Abstract: Black carbon (BC) plays a significant role in climate change, which has attracts increasing research interest. Single Particle Soot Photometer (SP2) is used at Wuqing Meteorological Station in Tianjin. SP2 utilizes the high optical power available intra-cavity from a Nd:YAG laser as the analytical technique. Light absorbing particles, mainly BC in atmosphere, absorb sufficient energy and are heated to the point of incandescence. The energy emitted in this incandescence is measured, and quantitatively determine the mass of the particle. SP2 operates in a single particle mode, measuring the light scattering and incandescence of each particle. Through the time delay between the two signals, the mixing state of BC particles can be obtained. SP2 is different from the traditional filter-based method which could provide more accurate information on single BC particle properties. First, SP2 could count the BC particles individually, so BC number concentration could be given; second, SP2 measures the mass of each BC particle, which could be converted to particle size; finally, SP2 could give the information on BC mixing state, which is important for estimating the aerosol effect on climate change.The observation taken at Wuqing in December 2009 shows that the average number concentration of BC is 1504 cm^-3, with the maximum 5050 cm^-3and the minimum 46.8 cm^-3. The number of BC particles occupies 57.2% of the aerosol particles measured by SP2. The average number concentration of non-absorbing aerosol is 1124 cm^-3, with the maximum 3311 cm^-3and the minimum 70.7 cm^-3. The average mass concentration of BC is 8.15 μg/m³. 51.5% of BC particles are thickly coated. On a clear windy day, the daily average number concentration is 215 cm^-3, the mass concentration is 1.17 μg/m³, and 40.2% of them are mixed. However in the seriously polluted case, the daily average number concentration is 3169 cm^-3, the mass concentration reaches 17.2 μg/m³, and the ratio of mixed BC also increases to 78.7%.