Study on vehicular emissions of VOCs and OVOCs by using multiple measurement techniques in Hong Kong

Abstract

Hong Kong is one of the most densely populated cities all over the world. It is facing serious air pollution problems during the past twenty years such as the increasing ground level ozone trend and the reduction of visibility year by year. Volatile organic compounds (VOCs) which can be emitted from multiple sources are important precursors of tropospheric ozone and secondary organic aerosols. Vehicular emissions is one of the major contributors to ambient VOCs in Hong Kong. The Hong Kong Special Administrative Region Government (HKSARG) has implemented a series of control measures to reduce local air pollutions from on-road vehicles. In order to investigate the changes of characteristics and source emission of VOCs and oxygenated VOCs (OVOCs), one roadside campaign at Mong Kok Air Quality Monitoring Station (MK AQMS) and one inside tunnel campaign at Shing Mun (SM) Tunnel were undertaken in this study. The proton transfer reaction-mass spectrometry (PTR-MS) technique was firstly introduced to measure OVOCs and VOCs in an urban roadside area of Hong Kong. The integrated effect of ambient relative humidity (RH) and temperature (T) on formaldehyde measurements by PTR-MS was explored in this study. A Poly 2-D regression was found to be the best nonlinear surface simulation (r = 0.97) of the experimental reaction rate coefficient ratio, ambient RH, and T for formaldehyde measurement. Inter-comparisons were performed between PTR-MS and other traditional VOC and OVOC measurement techniques, namely, (1) offline 2,4-dinitrophenylhydrazine (DNPH) cartridge sampling followed by high-performance liquid chromatography (HPLC) analysis; (2) online gas chromatography (GC) with flame ionization detection (FID); and (3) offline canister sampling followed by GC with mass spectrometer detection (MSD), FID, and electron capture detection (ECD). In general, good agreements were found between PTR-MS and other analytical techniques for OVOC species (i.e. formaldehyde, acetaldehyde, and acetone etc.) and aromatic hydrocarbons with correlation coefficients no less than 0.75. The characteristics and source apportionment of roadside VOCs and OVOCs were investigated and analyzed through the field study at MK AQMS. The total mixing ratio of VOCs and OVOCs at MK AQMS was found to be decreased by about 37% after the liquefied petroleum gas (LPG) catalytic convertor replacement programme (CCRP), which was launched by Hong Kong Environmental Protection Department (EPD) from September 2013 and finished by the end of April 2014. LPG vehicular emissions was found to be one of the major sources of roadside VOCs and OVOCs both before (45%) and after (38%) the LPG CCRP, however, the total contribution from LPG vehicular emissions decreased from 30.5 ppbv to 13.8 ppbv. The sum of OFP for the total target VOCs and OVOCs also decreased by 44% after the LPG CCRP. Updated vehicular VOC and OVOC emission profiles and emission factors were obtained from the tunnel study in 2015. Net concentrations and emission factors of nearly all gas species decreased a lot compared with previous tunnel study at the same sampling site in 2003. Emission factors of VOCs and OVOCs from diesel-fueled, gasoline-fueled, and LPG-fueled vehicles were reconstructed in this study. And the decrease of emission factors of those different vehicle types' tracers could reflect the controlling effectiveness on vehicular air pollutants emissions in Hong Kong. Overall, this study firstly introduced PTR-MS to measure urban roadside VOCs and OVOCs in Hong Kong, and explored the integrated effect of ambient RH and T on formaldehyde measurements by PTR-MS. The results of the large decrease of VOCs and OVOCs both at the roadside sampling site and the inside tunnel site could demonstrate the effectiveness of the series of air pollution control measures undertaken during the past twenty years by the Hong Kong government.