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 5-day period in December 2013. The refractory black carbon (rBC) mass concentrations measured by SP2 averaged 3.2aEuro-A mu gaEuro-m(-3), with the peak value of 12.1aEuro-A mu gaEuro-m(-3) at 04:26aEuro-LT on 7 December. The number of BC-containing particles captured by SPAMS in the size range 200-1200aEuro-nm agreed very well with that detected by SP2 (R-2 = 0.87). A cluster analysis of the single particle mass spectra allowed for the separation of BC-containing particles into five major classes: (1) Pure BC; (2) BC attributed to biomass burning (BBBC); (3) K-rich BC-containing (KBC); (4) BC internally mixed with OC and ammonium sulfate (BCOC-SOx); (5) BC internally mixed with OC and ammonium nitrate (BCOC-NOx). The size distribution of internally mixed BC particles was bimodal. Detected by SP2, the condensation mode peaked around aEuro-230aEuro-nm and droplet mode peaked around aEuro-380aEuro-nm, with a clear valley in the size distribution around aEuro-320aEuro-nm. The condensation mode mainly consisted of traffic emissions, with particles featuring a small rBC core ( aEuro-60-80aEuro-nm) and a relatively thin absolute coating thickness (ACT, aEuro-50-130aEuro-nm). The droplet mode included highly aged traffic emission particles and biomass burning particles. The biomass burning particles had a larger rBC core ( aEuro-80-130aEuro-nm) and a thick ACT ( aEuro-110-300aEuro-nm). The highly aged traffic emissions had a smaller core ( aEuro-60-80aEuro-nm) and a very thick ACT ( aEuro-130-300aEuro-nm), which is larger than reported in any previous literature. A fast growth rate ( aEuro-20aEuro-nmaEuro-h(-1)) of rBC with small core sizes was observed during the experiment. High concentrations pollutants like NO2 likely accelerated the aging process and resulted in a continuous size growth of rBC-containing particles from traffic emission.