The impacts of transport from South and Southeast Asia on O<sub>3</sub> concentrations in China from 2015 to 2050

Abstract

<p indent="0mm">Although the significant decrease in PM_2.5 concentrations across China in response to the Air Pollution Prevention and Control Action Plan during 2013–2017, the ozone (O₃) concentrations have increased rapidly. Previous studies have revealed that long-range transboundary transport from South Asia (SA) and Southeast Asia (SEA) greatly influences the O₃ concentrations in China; however, these studies majorly focused on biomass burning events. The impact of anthropogenic emissions from neighboring countries on China’s O₃ pollution has not been comprehensively and quantitatively investigated. In addition, the analysis of the impact of future transport from SA and SEA on O₃ concentrations in China is required. Based on (Shared Socioeconomic Pathways) SSPs, a new set of future emission inventories, this study estimated the MDA8 O₃ concentrations in China, SA, and SEA under different SSPs scenarios from 2015 to 2050 with varying anthropogenic and open burning emissions using a three-dimensional chemical transport model (GEOS-Chem). Several sensitivity experiments were simulated to quantitatively assess the impact of anthropogenic and open burning emission changes in SA and SEA on the future atmospheric O₃ concentrations in China. Transport from SA and SEA can influence MDA8 O₃ concentrations in China widely, covering the Pearl River Delta (PRD), Guangxi, Yunnan, Guizhou, Sichuan Basin (SCB), Qinghai, Tibet and parts of Xinjiang. It contributed to an increase in annual mean MDA O₃ concentrations over above regions by <sc>3.0–19.0 μg m⁻³</sc> in 2015, and led to an increase in MDA8 O₃ concentration in China by <sc>4.0 μg m⁻³.</sc> Monthly variation characteristics of the impacts of SA and SEA transport on concentrations of MDA O₃ in SCB, PRD, and YRD regions were further explored in this study. In 2015, the regional transport from SA and SEA had the highest impact on the MDA8 O₃ concentrations in SCB, increased its annual mean concentration by <sc>6.2 μg m⁻³,</sc> followed by PRD <sc>(+4.7 μg m⁻³),</sc> and YRD <sc>(+0.6 μg m⁻³).</sc> Accompanied by the strong westerly and southwesterly winds on the surface and 850 hPa near 30°N in spring, the influence of transport from SA and SEA on MDA8 O₃ concentrations was highest in SCB in March 2015 <sc>(+10.1 μg m⁻³,</sc> +7.2%), followed by April <sc>(+9.4 μg m⁻³,</sc> +6.4%). Due to the prevailing southwesterly winds at 10°–30°N on the surface and 850 hPa in summer, the contribution attained its peak in July <sc>(+14.0 μg m⁻³,</sc> +10.4%), followed by August <sc>(+9.1 μg m⁻³,</sc> +6.5%) in PRD, while it peaked in July <sc>(+2.5 μg m⁻³,</sc> +1.7%), followed by June <sc>(+1.4 μg m⁻³,</sc> +0.9%) in YRD. Based on medium and long-term perspectives (2015–2030), the impact of transport from SA and SEA on MDA8 O₃ concentrations in SCB, PRD, and YRD from 2015 to 2030 were simulated. Substantive reductions under SSP1, especially under SSP1-1.9 pathway were observed (the largest decline in SCB was <sc>−3.5 μg m⁻³</sc> (−36.5%) in April, and <sc>−3.0 μg m⁻³</sc> (−21.1%) and <sc>−1.1 μg m⁻³</sc> (−43.1%) in PRD and YRD, respectively, in July). The MDA8 O₃ concentrations in SCB, PRD, and YRD during 2015–2030 were projected to increase under SSP3-7.0 scenario (the highest increase in the SCB will be <sc>+1.5 μg m⁻³</sc> (+20.1%) in May, and <sc>+1.2 μg m⁻³</sc> (+8.5%) and <sc>0.2 μg m⁻³</sc> (+7.5%) in July, respectively, in PRD and YRD). Therefore, controlling O₃ pollution due to the transport from SA and SEA in China during 2015–2030 will be easier year by year under the SSP1-1.9 scenario, while greater efforts would be required under the SSP3-7.0 pathway. The long-term (2015–2050) trend of MDA8 O₃ concentrations in the above three regions affected by the transport from SA and SEA is similar to that of the mid-to-long term.