South Asia, notably the vast Indo–Gangetic plains (IGP), is one of the most heavily populated and most polluted regions of the world, and it is getting worse, for example, the region has been projected to have the worst ozone pollution in the world by 2030. The region is directly upwind of vulnerable ecosystems in the Himalayan-Tibetan region, one of the least sampled regions of the globe. Recent field experiments and modeling studies focused on northern South Asia have demonstrated that human activities have had dramatic impacts on atmospheric chemistry and physics in this region, with important implications for air quality, human health, agriculture, the cryosphere, and climate in the broader region. In order to characterize the air pollutant emissions, ambient distributions, atmospheric processes, and potential impacts in this region, the Institute for Advanced Sustainability Studies (IASS), Potsdam, Germany, and the International Centre for Integrated Mountain Development (ICIMOD), Kathmandu, Nepal, led the SusKat-ABC (Sustainable Atmosphere for the Kathmandu Valley – atmospheric brown cloud) international air pollution measurement campaign during December 2012–June 2013 in Nepal. In the course of this campaign, high-quality atmospheric data were collected at six sites (including one supersite) in the highly polluted Kathmandu Valley as well as at six sites in the surrounding regions, including the northern part of the IGP, the Himalayan foothills, and the base of Mt. Everest. With the participation of over 40 scientists from 18 research groups in 9 countries, SusKat-ABC was the second largest international air pollution measurement campaign ever conducted in South Asia (after INDOEX in 1999), and it provides detailed air pollution data for the first time for the Himalayan foothills. The campaign was endorsed by the UNEP’s Project ABC (atmospheric brown cloud). Key features of the measurements include the first deployment of a proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) in South Asia for the speciated measurement of ambient volatile organic compounds (VOCs), a 3-year time series of black carbon (BC) monitored with the new generation Aethalometer AE33 (the only such time series worldwide acquired with an AE33 in a highly polluted environment), an isotopic analysis of elemental carbon (EC) and organic carbon (OC) and quantification of BC and brown carbon (BrC), seasonal variations of several air pollutants at multiple sites, particle-bound polycyclic aromatic hydrocarbons (PAHs) at six sites across the Himalayan foothills, as well as a detailed analysis of the interplay between meteorology and emissions in determining ambient air pollution concentrations in the Kathmandu Valley. This special issue will highlight the field studies carried out under the SusKat-ABC field campaign and associated modeling investigations. These studies address scientific questions on the emissions, atmospheric distributions, and atmospheric chemical composition, processing, regional trans-boundary transport, and impacts of air pollutants, for example, on air quality, human health, weather, the cryosphere, and climate in the IGP and Himalayan regions. Furthermore, this special issue is open to the submission of closely related manuscripts that are generally focused on these topics relating to IGP-Himalayan-Tibetan region. The findings of the papers in this special issue are expected to be of a great interest to the international scientific community, policy and decision makers, and the general public alike.