Characteristics of gaseous HNO2, HNO3, NH3 and particulate ammonium nitrate in an urban city of Central Taiwan

Simultaneous measurements of ambient gaseous species HNO2, HNO3, NH3 and particulate NO3− and NH4+ were carried out during 2002 in the city of Taichung, Taiwan. An annular denuder system (ADS) was used for the measurements. The annual mean concentrations of HNO2, HNO3, NH3, NO3− and NH4+ were 2.9, 1.9, 8.5, 6.0 and 4.6 μg m−3, respectively. Daily and seasonal patterns were observed. The concentration of HNO2 was found to be lower during the summer, whereas HNO3 and NH3 displayed higher concentrations. On the other hand, particulate NO3− exhibited significant seasonal variation with lower levels observed in autumn and summer. The average nitrogen conversion ratios (Fn) in the spring and summer were 21%, approximately 1.5 factor higher than those in the autumn and winter. Moreover, the photochemical reaction played an important role for the oxidation of NO2 to nitrate since Fn was well correlated with O3 during daytime in all seasons. During nighttime, Fn correlated well with relative humidity. These results suggest that the factors affecting nitrogen conversion ratio are different during daytime and nighttime. The present findings are compared with similar measurements worldwide, and the prevailing mechanisms are discussed.