Date of Publication : May

Abstract: Significant warming has been observed in the Northwestern Himalaya (NWH) in recent decades, but little is known about sulphate aerosols, a key cooling agent that affects regional climate. This study uses sulphate aerosol optical depth (SO₄ AOD) MERRA-2 reanalysis data from 1980 to 2023 to investigate altitude-dependent patterns and their relationships to regional warming. Trend analysis reveals a clear SO₄ AOD decline at high elevations and a modest increase in the foothills. Spatial mapping shows the strongest negative trends along the mountain front, with weaker or positive trends in lower valleys. The high-altitude decline is attributed to the absence of local sulphate sources and inefficient vertical transport of hygroscopic sulphate particles. Under humid boundary-layer conditions, these aerosols absorb water, increasing mass and settling velocity, thereby hindering their ascent across steep Himalayan terrain. Additionally, lower relative humidity at higher altitudes further decreases aerosol growth and scattering. Trends in black carbon and dust AOD are also examined. The decrease in sulphate cooling, combined with stable or increasing warming agents, likely contributes to accelerated warming in the region. These data-driven insights clarify how aerosol distribution and mountain meteorology together influence climate change across the NWH.

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