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Photoacoustic Measurements of Amplification of theAbsorption Cross Section for Coated Soot AerosolsPedro A. Bueno a b , Daniel K. Havey b c , George W. Mulholland a b , Joseph T. Hodges b ,Keith A. Gillis b , Russell R. Dickerson a & Michael R. Zachariah a ba Department of Chemistry and Biochemistry, University of Maryland, College Park,Maryland, USAb National Institute of Standards and Technology, Gaithersburg, Maryland, USAc Department of Chemistry and Biochemistry, James Madison University, Harrisonburg,Virginia, USA
Available online: 13 May 2011
To cite this article: Pedro A. Bueno, Daniel K. Havey, George W. Mulholland, Joseph T. Hodges, Keith A. Gillis, Russell R.Dickerson & Michael R. Zachariah (2011): Photoacoustic Measurements of Amplification of the Absorption Cross Section forCoated Soot Aerosols, Aerosol Science and Technology, 45:10, 1217-1230
To link to this article: http://dx.doi.org/10.1080/02786826.2011.587477
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Aerosol Science and Technology, 45:12171230, 2011Copyright C American Association for Aerosol ResearchISSN: 0278-6826 print / 1521-7388 onlineDOI: 10.1080/02786826.2011.587477
Photoacoustic Measurements of Amplification of theAbsorption Cross Section for Coated Soot Aerosols
Pedro A. Bueno,1,2 Daniel K. Havey,2,3 George W. Mulholland,1,2 Joseph T.Hodges,2 Keith A. Gillis,2 Russell R. Dickerson,1 and Michael R. Zachariah1,21Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland, USA2National Institute of Standards and Technology, Gaithersburg, Maryland, USA3Department of Chemistry and Biochemistry, James Madison University, Harrisonburg, Virginia, USA
A quantitative understanding of the absorption and scatteringproperties of mixed soot and aerosol particles is necessary for eval-uating the Earths energy balance. Uncertainty in the net radiativeforcing of atmospheric aerosols is relatively large and may be lim-ited by oversimplified models that fail to predict these properties forbare and externally mixed soot particles. In this laboratory study offlame-generated soot, we combine photoacoustic spectroscopy, par-ticle counting techniques, and differential mobility analysis to ob-tain high-precision measurements of the size-dependent absorptioncross section of uncoated and coated soot particles. We investigatehow the coating of soot by nonabsorbing films of dibutyl phthalate(chosen as a surrogate for sulfuric acid) affects the particles mor-phology and optical properties. Absorption measurements weremade with photoacoustic spectroscopy using a laser at = 405 nm.We report measurements and model calculations of the absolutecross section, mass absorption coefficient, and amplification of theabsorption cross section. The results are interpreted and modeledin terms of a coreshell geometry and LorenzMie theory of scat-tering and absorption. We discuss evidence of soot particle andcollapse as a result of the coating process and we demonstrate theability to resolve changes in the coating thickness as small as 2 nm.
INTRODUCTIONSoot is the principal light-absorbing atmospheric aerosol
(Jacobson 2001). Sometimes referred to as black carbon, sootis composed of graphitic carbon, as well as other organic com-
Received 2 December 2010; accepted 18 March 2011.The authors would like to acknowledge the support of the Depart-
ment of Energys National Institute for Global Environmental ChangeAward #DE-FC02-03ER63613 and the support of NISTs GreenhouseGas Measurements Program. We also thank the University of Mary-lands Nanoscale Imaging Spectroscopy and Properties Lab for theuse of their facilities and Dr. Wen-An Chiou for producing the TEMmicrographs.
Address correspondence to Michael R. Zachariah, 2125 Glenn L.Martin Hall, University of Maryland, 2125 Martin Hall, College Park,MD 20742, USA. E-mail: [email protected]
pounds. Since soot is not uniquely defined in terms of chemicalcomposition, size or morphology, it is difficult to accuratelyassess its role in atmospheric chemistry and global warming.While the exact impact of soot on radiative transfer in the atmo-sphere is still under investigation, it is well established that allforms of soot absorb incoming solar radiation. This lightmatterinteraction contributes to warming of the atmosphere (Inter-governmental Panel on Climate Change [IPCC] 2009) and itsmagnitude is typically quantified in terms of a positive radia-tive forcing expressed as a radiant power per unit area. Theradiative forcing is directly related to the spectral dependenceof the absorption and scattering cross sections of the soot par-ticles (optical properties). However, the exact contribution ofsoot to the overall energy balance in the atmosphere is an un-resolved question that motivates this study. The IPCC reports arelatively large uncertainty in the radiative forcing of soot andother light-absorbing particles (IPCC 2009). The positive radia-tive forcing of light-absorbing atmospheric aerosols is maskedby the negative radiative forcing of most other particles presentin the atmosphere. The warming effect (positive radiative forc-ing) is effectively unique to soot, which makes it important asa greenhouse species. With a radiative forcing ranging from0.5 W m-2 to 0.9 W m-2, soot is one of the largest contributorsto atmospheric warming (Schmid et al. 2004; Ramanathan andCarmichael 2008).
It is understood that soot ages when released into the at-mosphere (Dickerson et al. 2002; Net et al. 2009). This pro-cess usually involves the oxidation of the particles and eventualcoating by chemicals found in the atmosphere. One of the mostcommon coatings found on soot is sulfuric acid (Hallett et al.1989; Schumann et al. 1996; Saathoff, Naumann, et al. 2003),which while weakly absorbing in the visible, may influencethe effective absorption efficiency of soot (Chylek et al. 1995;Jacobson 2000, 2001; Lack and Cappa 2010). The aging of thesoot leads to several possible mixing states: externally mixedstates, where soot is coarsely mixed with the coating material inseparate phases; coreshell states, where there is a well defined
1218 P. A. BUENO ET AL.
core and coating in a single particle; and internally mixed states,where the soot and coating material are well mixed in a near ho-mogenous manner in a single phase particle. It is the coreshellstate that appears to be the most prevalent for aged soot (Jacob-son 2000) and the system considered in this study. The coatingof soot with sulfuric acid and subsequent hygroscopic growthdue to interactions in the atmosphere lead to enhancement of theabsorption cross section of the soot particles; recent work hasshown that the coating processes can nearly double the amountof light absorption by soot (Zhang et al. 2008).
More accurate measurements of soot absorption cross sec-tion (Cabs) and radiative forcing are required by global climatemodels (Haywood and Ramaswamy 1998; Cooke et al. 1999;Haywood and Boucher 2000). However, such data are difficultto obtain because of variability in morphology, composition,mixing state, and conditioning of soot. Since soot cannot bedefined by a unique chemical formula (e.g., silica) and its com-position is source dependent (Cartwright et al. 1956; Schneider1972), it presents an initially difficult characterization problem,which becomes more complex based on atmospheric aging.These complexities contribute to the highly uncertain values forthe radiative forcing of soot. Accurate measurement of the op-tical properties of soot in the atmosphere is difficult because ofthe large number of uncontrolled variables, such as temperature,pressure, and humidity, as well as the soots source, composi-tion, history, and age (Saathoff, Mohler, et al. 2003; Weingartneret al. 2003; Schnaiter et al. 2005). Another factor that hindersmany optical measurements of soot is the traditional use offilter-based measurements such as particle soot absorption pho-tometers and aethalometers (Weingartner et al. 2003; Cappaet al. 2008; Chow et al. 2009; Lack et al. 2009). While filter-based measurements are robust and involve simple applicationsof Beers law, there are many drawbacks to those methods.These include: (1) an artificially high absorption reading if lightis scattered off the particles on th