Long-term AERONET observations reveal recurring aerosol regimes and contrasting radiative effects in the central Himalayan foothills

Abstract. We analyzed AERONET Version 3 Level 2.0 observations over Pokhara, Nepal, from 2010 to 2024 to assess aerosol trends, regimes, and radiative effects. Theil–Sen and Mann–Kendall analyses showed no significant trend in AOD₄₄₀ annual medians (slope = −0.0024 yr⁻¹; τ = −0.08, p = 0.747) or deseasonalized monthly anomalies. AOD₄₄₀ was highest during pre-monsoon and lowest during monsoon. Gaussian mixture modeling identified four recurring, partially overlapping aerosol regimes with pronounced seasonal differences in occurrence. The mixed-size regime produced the strongest median surface dimming, while strongly absorbing regimes enhanced atmospheric forcing. Surface forcing was closely related to AOD₄₄₀ (R² = 0.817), whereas atmospheric forcing was most strongly related to absorption AOD (R² = 0.892). Almucantar and Hybrid retrievals agreed on 89.0% of regime assignments (κ = 0.836). Aerosol variability over Pokhara was therefore governed mainly by seasonal changes and recurring aerosol mixtures rather than by a persistent change in column loading. More broadly, the results show that a stable long-term AOD record does not necessarily imply stable aerosol radiative effects, because changes in particle size and absorption can substantially alter the radiative influence of the aerosol column.