Articles | Volume 16, issue 10
Research article
31 May 2016
Research article |  | 31 May 2016

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

Xiaopu Lyu, Hai Guo, Isobel J. Simpson, Simone Meinardi, Peter K. K. Louie, Zhenhao Ling, Yu Wang, Ming Liu, Connie W. Y. Luk, Nan Wang, and Donald R. Blake

Abstract. 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 (NOx) 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, NOx and nitric oxide (NO) in the atmosphere. Receptor modeling results further revealed that propane, propene, i-butane, n-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 NOx, 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–NOx sensitivity in ambient air indicated VOC-limited regimes in the O3 formation before and during the program. Moreover, a maximum reduction percentage of NOx (i.e., 69 %) and the lowest reduction ratio of VOCs ∕ NOx (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.

Short summary
In this study, the effectiveness of a LPG converter replacement program was evaluated. It was found that LPG-related VOCs and NOx decreased significantly due to the implementation of the program. Source apportionment also revealed the reduction of VOCs and NOx in LPG-fueled vehicle exhaust. From before to during the program, O3 increased slightly, mainly due to the reduction of NOx in LPG-fueled vehicle exhaust. To retain zero O3 increment, the lowest reduction ratio of VOCs / NOx was determined.
Final-revised paper