Real-Time Measurement of NO and NH₃ Concentration Variations Using Direct Absorption Spectroscopy in the Smog Chamber to Analyze NH₄NO₃ Photochemical Formation Characteristics

Abstract. In urban atmospheric chemistry, NOx and NH₃ in the atmosphere are major species participating in the secondary aerosol formation process, causing severe environmental problems such as decreased visibility and acid rain. In order to respond effectively to particulate matter problems, the correlation of precursors should be identified in detail. This study used UV-C light to convert gaseous substances into particulate substances in the small-scale smog chamber to simulate the photochemical reaction. The effects of several operating variables, such as UV-C light intensity, relative humidity, and initial concentrations of O₂, NO, and NH₃, on the NH₄NO₃ formation were investigated. Since atmospheric gas species are short-lived, they require a measurement technique with an ultra-fast response and high sensitivity. Therefore, the concentrations of NO and NH₃ were measured using Direct Absorption Spectroscopy techniques with the wavenumber regions of 1926 and 6568 cm^-1, respectively. NO and NH₃ were precisely measured with an error rate of less than 3 % with the reference gas. The results show that NO and NH₃ were converted over 98 % when UV-C light intensity was 24 W and relative humidity was about 30 % at 1 atm, 296 K. It also showed that higher UV-C light intensity, O₃ concentration, and relative humidity induced higher conversion rates and secondary aerosol generation. In particular, it was experimentally confirmed that the secondary aerosol generation and growth process was greatly influenced by relative humidity.