ATMOSPHERIC AEROSOLS AND THEIR IMPACTS

Atmospheric aerosols are suspended solid and liquid particles of natural and anthropogenic origin and are among the most complex and influential components of the Earth system, exerting profound impacts on air quality, human health, climate processes, and the cryosphere. Despite constituting a small fraction of the total atmospheric mass, aerosols play a disproportionately large role in regulating radiative balance, cloud microphysics, visibility, and biogeochemical interactions. They originate from a wide range of sources, including fossil fuel combustion, vehicular emissions, industrial activities, biomass burning, agricultural practices, mineral dust uplift, sea spray, volcanic eruptions, and wildfires, while secondary aerosols form through atmospheric oxidation of precursor gases such as sulphur dioxide, nitrogen oxides, ammonia, and volatile organic compounds. Depending on their size, composition, and atmospheric lifetime, aerosols can remain suspended for days to weeks and be transported over long distances, thereby influencing regions far from their sources. Globally, aerosol distributions exhibit strong spatial and temporal variability, with South and East Asia emerging as major hotspots of fine particulate pollution. The Himalayan region is particularly vulnerable due to its proximity to the Indo-Gangetic Plain, complex topography, and unique meteorological conditions that favor pollutant accumulation and long-range transport. Seasonal temperature inversions, shallow boundary layers, calm winter conditions, and valley-scale circulations lead to enhanced aerosol loading, especially during winter and autumn. Both local emissions—such as domestic fuel use, transportation, industries, and biomass burning—and transboundary transport contribute significantly to the regional aerosol burden. These elevated concentrations pose serious risks to human health, as fine particulate matter (PM2.5) can penetrate deep into the respiratory system and bloodstream, increasing the incidence of cardiovascular and respiratory diseases, lung cancer, asthma, and premature mortality. In densely populated regions of northern India, aerosol pollution is also associated with substantial economic losses due to increased healthcare costs and reduced productivity. From a climatic perspective, aerosols exert both cooling and warming influences. Scattering aerosols such as sulphates and nitrates reduce surface solar radiation and induce negative radiative forcing, whereas absorbing aerosols, particularly black carbon, warm the atmosphere and alter cloud properties through direct and semi-direct effects. Aerosol–cloud interactions remain one of the largest uncertainties in climate projections. In cryospheric regions, the deposition of light-absorbing aerosols on snow and glaciers reduces surface albedo, accelerates melting, and enhances glacier retreat, posing a significant threat to water resources in the Hindu Kush–Himalaya–Tibetan Plateau, often referred to as the “Third Pole”. High aerosol concentrations also degrade air quality and visibility, leading to haze formation that disrupts transportation, aviation, agriculture, and daily life, particularly in mountain basins such as Kashmir. In regions where ground-based monitoring is sparse, satellite remote sensing has emerged as an indispensable tool, providing near-global observations of aerosol properties, transport pathways, and long-term trends. Overall, atmospheric aerosols represent a critical environmental challenge whose multifaceted impacts underscore the urgent need for strengthened emission controls, cleaner energy transitions, expanded monitoring networks, and coordinated regional and global mitigation strategies.