Color of brown carbon: A model for ultraviolet and visible light absorption by organic carbon aerosol

Abstract

We recommend ultraviolet and visible absorption spectra to represent particular types of atmospheric organic particles. Spectra of liquids and particles can be compared using the absorption coefficient of bulk material divided by material density. Reported absorption by absorbing organic aerosol from combustion is greater than that of organic material isolated by humic acid extraction. We examine ultraviolet and visible spectra of 200 organic compounds, concluding that visible absorption may be attributable to n → π* electronic transitions in a small fraction of oxygenated compounds. Absorption spectra can be communicated using the band‐gap and Urbach relationships instead of the absorption Angstrom exponent. Water‐soluble atmospheric aerosol has a band‐gap of about 2.5 eV; insoluble aerosol may have a lower band‐gap and higher absorption. Although different types of organic carbon may exhibit a continuum in absorption, there is a sharp distinction between the most‐absorbing organic carbon and black carbon.

Inferring absorbing organic carbon content from AERONET data

“…7) give optical constants of several organic components, most of which taken from Jacobson (1999), but all these imaginary indices are around 350 nm or lower. However, in order to carry out a sensitivity of our retrieval approach, we estimated absorbing OC concentrations using imaginary index values reported by Sun et al (2007) and Chen and Bond (2010). Chen and Bond (2010) values are smaller than those of Kirchstetter et al (2004) used in our study, therefore our retrieved absorbing OC concentrations in turn are lower.…”

“…All the components are assumed to be spectrally constant, except for OC whose imaginary index at 440 nm and 670 nm is by Kirchstetter et al (2004) and zero at 870 nm and 1020 nm. Shown are also the imaginary index values for OC, based on Chen and Bond (2010) and Sun et al (2007), that were used to assess the sensitivity of the OC retrieval for the assumed imaginary index. Kirchstetter et al (2004) 1.53 0.073/0.0034 (440 nm/670 nm) OC, Chen and Bond (2010) to be larger than the mean of the imaginary indices at 670, 870 and 1020 nm.…”

Truncation and Angular-Scattering Corrections for Absorbing Aerosol in the TSI 3563 Nephelometer

“…These aerosols were assumed to be homogeneous, that is, there were no differences in refractive index within each particle. For realistic values of k, we used two wavelength-dependent refractive indices for brown carbon suggested by Sun et al (2007;termed "humic" and "polymerized"). The Sun et al paper fit relationships to absorption spectra from atmospheric light-absorbing carbon that had appeared in the literature to date, including reports from Havers et al (1998), Kirchstetter et al (2004), and Hoffer et al (2006).…”

“…The resulting error in calculated scattering and absorption is also given. The refractive indices were chosen to correspond roughly to properties of polluted air, air near roadways, and brown carbon (Sun et al 2007), but these values are only illustrative. Uncertainties introduced by the AO1998 truncation correction are not large except for Case 3, where the error in co-albedo (absorption) is in error by 20%.…”

Estimation of aerosol water and chemical composition from AERONET Sun–sky radiometer measurements at Cabauw, the Netherlands

“…For most compounds, especially OC, RI decreases with wavelength (Table 4), but on several occasions AERONET RI increases with wavelength, which leads to discrepancies between modeled and observed aerosol optical properties, especially at 440 nm. Since IRI strongly increases towards the ultraviolet wavelengths for OC, contrary to that of BC (Jacobson, 1999;Kirchstetter et al, 2004;Sun et al, 2007;and Chen and Bond, 2010), this hampers the method's ability to distinguish black carbon from absorbing organic matter ("brown carbon") (e.g., Bergstrom et al, 2007;Russell et al, 2010;Arola et al, 2011).…”