How transboundary haze affects Nepal

A NASA satellite image showing Central Nepal's river valleys and how they act as funnels for pollution from the Indian plains to travel up to the mountains.

The Himalayan mountains act as a barrier separating the clean air of the Tibetan Plateau from the polluted Indo-Gangetic plains. But Nepal’s rivers cut through the mountains, and their valleys allow smog from the plains to penetrate deep into the mountains.

This haze intrusion goes up to elevations of 4,000m, and the pollution layer is more shallow and flat in winter, and inclined and parallel to the ridgeline in the pre-monsoon. The depth of this pollution layer up the valleys also decreases with elevation.

These visible air pollution images are captured regularly by the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument aboard the two NASA satellites Terra and Aqua orbiting the Earth. The dull bluish haze is mostly made up of fine aerosol particles and extends right across the Indo-Gangetic plains from Pakistan to Bangladesh, as well as the Tarai in Nepal.

While human-generated emissions from agricultural burning, urban, rural and industrial emissions are mainly responsible for this region-wide pollution, weather conditions and the blocking effect of the Siwalik Range also affect its spread. More interestingly, if we zoom in on the images, we can see that the haze does not just stay in the plains, but flows up into the inner Himalaya along river valleys.

These satellite images provide valuable insight for researchers looking into the geographical spread and long-range transport of aerosol particles. Scientists at Duke University in the United States have developed a novel technique to use the high-resolution satellite images from MODIS and elevation data from SRTM (Shuttle Radar Topography Mission) to extract the daytime haze run-up that measure intrusion length and height over the Arun Valley in eastern Nepal.

The Arun River starts in Tibet, snakes around Mt Everest to find a gap between Makalu and Kangchenjunga to tumble down Himalayan gorges to meet up with the Sun Kosi and Tamor— flowing out into the Tarai at Chhatra and then through the Kosi Barrage to meet the Ganges in India. This also opens up the entry gate for haze to flow from the Indo-Gangetic Plains into the inner Himalayas.

In winter, the surface temperature in the plains plummets, and the air is dry. The southern edge of the Tibetan Plateau at upper levels is also battered by the sub-tropical jetstream, with weak westerly wind below it. The low surface temperature in the plains confines the pollution to a shallow boundary layer that is lower than the Siwalik Range. Up-valley winds along the river valleys then transport the haze into the Inner Himalaya.

The river valley-plains temperature contrast produces a pressure gradient that drives this flow up the valley during the day and down the valley during the night. Besides, the topography, vegetation cover and groundwater level in the slopes can also contribute to warming of valley air which could further amplify the up-valley wind.

In addition, the daytime up-slope and night-time down-slope winds developing in the river valleys also play and important role in ventilating the air pollution from the valley surface.

Measurements of the nature and size of fine aerosols were conducted for the first time in the middle Himalayan region in the pre-monsoon. In general, the aerosol size distribution peaks around 100nm, which is usually the oldest, most processed background aerosol and about the size of the Covid-19 virus.

Another smaller peak around 20 nm, is indicative of fresh but not necessarily local aerosols. The chemical composition of PM2.5 (particulate matter below 2.5 microns) is dominated by organic matter. Organic carbon (OC) comprises the major fraction (64–68%) of the aerosol concentration followed by ionic species (24–26%), Elemental Carbon (EC) compromises 7–10% of the total composition and 27% of OC is water soluble.

Elemental Carbon and Organic Carbon together are aerosols contributed by burning of wood fuels, crop-residue, coal including vehicular emission and others. The light absorbing properties of elemental carbon play a crucial role in atmospheric warming and surface cooling due to their interaction with the daytime solar heating.

Industrial and vehicular emissions, biomass burning, soil dust and others including chemical transformations in the atmosphere contribute to the ionic species in aerosols. These are readily water-soluble parts of the aerosol which along with organic particles aid in their growth in presence of moisture, contributing to increase in optical thickness of haze.

The aerosol concentration also changes over the day. It increases in the morning (05:00-10:00) and in the evening (17:00-22:00). The lower values in the afternoon could be attributed partly to more mixing, and increase in the boundary layer height.

During the night, the downslope winds lift the warm air mass in the valley, resulting in the decrease of the aerosol concentration near the surface. On the other hand, aerosol loading can also be eventually washed away by local rainfall during the pre-monsoon and winter.

While pollution haze is visible even in satellite images, its quantitative measurements can also be made by remote sensing. Haze and dust particles in the air block sunlight reaching the ground by absorbing and scattering it. This property of aerosols is used to estimate the Aerosol Optical Depth (AOD).

A long-term average of winter AOD from December-February over the last 20 years in the region shows that winter haze extends throughout the Indo-Gangetic plains, with relatively higher readings for the eastern regions of Bihar, West Bengal and Bangladesh, across Nepal’s southeastern border. The regional average winter AOD also shows an alarming increase in the past two decades due to increase in emissions from increased crop-residue burning, rapid urbanisation, industrialisation, etc.

Aside from transboundary haze, this winter has also seen an increase in wildfires that have exacerbated the pollution in Nepal’s Inner Himalayan valleys. The Pathivara and Annapurna Conservation Area fires in December, followed by uncontrolled wildfires in Manang, Lamjung and Rasuwa in January were clearly visible in NASA satellite images.

The very high pollution levels in Kathmandu Valley on 6-7 January was also aggravated by overcast skies which subdued the ventilation mechanism of the valley, but smoke from these wildfires also acted as contributors. Since the daily satellite data is in the public domain, they could be used along with global models by Nepal’s met office to nowcast air pollution episodes over the Himalaya.

While local air pollution can be brought under control in the interest of protecting public health by the local governments with appropriate policy and regulation, it is trans-boundary pollution that cannot be easily addressed. It requires regional collaboration to adopt and implement policies to reduce sources of pollution like crop residue burning, vehicular and industrial emissions. The atmospheric initiative at the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD) could be one such mechanism.

More immediately, there is an urgent need to set up permanent air quality monitoring stations along Nepal’s major river valley outlets to monitor trans-boundary air pollution and more research by universities in Nepal to improve our knowledge of local and regional pollution so that policy makers can make better decisions.

Prabhakar Shrestha is a research scientist working at Bonn University, Germany.

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