Size distribution and mixing state of black carbon particles during a heavy air pollution episode in Shanghai
A Single Particle Aerosol Mass Spectrometer (SPAMS), a Single Particle Soot Photometer (SP2) and various meteorological instruments were employed to investigate the chemical and physical properties of black carbon (BC) aerosols during a regional air pollution episode in urban Shanghai over a five-day period in December 2013. The average PM2.5 and BC mass concentrations were 221 and 3.2 μg m-3, respectively, with the PM2.5 peak value of 636 μg m-3 at noon of 6 December and the BC peak value of 12.1 μg m-3 at 04:26 LT on 7 December. The number size of BC cores was distributed over ~ 60-400 nm, with a peak around ~ 60 nm. The BC core mass size distribution was within ~ 70-500 nm, with a peak around ~ 200 nm.
The number concentration of BC-containing particles captured by SPAMS in the size range 200-1200 nm agreed very well with that detected by SP2 (R2 = 0.87). A cluster analysis of the single particle mass spectra allowed for the separation of BC-containing particles into seven classes. Pure BC accounted for 0.53 % of BC-containing particles; BC attributed to biomass burning (BBBC) accounted for 22.60 %; K-rich BC-containing (KBC), NaK-rich BC-containing (NaKBC), BC internally-mixed with OC and ammonium sulfate (BCOC-SOx), BC internally-mixed with OC and ammonium nitrate (BCOC-NOx) were all attributed to traffic emissions and accounted for 73.24 %; unidentified particles accounted for 3.63 %.
The size distribution of internally-mixed BC particles was bimodal. Detected by SP2, the condensation mode peaked around ~ 230 nm and droplet mode peaked around ~ 380 nm, with a clear valley in the size distribution around ~ 320 nm. The condensation mode mainly consisted of traffic emissions, with particles featuring a small BC core (~ 60-80 nm) and a relatively thin absolute coating thickness (ACT, ~ 50-130 nm). The droplet mode included highly aged traffic emission particles and biomass burning particles. The highly aged traffic emissions had a small core (~ 60-80 nm) and a very thick ACT (~ 130-300 nm), which is larger than reported in any previous literature. The biomass burning particles had a larger BC core (~ 80-130 nm) and a thick ACT (~ 110-300 nm). High concentration gaseous pollutants like NO2 were found to accelerate the aging process and resulted in a continuous size growth of BC-containing particles from traffic emission. The condensation of gaseous pollutants made a significant contribution to the extremely high particulate matter during heavy pollution episode in the urban area.