Abstract

We deployed a Single Particle Soot Photometer (SP2) and Single Particle Aerosol Mass Spectrometer (SPAMS) to investigate the size distribution and mixing state of ambient black carbon (BC) particles in Shanghai during the winter of 2017. The mixing state of the particles changed drastically under different meteorological conditions, and higher concentrations often occurred during haze episodes. The BC particles existed in two size modes: the condensation mode, which mainly consisted of fresh traffic-emitted particles, and the droplet mode, which encompassed heavily aged traffic-emitted particles, biomass burning particles and heavy-duty diesel engine emission particles. Whereas the heavily aged traffic-emitted particles exhibited relatively small BC cores (60–80 nm in diameter) surrounded by a thick coating (100–160 nm), the biomass burning particles displayed slightly larger cores (80–130 nm) covered by an even thicker layer (160–300 nm). However, we observed a large number of particles with large cores (150–200 nm) but a relatively thin coating (40–80 nm) on clean days. Trajectory analyses of the air masses showed that they usually originated in local areas or on the North China Plain during haze episodes but mostly migrated from the East China Sea during clean episodes. Interestingly, the BC particles in the clean air masses contained V more frequently and generated intense mass spectrum signals for Ca+, suggesting that these particles probably arose from heavy machinery emissions near ports. Our real-time single particle data indicates that the sources of BC aerosol can be easily identified by measuring the size distribution and mixing state of the particles.

Highlights

  • black carbon (BC) aerosols, one of the strongest light-absorbing components in the atmosphere and a major contributor to positive radiative forcing, are produced by incomplete combustion of fossil fuels or biomass (Jayne et al, 2000; Bond et al, 2013)

  • The results show that BC coatings were positively correlated with the pollution level)

  • We characterized the ambient BC particles during both clean and polluted periods in wintertime Shanghai by analyzing their size distribution and mixing state with an SP2 and a Single Particle Aerosol Mass Spectrometer (SPAMS), which can detect the physical and chemical properties of BC aerosol at the resolution of a single particle

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Summary

INTRODUCTION

BC aerosols, one of the strongest light-absorbing components in the atmosphere and a major contributor to positive radiative forcing, are produced by incomplete combustion of fossil fuels or biomass (Jayne et al, 2000; Bond et al, 2013). Liu et al (2019) studied the size-resolved mixing state of BC-containing particles by an SP2 and their mass spectra was measured by a Soot Particle Aerosol Mass Spectrometer (SP-AMS). They compared two online source apportionment methods using simultaneous measurements made by the SP2, and the chemical approach using the positive matrix factorization (PMF) of mass spectra from the SP-AMS for the first time (Liu et al, 2019). The size distribution and mixing state of BC were analyzed where the observation site is near the port with increased emissions by various heavy machinery in urban Shanghai based on the measurements combining SP2 and SPAMS during a wintertime in 2017 when haze occurred frequently. We compared the results from this study to Gong et al (2016) and showed how fresh industrial emissions affected BC particle mixing states

METHODS
Overview of the Sampling Conditions
BC Size Distribution and Number Concentration Measured by SP2
BC Particle Classification by SPAMS
Size distribution and source identification of BC particles
60 Case 1 Case 2 Case 3 Case 4
Findings
CONCLUSIONS
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