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Abstract: Black carbon (BC) aerosols alter the radiation budget both directly (by absorbing solar visible radiation) and indirectly (by acting as cloud condensation nuclei) and cause adverse health effects. The absorbing efficiency and direct radiative effect of BC strongly depend on its mass concentration (M BC) and type of emission source. In the present study, we report measurements of M BC at the Research Center for Advanced Science and Technology (RCAST), located at the center of the urban boundary of Tokyo, using an EC-OC analyzer from 2003 to 2005 and a filter-based Continuous Soot Monitoring System (COSMOS) during 2007–2010. The results indicate that M BC have decreased significantly from 2.6 μg m−3 to 0.5 μg m−3 (∼80% reduction) between 2003 and 2010. Vehicular emissions are the dominant source of BC in Tokyo, and the observed reduction in M BC is mainly attributed to the stringent regulations of particulate matter exhaust from vehicles imposed by the Japanese government. In addition, this observation is also supported from emission estimates using diurnal-weekly variations of M BC in Tokyo and explains the observed reduction to within about 20%. This is the first clear evidence of a significant reduction in BC emissions in Tokyo and shows that measures taken to reduce BC emissions from traffic sources have a strong effect on air quality in a mega-city and also reduce the climate impact of traffic emissions. We highlight the importance of long-term and reliable measurements in detecting BC trends and for the validation and regulation of emission control measures in mega-cities. [Copyright &y& Elsevier]