Determination of HONO concentrations in Seoul, Korea using CIMS and HONO formation

Abstract

In Seoul, nitrate (NO3-) was the major ion component of PM2.5, accounting for 44%, followed by SO42- (30%), NH4+ (21%), and Cl- (2%) (Seoul Metropolitan Government Research Institute of Public Health & Environment, 2022). Therefore, in order to effectively manage PM2.5 in Seoul, it is necessary to understand the precursor of nitrate. In the presence of NH3, HNO3 can generate an ammonium nitrate (NH4NO3) aerosol in the troposphere (Seinfeld and Pandis, 2016). Due to high level of NH3 observed in South Korea, NH4NO3 is sensitive to the availability of HNO3 (Travis et al., 2022). The gas phase reaction of NO2 and OH is the main production pathway of HNO3 during the daytime, and this reaction appeared to contribute up to 82% of HNO3 formation during the daytime (Xue et al., 2022). HONO is a reservoir and source of OH. The photolysis of HONO is the major source of OH radical formation in the morning in urban areas (Mao et al., 2010; Czader et al., 2012) and OH radicals promote the formation of oxidation products such as O3 and HNO3 (Alicke et al., 2003). Therefore, understanding the reaction and formation characteristics of HONO and HNO3 is essential to predict the oxidation process and secondary pollution in the urban atmospheric environment. In this study, Chemical Ionization Mass Spectrometry (CIMS) and Time-of-Flight Chemical Ionization Mass Spectrometry (ToF-CIMS) was used to measure HONO and HNO3 in Seoul, Korea, during the summer of 2021 and spring of 2022, respectively. The mean concentration of HONO in summer and spring were 0.54 ± 0.51 ppb and 0.11 ± 0.12 ppb, respectively. The average concentration of HNO3 was 0.31 ± 0.46 ppb in summer and 0.10 ± 0.08 ppb in spring, respectively. The average concentration of these species for the summer of 2021 was significantly higher than in the spring of 2022, and the difference was about 4.9 times for HONO and 3.1 times for HNO3. In the case of the summer measurement in 2021, to examine the relation of HONO with a high concentration of PM2.5, we classified the period based on PM2.5 and HONO concentration: the high PM2.5 episode with the high HONO concentration (Period 1) and the non-episode of PM2.5 with the high HONO concentration (Period 2). The average concentration of HONO and HNO3 in period 1 were 0.76 ppb and 0.617 ppb, respectively. In period 2, the mean concentration of HONO and HNO3 were 1.19 ppb and 0.545 ppb, respectively. With the implementation of the F0AM model coupled with the MCM v.3.3.1, the estimation of OH production in the presence of HONO and formation characteristics of HONO were conducted. When the measured HONO was present in the F0AM model, the daily average OH production enhanced 2.5 times in the whole experiment during 2021 summer, 2.0 times in the period 1, 4.4 times in the period 2, and 1.2 times in the whole experiment during 2022 spring, respectively. The simulated OH concentration without the measured HONO in spring was 5.74 times lower than that of summer, suggesting that it is associated with the low concentration of secondary pollutants in spring. It was simulated to generate more OH radicals during the highest concentration of HONO episode (period 2), implicating that high concentrations of HONO can enhance the oxidation capacity of the atmosphere and affect the formation of secondary pollutants. In both seasons of summer and spring, the most influential source for the formation of HONO was the heterogeneous conversion of NO2 on the ground surface. The combined contribution ratios of the default reaction and the light-enhanced reaction were found to be 39.1% and 53.5% in summer and spring, respectively. During the period 1 in summer, when there was the high concentration of PM2.5 episode, the contributions of the heterogeneous reaction on the aerosol surface and photolysis of HNO3 were increased compared to the entire measurement period in summer, whereas the homogeneous reaction of NO and OH was significantly decreased from 22.0% to 12.8%. On the other hand, during the period 2, there was no clear difference in the average daily contribution of HONO formation. ;서울의 PM2.5 중 가장 많은 부분을 차지하는 것은 2차 오염물질인 이온 물질이며, 그 중 NOy를 전구체로 갖는 질산염이 서울시 PM2.5의 주요한 이온 성분인 것으로 알려져 있다. HONO와 HNO3는 대기 중 질소산화물의 총칭인 NOy에 속하는 물질로, HNO3는 NH3와의 반응을 통해 NH4NO3을 생성한다. HONO는 NOx 및 HOx의 반응에 관여하며, 광분해를 통하여 대기의 산화 능력을 담당하는 OH radicals을 생성함으로써 HNO3, O3 등 2차 오염물질 형성에 영향을 준다. 따라서 대기 중 광화학 반응을 파악하고 2차 오염물질을 저감하기 위해서는 HONO 및 HNO3에 대한 이해가 필요한 상황이다. 본 연구에서는 2021년 여름, 2022년 봄철 총 2회에 걸쳐 서울에 위치한 수도권 대기오염집중측정소에서 HONO와 HNO3를 비롯한 대기 중의 가스상 성분 측정을 진행하였다. 2021년 6월 8일부터 6월 30일까지 여름철 측정 기간 동안에는 CIMS(Chemical Ionization Mass Spectrometry)를 사용하였으며, 2022년 3월 31일부터 4월 15일까지 진행된 봄철 측정 기간 동안에는 ToF-CIMS(Time-of-Flight Chemical Ionization Mass Spectrometry) 장비를 이용하였다. 측정 결과 2021년 여름철 HONO 및 HNO3의 평균 농도는 각각 0.54 ± 0.51 ppb, 0.31 ± 0.46ppb이었으며, 2022년 봄철 HONO 및 HNO3의 평균 농도는 각각 0.12 ± 0.13 ppb, 0.10 ± 0.09 ppb 로 관측되었다. 또한 0차원 박스 모델인 F0AM(Framework for 0-dimensional Atmospheric Modeling) 모델과 MCM (Master Chemical Mechanism) v.3.3.1을 이용하여 HONO 농도 수준에 따른 OH radical 생성 변화량을 모사하였으며, 계절별 HONO의 생성 원인 별 기여도를 계산하였다. 그 결과 HONO 농도가 가장 높았던 2021년 여름철의 Period 2 기간 동안에 가장 많은 OH radical이 발생하는 것으로 계산되었으며, 이는 고농도의 HONO가 대기 중 산화 능력을 강화시키고, 2차 오염물질 형성에 영향을 줄 수 있음을 의미한다. HONO의 생성 기작 별 기여도를 분석한 결과, 여름과 봄 두 계절동안 HONO의 생성에 가장 많은 영향을 준 요소는 지표면상에서의 NO2의 비균질 반응이었으며, 각각 39.1%와 53.5%의 기여도를 나타내었다.