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

Abstract

This paper investigates the concentrations and formation rates of ambient nitrous acid (HNO2) in the urban atmosphere. Simultaneous measurements of HNO2 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 NOx to HNO2 were assessed by evaluating potential reactions. The HNO2 concentrations ranged from 0.1 to 15.3 mu g m(-3) with a mean value of 2.9 mu g m(-3). The lower levels of HNO2 found in the 10-14 and 14-18-h sampling intervals were due to photolytic reactions. Moreover, heterogeneous reactions of NO2 and H2O on the aerosol surface contributed to the numerous production pathways of HNO2 during nighttime, which was verified by experimental observations and theoretical evaluations. The formation rates of HNO2 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 HNO2 was significantly dependent on NO concentration during daytime, whereas NO2 was the major factor influencing the formation of HNO2 during nighttime.