Abstract
ABSTRACTAerosol light absorption is important to radiation balance, but it is difficult to accurately quantify using measurements. An intercomparison experiment for the measurement of the aerosol absorption coefficient (bap) was performed at an urban site in Beijing during the summer of 2012, including the filter-based particle soot absorption photometer (PSAP) and aethalometer (AE-31), and the reference photoacoustic extinctiometer (PAX), CRDS-Neph (cavity-ring down spectroscopy/nephelometer) system, and multi-angle absorption photometer (MAAP). The CRDS-Neph system and PAX performed poorly due to unexpected reasons. The corrected bap of the PSAP agreed well with the reference values determined by the MAAP, implying the applicability of this correction scheme as well as the credibility of the reference bap of the MAAP. A new conversion factor with a value of ~7.1 ± 0.05 m2/g at ~530 nm was established by regressing the reference bap against the AE-31 recorded black carbon (BC) concentrations, which is lower than the previously used value (8.28 m2/g). Accordingly, the absorption Angstrom exponent (αap) was estimated as 0.85 ± 0.21 on average. It was ~1 on clean days but significantly lower during pollution episodes, implying the main contributor to aerosol light absorption is freshly-emitted BC on clean days but aged BC during pollution. BC core sizes and the coating are likely to have a great impact on the αap, which needs further investigation. The mass absorption efficiency of BC was estimated by regressing the bap against the filter-analyzed elemental carbon (EC) concentrations, resulting in a mean of 9.2 ± 0.5 m2/g at 670 nm. It was remarkably higher during pollution episodes than on clean days, implying a high variation of aerosol properties, such as the mixing state, with pollution levels.
Highlights
Aerosol light absorption is important for its direct influence on radiation balance (Charlson et al, 1992; Jacobson, 2001) and for its semi-direct effect on the enhanced dissipation of clouds by heating the vicinal atmosphere (Ackerman et al, 2000)
Aerosol light absorption is important to radiation balance, but it is difficult to accurately quantify using measurements
To reduce the uncertainties in measurements caused by the multiply light scattering within the filter, and the scattering induced by embedded aerosols, a new filter-based instrument, the multi-angle absorption photometer (MAAP), has been developed to detect the backscattered light at two angles at the same time as when the transmission is measured
Summary
Aerosol light absorption is important for its direct influence on radiation balance (Charlson et al, 1992; Jacobson, 2001) and for its semi-direct effect on the enhanced dissipation of clouds by heating the vicinal atmosphere (Ackerman et al, 2000). Accurate measurements of the bep (e.g., as measured by cavity-ring down spectroscopy, CRDS) (O’Keefe and Deacon, 1988) and the synchronous bsp (e.g., as measured by an integrating nephelometer, Neph) (Anderson et al, 1996) can provide reference values for bap These reference instruments can provide more reliable bap, it is not feasible for them to be widely used in long-term monitoring studies because of their high cost and/or the difficulty involved in their operation and maintenance, especially at remote sites. They usually only provide information on aerosol absorption at a single wavelength (e.g., ~530 nm) and cannot indicate the light absorption spectral dependence of aerosols, which is important in climate models (Bond, 2001). Because of the high spatial variation in aerosol properties, our work is meaningful for the acquisition of relatively accurate aerosol absorption properties in this region
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