Wintertime Air Quality across the Kathmandu Valley, Nepal: Concentration, Composition, and Sources of Fine and Coarse Particulate Matter

Abstract

The Kathmandu Valley in Nepal experiences poor air quality, especially in the dry winter season. In this study, we investigated the concentration, chemical composition, and sources of fine and coarse particulate matter (PM2.5, PM10, and PM10–2.5) at three sites within or near the Kathmandu Valley during the winter of 2018 as part of the second Nepal Ambient Monitoring and Source Testing Experiment (NAMaSTE 2). Daily PM2.5 concentrations were very high throughout the study period, ranging 72–149 μg m–3 at the urban Ratnapark site in Kathmandu, 88–161 μg m–3 at the suburban Lalitpur site, and 40–74 μg m–3 at rural Dhulikhel on the eastern rim of the Kathmandu Valley. Meanwhile, PM10 ranged 194–309, 174–377, and 64–131 μg m–3, respectively. At the Ratnapark site, crustal materials from resuspended soil contributed an average of 11% of PM2.5 and 34% of PM10. PM2.5 was largely comprised of organic carbon (OC, 28–30% by mass) and elemental carbon (EC, 10–14% by mass). As determined by chemical mass balance source apportionment modeling, major PM2.5 OC sources were garbage burning (15–21%), biomass burning (10–17%), and fossil fuel (14–26%). Secondary organic aerosol (SOA) contributions from aromatic volatile organic compounds (13–23% OC) were larger than those from isoprene (0.3–0.5%), monoterpenes (0.9–1.4%), and sesquiterpenes (3.6–4.4%). Nitro-monoaromatic compoundsof interest due to their light-absorbing properties and toxicityindicate the presence of biomass burning-derived SOA. Knowledge of primary and secondary PM sources can facilitate air quality management in this region.

Submicron Aerosol Composition and Source Contribution across the Kathmandu Valley, Nepal, in Winter

“…The complementary approach used offline molecular marker-based chemical mass balance analysis of PM 2.5 from colocated filter sampling. 58 As detailed next, both approaches identified the same major contributors. We examine the agreement to assess our overall confidence in sources and ultimately inform mitigation strategies.…”

“…On the other hand, SOC derived from the sum of OOA shows a positive mass bias toward the OMM-CMB factor, with a slope of 0.5 and R 2 of 0.07. The OMM-CMB calculation of SOC does not capture all SOA because molecular markers exist only for SOA from specific precursors . Thus, we have added the unassigned OC from filters to measure SOC in this comparison.…”

“…The OMM-CMB calculation of SOC does not capture all SOA because molecular markers exist only for SOA from specific precursors. 58 Thus, we have added the unassigned OC from filters to measure SOC in this comparison. This improves the comparison between AMS SOC and filter-based SOC.…”

“…PM 2.5 filters coincident with the mAMS were collected twice a day for 11-h sample times by the University of Iowa. 58 Analysis of the filters following collection provided measurements of elemental carbon (EC) and organic carbon (OC) and numerous tracer species used for organic molecular marker-based chemical mass balance (OMM-CMB) source apportionment as described in Islam et al 31 A dual-spot aethalometer (AE33, Magee Scientific) measured light-absorbing carbonaceous species in PM. This instrument measures light attenuation by particles on Teflon filter tapes to calculate BC concentration and estimate brown carbon (BrC) absorption.…”

“… 64 In all cases, the inlet line and laboratory were electrically grounded with a 1-m-long steel rod. A separate system located within 15 m of the mAMS inlet collected PM 2.5 and PM 10 filters as discussed in Islam et al 58 Gas-phase instrumentation sampled from a dedicated Teflon inlet.…”