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

Abstract Concentrations of PM 2.5 in Tokyo, a densely populated megacity, often increase because of NH 4 NO 3 formation under low-wind conditions during winter. To obtain source information of NH 3 as a NH 4 NO 3 precursor, hourly NH 3 and NH 4 + concentrations were measured at an urban site in Tokyo in December 2017. Results show that PM 2.5 and NH 4 + concentrations increased simultaneously under low-wind and low-temperature conditions along with NH 3 , NOx, CO, and optical black carbon (OBC) concentrations. The remaining NH 4 + (herein, ΔNH 4 +) subtracted from observed NH 4 + to equivalent SO 4 2− concentration correlated well with NO 3 − in PM 2.5 , indicating the existence of fine NH 4 NO 3 particles. Regression analysis of hourly NH 3 + ΔΝH 4 + concentrations with CO, NOx, and OBC showed significant correlation. Virtual emission rates (VER) of NH 3 per motor vehicle in Tokyo were estimated using observed relations between NH 3 + ΔΝH 4 + and OBC, etc. with data from vehicular statistics: they were 3.7–32 mg/km. The regression of NH 3 + ΔΝH 4 + 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 NH 3 during winter in Tokyo. Highlights • Considering excess NH 4 + concentrations over SO 4 2− was useful to study urban NH 3. • Virtual emission rates of NH 3 per vehicle in Tokyo were 3.7–32 mg/km. • Non-vehicular NH 3 sources contributed almost equally to winter vehicular emissions. [ABSTRACT FROM AUTHOR]