Contribution of ozonation forming the particulate nitrosodi-methylamine (NDMA) in the ambient air 

Nitrosamines, organic compounds featuring the nitroso functional group (N-NO), are found in both gas and particle phases in the ambient air, known for their carcinogenic properties. The International Agency for Research on Cancer (IARC) has classified most nitrosamines as likely carcinogenic to humans. Given their carcinogenic nature, it is crucial to manage ambient concentrations of nitrosamines. Nevertheless, the concentrations of nitrosamines in Seoul, South Korea, surpass the recommended level of 10 ng/m3 set by the Norwegian Institute of Public Health (NIPH). In the previous study, the contributions of primary emissions and atmospheric reactions were investigated using field measurement data from Seoul, Korea, along with a kinetic box. It was estimated that there was a mixed contribution of the atmospheric reaction and primary emission. In addition, the model estimation result showed that nitrosamine formation was enhanced by nitrosation under higher concentrations of NOx in Seoul. However, there was a large discrepancy between the measured and estimated concentrations of particulate nitrosamines in Seoul. Therefore, further identification of the hidden reaction forming nitrosamines was necessary. This study aimed to identify whether ozonation could be an unknown reaction forming particulate nitrosodi-methylamine (NDMA) to reduce the discrepancy between the measured and estimated concentration of NDMA in the previous study. Ozonation of dimethylamine (DMA) can form dimethylhydrazine (UDMH), subsequently, UDMH reaction with O3 and O2 can produce NDMA in the ambient air. In order to quantify the contribution of ozonation, the ozonation mechanism was added to the kinetic box model developed in the previous study. The model simulation results showed that (1) the ozonation contributed to the ambient concentration of NDMA (7.9 ± 3.8% (winter); 1.9 ± 3.0% (spring); 10.0 ± 0.77% (summer); 3.6 ± 7.3% (autumn)), (2) the relatively higher O3/NOx ratio in summer (1.63 ± 0.69; 0.64 ± 0.52 (winter); 1.14 ± 0.92 (spring); 0.52 ± 0.54 (autumn)) could enhance ozonation, and (3) relatively lower pH in summer (2.2 ± 0.4; 5.3 ± 1.2 (winter); 3.9 ± 1.2 (spring); 3.9 ± 0.7 (autumn)) could hinder nitrosation compared to that in other seasons.