Drivers of 2013-2020 ozone trends in the Sichuan Basin, China: Impacts of meteorology and precursor emission changes.

ABSTRACT

The Sichuan Basin (SCB) of China is known for excessive ozone (O3) pollution owing to high anthropogenic emissions combined with terrain-induced poor ventilation and weak wind fields against the surrounding mountains. While O3 pollution has emerged as a prominent concern in southwestern China yet variations in O3 levels during 2013-2020 are still unclear and the dominant factor in explaining the long-term O3 trend throughout the SCB remains elusive due to uncertainties in emission inventory and variability associated with meteorological conditions. Here, we use extensive basin-wide ambient measurements to examine the spatial pattern and trend of O3 and leverage OMI and TROPOMI satellites in conjunction with MEIC emission inventory to track emission changes. Sensitivity simulations are conducted by using WRF-CMAQ model to investigate the impacts of meteorological variability and emission changes on O3 changes over 2013-2020. O3 concentrations exhibit obvious interannual increases during 2013-2019 and a slight decrease in 2020. Both decreases in the MEIC emission inventory (-2.9% yr-1) and OMI NO2 column density (-3.1% yr-1) reflects the declining trend in NOx emissions over 2013-2020, while anthropogenic VOCs were not adequately regulated during 2013-2017, which explained the majority of deteriorated O3 pollution from 2013 to 2017. Furthermore, attribution analysis based on CMAQ simulations indicate that the unexpected aggravated O3 levels in 2019 is not only modulated by disproportional reductions in VOCs and NOx emissions, but also associated with unfavorable meteorological conditions featured by profound heatwaves and frequent stagnant conditions. In 2020, the abnormal meteorological conditions in May leads to substantial increase of O3 by 26.8 µg m-3 as compared to May 2019, while the considerable enhancement was fully offset by low O3 levels over the whole period which attributes to substantial emission reductions. This study reveals the long-term trend of O3 levels and precursor emissions and highlights the effects of meteorological variability and emission changes on O3 pollution over the SCB, with strong implications for designing effective O3 control measures.