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 3.2ĝ€μgĝ€mĝ'3, with the peak value of 12.1ĝ€μgĝ€mĝ'3 at 04:26ĝ€LT on 7 December. The number of BC-containing particles captured by SPAMS in the size range 200-1200ĝ€nm agreed very well with that detected by SP2 ( R 2 Combining double low line 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 ĝ1/4 ĝ€230ĝ€nm and droplet mode peaked around ĝ1/4 ĝ€380ĝ€nm, with a clear valley in the size distribution around ĝ1/4 ĝ€320ĝ€nm. The condensation mode mainly consisted of traffic emissions, with particles featuring a small rBC core (ĝ1/4 ĝ€60-80ĝ€nm) and a relatively thin absolute coating thickness (ACT, ĝ1/4 ĝ€50-130ĝ€nm). The droplet mode included highly aged traffic emission particles and biomass burning particles. The biomass burning particles had a larger rBC core (ĝ1/4 ĝ€80-130ĝ€nm) and a thick ACT (ĝ1/4 ĝ€110-300ĝ€nm). The highly aged traffic emissions had a smaller core (ĝ1/4 ĝ€60-80ĝ€nm) and a very thick ACT (ĝ1/4 ĝ€130-300ĝ€nm), which is larger than reported in any previous literature. A fast growth rate (ĝ1/4 ĝ€20ĝ€nmĝ€hĝ'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.