Mixing state of carbonaceous aerosol in an urban environment: single particle characterization using the soot particle aerosol mass spectrometer (SP-AMS)

A K Y Lee,T B Onasch,M D Willis,R M Healy,J P D Abbatt

doi:10.5194/acp-15-1823-2015

Abstract

Abstract. Understanding the impact of atmospheric black carbon (BC)-containing particles on human health and radiative forcing requires knowledge of the mixing state of BC, including the characteristics of the materials with which it is internally mixed. In this study, we examine the mixing state of refractory BC (rBC) and other aerosol components in an urban environment (downtown Toronto) utilizing the Aerodyne soot particle aerosol mass spectrometer equipped with a light scattering module (LS-SP-AMS). k-means cluster analysis was used to classify single particle mass spectra into chemically distinct groups. One resultant particle class is dominated by rBC mass spectral signals (C1&amp;plus; to C5&amp;plus;) while the organic signals fall into a few major particle classes identified as hydrocarbon-like organic aerosol (HOA), oxygenated organic aerosol (OOA), and cooking emission organic aerosol (COA). A gradual mixing is observed with small rBC particles only thinly coated by HOA (~ 28% by mass on average), while over 90% of the HOA-rich particles did not contain detectable amounts of rBC. Most of the particles classified into other inorganic and organic particle classes were not significantly associated with rBC. The single particle results also suggest that HOA and COA emitted from anthropogenic sources were likely major contributors to organic-rich particles with vacuum aerodynamic diameter (dva) ranging from ~ 200 to 400 nm. The similar temporal profiles and mass spectral features of the organic classes identified by cluster analysis and the factors from a positive matrix factorization (PMF) analysis of the ensemble aerosol data set validate the interpretation of the PMF results.

Highlights

Atmospheric black carbon (BC) particles play an important role in regional air quality and introduce large uncertainties into radiative forcing estimates of Earth’s atmosphere (Bond et al, 2013)
Organics dominated the particulate mass, whereas refractory BC (rBC) contributed 4– 9 % of the average particle mass, assuming the collection efficiency (CE) for ambient rBC varies between 0.6 and 1 (i.e., CE range for bare and heavily coated Regal Black particles, respectively) (Willis et al, 2014) and the CE for all non-refractory particulate matter (NR-PM) evaporated from the tungsten vaporizer varies between 0.5 and 1 depending on chemical composition (Middlebrook et al, 2012)
Willis et al (2014) reported that the particle beam width of ambient rBC particles measured at the same location of downtown Toronto is similar to that of heavily coated Regal Black particles generated in the laboratory

Summary

Introduction

Atmospheric black carbon (BC) particles play an important role in regional air quality and introduce large uncertainties into radiative forcing estimates of Earth’s atmosphere (Bond et al, 2013). Coatings on BC aerosol surfaces with varying morphology (e.g., partly coated and embedded) and thickness have been observed using electron microscopy (China et al, 2013, 2014). These coatings can be formed through condensation and coagulation of low-volatility materials co-emitted from combustion sources (e.g., unburned organics) and produced via photochemical processing during transport. Current research is evaluating whether particle coatings can significantly enhance the light absorption efficiency of ambient BC (Cappa et al, 2012; Jacobson 2001; Lack et al, 2012; Metcalf et al, 2013).

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