Hong Kong vehicle emission changes from 2003 to 2015 in the Shing Mun Tunnel

This study characterized motor vehicle emission rates and compositions in Hong Kong's Shing Mun tunnel (SMT) during 2015 and compared them to similar measurements from the same tunnel in 2003. Average PM_2.5 concentrations in the SMT decreased by ∼70% from 229.1 ± 22.1 µg/m³ in 2003 to 74.2 ± 2.1 µg/m³ in 2015. Both PM_2.5 and sulfur dioxide (SO₂) emission factors (EF_D) were reduced by ∼80% and total non-methane (NMHC) hydrocarbons EF_D were reduced by 44%. These reductions are consistent with long-term trends of roadside ambient concentrations and emission inventory estimates, indicating the effectiveness of emission control measures. EF_D changes between 2003 and 2015 were not statistically significant for carbon monoxide (CO), ammonia (NH₃), and nitrogen oxides (NO_x). Tunnel nitrogen dioxide (NO₂) concentrations and NO₂/NO_x volume ratios increased, indicating an increased NO₂ fraction in the primary vehicle exhaust emissions. Elemental carbon (EC) and organic matter (OM) were the most abundant PM_2.5 constituents, with EC and OM, respectively, contributing to 51 and 31% of PM_2.5 in 2003, and 35 and 28% of PM_2.5 in 2015. Average EC and OM EF_D decreased by ∼80% from 2003 to 2015. The sulfate EF_D decreased to a lesser degree (55%) and its contribution to PM_2.5 increased from 10% in 2003 to 18% in 2015, due to influences from ambient background sulfate concentrations. The contribution of geological materials to PM_2.5 increased from 2% in 2003 to 5% in 2015, signifying the importance of non-tailpipe emissions.