Abstract
Surface ozone (O3) increased by ∼20% in the Yangtze River Delta (YRD) region of China during 2014–2020, but the aggravating trend is highly variable on interannual time and city-level space scales. Here, we employed multiple air quality observations and numerical simulation to describe the increasing but variable trend of O3 and to reveal the main driving factors behind it. In 2014–2017, the governmental air pollution control action plan was mostly against PM2.5 (mainly to control the emissions of SO2, NOx, and primary PM2.5) and effectively reduced the PM2.5 concentration by 18%–45%. However, O3 pollution worsened in the same period with an increasing rate of 4.9 μg m−3 yr−1, especially in the Anhui province, where the growth rate even reached 14.7 μg m−3 yr−1. After 2018, owing to the coordinated prevention and control of both PM2.5 and O3, volatile organic compound (VOC) emissions in the YRD region has also been controlled with a great concern, and the O3 aggravating trend in the same period has been obviously alleviated (1.1 μg m−3 yr−1). We further combined the precursor concentration and the corresponding O3 formation regime to explain the observed trend of O3 in 2014–2020. The leading O3 formation regime in 2014–2017 is diagnosed as VOC-limited (21%) or mix-limited (58%), with the help of a simulated indicator HCHO/NOy. Under such condition, the decreasing NO2 (2.8% yr−1) and increasing VOCs (3.6% yr−1) in 2014–2017 led to a rapid increment of O3. With the continuous reduction in NOx emission and further in ambient NOx/VOCs, the O3 production regime along the Yangtze River has been shifting from VOC-limited to mix-limited, and after 2018, the mix-limited regime has become the dominant O3 formation regime for 55% of the YRD cities. Consequently, the decreases of both NOx (3.3% yr−1) and VOCs (7.7% yr−1) in 2018–2020 obviously slowed down the aggravating trend of O3. Our study argues that with the implementation of coordinated regional reduction of NOx and VOCs, an effective O3 control is emerging in the YRD region.
Highlights
IntroductionA high concentration of O3 may degrade air quality and pose health risks to humans, such as respiratory illness and premature birth (Barry et al, 2019; Ekland et al, 2021; Guan et al, 2021)
Tropospheric ozone (O3) is a critical secondary air pollutant featured with strong oxidation (Thompson, 1992), which is produced via complicated photochemical reactions with volatile organic compounds (VOCs) and nitrogen oxides (NOx, x = 1 and 2) acting as its precursors (Seinfeld and Pandis, 2006; Wang et al, 2017)
Our study differs from previous studies in that 1) we combined surface air quality and meteorology observations, satellite remote sensing observations, numerical model simulation and an indicatorbased approach to uncover the increasing but variable pattern of O3 in the Yangtze River Delta (YRD) region, and 2) we further explored and compared the chemical mechanisms and driving factors in different regions and at different stages
Summary
A high concentration of O3 may degrade air quality and pose health risks to humans, such as respiratory illness and premature birth (Barry et al, 2019; Ekland et al, 2021; Guan et al, 2021). A severe O3 pollution episode leads to premature mortality as well. A rise of 10 μg m−3 in O3 concentration would increase mortality by 2% (Lei et al, 2019). Agricultural yields are negatively impacted by O3 with losses ranging from 6% to 15% for wheat products in China (Feng et al, 2015). As an oxidant, the elevated O3 promotes the formation of secondary aerosols (Chen et al, 2020b; Feng et al, 2021). The enhanced O3 and secondary aerosols induced an unexpected haze event in Beijing during the COVID-19 lockdown (Huang et al, 2020; Le et al, 2020)
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