Significant parameter for controlling the partition of ambient nitrate species between HNO₃(g) and NH₄NO₃(p)

The equilibrium between nitric acid gas (HNO₃(g)) and ammonium nitrate aerosol (NH₄NO₃(p)) in ambient air was studied based on the monitoring data obtained using a five-stage filter-pack system, in which the fine aerosol and the coarse aerosol were separately collected; this made it possible to evaluate the actual situation of the equilibrium more accurately. The partition between HNO₃(g) and coarse particulate nitrate (c-NO₃^-(p)), as well as that between HNO₃(g) and fine particulate nitrate (f-NO₃^-(p)), could be evaluated individually thanks to the classification separation of the aerosol by size. The c-particle proportion c-NO₃^-(p)/(c-NO₃^-(p) + HNO₃(g)) between HNO₃(g) and c-NO₃^-(p) had a weak negative correlation (r = -0.46, p<0.001) with air temperature; in contrast, the f-particle proportion f-NO₃^-(p)/(f-NO₃^-(p) + HNO₃(g)) between HNO₃(g) and f-NO₃^-(p) had a moderate negative correlation (r = -0.80, p<0.001) with air temperature in total; furthermore, the f-particle proportion had an interesting and discriminative dependence on air temperature which could be divided into two regions by an air temperature around 15°C. The condition of high air temperature accompanied by high relative humidity frequently resulted in the deliquescent state of NH₄NO₃(p), providing the disconnect from the theoretical prediction for the products of [NH₃(g)] and [HNO₃(g)] ([NH₃(g)][HNO₃(g)]) by Seinfeld and Pandis (1998).