Size distribution and mixing state of black carbon particles during a heavy air pollution episode in Shanghai

Abstract. 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 3.2 µg m⁻³, with the peak value of 12.1 µg m⁻³ at 04:26 LT on 7 December. The number 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 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-SO_x); (5) BC internally mixed with OC and ammonium nitrate (BCOC-NO_x). 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 rBC 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 biomass burning particles had a larger rBC core (∼  80–130 nm) and a thick ACT (∼  110–300 nm). The highly aged traffic emissions had a smaller core (∼  60–80 nm) and a very thick ACT (∼  130–300 nm), which is larger than reported in any previous literature. A fast growth rate (∼  20 nm 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.