Analysis of Wintertime Nitrate Formation in Seoul Using Box Model Simulations

The concentrations of PM_2.5 (particulate matter with an aerodynamic diameter less than or equal to 2.5 µm) components and their gaseous precursors measured during the Fine Particle Research Initiative in East Asia considering National Differences project in winter at the Seoul site were used for box modeling to investigate the characteristics of high PM_2.5 concentration episodes. The measurement data showed that an ammonia-rich condition prevailed during the studied period and the secondary inorganic aerosol species, particularly ammonium nitrate, were the major contributors to the high PM_2.5 concentration. A box model OCABOX (Observation Constrained Atmospheric BOX model) was used for the simulation of photochemical reactions and EKMA (Empirical Kinetic Modeling Approach) analysis. OCABOX underestimated considerably the concentration of particulate nitrate particularly during the nighttime of the high PM_2.5 period. Constraining the ozone concentration with measured data mitigated the underestimation of nighttime nitrate significantly. The model-predicted nighttime ozone concentration that was much lower than observation owing to the excessive titration by NO was pointed out to be the main reason for the serious underestimation of nocturnal nitrate formation. The EKMA analysis showed that the formations of ozone and nitrate were in the NO_x-saturated and the transition regimes, respectively. The NO_x-saturation characteristics of the ozone formation appeared even more severe during the high PM_2.5 episode period because the high PM_2.5 concentration was led by high NO_x concentration. Nitrate formation did not show the NO_x-saturation characteristics because of the underestimated nocturnal formation and the HONO constraint provided proportionally to the NO_x concentration in this study. Further investigation on the effects of the method of HONO constraining on the nitrate formation sensitivity is warranted.