Volume 15, No. 4, August 2015, Pages 1178-1187 PDF(491 KB)
Spectral Light Absorption of Ambient Aerosols in Urban Beijing during Summer: An Intercomparison of Measurements from a Range of Instruments
Yunfei Wu1, Peng Yan2, Ping Tian1, Jun Tao3, Ling Li4, Jianmin Chen4, Yangmei Zhang5, Nianwen Cao6, Chong Chen6, Renjian Zhang1
1 RCE-TEA, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
2 CAWAS, Meteorological Observation Center of CMA, Beijing 100081, China
3 South China Institute of Environmental Sciences, Ministry of Environmental Protection, Guangzhou 510655, China
4 Center of Atmospheric Chemistry Study, Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China
5 Chinese Academy of Meteorological Sciences of CMA, Beijing 100081, China
6 School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing 210044, China
- Intercomparisons of the bap determined by different methods were performed.
- The corrected bap of PSAP agreed well with the reference determined by the MAAP.
- A new conversion factor between the bap and the BC recorded by AE-31 was presented.
- The αap were interestedly lower than 1, especially during the pollution.
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 (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 Ångström 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.
Ambient aerosol; Black carbon; Spectral light absorption.