Spectral absorption properties of atmospheric aerosols

Abstract

Abstract. We have determined the solar spectral absorption optical depth of atmospheric aerosols for specific case studies during several field programs (three cases have been reported previously; two are new results). We combined airborne measurements of the solar net radiant flux density and the aerosol optical depth with a detailed radiative transfer model for all but one of the cases. The field programs (SAFARI 2000, ACE Asia, PRIDE, TARFOX, INTEX-A) contained aerosols representing the major absorbing aerosol types: pollution, biomass burning, desert dust and mixtures. In all cases the spectral absorption optical depth decreases with wavelength and can be approximated with a power-law wavelength dependence (Absorption Angstrom Exponent or AAE). We compare our results with other recent spectral absorption measurements and attempt to briefly summarize the state of knowledge of aerosol absorption spectra in the atmosphere. We discuss the limitations in using the AAE for calculating the solar absorption. We also discuss the resulting spectral single scattering albedo for these cases.

Aerosol optical properties and direct radiative forcing based on measurements from the China Aerosol Remote Sensing Network (CARSNET) in eastern China

“…The mean AAE values at Xiaoshan and Fuyang were < 1.00, which suggests that coated black carbon particles dominated at these two sites. However, this also could be due to measurement uncertainties due to smaller numbers of samples from those sites or to slightly larger values of the imaginary part of the refractive index at longer wavelengths for certain particles (Bergstrom et al, 2007). There would need to be more observations to confirm the low AAEs at these two sites.…”

“…It should also be mentioned that uncertainties in the AAE calculations due to uncertainties in the SSAs may have contributed to the observed differences between sites (Bergstrom et al, 2007;Giles et al, 2011). Ideally, the aerosol physical and chemical characteristic measurements will be combined at some point to more definitively classify the aerosol types.…”

“…Generally, an AAE < 1 indicates mixing, coating and coagulation of black carbon with organic and inorganic materials; an AAE close to 1 indicates absorbing black carbon aerosols from the fossil fuel burning; and an AAE > 1.10 indicates absorbing aerosols mainly from biomass burning or mineral dust (Russell et al, 2010;Bergstrom et al, 2007;Lack and Cappa, 2010). The annual mean AAEs at Hangzhou, Xiaoshan, Fuyang, LinAn, Tonglu, Jiande and ChunAn were 1.13 ± 0.46, 0.88 ± 0.42, 0.85 ± 0.43, 0.98 ± 0.35, 1.11 ± 0.49, 1.16 ± 0.44 and 0.93 ± 0.31, respectively (Table 1), and for discussion purposes, the seven sites were grouped into three categories based on their average AAEs.…”

Aethalometer multiple scattering correction &lt;i&gt;C&lt;/i&gt;&lt;sub&gt;ref&lt;/sub&gt; for mineral dust aerosols

“…The SSA decreased with increasing wavelength, as expected for pollution aerosols (e.g. Bergstrom et al, 2007;Di Biagio et al, 2016). The wide range of values indicates the occurrence of particles with very different absorption properties, hence chemical composition (or complex refractive index) and/or different size distribution (e.g.…”

“…This is particularly true when compared to other aerosol species, such as soot, for which the SSA in the visible may be as low as 0.2 (Bergstrom et al, 2007). Nonetheless, because of its elevated atmospheric concentration (∼ 100-100 000 µg m −3 close to sources and ∼ 0.1-100 µg m −3 after mid-to intercontinental transport; e.g.…”