A brown cloud is an atmospheric pollution phenomenon characterized by a persistent, regionally extensive layer of brownish haze composed of fine particulate matter (PM₂.₅ and PM₁₀), black carbon, organic aerosols, ozone, and nitrogen oxides, often mixed with secondary inorganic aerosols such as sulfates and nitrates. It typically forms under stagnant meteorological conditions that inhibit vertical mixing, allowing pollutants from combustion sources (e.g., biomass burning, fossil fuel use) to accumulate. Brown clouds alter radiative forcing by absorbing and scattering solar radiation, perturb cloud microphysics via aerosol–cloud interactions, reduce surface insolation, and degrade air quality, with documented impacts on atmospheric chemistry, hydrological cycles, and human health.
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