Wintertime Air Quality in Lumbini, Nepal: Sources of Fine Particle Organic Carbon

Md Robiul Islam, Tianyi Li, Khadak Mahata, Nita Khanal, Benjamin Werden, Michael R. Giordano, P. S. Praveen, Narayan Babu Dhital, Anobha Gurung, Arnico K. Panday, Indu Bikram Joshi, Shankar Prasad Poudel, Yanbo Wang, Eri Saikawa, Robert J. Yokelson, Peter F. Decarlo, Elizabeth A. Stone

Research output: Contribution to journalArticlepeer-review

Abstract

The Indo-Gangetic Plains (IGP) experience high levels of airborne particulate matter (PM), especially during the dry season. Contributing to PM are natural and anthropogenic emissions and the atmospheric transformation of gases to form particles. Regional smog events occur frequently during wintertime and provide an atmospheric medium for aerosol processing. Here, we investigate the chemical composition and sources of PM at a representative site in the northern IGP during the second Nepal Ambient Monitoring and Source Testing Experiment (NAMaSTE 2). In Lumbini, Nepal, the 24 h average PM2.5 and PM10 concentrations ranged 48-295 and 60-343 μg m-3, respectively, from December 20, 2017, to January 1, 2018. On average (± standard deviation), PM2.5 was composed of 39 ± 7% organic carbon (OC), 5 ± 2% elemental carbon (EC), and 20 ± 6% secondary inorganic ions (ammonium, nitrate, and sulfate), 2.0% chloride, and 1.3% potassium. Biomass burning was a major PM source, indicated by a median levoglucosan concentration of 3.5 μg m-3. Secondary organic aerosol (SOA) derived from biomass burning was indicated by high concentrations of nitromonoaromatic compounds (e.g., 4-nitrocatechol peaking at 435 ng m-3). During periods of fog, characterized by high relative humidity (RH) and relatively low solar radiation, nitroaromatic concentrations dropped despite levoglucosan remaining high, indicating that their formation was suppressed. Chemical signatures of SOA indicated that volatile organic compound (VOC) precursors were primarily combustion-derived, with small contributions from biogenic VOC. Through molecular markers and chemical mass balance (CMB) modeling, sources of PM2.5 OC were identified as cow dung burning (24 ± 16%), other biomass burning (20 ± 7%), plastic/garbage burning (4.7 ± 3.2%), vehicle emissions (3.1 ± 1.4%), coal combustion (0.3 ± 0.2%), and SOA from monoaromatic VOC (4.1 ± 0.8%), diaromatic VOC (8.9 ± 4.0%), cresol (0.3 ± 0.4%), isoprene (0.4 ± 0.2%), monoterpenes (1.5 ± 0.6%), and sesquiterpenes (3.2 ± 0.7%). Understanding the levels of PM in Lumbini, along with its chemical composition and sources of OC, contributes to a better understanding of regional air quality episodes in the IGP.

Original languageEnglish
Pages (from-to)226-238
Number of pages13
JournalACS Earth and Space Chemistry
Volume5
Issue number2
DOIs
StatePublished - Feb 18 2021

Keywords

  • Indo-Gangetic plains
  • atmospheric aerosols
  • biomass burning
  • carbonaceous aerosol
  • chemical mass balance
  • garbage burning
  • molecular markers
  • source apportionment

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