Observation-based modeling of ozone chemistry in the Seoul metropolitan area during the Korea-United States Air Quality Study (KORUS-AQ)

Abstract

The Seoul Metropolitan Area (SMA) has a population of 24 million and frequently experiences unhealthy levels of ozone (O 3). In this work, measurements taken during the Korea-United States Air Quality Study (KORUS-AQ, 2016) are used to explore regional gradients in O 3 and its chemical precursors, and an observationally-constrained 0-D photochemical box model is used to quantify key aspects of O 3 production including its sensitivity to precursor gases. Box model performance was evaluated by comparing modeled concentrations of select secondary species to airborne measurements. These comparisons indicate that the steady state assumption used in 0-D box models cannot describe select intermediate species, highlighting the importance of having a broad suite of trace gases as model constraints. When fully constrained, aggregated statistics of modeled O 3 production rates agreed with observed changes in O 3 , indicating that the box model was able to represent the majority of O 3 chemistry. Comparison of airborne observations between urban Seoul and a downwind receptor site reveal a positive gradient in O 3 coinciding with a negative gradient in NO x , no gradient in CH 2 O, and a slight positive gradient in modeled rates of O 3 production. Together, these observations indicate a radical-limited (VOClimited) O 3 production environment in the SMA. Zero-out simulations identified C 7+ aromatics as the dominant VOC contributors to O 3 production, with isoprene and anthropogenic alkenes making smaller but appreciable contributions. Simulations of model sensitivity to decreases in NO x produced results that were not spatially uniform, with large increases in O 3 production predicted for urban Seoul and decreases in O 3 production predicted for far-outlying areas. The policy implications of this work are clear: Effective O 3 mitigation strategies in the SMA must focus on reducing local emissions of C 7+ aromatics, while reductions in NO x emissions may increase O 3 in some areas but generally decrease the regional extent of O 3 exposure.

Characterization, sources and reactivity of volatile organic compounds (VOCs) in Seoul and surrounding regions during KORUS-AQ

“…As a group, aromatics contributed most to VOC OH reactivity in Seoul during KORUS-AQ (44%), followed by nonisoprene alkenes (19%), isoprene (17%), heavy alkanes ≥C 4 (16%) and light C 2 -C 3 alkanes (5%) (Figure 8b). These results show generally good agreement with 0-D photochemical box modeling estimates of O 3 production by VOCs over Seoul during KORUS-AQ, which were constrained by DC-8 observations (Schroeder et al, 2020). The box model used zero-out simulations, whereby selected VOCs or VOC groups were removed from the model to test the model sensitivity to these VOCs.…”

“…Rate constants were taken from Atkinson and Arey (2003) and Atkinson et al (2004Atkinson et al ( , 2006Atkinson et al ( , 2008, and average VOC mixing ratios over Seoul were used (Table S-2). Even though formaldehyde and acetaldehyde were present at appreciable mixing ratios in Seoul (Table S-2), they were not included in the O 3 analysis below because they were found to have a dominant photochemical source during KORUS-AQ (Schroeder et al, 2020). VOCs that are both abundant and reactive with OH have a high potential to form O 3 .…”

“…The model found that O 3 production in Seoul is VOC-limited and most sensitive to changes in aromatics, followed by isoprene and anthropogenic alkenes. For example, removing aromatics caused a 32% reduction in mean ozone production in the Seoul region, and removing isoprene and non-isoprene alkenes caused 15% and 14% reductions, respectively (Schroeder et al, 2020). When expressed in pie chart format, to directly compare with the OH reactivity calculations, the model was most sensitive to aromatics (49%) followed by isoprene (23%), other alkenes (22%), heavy alkanes (5%) and light alkanes (2%) (Figure 8c).…”

“…The box modeling also assessed the impact of methanol on O 3 formation (Schroeder et al, 2020). Removing methanol caused a 6% reduction in O 3 production in the zero-out calculations, or 8% when expressed as a pie chart together with the VOC groups ( Figure S-7b).…”

Measurement report: Characterization of severe spring haze episodes and influences of long-range transport in the Seoul metropolitan area in March 2019

“…O 3 is an important air pollutant itself and contributes to the secondary formation of PM 2.5 . Owing to the growing concern on haze pollution, extensive studies have been conducted in recent years to investigate the sources and formation mechanisms of haze in Korea (Schroeder et al, 2020;Peterson et al, 2019;Kim and Zhang, 2019;Nault et al, 2018;Kim et al, 2017;Kim et al, 2018). Previous studies have shown that stagnant meteorological conditions, primary emissions, secondary formation, and regional transport are the major factors leading to the formation and evolution of severe haze episodes in Korea (Nault et al, 2018;Kim et al, 2018Kim et al, , 2017.…”