Vehicular exhaust contributions to high NH3 and PM2.5 concentrations during winter in Tokyo, Japan

Concentrations of PM2.5 in Tokyo, a densely populated megacity, often increase because of NH4NO3 formation under low-wind conditions during winter. To obtain source information of NH3 as a NH4NO3 precursor, hourly NH3 and NH4+ concentrations were measured at an urban site in Tokyo in December 2017. Results show that PM2.5 and NH4+ concentrations increased simultaneously under low-wind and low-temperature conditions along with NH3, NOx, CO, and optical black carbon (OBC) concentrations. The remaining NH 4 + (herein, Delta NH4+) subtracted from observed NH4+ to equivalent SO42- concentration correlated well with NO3- in PM2.5, indicating the existence of fine NH4NO3 particles. Regression analysis of hourly NH3 + Delta NH4+ concentrations with CO, NOx, and OBC showed significant correlation. Virtual emission rates (VER) of NH3 per motor vehicle in Tokyo were estimated using observed relations between NH3 + Delta NH4+ and OBC, etc. with data from vehicular statistics: they were 3.7-32 mg/km. The regression of NH3 + Delta NH4+ with OBC concentrations indicated an intercept of about 3.2 ppb, which is about half of the monthly average in December. This result implies that the non-vehicular source strength was nearly the same strength as the bulk vehicular emissions of NH3 during winter in Tokyo.