Spectral absorption properties of organic carbon aerosol during a polluted winter in Beijing, China

Yunfei Wu; Jiwei Li; Chen Jiang; Yunjie Xia; Jun Tao; Ping Tian; Chang Zhou; Chaoying Wang; Xiangao Xia; Ru-Jin Huang; Renjian Zhang

doi:10.1016/j.scitotenv.2020.142600

Spectral absorption properties of organic carbon aerosol during a polluted winter in Beijing, China

Sci Total Environ. 2021 Feb 10;755(Pt 2):142600. doi: 10.1016/j.scitotenv.2020.142600. Epub 2020 Oct 2.

Authors

Yunfei Wu¹, Jiwei Li², Chen Jiang², Yunjie Xia², Jun Tao³, Ping Tian⁴, Chang Zhou², Chaoying Wang², Xiangao Xia⁵, Ru-Jin Huang⁶, Renjian Zhang⁵

Affiliations

¹ Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China. Electronic address: wuyf@mail.iap.ac.cn.
² Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; University of Chinese Academy of Sciences, Beijing 100049, China.
³ South China Institute of Environmental Sciences, Ministry of Ecology and Environment, Guangzhou 510530, China.
⁴ Beijing Weather Modification Office, Beijing 100089, China.
⁵ Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China; Xianghe Observatory of Whole Atmosphere, Institute of Atmospheric Physics, Chinese Academy of Sciences, Xianghe 065400, China.
⁶ Key Laboratory of Aerosol Chemistry and Physics, State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China.

PMID: 33045607
DOI: 10.1016/j.scitotenv.2020.142600

Abstract

A fraction of organic carbon (OC) is found to exhibit the capability to absorb solar radiation. However, the absorption properties of OC remain poorly characterized partly due to uncertainties in determination methods. In this study, the absorption coefficient (b_ap) of OC (b_ap,OC) in Beijing during a polluted winter was estimated on the basis of the combined measurements of black carbon (BC) size distribution and total aerosol b_ap (b_ap,meas). The bare BC b_ap (b_ap,bareBC) calculated using Mie theory on the basis of measured size distribution exhibited weak wavelength dependence, with a mean absorption Ångström exponent (AAE) of 0.56 ± 0.04 within the 470-660 nm wavelength range, which was lower than the value of 1 commonly used for freshly emitted BC. The calculated b_ap,bareBC was compared with b_ap,meas at 950 nm to derive the coating thickness of BC, from which the calculation of coated BC b_ap (b_ap,coatBC) within 370-660 nm was based using the core-shell Mie model. Given the thick coatings, the AAE of coated BC, with a mean of 0.53 ± 0.12, was slightly lower than that of bare BC. Subsequently, b_ap,OC was obtained by subtracting b_ap,coatBC from b_ap,meas, accounting for 59.57 ± 4.82% of b_ap,meas at 370 nm on average. The average mass absorption efficiency of OC was estimated to be 1.48 ± 0.36 m² g^-1 at 370 nm. b_ap,OC significantly decreased as wavelength increased, deriving an AAE of OC with a mean of 2.72 ± 0.32 within the 370-660 nm range. The level of b_ap,OC estimated on the basis of a widely used attribution method assuming a constant BC AAE of 1 was ~60% lower than the currently presented value, probably underestimating OC radiative effect by a factor of >3. More accurate estimations of b_ap,OC based on more advanced measurements and suitable theory calculations are recommended to provide more reliable assessments of OC radiative effects.

Keywords: Black carbon; Organic carbon; Size distribution; Spectral absorption properties.