Evaluation of formation rates of NO2 to gaseous and particulate nitrate in the urban atmosphere

This work investigates the formation rates of gaseous and particulate nitrate in an urban atmosphere. This investigation is based on theoretical evaluations regarding potential NO2 oxidant reactions, leading to calculated values compared with field observations in Taichung City, Taiwan in 2002. The results show that the correlation coefficient (r) between the calculated value and field observation for HNO3 was 0.83 and that for NO3- was 0.66, suggesting that the theoretical calculations are suitable for assessing the formation rate of nitrate species. The 12-h average conversion rate of NO2 to HNO3 was approximately 3.4±3.1% h−1, while the formation rate of particulate NO3- was 2.4±1.8% h−1. Seasonal variations in gaseous and particulate production rates were obtained. The formation rate of HNO3 was found to be higher in summer, whereas the NO3- rate was lowest. The factors affecting the formation rate were also investigated using the model sensitivity test. The results showed that both HNO3 and NO3- were significantly dependent upon the ambient temperature and relative humidity in a moist atmosphere.