Hong Kong has long been suffering from deteriorating air quality due to exacerbating vehicular emission problem. NOx, NO2, PM2.5 and PM10, which have detrimental health impact to pedestrians on street levels, have significant contribution to roadside pollution in Hong Kong during the past decades. In order to alleviate the escalating emission of these pollutants, the government has subsidized a lot of vehicular emission control programs recently, but the effectiveness of some programs in air pollutant reduction has not been evaluated yet.
This study aims to quantify the impact of three vehicular emission control programs: Catalytic Converter Replacement Scheme, Pre-Euro IV Diesel Commercial Vehicles (DCVs) Replacement Program and Selective Catalytic Reduction (SCR) retrofit on Euro II and III franchised buses on vehicular emission by conducting integrated assessments. The effectiveness of the three policies was evaluated by investigating the concentration changes of five target pollutants: NOx, NO, NO2, PM10 and PM2.5 at all stations from pre-control period (April 2012 to March 2013) to post-control period (April 2015 to March 2016) by employing six assessments: spatiotemporal analysis, wind-pollution decomposition, Elemental Carbon (EC) analysis, source apportionment analysis and air quality modelling. An innovative hybrid method combining the results of Positive Matrix Factorization (PMF) and wind-pollution-decomposition analysis was also adopted to further quantify the change in PM mass contribution from vehicle exhaust driven by non-wind factors.
Results of integrated assessments indicate that non-wind factors contributed to substantial reduction in all target pollutants, and NOx, NO and NO2 reduction at roadside was significantly greater than other stations. As PM10 and PM2.5 have a relatively complex source (both local and regional), the difference between the drop in PM level at roadside and other stations was not obvious. Further, results of EC and PMF analysis show that there was tremendous reduction in PM10 and PM2.5 mass contribution from vehicle exhaust due to non-wind effect during the study periods, and the result was consistent with that of air quality modelling, suggesting that the phase out of pre-Euro IV DCVs was effective in reducing PM emission from diesel fuel engine.
To conclude, the results of integrated assessments were in line with the hypothesis that the three vehicle emission control programs were effective in roadside emission reduction. In the near future, this study will serve as a springboard to future strategy for vehicular emission control in Hong Kong. Further questions like whether the timely replacement of worn-out catalytic converter or phasing out older vehicles with outdated Euro standard is the best and most sustainable available solution should be addressed in future.