Elucidating the Impacts of Various Atmospheric Ventilation Conditions on Local and Transboundary Ozone Pollution Patterns: A Case Study of Beijing, China

The ventilation condition in the atmospheric boundary layer, which varies with the synoptic pattern, is a crucial factor affecting the transport and dispersion of air pollutants. In this study, taking Beijing as an example, local ozone (O3) accumulation and the transboundary O3 pollution (TOP) patterns during the warm season were explored under different boundary layer ventilation conditions by using integrated objective weather classification, non-negative matrix factorization, and backward trajectory model. Results show that local sources are a major contributor to O3 pollution in the whole of Beijing, accounting for ~73.36% of the O3 concentration on average. The local accumulation is mainly facilitated by poor ventilation conditions with weak wind speeds (<3 m/s) under favorable photochemical reaction conditions and abundant precursors, resulting in local O3 events. In contrast, the occurrence of synoptic patterns associated with TOP is more frequent, so the TOP contribution cannot be ignored, especially for the northern regions, where it may exceed 50%. Horizontal wind vector variations play a marked role in driving the TOP, affecting not only the location of the source region and transport pathway but also the transport distance and volume. In addition, a strong nocturnal low-level jet (LLJ) results in horizontal transport and a temporary rise in O3 concentration, while a daytime LLJ stimulates a peak in O3 concentration over the downwind area, lagging by about 1 hr. Our findings provide new perspectives on the effects of boundary layer ventilation on the regulation of O3 pollution, as well as other air pollutants.