Nighttime NO3 radical variations near the ground in the summertime Seoul, Korea

NO­3 radical plays an important role in nighttime chemistry in the urban airs. It reacts fast with numerous unsaturated hydrocarbons and forms N2O5, which can yield secondary organic aerosol and aerosol nitrate, respectively. NO3 is also a precursor of chlorine radical reservoir ClNO2 and Cl2 which stimulate the further oxidation of hydrocarbons and ozone production during the early morning hours. Despite of its importance to aerosol and ozone chemistry, its field measurements of NO3 radical were very scarce in the urban conditions, especially during the summer in Korea.

Summertime NO3 concentrations were measured at the campus of Korea University in Seoul with a prototype long-path DOAS system built at Hankuk University of Foreign Studies for a month from May 12 to June 13, 2022. The collimated light from a LED source lamp on 30 m above the ground was bounced with retroreflector at 980 m distance on 20 m above the ground. A convex lens was used to focus returned light onto the fiber cable connected to peltier cooled CCD spectrometer. Major air pollutant species, such as NO, NO2, O3, CO, SO2, PM2.5 mass, BC, EC/OC, VOCs speciation and meteorological parameters were measured simultaneously at the site with DOAS system. Observed nighttime NO3 concentrations ranged from detection limit of 2 pptv to 30 pptv with an average of 9 pptv, which was lower compared to those in other urban studies. NO3 concentrations were analyzed on the basis of its production with NO2 and O3 reaction, and loss with VOCs according to various the meteorological and chemical conditions. Chemical loss, deposition near the ground and frequent precipitations during the experiment period were more likely for the relatively low NO3 concentrations observed in this study. The importance of NO3 to aerosol and ozone production in Seoul will be also presented in the conference.