Changes in black carbon and PM2.5 in Tokyo in 2003-2017

Black carbon (BC) particles cause adverse health effects and contribute to the heating of the atmosphere by absorbing solar visible radiation. Efforts have been made to reduce BC emissions, especially in urban areas; however, long-term measurements of the mass concentration of BC (M_BC) are very limited in Japan. We report measurements of MBC in Tokyo from 2003 to 2017 showing that MBC decreased by a factor of 3 from 2003 to 2010 and was stable from 2010 to 2017. The decrease in PM2.5 concentrations during 2003-2010 was much smaller. The diurnal variations of BC size distributions suggest that the BC in Tokyo originates mainly from local sources, even after 2010. Our 3-dimensional model calculations show that BC from the Asian continent contributes a small portion (about 20%) of the annual average M_BC in the Kanto region of Japan, which includes Tokyo. This indicates that continued reduction of BC emissions inside Japan should be effective in further decreasing M_BC.