Evaluation of simulated O₃ production efficiency during the KORUS-AQ campaign: Implications for anthropogenic NO_x emissions in Korea

We examine O₃ production and its sensitivity to precursor gases and boundary layer mixing in Korea by using a 3-D global chemistry transport model and extensive observations during the KORea-US cooperative Air Quality field study in Korea, which occurred in May-June 2016. During the campaign, observed aromatic species onboard the NASA DC-8 aircraft, especially toluene, showed high mixing ratios of up to 10 ppbv, emphasizing the importance of aromatic chemistry in O₃ production. To examine the role of VOCs and NO_x in O₃ chemistry, we first implement a detailed aromatic chemistry scheme in the model, which reduces the normalized mean bias of simulated O₃ mixing ratios from -26% to -13%. Aromatic chemistry also increases the average net O₃ production in Korea by 37%. Corrections of daytime PBL heights, which are overestimated in the model compared to lidar observations, increase the net O₃ production rate by ~10%. In addition, increasing NO_x emissions by 50% in the model shows best performance in reproducing O₃ production characteristics, which implies that NO_x emissions are underestimated in the current emissions inventory. Sensitivity tests show that a 30% decrease in anthropogenic NO_x emissions in Korea increases the O₃ production efficiency throughout the country, making rural regions ~2 times more efficient in producing O₃ per NO_x consumed. Simulated O₃ levels overall decrease in the peninsula except for urban and other industrial areas, with the largest increase (~6 ppbv) in the Seoul Metropolitan Area (SMA). However, with simultaneous reductions in both NO_x and VOCs emissions by 30%, O₃ decreases in most of the country, including the SMA. This implies the importance of concurrent emission reductions for both NO_x and VOCs in order to effectively reduce O₃ levels in Korea.