Uncertainties in thermal-optical measurements of black carbon: Insights from source and ambient samples

Jiu-Meng Liu; Zhen-Yu Du; Lin-Lin Liang; Qin-Qin Yu; Guo-Feng Shen; Yong-Liang Ma; Mei Zheng; Yuan Cheng; Ke-Bin He

doi:10.1016/j.scitotenv.2018.11.353

Uncertainties in thermal-optical measurements of black carbon: Insights from source and ambient samples

Sci Total Environ. 2019 Mar 15:656:239-249. doi: 10.1016/j.scitotenv.2018.11.353. Epub 2018 Nov 24.

Authors

Jiu-Meng Liu¹, Zhen-Yu Du², Lin-Lin Liang³, Qin-Qin Yu¹, Guo-Feng Shen⁴, Yong-Liang Ma⁵, Mei Zheng⁶, Yuan Cheng⁷, Ke-Bin He⁵

Affiliations

¹ School of Environment, Harbin Institute of Technology, Harbin, China.
² National Research Center for Environmental Analysis and Measurement, Beijing, China.
³ State Key Laboratory of Severe Weather & Key Laboratory for Atmospheric Chemistry of CMA, Chinese Academy of Meteorological Sciences, Beijing, China.
⁴ College of Urban and Environmental Sciences, Peking University, Beijing, China.
⁵ State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, China.
⁶ College of Environmental Sciences and Engineering, Peking University, Beijing, China.
⁷ School of Environment, Harbin Institute of Technology, Harbin, China. Electronic address: ycheng@hit.edu.cn.

PMID: 30504024
DOI: 10.1016/j.scitotenv.2018.11.353

Abstract

Black carbon (BC) is important due to its complex influences on the environment and on climate in particular. However, reported BC data are largely dependent on measurement techniques due to the multitude of measurement principles. Here we focused on thermal-optical method which has been widely used to determine BC mass (as elemental carbon, EC). Several factors influencing EC measurement were investigated. Results from source samples representing vehicle engine emissions pointed to a continuum of EC components in thermal stability and provided direct observational evidence for the premature evolution of EC in inert atmosphere. It was also found that EC masses may be substantially underestimated for the vehicle exhaust samples if the adopted protocol requires an oxidizing atmosphere to define the split point between organic carbon (OC) and EC. Results from a field campaign conducted during winter in Beijing showed that the optical attenuation (ATN; i.e., the filter transmittance signal, I) was largely saturated for the samples with relatively high loadings, indicating their EC results were unreliable. Improved measurement of EC was achieved by extracting these heavily loaded filters using methanol, given that ATN was considerably reduced by the extraction and, moreover, saturation of ATN (or I) became not evident for the extracted samples. The methanol extraction also significantly reduced the transformation of OC to char-OC, by removing the majority (i.e., ~85%) of the deposited organic aerosols. Higher EC were measured for the extracted samples compared with the untreated ones, indicating that EC tends to be underestimated due to the charring-induced uncertainties. In addition, the methanol extraction largely reduced the inter-protocol discrepancy in the EC measurement results. Similar effects of methanol extraction have been observed during summer in Beijing, despite the seasonal variations of aerosol sources and compositions. This study indicates the potential benefits of methanol extraction for EC measurement.

Keywords: Elemental carbon; Methanol extraction; Optical attenuation; Organic carbon; Thermal-optical method.