Worsening summertime ozone pollution in the Guanzhong Basin, China from 2014 to 2018: Impacts of synoptic conditions and anthropogenic emissions

The ozone (O₃) pollution in the Guanzhong Basin (GZB), China has progressively deteriorated from 2014 to 2018, with the summertime near-surface maximum daily 8 h average (MDA8) O₃ concentration ([O₃]) increasing from about 104 to 141 μg m⁻³, although the emission mitigation strategies have been carried out since 2013. However, it remains elusive about whether anthropogenic emissions or meteorological conditions contribute to the worsening of O₃ pollution in the GZB. A subjective classification method is first used to classify the synoptic patterns influencing the GZB into favorable and unfavorable conditions. The unfavorable synoptic conditions generally correspond to less precipitation, more solar radiation and higher near-surface temperature, which facilitate formation of the O₃ pollution, and it is opposite for the favorable conditions. The increasing trend of MDA8 [O₃] is correlated well with the increase in occurrence of unfavorable synoptic conditions and near-surface temperature from 2014 to 2018. Sensitivity experiments using the WRF-Chem model reveals that the emission mitigation strategies from 2014 to 2018 enhance MDA8 [O₃] by 19.2% on average in the GZB, which counts about 54% of the observed MDA8 O₃ increasing trend. Particularly, the particulate matter decrease due to the emission mitigation contributes about 11.5% of the observed trend, caused by the reduced HO₂ heterogeneous uptake and enhanced photolysis. The rest 46% of the observed MDA8 O₃ increasing trend can be mainly contributed by the variation of occurrence days of unfavorable synoptic situations.