Light absorption properties of elemental carbon (EC) and water-soluble brown carbon (WS-BrC) in the Kathmandu Valley, Nepal: A 5-year study

Pengfei Chen; Shichang Kang; Lekhendra Tripathee; Kirpa Ram; Maheswar Rupakheti; Arnico K Panday; Qianggong Zhang; Junming Guo; Xiaoxiang Wang; Tao Pu; Chaoliu Li

doi:10.1016/j.envpol.2020.114239

Light absorption properties of elemental carbon (EC) and water-soluble brown carbon (WS-BrC) in the Kathmandu Valley, Nepal: A 5-year study

Environ Pollut. 2020 Jun:261:114239. doi: 10.1016/j.envpol.2020.114239. Epub 2020 Feb 20.

Authors

Affiliations

¹ State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences (CAS), Lanzhou, 730000, China.
² State Key Laboratory of Cryospheric Science, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences (CAS), Lanzhou, 730000, China; CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, 100101, China; Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, CAS, Beijing, 100101, China. Electronic address: shichang.kang@lzb.ac.cn.
³ Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, CAS, Beijing, 100101, China; Institute of Environment and Sustainable Development, Banaras Hindu University, Varanasi, 221005, India.
⁴ Institute for Advanced Sustainability Studies, Potsdam, 14467, Germany.
⁵ International Centre for Integrated Mountain Development, 44700, Kathmandu, Nepal.
⁶ CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing, 100101, China; Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, CAS, Beijing, 100101, China.

PMID: 32114124
DOI: 10.1016/j.envpol.2020.114239

Abstract

This study presents a comprehensive analysis of organic carbon (OC), elemental carbon (EC), and particularly the light absorption characteristics of EC and water-soluble brown carbon (WS-BrC) in total suspended particles in the Kathmandu Valley from April 2013 to January 2018. The mean OC, EC, and water-soluble organic carbon (WSOC) concentrations were 34.8 ± 27.1, 9.9 ± 5.8, and 17.4 ± 12.5 μg m^-3, respectively. A clear seasonal variation was observed for all carbonaceous components with higher concentrations occurring during colder months and lower concentrations in the monsoon season. The relatively low OC/EC ratio (3.6 ± 2.0) indicates fossil fuel combustion as the primary source of carbonaceous components. The optical attenuation (ATN) at 632 nm was significantly connected with EC loading (EC_S) below 15 μg cm^-2 but ceased as EC_S increased, reflecting the increased influence of the shadowing effect. The derived average mass absorption cross-section of EC (MAC_EC) (7.0 ± 4.2 m² g^-1) is comparable to that of freshly emitted EC particles, further attesting that EC was mainly produced from local sources with minimal atmospheric aging processes. Relatively intensive coating with organic aerosols and/or salts (e.g., sulfate, nitrate) was probably the reason for the slightly higher MAC_EC during the monsoon season, whereas increased biomass burning was a major factor leading to lower MAC_EC in other seasons. The average MAC_WS-BrC at 365 nm was 1.4 ± 0.3 m² g^-1 with minimal seasonal variations. In contrast to MAC_EC, biomass burning was the main reason for a higher MAC_WS-BrC in the non-monsoon season. The relative light absorption contribution of WS-BrC to EC was 9.9% over the 300-700 nm wavelength range, with a slightly higher ratio (13.6%) in the pre-monsoon season. Therefore, both EC and WS-BrC should be considered in the study of optical properties and radiative forcing of carbonaceous aerosols in this region.

Keywords: Elemental carbon; Kathmandu valley; Light absorption; Radiative forcing; Water-soluble brown carbon.

MeSH terms

Aerosols
Biomass
Carbon* / chemistry
Environmental Monitoring*
Light*
Nepal
Particulate Matter* / chemistry
Seasons
Water / chemistry

Substances

Aerosols
Particulate Matter
Water
Carbon