Organic molecular tracers in the atmospheric aerosols from Lumbini, Nepal, in the northern Indo-Gangetic Plain: influence of biomass burning

摘要

To better understand the characteristics of biomass burning in thenorthern Indo-Gangetic Plain (IGP), total suspended particles were collectedin a rural site, Lumbini, Nepal, during April 2013 to March 2014 and analyzedfor the biomass burning tracers (i.e., levoglucosan, mannosan, vanillic acid). The annual average concentration of levoglucosan was734 ± 1043 ng m with the maximum seasonal mean concentrationduring post-monsoon season (2206 ± 1753 ng m), followed bywinter (1161 ± 1347 ng m), pre-monsoon(771 ± 524 ng m) and minimum concentration during monsoonseason (212 ± 279 ng m). The other biomass burning tracers(mannosan, galactosan, -hydroxybenzoic acid, vanillic acid, syringic acidand dehydroabietic acid) also showed the similar seasonal variations. Therewere good correlations among levoglucosan, organic carbon (OC) and elementalcarbon (EC), indicating significant impact of biomass burning activities oncarbonaceous aerosol loading throughout the year in Lumbini area. Accordingto the characteristic ratios, levoglucosan ∕ mannosan (lev ∕ man) andsyringic acid ∕ vanillic acid (syr ∕ van), we deduced that the highabundances of biomass burning products during non-monsoon seasons were mainlycaused by the burning of crop residues and hardwood while the softwood hadless contribution. Based on the diagnostic tracer ratio (i.e., lev ∕ OC),the OC derived from biomass burning constituted large fraction of total OC,especially during post-monsoon season. By analyzing the MODIS fire spotproduct and 5-day air-mass back trajectories, we further demonstrated thatorganic aerosol composition was not only related to the local agriculturalactivities and residential biomass usage but also impacted by theregional emissions. During the post-monsoon season, the emissions from riceresidue burning in western India and eastern Pakistan could impactparticulate air pollution in Lumbini and surrounding regions in southernNepal. Therefore, our finding is meaningful and has a great importance foradopting the appropriate mitigation measures, not only at the local level butalso by involving different regions and nations, to reduce the biomassburning emissions in the broader IGP region nations.