Ambient air quality in the Kathmandu Valley, Nepal, during the pre-monsoon: concentrations and sources of particulate matter and trace gases

Abstract. The Kathmandu Valley in Nepal is a bowl-shaped urban basin that experiences severe air pollution that poses health risks to its 3.5 million inhabitants.

As part of the Nepal Ambient Monitoring and Source Testing Experiment

(NAMaSTE), ambient air quality in the Kathmandu Valley was investigated from

11 to 24 April 2015, during the pre-monsoon season. Ambient concentrations

of fine and coarse particulate matter (PM2.5 and PM10,

respectively), online PM1, inorganic trace gases (NH3, HNO3,

SO2, and HCl), and carbon-containing gases (CO2, CO, CH4, and

93 non-methane volatile organic compounds; NMVOCs) were quantified at a

semi-urban location near the center of the valley. Concentrations and ratios

of NMVOC indicated origins primarily from poorly maintained vehicle

emissions, biomass burning, and solvent/gasoline evaporation. During those

2 weeks, daily average PM2.5 concentrations ranged from 30 to 207 µg m−3, which exceeded the World Health Organization 24 h

guideline by factors of 1.2 to 8.3. On average, the non-water mass of

PM2.5 was composed of organic matter (48 %), elemental carbon

(13 %), sulfate (16 %), nitrate (4 %), ammonium (9 %), chloride

(2 %), calcium (1 %), magnesium (0.05 %), and potassium (1 %). Large

diurnal variability in temperature and relative humidity drove corresponding

variability in aerosol liquid water content, the gas–aerosol phase

partitioning of NH3, HNO3, and HCl, and aerosol solution pH. The

observed levels of gas-phase halogens suggest that multiphase

halogen-radical chemistry involving both Cl and Br impacted regional air

quality. To gain insight into the origins of organic carbon (OC), molecular

markers for primary and secondary sources were quantified. Levoglucosan

(averaging 1230±1154 ng m−3), 1,3,5-triphenylbenzene (0.8±0.6 ng m−3), cholesterol (2.9±6.6 ng m−3), stigmastanol (1.0

±0.8 ng m−3), and cis-pinonic acid (4.5±1.9 ng m−3)

indicate contributions from biomass burning, garbage burning, food cooking,

cow dung burning, and monoterpene secondary organic aerosol, respectively.

Drawing on source profiles developed in NAMaSTE, chemical mass balance (CMB)

source apportionment modeling was used to estimate contributions to OC from

major primary sources including garbage burning (18±5 %), biomass

burning (17±10 %) inclusive of open burning and biomass-fueled

cooking stoves, and internal-combustion (gasoline and diesel) engines (18±9 %). Model sensitivity tests with newly developed source profiles

indicated contributions from biomass burning within a factor of 2 of

previous estimates but greater contributions from garbage burning

(up to three times), indicating large potential impacts of garbage burning

on regional air quality and the need for further evaluation of this source.

Contributions of secondary organic carbon (SOC) to PM2.5 OC included

those originating from anthropogenic precursors such as naphthalene (10±4 %) and methylnaphthalene (0.3±0.1 %) and biogenic

precursors for monoterpenes (0.13±0.07 %) and sesquiterpenes (5±2 %). An average of 25 % of the PM2.5 OC was unapportioned,

indicating the presence of additional sources (e.g., evaporative and/or

industrial emissions such as brick kilns, food cooking, and other types of

SOC) and/or underestimation of the contributions from the identified source

types. The source apportionment results indicate that anthropogenic

combustion sources (including biomass burning, garbage burning, and

fossil fuel combustion) were the greatest contributors to PM2.5 and, as

such, should be considered primary targets for controlling ambient PM

pollution.