Spectral Light Absorption of Ambient Aerosols in Urban Beijing during Summer: An Intercomparison of Measurements from a Range of Instruments

• Published in: Aerosol and Air Quality Research 2015-08, Vol.15 (4), p.1178-1187+i-ix
• Main Authors: Wu, Yunfei, Yan, Peng, Tian, Ping, Tao, Jun, Li, Ling, Chen, Jianmin, Zhang, Yangmei, Cao, Nianwen, Chen, Chong, Zhang, Renjian
• Format: Article
• Language: English
• ISSN: 1680-8584; 2071-1409
• DOI: 10.4209/aaqr.2014.09.0224

Aerosol 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 ( b ap ) 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 b ap 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 b ap of the MAAP. A new conversion factor with a value of ~7.1 ± 0.05 m 2 /g at ~530 nm was established by regressing the reference b ap against the AE-31 recorded black carbon (BC) concentrations, which is lower than the previously used value (8.28 m 2 /g). Accordingly, the absorption Ångström exponent ( b ap ) 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 b ap , which needs further investigation. The mass absorption efficiency of BC was estimated by regressing the b ap against the filter-analyzed elemental carbon (EC) concentrations, resulting in a mean of 9.2 ± 0.5 m 2 /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.