An ensemble assessment of the effectiveness of vehicular emission control programs for air quality improvement in Hong Kong

Abstract Evaluation of applied emission control programs for ambient air quality improvement in the short term is important for timely policy adjustment and air quality protection. However, this is generally a non-trivial task because of the possible contribution of various factors, such as the regional background effect, meteorological impact, and other source impacts, which must be quantified and eliminated from the observed concentration changes. In this study, we developed an ensemble assessment approach employing multiple methods: spatiotemporal analysis, wind pollution decomposition (WPD) analysis, elemental carbon (EC)-WPD hybrid analysis, and air quality modelling. By combining these methods, we showed that the contributions of different factors could be resolved, and the combined results of all methods formed a consistent result clearly demonstrating that a significant reduction in NOx (112.4 ± 25.9 μg/m3 reduction), NO2 (31.5 ± 6.0 μg/m3 reduction), and PM10 and PM2.5 (reduced by 6.8 μg/m3 and 4.5 μg/m3, respectively) occurred due to the impact of the applied control programs. This study could serve as a springboard for future vehicular emission control strategies in Hong Kong in the near future. Moreover, this holistic analysis could also provide an independent method for estimating the emission control impact in other megacities.