Sources of atmospheric particulate matter in Nepal

Md Robiul Islam

doi:10.17077/etd.005936

Airborne particulate matter (PM) causes respiratory and cardiopulmonary diseases and results in millions of premature deaths each year. The majority of these deaths occur in low-and middle-income countries such as Nepal that experiences severe pollution problems, including elevated levels of atmospheric PM. Reducing PM requires understanding the major contributing sources. The research presented herein is based upon the Nepal Ambient Monitoring and Source Testing Experiment (NAMaSTE) and focuses on advancing analytical methodology to assess the impact of garbage burning in the atmosphere and to characterize of the composition, sources, and spatiotemporal variability of ambient PM in Nepal. A molecular marker of PM derived from garbage burning is 1,3,5-triphenylbenzene (TPB) that is produced by plastic combustion. As part of NAMaSTE, gas chromatography and mass spectrometry (GCMS) were applied to quantify TPB in emissions from 35 different combustion samples, including 5 garbage burning combustion and 105 samples of atmospheric fine particulate matter with aerodynamic diameter less than 2.5 µm (PM2.5). The specificity of TPB to garbage burning emission was confirmed by its absence in emissions from cook stoves, brick kilns, generators, groundwater pumps, motorcycles, crop residue, etc. The fuel-based emission factor (as mg of TPB emitted per kg of garbage burned) ranged 0.27-1.87 mg kg-1. The chemical profiles of PM emitted from garbage burning will also support estimating garbage burning contributions to atmospheric PM. To support more sensitive detection of TPB while implementing green chemical principles on reducing solvent use, we developed a thermal desorption GCMS quantification method. A quartz fiber filter containing PM2.5 is loaded into the GC inlet liner, heated to 275 oC to desorb the analyte, and then focused onto the GC column. The thermal desorption GCMS method had 6 times lower detection limit than traditional solvent-extraction GCMS techniques for the quantification of TPB, indicating that this method can quantify smaller level of TPB and will aid in further assessment of garbage burning impacts on ambient PM. Atmospheric PM samples were collected in the Kathmandu Valley of Nepal, including at the sub-urban Bode site during the pre-monsoon season in April 2015 and at urban Kathmandu, sub-urban Lalitpur, and rural Dhulikhel sites during the dry winter season in January-February 2018. Daily average concentrations of PM2.5 and PM10 exceeded the World Health Organization (WHO) 24-hour guidelines (of 25 µg m-3 and 50 µg m-3, respectively) on all study days. At urban and sub-urban sites, PM2.5 ranged: 30-207 µg m-3 and PM10 ranged: 52-377 µg m-3. Seasonally, PM concentrations were highest during the dry winter season compared to the pre-monsoon. Spatially, PM concentrations were highest at the urban and sub-urban sites, and were approximately half at the rural site. On average, major components of PM2.5 mass included organic matter (48-50%) and elemental carbon (10-14%). Resuspended dust accounted for 11% of PM2.5 and 33% of PM10 at the urban Kathmandu site. Molecular markers and chemical mass balance source apportionment modeling revealed major PM2.5 organic carbon (OC) sources to be garbage burning (15-21%), biomass burning (10-17%), and gasoline and diesel-fueled internal combustion engines (12-23%). Contribution of internal-combustion engines was approximately double at urban Kathmandu compared to rural Dhulikhel. Contributions of secondary organic aerosol (SOA) to PM2.5 OC were minor compared to primary sources, and were more strongly influenced by anthropogenic precursors compared to biogenic precursors. Nitro-aromatic compounds that are strongly light-absorbing, toxic, and form by oxidation of cresol were measured for the first time in the Kathmandu Valley. Following that cresol largely originates from biomass burning, we estimate that biomass burning derived SOA source contributed at least 1% of the measured OC in Nepal. Additional PM samples were collected in Lumbini, located in the Indo-Gangetic Plains region during winter of 2017-18. Here, the daily average PM2.5 concentrations (48-295 µg m-3) exceeded the WHO 24-hour guideline by factors of 2-12. On average, PM2.5 was composed of 66% organic matter, 5% elemental carbon, 20% secondary inorganic ions (ammonium, nitrate, and sulfate), 2% chloride, and 1.3% potassium. Biomass burning, including wood burning and dung burning was the largest sources of PM2.5 OC, accounting for on average 44%. Meanwhile, garbage burning (5%) and vehicle emissions (3%) made smaller contributors to PM2.5 OC compared to the Kathmandu Valley sites. Nitro-aromatic compounds were among the highest levels observed worldwide, indicating biomass burning impacts on SOA formation. The NACs and biomass burning SOA were highly sensitive to regional fog episodes. Overall, the work presented in this dissertation advances the knowledge of the chemical composition and sources of PM2.5 in Nepal. By incorporating recently updated regional source profiles for combustion sources developed in NAMaSTE and new tracers for SOA derived from anthropogenic and biomass burning precursors, the source apportionment results of this study provide a more complete understanding of the sources of PM2.5 OC. The results showed that garbage burning was a consistent and one of the major sources of PM2.5 OC that holds potential for improving air quality. The estimated contributions of biomass burning and anthropogenic SOA through the measured tracers indicated that additional SOA from these sources are likely to contribute to the unapportioned OC. Through the study of seasonal and spatial variations in PM, it was observed that the air quality in Nepal was worse in urban locations compared to the rural counterparts especially during winter. The advanced understanding of the major sources of PM can aid in developing and assessing future air pollution mitigation strategies in Nepal. PUBLIC ABSTRACT:There are millions of invisible particles in the air around us that cause lung and heart diseases and millions of premature deaths each year. Nepal, located in South Asia, experiences severe particle air pollution consistently exceeding health guidelines. Reducing air pollution requires an understanding of the major contributing sources. The goals of this dissertation are to characterize the chemical composition of atmospheric particles in Nepal and determine their sources. A highly sensitive and accurate method was evaluated and applied to measure a chemical named 1,3,5-triphenylbenzene that is used to track garbage burning. This method was further improved to make it more environmentally friendly. At the urban and sub-urban locations in the Kathmandu Valley, garbage burning (15-21%), biomass burning (10-17%), and vehicle and other engines (12-23%) were the major sources of organic chemicals in fine particles. In contrast, dung burning (24%) and other biomass burning (20%) were major sources in the rural Lumbini, located in the agricultural Indo Gangetic Plains. The new findings on the major sources of atmospheric particles can aid in developing and assessing air pollution control strategies in Nepal and reduce the negative impacts of particle air pollution on human health.

Sources of atmospheric particulate matter in Nepal

Files and links (1)

Abstract

Details

Metrics