Concentrations and Formation Rates of Ambient Nitrous Acid in Taichung City, Taiwan

摘要

This paper investigates the concentrations and formation rates of ambient nitrous acid (HNO_2) in the urban atmosphere. Simultaneous measurements of HNO_2 and aerosol particles were carried out from July 2005 to January 2006 by using an annular denuder system (ADS). In addition, the formation rates of NO_x to HNO_2 were assessed by evaluating potential reactions. The HNO_2 concentrations ranged from 0.1 to 15.3 μg m~(-3) with a mean value of 2.9 μg m~(-3). The lower levels of HNO_2 found in the 10-14 and 14-18-h sampling intervals were due to photolytic reactions. Moreover, heterogeneous reactions of NO_2 and H_2O on the aerosol surface contributed to the numerous production pathways of HNO_2 during nighttime, which was verified by experimental observations and theoretical evaluations. The formation rates of HNO_2 ranged from 0.5-2.6 % h~(-1), while the highest values were found at nighttime. Finally, the factors affecting the formation rate were investigated using sensitivity analysis. The results showed that HNO_2 was significantly dependent on NO concentration during daytime, whereas NO_2 was the major factor influencing the formation of HNO_2 during nighttime.