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

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 NO_x to HNO₂ were assessed by evaluating potential reactions. The HNO₂ 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 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. [ABSTRACT FROM AUTHOR]