Particle number emission factors from diesel trucks at a traffic intersection: Long-term trend and relation to particle mass-based emission regulation

Abstract

Abstract Particulate matter (PM) in automobile tailpipe exhaust is regulated by PM mass but not particle number (PN) in most parts of the world, including Japan, so the effect of PM mass regulations on the atmospheric PN concentration (PNC) is of interest from the point of view of whether a PNC regulation should be introduced to reduce PNCs. This study examined long-term (2004–2017) monitoring of PNCs and other pollutants at a traffic intersection and other sites in Japan. The decay rate of elemental carbon concentrations at the traffic intersection excellently matched the reductions of PM mass in tailpipe emissions from diesel trucks, indicating that the reduction in atmospheric concentrations directly reflected cleaner vehicle exhausts. Both the reduction of traffic volume and the replacement of older vehicles with newer models that comply with tighter regulations were found to reduce emissions. However, a slower reduction trend was found for exhaust-related PNCs (after excluding photochemically formed aerosols) with particle diameter Dp