Variability of ambient black carbon concentration in the Central Himalayaand its assessment over the Hindu Kush Himalayan region br

During 2015-2018, eight black carbon (BC) monitoring sites were established in Nepal and Bhutan tofill a significantdata gap regarding BC measurement in Central Himalaya. This manuscript analyzes and presents data from these eightstations and one additional station on the Tibetan plateau (TP). Complex topography, varied emission sources, and at-mospheric transport pathways significantly impacted the BC concentrations across these stations, with annual meanconcentrations varying from 36 ng m-3to 45,737 ng m-3. Higher annual mean concentrations (5609 +/- 4515 ngm-3) were recorded at low-altitude sites than in other locations, with seasonal concentrations highest in the winter(7316 +/- 2541 ng m-3). In contrast, the annual mean concentrations were lowest at high-altitude sites (376 +/- 448 ng m-3); the BC concentrations at these sites peaked during the pre-monsoon season (930 +/- 685 ng m-3).Potential source contributions to the total observed BC were analyzed using the absorption angstrom exponent(AAE). AAE analysis showed the dominance of biomass burning sources (>50 %), except in Kathmandu. By combiningour data with previously published literature, we put our measurements in perspective by presenting a comprehensiveassessment of BC concentrations and their variability over the Hindu Kush Himalayan (HKH) region. The BC levelsin all three geographic regions, high, mid, and low altitude significantly influenced by the persistent seasonalmeteorology. However, the mid-altitude stations were substantially affected by valley dynamics and urbanization.The low-altitude stations experienced high BC concentrations during the winter and post-monsoon seasons. Concen-tration weighted trajectory (CWT) and frequencyanalyses revealed the dominanceof long-range transportedpollution during winter over HKH, from west to east. South Asian sources remained significant during the monsoon season.During pre- and post-monsoon, the local, regional, and long-distance pollution varied depending on the location ofthe receptor site

Automated summary

During the period of 2015-2018, eight black carbon (BC) monitoring sites were established in Nepal and Bhutan to address the lack of BC measurement data in the Central Himalaya region. This study analyzed the data from these eight stations and an additional station on the Tibetan plateau. The BC concentrations varied greatly across the stations due to complex topography, diverse emission sources, and atmospheric transport pathways. Potential source contributions were identified using absorption angstrom exponent (AAE) analysis, with biomass burning being the dominant source. By combining the data with existing literature, a comprehensive assessment of BC concentrations and variability in the Hindu Kush Himalayan (HKH) region was presented. The BC levels in different altitude regions were significantly influenced by seasonal meteorology and other factors such as valley dynamics and urbanization.