title: Measured Wavelength-Dependent Absorption Enhancement of Internally Mixed Black Carbon with Absorbing and Nonabsorbing Materials.
abstract: Optical absorption spectra of laboratory generated aerosols consisting of black carbon (BC) internally mixed with nonabsorbing materials (ammonium sulfate, AS, and sodium chloride, NaCl) and BC with a weakly absorbing brown carbon surrogate derived from humic acid (HA) were measured across the visible to near-IR (550 to 840 nm). Spectra were measured in situ using a photoacoustic spectrometer and step-scanning a supercontinuum laser source with a tunable wavelength and bandwidth filter. BC had a mass-specific absorption cross section (MAC) of 7.89 ± 0.25 m(2) g(-1) at λ = 550 nm and an absorption Ångström exponent (AAE) of 1.03 ± 0.09 (2σ). For internally mixed BC, the ratio of BC mass to the total mass of the mixture was chosen as 0.13 to mimic particles observed in the terrestrial atmosphere. The manner in which BC mixed with each material was determined from transmission electron microscopy (TEM). AS/BC and HA/BC particles were fully internally mixed, and the BC was both internally and externally mixed for NaCl/BC particles. The AS/BC, NaCl/BC, and HA/BC particles had AAEs of 1.43 ± 0.05, 1.34 ± 0.06, and 1.91 ± 0.05, respectively. The observed absorption enhancement of mixed BC relative to the pure BC was wavelength dependent for AS/BC and decreased from 1.5 at λ = 550 nm with increasing wavelength while the NaCl/BC enhancement was essentially wavelength independent. For HA/BC, the enhancement ranged from 2 to 3 and was strongly wavelength dependent. Removal of the HA absorption contribution to enhancement revealed that the enhancement was ≈1.5 and independent of wavelength.
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.|Department of Chemistry and Biochemistry, University of Maryland , College Park, Maryland 20742, United States.
authors.forename: Rian
authors.initials: R
authors.lastname: You
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.
authors.forename: James G
authors.initials: JG
authors.lastname: Radney
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.|Department of Chemistry and Biochemistry, University of Maryland , College Park, Maryland 20742, United States.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Zachariah
authors.affiliation: Material Measurement Laboratory, National Institute of Standards and Technology , Gaithersburg, Maryland 20899, United States.
authors.forename: Christopher D
authors.initials: CD
authors.lastname: Zangmeister
mesh_terms: D000336:Aerosols / Q000032:analysis; D001272:Atmosphere; D002244:Carbon; D006812:Humic Substances; D053260:Soot / Q000032:analysis; D013057:Spectrum Analysis
chemical_list: D000336:Aerosols; D006812:Humic Substances; D053260:Soot; D002244:Carbon