Effectiveness of replacing catalytic converters in LPG-fueled vehicles in Hong Kong

Many taxis and public buses are powered by liquefied petroleum gas (LPG) in Hong Kong. With more vehicles using LPG, they have become the major contributor to ambient volatile organic compounds (VOCs) in Hong Kong. An intervention program which aimed to reduce the emissions of VOCs and nitrogen oxides (NO_<i>x</i>) from LPG-fueled vehicles was implemented by the Hong Kong government in September 2013. Long-term real-time measurements indicated that the program was remarkably effective in reducing LPG-related VOCs, NO_<i>x</i> and nitric oxide (NO) in the atmosphere. Receptor modeling results further revealed that propane, propene, <i>i</i>-butane, <i>n</i>-butane and NO in LPG-fueled vehicle exhaust emissions decreased by 40.8 ± 0.1, 45.7 ± 0.2, 35.7 ± 0.1, 47.8 ± 0.1 and 88.6 ± 0.7 %, respectively, during the implementation of the program. In contrast, despite the reduction of VOCs and NO_<i>x</i>, O₃ following the program increased by 0.40 ± 0.03 ppbv (∼ 5.6 %). The LPG-fueled vehicle exhaust was generally destructive to OH and HO₂. However, the destruction effect weakened for OH and it even turned to positive contribution to HO₂ during the program. These changes led to the increases of OH, HO₂ and HO₂ ∕ OH ratio, which might explain the positive O₃ increment. Analysis of O₃–VOCs–NO_<i>x</i> sensitivity in ambient air indicated VOC-limited regimes in the O₃ formation before and during the program. Moreover, a maximum reduction percentage of NO_<i>x</i> (i.e., 69 %) and the lowest reduction ratio of VOCs ∕ NO_<i>x</i> (i.e., 1.1) in LPG-fueled vehicle exhaust were determined to give a zero O₃ increment. The findings are of great help to future formulation and implementation of control strategies on vehicle emissions in Hong Kong, and could be extended to other regions in China and around the world