Vehicle exhaust: An overstated cause of haze in China.

Overall, total exhaust emissions of NOX, VOC and particulate matter (PM) declined, though vehicles in use continued to increase in China. This suggested that contribution of motor vehicle exhaust to haze in China may be exaggerated. A higher frequency of haze episodes in China with lower total emissions fraction from vehicle exhaust compared with those in the USA confirmed that there exists no strict causality between vehicle and haze. No significant correlation (p<0.05) between haze days and the contribution of vehicles to airborne PM2.5 (particulates that are <2.5μm in aerodynamic diameter) or car ownerships. All results revealed again that vehicle exhaust is an overstated cause for haze formation in China.