From Aerosol Columns to Breathing-Zone Exposure: Meteorological Controls, Vertical Mixing, and Measurement Uncertainty in Nepal’s Mountain Valleys

Aerosol optical depth (AOD) and ground-level particulate matter (PM) are frequently interpreted together in Himalayan air-pollution research, although they measure different atmospheric quantities. In Nepal, sharp elevation gradients, mountain–valley circulations, monsoon rainfall, combustion sources, and transboundary transport can decouple column-integrated optical loading from breathing-zone exposure. This critical integrative review asks how meteorological processes connect emissions, aerosol optics, surface PM2.5, and exposure, and which monitoring designs can test those links. Nepalese sun-photometer, surface, chemical, airborne, transport, and health evidence is synthesized across urban valleys, the Terai, and high-elevation sites. The analysis gives particular attention to multiyear optical measurements in Lumbini and draws methodological lessons from neighborhood-level mobile monitoring in Massachusetts and a 2026 review of remote-sensing technologies in the United States. Those international studies inform method selection rather than supply Nepal-specific effect estimates. The synthesis shows that vertical mixing, aerosol water uptake, particle composition, sampling gaps, and neighborhood heterogeneity can produce different trends in AOD and surface PM. It proposes a cross-platform validation strategy linking reference PM monitoring, calibrated mobile sensors, ground-based and satellite optical observations, vertical profiles, meteorology, and uncertainty-aware data fusion. This approach could improve exposure-relevant estimates and event-specific forecasting in complex Himalayan terrain.