Light Absorption by Ambient Black and Brown Carbon and its Dependence on Black Carbon Coating State for Two California, USA, Cities in Winter and Summer

Christopher D Cappa,Douglas R Worsnop,Xiaolu Zhang,Kevin J Sanchez,Leah R Williams,Derek J Price,Jun Liu,Chia‐Li Chen,Lynn M Russell,Timothy B Onasch,Sonya Collier,Alex K Y Lee,Sijie Chen,Qi Zhang,Jon Abbatt,Raghu Betha,Gavin R Mcmeeking

doi:10.1029/2018jd029501

Abstract

AbstractObservations from a wintertime and summertime field campaign are used to assess the relationship between black and brown carbon (BC and BrC, respectively) optical properties and particle composition and coating state. The wintertime campaign, in Fresno, CA, was impacted by primary emissions from residential wood burning, secondary organic and inorganic particle formation, and BC from motor vehicles. Two major types of BrC were observed in wintertime. One occurred primarily at night—the result of primary biomass burning emissions. The second was enhanced in daytime and strongly associated with particulate nitrate and the occurrence of fog. The biomass‐burning‐derived BrC absorbed more strongly than the nitrate‐associated BrC but had a weaker wavelength dependence. The wintertime BC‐specific mass absorption coefficient (MACBC) exhibited limited dependence on the ensemble‐average coating‐to‐BC mass ratio (Rcoat‐rBC) at all wavelengths, even up to Rcoat‐rBC of ~5. For the summertime campaign, in Fontana, CA, BC dominated the light absorption, with negligible BrC contribution even after substantial photochemical processing. The summertime MACBC exhibited limited dependence on Rcoat‐rBC, even up to ratios of >10. Based on the four classes of BC‐containing particles identified by Lee et al. (2017, https://doi.org/10.5194/acp‐17‐15055‐2017) for the summertime measurements, the general lack of an absorption enhancement can be partly—although not entirely—attributed to an unequal distribution of coating materials between the BC‐containing particle types. These observations demonstrate that in relatively near‐source environments, even those impacted by strong secondary aerosol production, the ensemble‐average, mixing‐induced absorption enhancement for BC due to coatings can be quite small.

Highlights

Light absorbing aerosol particles influence climate through their ability to absorb solar and, to a lesser extent, longwave radiation
The mixing-induced absorption enhancement can be defined as Eabs = babs,coat/babs,black carbon (BC), where babs,coat and babs,BC are the absorption coefficients for coated 87 and uncoated BC, respectively
We report on measurements of light absorption by ambient particles made concurrent with BC ensemble-average coating state and ensemble-average total particle composition

Summary

Introduction

Light absorbing aerosol particles influence climate through their ability to absorb solar and, to a lesser extent, longwave radiation. When BC is internally mixed with (i.e., “coated” by) non-absorbing materials, the absorption by the BC is increased (Bond et al, 2006; Fuller et al, 1999; Lack and Cappa, 2010) This mixing-induced enhancement, commonly (yet inaccurately) referred to as the “lensing” based absorption enhancement, has been observed in various laboratory studies using controlled, typically mono-disperse, BC sources (Cappa et al, 2012; Cross et al, 2010; Lack et al, 2009; Metcalf et al, 2013; Peng et al, 2016; Schnaiter et al, 2005; Shiraiwa et al, 2010). The mixing-induced absorption enhancement can be defined as Eabs = babs,coat/babs,BC, where babs,coat and babs,BC are the absorption coefficients for coated and uncoated BC, respectively This definition implies no contributions from other absorbing components (e.g., BrC), it should be noted that the observable Eabs can be influenced by absorbing components that are either internally or externally mixed from BC.

Methods

Results

Conclusion