Abstract. Organic atmospheric aerosols in the Hindu
Kush–Himalayas–Tibetan Plateau region are still poorly characterized. To
better understand the chemical characteristics and sources of organic
aerosols in the foothill region of the central Himalaya, the atmospheric
aerosol samples were collected in Bode, a suburban site of the Kathmandu
Valley (KV) over a 1-year period from April 2013 to April 2014. Various
molecular tracers from specific sources of primary organic aerosols (POAs)
and secondary organic aerosols (SOAs) were determined. Tracer-based
estimation methods were employed to apportion contributions from each source.
The concentrations of organic carbon (OC) and elemental carbon (EC) increased
during winter with a maximum monthly average in January. Levoglucosan (a
molecular tracer for biomass burning, BB) was observed as the dominant
species among all the analyzed organic tracers and its annual average
concentration was 788±685 ng m−3 (ranging from 58.8 to
3079 ng m−3). Isoprene-SOA (I-SOA) represented a high concentration
among biogenic-SOA tracers. For the seasonality, anhydrosugars, phenolic
compounds, resin acid, and aromatic SOA tracer showed similar seasonal variations with OC and EC while
monosaccharides, sugar alcohols, and I-SOA tracers showed lower levels during
winter. BB contributed a significant fraction to OC, averaging 24.9 %±10.4 % during the whole year, and up to 36.3 %±10.4 % in
the post-monsoon season. On an annual average basis, anthropogenic
toluene-derived secondary OC accounted for 8.8 % and biogenic secondary
OC contributed 6.2 % to total OC. The annual contribution of fungal
spores to OC was 3.2 % with a maximum during the monsoon season
(5.9 %). For plant debris, it accounted for 1.4 % of OC during the
monsoon. Therefore, OC is mainly associated with BB and other anthropogenic
activity in the KV. Our findings are conducive to designing effective
measures to mitigate the heavy air pollution and its impacts in the KV and
surrounding area.