Satellites Reveal Air Pollution Rising Again Across the Indian Himalaya

The Indian Himalayan Region, long imagined as a sanctuary of clean mountain air, is quietly losing that reputation. A new study published in the journal Air Quality, Atmosphere & Health has assembled one of the most detailed pictures yet of how air pollution has shifted across this vast mountain arc, tracking the years from the 2020 COVID-19 lockdown through the post-pandemic recovery to 2024. The findings are sobering: after the brief, unprecedented respite of the lockdown, concentrations of carbon monoxide, fine particulate matter, ozone, sulfur dioxide and nitrogen dioxide have climbed again across the Himalayan foothills, and the rate of increase has accelerated since 2021 in some of the range’s most vulnerable valleys.

The research, carried out by Diksha and Soora Naresh Kumar of the Division of Environmental Sciences at ICAR-Indian Agricultural Research Institute in New Delhi, was funded through India’s National Mission for Sustaining the Himalayan Ecosystem. Its central challenge was one that has long frustrated atmospheric scientists: the Himalaya is a region of extreme topographic complexity, where deep valleys, steep slopes and sharp elevation gradients make ground-based monitoring sparse and satellite retrievals unreliable. Mountains distort the assumptions built into standard satellite products, and the few air quality stations that exist are concentrated in towns and cities, leaving vast stretches of the range effectively unmeasured.

To overcome this, the team fused data from multiple satellite instruments with a state-of-the-art atmospheric model. Aerosol Optical Depth measurements from the Moderate Resolution Imaging Spectroradiometer, processed through the Multi-Angle Implementation of Atmospheric Correction algorithm, provided the aerosol backbone, while the Sentinel-5P TROPOMI instrument supplied column measurements of ozone, carbon monoxide, sulfur dioxide and nitrogen dioxide. These observations were combined with reanalysis data from the Goddard Earth Observing System-Composition Forecast model, which allowed the researchers to estimate near-surface concentrations from satellite-observed columns using modelled vertical scaling ratios and conversions based on other significant atmospheric variables.

The processing pipeline was as important as the data itself. Monthly composites were generated on the Google Earth Engine platform, while annual mosaics were enhanced to a one-kilometre resolution using ArcGIS Pro. Crucially, the raw satellite estimates were then bias-corrected against ground station measurements using three machine learning approaches: Gradient Boosting, Random Forest and Linear Regression. The corrected estimates showed strong agreement with observed data, with correlation coefficients ranging from 0.85 to 0.95, a level of performance that gives considerable confidence in the spatial patterns the study reveals across terrain where no ground instruments exist.

The choice of the 2020 lockdown as a benchmark period gives the analysis an unusual analytical power. The COVID-19 restrictions of 2020 produced an abrupt, near-uniform suppression of traffic, industry and construction across India, creating something close to a natural experiment in emission reduction. Earlier work, including studies of the Indo-Gangetic Plain’s influence on Himalayan air quality, documented how the lockdown visibly cleared the air even at high-altitude sites. By anchoring their timeline to that exceptional year, the researchers could measure precisely how much of the subsequent deterioration represents a rebound of economic activity rather than a continuation of longer-term trends.

The answer, in much of the region, is a great deal. In the Himalayan foothills, levels of carbon monoxide, PM2.5, ozone, sulfur dioxide and nitrogen dioxide all increased in the post-pandemic period. Rate-of-change analyses pinpoint the acceleration after 2021, with the steepest rises concentrated in the valleys of Jammu and Kashmir and in the foothills of Uttarakhand and Himachal Pradesh. These are precisely the areas where post-pandemic recovery of economic activity, expanding transport and growing industrial presence drove pollutant concentrations upward over the study period. The valleys, which trap air under temperature inversions especially in winter, act as natural holding chambers for whatever is emitted into them.

Not every pollutant followed the same trajectory, and the exceptions are scientifically revealing. Oxides of nitrogen, reported as NOx, remained comparatively stable in high-altitude regions throughout the study period. This stability suggests that the most remote parts of the range, far from roads and industry, have so far been buffered from the rebound affecting the lowlands, and that the pollution problem in the high Himalaya is driven less by local emissions than by transport of pollutants from the plains. The Indo-Gangetic Plain to the south is one of the most densely populated and polluted regions on Earth, and its plumes routinely ride upslope winds into the mountains, a dynamic that earlier studies of black carbon on Himalayan glaciers have documented in detail.

The distinction between foothill and high-altitude behaviour matters enormously for what happens next. The Himalaya is often called the water tower of Asia, and its glaciers feed rivers that sustain hundreds of millions of people. Deposited dark particles, whether black carbon or mineral dust mixed with combustion products, darken snow and ice surfaces and accelerate melting, linking air quality directly to water security. Aerosol pollution also alters regional radiation budgets and monsoon dynamics. A gradual increase of aerosol pollution over the so-called Third Pole over the past four decades has already been flagged as a driver of regional climate change, and the new study’s finding that air quality in the Indian Himalayan Region is worsening over time adds urgency to that warning.

There are also direct public health implications. Fine particulate matter and the gaseous pollutants tracked in the study are associated with respiratory and cardiovascular disease, adverse birth outcomes and aggravated asthma, with effects documented across age groups from children to the elderly. Mountain communities, often assumed to enjoy clean air, may face exposure patterns that mirror those of the plains, particularly in valley towns where tourism traffic, diesel generators, brick kilns and heating emissions accumulate under inversions. The foothill cities of Uttarakhand and Himachal Pradesh have expanded rapidly, and studies of Dehradun have already linked urbanisation to deteriorating air quality and changing temperature dynamics. The new regional maps put those local findings into a range-wide context.

Methodologically, the study demonstrates how far satellite-based air quality assessment has come. The combination of TROPOMI’s trace gas columns, MAIAC-corrected MODIS aerosol data, GEOS-CF reanalysis for vertical structure, and machine learning bias correction against sparse ground stations now yields surface concentration estimates at one-kilometre resolution even over terrain that defeats conventional monitoring. Comparable approaches have been applied successfully in Delhi, across the Arabian Peninsula, and in Thailand, suggesting the Himalayan workflow could be replicated for other mountain ranges, from the Andes to the Tibetan Plateau, where ground networks are thin but health and climate stakes are high.

What the study cannot yet say, and what the authors’ own framing makes clear, is where the region goes from here. The data end at 2024, and the post-2021 acceleration may continue as infrastructure projects, hydropower construction and tourism expand deeper into the mountains, or it may moderate as cleaner vehicle standards and cooking fuel transitions take hold. What the analysis does establish is a baseline and a method: a reproducible, validated way to watch pollution evolve across one of the planet’s youngest and most fragile mountain systems, season by season and valley by valley. For a region whose air was once its proudest asset, that watching has now become a necessity, and the satellites are already keeping score.

Subject of Research: Spatiotemporal variation of air pollutants across the Indian Himalayan Region from 2020 to 2024 using satellite data and machine learning bias correction

Article Title: High altitude alerts: temporal and spatial variations in air quality over the Indian Himalaya Region

Article References: High altitude alerts: temporal and spatial variations in air quality over the Indian Himalaya Region. (n.d.). https://doi.org/10.1007/s11869-026-02122-1

Image Credits: AI Generated

DOI: 10.1007/s11869-026-02122-1

Keywords: air pollution, Indian Himalayan Region, Sentinel-5P TROPOMI, MODIS, machine learning, bias correction, Google Earth Engine, GEOS-CF, PM2.5, nitrogen dioxide, COVID-19 lockdown, Himalayan valleys