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.