This study was conducted in the Central Himalayan middle hills to understand the nature of polycyclic aromatic hydrocarbons (PAHs) embedded in aerosol particles, their sources and human health risk assessments. The level of sum of 15 particlephase PAHs was between 9 and 335 ng/m(3), with an average concentration of 73 +/- 66 ng/m(3). There were strong seasonal differences in total suspended particles (TSP) and particle-bound PAH concentrations with higher concentrations in winter, followed by premonsoon and lowest in monsoon. The main contributor to the suspended particles was 5-ring PAHs (32%), followed by 4-ring (29%), 6-ring (28%), and 3-ring PAHs (11%). Conversely, the gas-phase PAHs showed that 3-ring PAHs contributed utmost to the total particles. The molecular ratios and principal component analysis indicated that both petrogenic and pyrogenic sources, particularly fossil fuel combustion, biomass combustion, and car exhausts, were the major sources of PAHs. The overall average Benzo (a)pyrene equivalent concentration of particulate PAHs was 11.71 ng/m(3), which substantially exceeded the WHO guideline (1 ng/m(3)), and indicated the potential health risks for local residents. The average lifetime inhalation cancer risk (ILCR) estimates associated with carcinogenic PAHs was 8.78x 10(-6) for adults, suggesting the possible cancer risk and 2.47 x10(-5) for children, signifying extreme carcinogenic effects of PAHs on children's health. Therefore, strict measures should be taken to reduce PAHs emissions in the region.

Debris-covered glaciers are ubiquitous in the Himalaya, and supraglacial debris significantly alters how glaciers respond to climate forcing. Estimating debris thickness at the glacier scale, however, remains a challenge. This study inverts a subdebris melt model to estimate debris thickness for three glaciers in the Everest region from digital elevation model difference-derived elevation change. Flux divergences are estimated from ice thickness and surface velocity data. Monte Carlo simulations are used to incorporate the uncertainties associated with debris properties, flux divergence, and elevation change. On Ngozumpa Glacier, surface lowering data from 2010 to 2012 and 2012 to 2014 are used to calibrate and validate the method, respectively. The debris thickness estimates are consistent with existing in situ measurements. The method performs well over both actively flowing and stagnant parts of the glacier and is able to accurately estimate thicker debris (>0.5m). Uncertainties associated with the thermal conductivity and elevation change contribute the most to uncertainties of the debris thickness estimates. The surface lowering associated with ice cliffs and supraglacial ponds was found to significantly reduce debris thickness, especially for thicker debris. The method is also applied to Khumbu and Imja-Lhotse Shar Glaciers to highlight its potential for regional application. Plain Language Summary Debris-covered glaciers are ubiquitous in the Himalaya, and this debris significantly alters the evolution of these glaciers. Estimating the thickness of debris on these glaciers, however, remains a challenge. This study develops a novel method for estimating the debris thickness on three glaciers in the Everest region of Nepal based on digital elevation models, surface velocity data, ice thickness estimates, and a debris-covered glacier energy balance model. The method was calibrated and validated on Ngozumpa Glacier, one of the largest debris-covered glaciers in Nepal, and was found to accurately estimate debris thickness. Specifically, this method was able to estimate thick debris (>0.5m), which has been a major limitation of previous studies. This is important because thick debris significantly reduces glacier melt rates by insulating the underlying ice. This study creates a step-change in our ability to model the past, present, and future evolution of debris-covered glaciers.