Overlooked crisis: cold temperature amplifies particle number emissions from gasoline vehicles

This study systematically investigates particle number (PN) emissions from light-duty gasoline vehicles through chassis dynamometer (60 emission test runs spanning -7 °C to 40 °C) combined with mechanistic analysis, revealing a severely underestimated "cold-temperature penalty" effect. While gasoline direct injection (GDI) engine technology improves fuel economy, it significantly amplifies PN emissions, particularly during cold startup mode, where GDI-equipped internal combustion engine vehicles (ICEVs) without gasoline particulate filters (GPFs) emitted PN levels up to 10 times higher than during hot startup operation at 23 °C. GPF installation demonstrated substantial PN reduction benefits, with its emission control efficacy surpassing the motor auxiliary mitigation effect of hybrid electric vehicles (HEVs). Notably, PFI-equipped HEVs without GPFs emitted around 7 times higher PN emissions than PFI-equipped ICEVs with GPFs at 23 °C conditions, challenging the conventional belief that "electrification equals cleanliness". Additionally, we quantified medium-speed emission migration in HEVs, exposing hidden pollution risks during motor-engine transition. A critical combustion efficiency threshold of 0.99 was identified as a regulatory pivot for the non-monotonic relationship between PN emissions and combustion efficiency, offering a new optimization target for emission control systems. An exponential correlation between equivalent mileage ratio (EMR) and ambient temperature was established, showing that even at 20 °C, GDI-equipped ICEVs without GPFs exhibited EMR values as high as 50, indicating that 1 km of cold-start PN emissions equals 50 km of hot-stabilized operation, highlighting a regulatory blind spot in current emission standards for cold regions. These findings provide scientific insights for combustion optimization and after-treatment design under extreme climates while underscoring the urgency of prioritizing GDI-equipped vehicles without GPFs in emission regulations for cold temperature zones.