter is controlled by reactions of dinitrogen pentoxide (N₂O₅) and is highly affected by the competition between heterogeneous atmospheric reactions and deposition to the snowpack. MISTRA (MIcrophysical STRAtus), a 1-D photochemical model, simulated an urban pollution plume from Fairbanks, Alaska to investigate this competition of N₂O₅ reactions and explore sensitivity to model parameters. It was found that dry deposition of N₂O₅ made up a significant fraction of N₂O₅ loss near the snowpack, but reactions on aerosol particles dominated loss of N₂O₅ over the integrated atmospheric column. Sensitivity experiments found the fate of NO_x emissions were most sensitive to NO emission flux, photolysis rates, and ambient temperature. The results indicate a strong sensitivity to urban area density, season and clouds, and temperature, implying a strong sensitivity of the results to urban planning and climate change. Results suggest that secondary formation of particulate (PM_2.5) nitrate in the Fairbanks downtown area does not contribute significant mass to the total PM_2.5 concentration, but appreciable amounts are formed downwind of downtown due to nocturnal NO_x oxidation and subsequent reaction with ammonia on aerosol particles.