Effects of mobile emission reduction on ozone under future carbon neutrality scenarios in Korea

Tropospheric ozone (O 3 ), formed through photochemical reactions between volatile organic compounds (VOCs) and nitrogen oxides (NO x ≡ NO + NO 2 ), poses detrimental risks to health and ecosystems. In South Korea, the fourth-highest maximum daily 8-h average (MDA8) O 3 concentrations have increased from 53 to 89 ppbv over the past three decades, and the implementation of the carbon neutrality strategy in South Korea is expected to alter emissions of O 3 precursors (NO x , VOCs). In particular, emission changes in the transportation sector can affect ozone concentrations in densely populated urban areas, thereby impacting public health outcomes. Therefore, we investigate O 3 responses to transportation decarbonization using a global 3-D chemical transport model (GEOS-Chem) under mobile NO x and VOCs emission reduction scenarios. Across all scenarios involving 40 % reductions in mobile NO x emissions in South Korea, with additional VOC reductions and expanded coverage to China in some cases, MDA8 O 3 concentrations increased. Furthermore, O 3 production regimes in South Korea remain NO x -saturated or transitional in major cities despite NO x reductions, while suburban and remote forested regions are becoming more limited by NO x availability. NO x -only mobile emission reductions led to substantial MDA8 O 3 increases (up to 4.7 ppbv) and additional six exceedance days compared to results from the control run, whereas concurrent VOCs reductions mitigated both the magnitude (2.7 ppbv) and the number of ozone exceedance days. Also, emission reductions for both NO x and VOC in South Korea and China resulted in reducing O 3 exceedance days in May–July compared to results implemented only in South Korea, highlighting the importance of transboundary VOC mitigation. • O 3 increased under 40 % NO x /VOC mobile emission reduction scenarios for the carbon neutrality. • NO x -saturated or transitional O 3 production regime in urban areas maintained despite 40 % reduction in mobile NO x emissions. • Mobile NO x and VOC reductions increased O 3 and exceedance days in the SMA. • Both NO x -VOC reductions in South Korea and China reduced O 3 exceedances more than Korea-only reduction.