Development of Semi-continuous Measurement System for Atmospheric HNO₃

Abstract

Nitric acid (HNO 3 ) is formed to ammonium nitrate (NH 4 NO 3 ) of secondary aerosols by the photochemical reaction. A semi-continuous HNO 3 measurement system was developed for the concentration of HNO 3 in the atmosphere through ion chromatography after it is sufficiently absorbed by deionized water. In order to correct the Nitrate (NO 3 - ) concentration, an experiment was conducted to simulate the concentration of atmospheric nitrogen dioxide (NO 2 ) and hydrogen peroxide (H 2 O 2 ) which produce additional NO 3 - . 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 HNO 3 measurement systems at high concentrations of HNO 3 using a standard gas. In addition, an experiment to measure HNO 3 in the atmosphere was conducted using both the denuder method and the semi-continuous HNO 3 system. The concentration of HNO 3 additionally generated by H 2 O 2 and NO 2 inside the measurement system was measured lower than the detection limit of the previously performed QC/QA, indicating that it was suitable for measuring HNO 3 . In the comparison experiment between the different equipment using a standard HNO 3 gas, a difference of 4.7% to 8.4% was observed, demonstrating that the results of HNO 3 concentration measurements were similar. When measuring HNO 3 in the air, the comparison experiment between the semi-continuous HNO 3 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 HNO 3 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 HNO 3 during the measurement period because the data is acquired in real-time.