The fate of NO _x emissions due to nocturnal oxidation at high latitudes: 1-D simulations and sensitivity experiments

The fate of nitrogen oxide pollution during high-latitude winter is controlled by reactions of dinitrogen pentoxide (N ₂O ₅) 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 N ₂O ₅ reactions and explore sensitivity to model parameters. It was found that dry deposition of N ₂O ₅ made up a significant fraction of N ₂O ₅ loss near the snowpack, but reactions on aerosol particles dominated loss of N ₂O ₅ 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.