Opposing trends in the peak and low ozone concentrations in eastern China: anthropogenic and meteorological influences

Abstract. Due to considerable reductions in nitrogen oxides (NOx), ozone trends and variations in eastern China remain inadequately understood. Long-term observations of ozone precursors were conducted to explore the factors influencing ozone trends in this region. Combined with satellite and surface measurements, we evaluated the trends in low (2nd percentile), typical (50th percentile), and peak (98th percentile) ozone concentrations in detail. Observations indicate a significant decrease in peak ozone concentrations (−0.5 % yr−1), alongside an increase in low ozone concentrations (0.3 % yr−1), across eastern China during May–September from 2017 to 2022. The decline in typical ozone concentrations is notably slower than that of peak ozone concentrations, which is approximately −0.02 ppb yr−1 (−0.0 % yr−1) during the same period. Anthropogenic emissions primarily drive trends in low and peak ozone concentrations in eastern China, though meteorological effects also play a role. Ozone formation sensitivity shifts from volatile organic compound (VOC)-limited or transitional regimes in the morning (08:00–11:00 local time, LT), when ozone concentrations rise sharply, to NOx-limited regimes around peak concentrations (∼ 14:00 LT). The reduction in NOx concentrations is identified as a key factor driving the decline in peak ozone concentrations, aiming to further reduce ozone exceedance days. Thus, controlling NOx concentrations emerges as crucial for mitigating peak ozone levels. Moreover, the increase in low ozone concentrations can also be attributed to both anthropogenic emissions and meteorological factors. Our findings underscore the beneficial impacts of NOx reduction on managing peak ozone levels. Regular changes in ozone formation sensitivity throughout the day should be considered when formulating effective ozone control policies.