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 non-absorbing 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 nm 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 m2 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.

Measurement and modeling of the multiwavelength optical properties of uncoated flame-generated soot

“…They did not report any notable size dependence for their MAC values. Our MAC values (especially for x > 0.5) compare well with the value of 8.16 m 2 g −1 at λ = 532 reported by You et al (2016) (extrapolated from 7.89 ± 0.25 m 2 g −1 at λ = 550 nm) for soot particles generated from the combustion of organic fuel stock over the d p,VED range ∼ 80 to ∼ 210 nm. They also compare favorably to the range of values 7.2 to 8.5 m 2 g −1 reported for λ = 532 nm for d p,VED ∼ 100 nm observed in Saliba et al (2016) for particles generated from a cookstove.…”

“…terial (Schnaiter et al, 2003(Schnaiter et al, , 2006Kirchstetter et al, 2004;Bergstrom et al, 2002;You et al, 2016;Sharma et al, 2013).…”

Measurement and modeling of the multi-wavelength optical properties of uncoated flame-generated soot

“…The measurements here are consistent with this expectation for the collapsed (coated-denuded) particles, but the nascent particles give an AAE that is somewhat larger than 1. The observed AAE values are similar to results from previous studies examining either freshly emitted soot particles or soot particles containing very little organic material (Bergstrom et al, 2002;Schnaiter et al, 2003;Kirchstetter 10 et al, 2004;Schnaiter et al, 2006;Sharma et al, 2013;You et al, 2016).…”

Light source effects on aerosol photoacoustic spectroscopy measurements

“…This can be demonstrated by assuming that each soot monomer behaves as an independent subunit of the whole (i.e. Rayleigh-Debye-Gans approximation, RDG) and has a refractive index of 1.77 + 0.8 i at λ = 660 nm – the value determined for flame-generated black carbon at λ = 660 nm in You, et al (2016) [10]. Therefore, each dry monomer contributes 2.23 × 10 −17 m 2 to the measured C abs ; for the roughly 639 monomers in an aggregate, this would correspond to an aggregate C abs of 1.42 × 10 −14 m 2 which is within reason of the values shown in Fig.…”

“…However, photoacoustic measurement uncertainties related to the type of laser used has received little attention. In previous investigations, pulsed [6–8], quasi-continuous wave [1, 9, 10] and continuous wave [2, 5, 11–14] lasers have been used with the latter being the most common.…”