Complex temperature dependence of vehicular emissions: Evidence from a global meta-analysis

The significant impact of low ambient temperature, which was less regulated, on vehicle exhaust emissions had garnered considerable attention. This study investigated the impact of ambient temperature on exhaust emissions based on the global meta-analysis. The estimated sizes (mean difference, MD_t) of 11 exhaust pollutants were quantified with 1795 observations at low ambient temperatures (LATs, -18 °C to -7 °C) versus warm ambient temperatures (WATs, 20 °C-30 °C). The results indicated a strong and positive effect of LATs on vehicular emissions, with the average ratio of vehicular emission factors at LATs to those at WATs (EF_LAT/EF_WAT) ranging from 1.14 to 3.84. Oil-based subgroup analysis indicated a quite large MD_t [NO_x] of diesel engines (12.42-15.10 mg km^-1·k^-1). Particulate emissions were 0.22-1.41 mg km^-1·k^-1 enhanced during cold-start tests at LATs. The application of particulate filters on motor vehicles greatly reduced the impact of ambient temperature on tailpipe particulate emissions, at the expense of induced NO_x emissions. During the Federal Test Procedure (FTP-75), exhaust emissions showed higher temperature dependence compared to the averaged levels (1.31-39.31 times). Locally weighted regression was used to determine exhaust temperature profiles, revealing that gasoline vehicles emitted more particulates at LATs, while diesel vehicles showed the opposite trend. Given the widespread use of motor vehicles worldwide, future motor vehicle emission standards should include tighter limits on exhaust emissions at LATs.