In the efforts at controlling automobile emissions, it is important to know in what extent air pollutants from on-road vehicles could be truly reduced. In 2014 we conducted tests in a heavily trafficked tunnel in south China to characterize emissions of volatile organic compounds (VOC) from on-road vehicle fleet and compared our results with those obtained in the same tunnel in 2004. Alkanes, aromatics, and alkenes had average emission factors (EFs) of 338, 63, and 42 mg km^-1 in 2014 against that of 194, 129, and 160 mg km^-1 in 2004, respectively. In 2014, LPG-related propane, n-butane and i-butane were the top three non-methane hydrocarbons (NMHCs) with EFs of 184 ± 21, 53 ± 6 and 31 ± 3 mg km^-1; the gasoline evaporation marker i-pentane had an average EF of 17 ± 3 mg km^-1; ethylene and propene were the top two alkenes with average EFs of 16 ± 1 and 9.7 ± 0.9 mg km^-1, respectively; isoprene had no direct emission from vehicles; toluene showed the highest EF of 11 ± 2 mg km^-1 among the aromatics; and acetylene had an average EF of 7 ± 1 mg km^-1. While EFs of total NMHCs decreased only 9% from 493 ± 120 mg km^-1 in 2004 to 449 ± 40 mg km^-1 in 2014, their total ozone formation potential (OFP) decreased by 57% from 2.50 × 10³ mg km^-1 in 2004 to 1.10 × 10³ mg km^-1 in 2014, and their total secondary organic aerosol formation potential (SOAFP) decreased by 50% from 50 mg km^-1 in 2004 to 25 mg km^-1 in 2014. The large drop in ozone and SOA formation potentials could be explained by reduced emissions of reactive alkenes and aromatics, due largely to fuel transition from gasoline/diesel to LPG for taxis/buses and upgraded vehicle emission standards.