Comparative Assessment of Urban and Rural PM2.5 Concentrations across Physiographic Regions of Nepal
The adverse health effects of exposure to ambient air pollution are well established; however, limited spatial coverage of regulatory monitoring networks constrains comprehensive assessment, particularly in regions with complex terrain. The increasing availability of low-cost sensors provides a promising approach to overcome these limitations. In this study, a network of TSI BlueSky low-cost PM2.5 monitors was deployed to investigate the spatial and temporal variability of PM2.5 concentrations across four physiographic regions of Nepal: Eastern Terai, Western Terai, Inner Terai, and Mid hills. The Eastern Terai consistently recorded the highest PM2.5 concentrations, followed by Western Terai, with winter levels at several sites far exceeding both national standards and WHO air quality guidelines. PM2.5 concentrations in the Inner Terai and Mid Hill regions were comparatively lower, though still of significant public health concern during winter months. A marked increase in PM2.5 concentrations across the Terai belt immediately following the monsoon season indicates the influence of transboundary pollution, with elevated levels persisting throughout the winter period. A sharp rise in PM2.5 concentrations across all study regions during April highlights the substantial contribution of forest fire activity to regional air quality degradation. Urban–rural paired site analysis revealed that PM2.5 pollution is not exclusively an urban phenomenon, as rural sites such as Lumbini and Sauraha recorded concentrations equal to or exceeding those of their urban counterparts. Coefficient of Divergence analysis confirmed that industrially influenced sites display markedly distinct pollution signatures compared to proximate paired sites. These findings underscore the necessity for strengthened monitoring networks and regionally tailored mitigation strategies that account for local emissions, transboundary pollution, and topographic influences on PM2.5 across Nepal.