Impacts of synoptic circulations on summertime ozone pollution in Guanzhong Basin, northwestern China

摘要

The Guanzhong Basin (GZB) has long been considered the most polluted city cluster in northwestern China with exposure to unhealthy levels of ozone (O-3) in summer, which is strongly associated with adverse meteorological conditions and massive percussor emissions. In this study, four typical synoptic circulation types (CTs) were identified in the summers of 2014-2019 via the T-mode PCA classification method, including two polluted (southeast high and northeast ridge), one lightly polluted (east high), and one clean (northwest high) type. The impact of CTs on the atmospheric transport and diffusion conditions gradually weakened with decreasing altitude, while local circulation played a critical role near the ground, resulting in dominant northeast and southwest surface winds, high proportions of stagnation conditions (50%). In particular, the meteorological conditions under the southeast high-type were marked by increasing influences of southerly airflow, high temperatures, low humidities, and the worst diffusion conditions, all of which were favorable for the local production of O-3. While, the northeast ridge-type was characterized by easterly airflows in the upper air, which transported O-3 and its precursors from upwind polluted areas. The regional transport model CAMx was applied to quantify the O-3 sources over the GZB. The heaviest O-3 pollution occurred under the southeast high-type, associated with the fast chemical formation rates and significant contributions from local emissions. The enhanced atmospheric oxidative capacity under the southeast high-type was suggested by elevated O-3 and Ox (O-3 + NO2) concentrations in the atmospheric boundary layer. Although the northeast ridge- and east high-type circulations led to similar chemical productions of O-3, the cross-regional transport of O-3 was increased under the northeast ridge-type and resulted in severe O-3 pollution. This would serve to maximize regional O-3 reduction under varying synoptic circulation conditions.