Development of Semi-continuous Measurement System for Atmospheric HNO3

Nitric acid (HNO3) is formed to ammonium nitrate (NH4NO3) of secondary aerosols by the photochemical reaction. A semi-continuous HNO3 measurement system was developed for the concentration of HNO3 in the atmosphere through ion chromatography after it is sufficiently absorbed by deionized water. In order to correct the Nitrate (NO3-) concentration, an experiment was conducted to simulate the concentration of atmospheric nitrogen dioxide (NO2) and hydrogen peroxide (H2O2) which produce additional NO3-. To evaluate the performance of the instrument, it compared the denuder method, Vocus PTR-TOF (Vocus proton-transfer-reaction time-of-flight mass spectrometer), and semi-continuous HNO3 measurement systems at high concentrations of HNO3 using a standard gas. In addition, an experiment to measure HNO3 in the atmosphere was conducted using both the denuder method and the semi-continuous HNO3 system. The concentration of HNO3 additionally generated by H2O2 and NO2 inside the measurement system was measured lower than the detection limit of the previously performed QC/QA, indicating that it was suitable for measuring HNO3. In the comparison experiment between the different equipment using a standard HNO3 gas, a difference of 4.7% to 8.4% was observed, demonstrating that the results of HNO3 concentration measurements were similar. When measuring HNO3 in the air, the comparison experiment between the semi-continuous HNO3 system and the denuder method showed a high correlation with a coefficient of about 0.98 and a difference of only 0.2%. The semi-continuous HNO3 measuring system was found to be hardly impacted by interference from other materials in the sample and produced results that were similar measured values to using other equipment. Also, it is easy to analyze the concentration and chemical properties of HNO3 during the measurement period because the data is acquired in real-time.