Quantification of black carbon mixing state from traffic: implications for aerosol optical properties

Megan D Willis,John C Wenger,Greg J Evans,Matthew West,Jonathan P D Abbatt,Robert M Healy,Nicole Riemer,Cheol-Heon Jeong,Jon M Wang,Alex K Y Lee

doi:10.5194/acp-16-4693-2016

Abstract

Abstract. The climatic impacts of black carbon (BC) aerosol, an important absorber of solar radiation in the atmosphere, remain poorly constrained and are intimately related to its particle-scale physical and chemical properties. Using particle-resolved modelling informed by quantitative measurements from a soot-particle aerosol mass spectrometer, we confirm that the mixing state (the distribution of co-emitted aerosol amongst fresh BC-containing particles) at the time of emission significantly affects BC-aerosol optical properties even after a day of atmospheric processing. Both single particle and ensemble aerosol mass spectrometry observations indicate that BC near the point of emission co-exists with hydrocarbon-like organic aerosol (HOA) in two distinct particle types: HOA-rich and BC-rich particles. The average mass fraction of black carbon in HOA-rich and BC-rich particle classes was < 0.1 and 0.8, respectively. Notably, approximately 90 % of BC mass resides in BC-rich particles. This new measurement capability provides quantitative insight into the physical and chemical nature of BC-containing particles and is used to drive a particle-resolved aerosol box model. Significant differences in calculated single scattering albedo (an increase of 0.1) arise from accurate treatment of initial particle mixing state as compared to the assumption of uniform aerosol composition at the point of BC injection into the atmosphere.

Highlights

Incomplete combustion emits teragram quantities of black carbon (BC) aerosol to the troposphere each year, resulting in a significant warming effect on climate that may be second only to carbon dioxide (Bond et al, 2013; Ramanathan and Carmichael, 2008; Jacobson, 2001)
Based on single particle soot-particle aerosol mass spectrometer (SP-AMS) measurements in both roadside and non-roadside studies, we identified two types of particles originating from vehicle exhaust, which are primarily composed of refractory black carbon (rBC) and hydrocarbon-like organic aerosol (Fig. 1a–d)
One type is dominated by hydrocarbon-like organic aerosol (HOA) mass and the other is dominated by rBC (“rBC-rich” particle class)

Summary

Introduction

Incomplete combustion emits teragram quantities of black carbon (BC) aerosol to the troposphere each year, resulting in a significant warming effect on climate that may be second only to carbon dioxide (Bond et al, 2013; Ramanathan and Carmichael, 2008; Jacobson, 2001). BC impacts on the global scale remain poorly constrained and are intimately related to its particle-scale physical and chemical properties (Bond et al, 2013). The majority of BC emissions in North America, Europe, and Latin America are derived from traffic-related sources, though the specific physical and chemical properties of BC-containing particles at emission depend greatly on the source (Bond et al, 2013). BC-containing particles are generally hydrophobic near emission and become mixed over time with hydrophilic species through condensation and coagulation (Johnson et al, 2005; Moteki et al, 2007; Moffet and Prather, 2009), with resulting impacts on particle hygroscopicity

Methods

Results

Conclusion