The impact of aerosols, especially the absorbing aerosols, in the Himalayan region is important for climate. We closely examine ground-based high-quality observations of aerosol characteristics including radiative forcing from several locations in the Indo-Gangetic Plain (IGP), the Himalayan foothills and the Tibetan Plateau, relatively poorly studied regions with several sensitive ecosystems of global importance, as well as highly vulnerable large populations. This paper presents a state-of-the-art treatment of the warming that arises from these particles, using a combination of new measurements and modeling techniques. This is a first-time analysis of its kind, including ground-based observations, satellite data, and model simulations, which reveals that the aerosol radiative forcing efficiency (ARFE) in the atmosphere is clearly high over the IGP and the Himalayan foothills (80-135 Wm(-2) per unit aerosol optical depth (AOD)), with values being greater at higher elevations. AOD is >0.30 and single scattering albedo (SSA) is similar to 0.90 throughout the year over this region. The mean ARFE is 2-4 times higher here than over other polluted sites in South and East Asia, owing to higher AOD and aerosol absorption (i.e., lower SSA). Further, the observed annual mean aerosol induced atmospheric heating rates (0.5-0.8 Kelvin/day), which are significantly higher than previously reported values for the region, imply that the aerosols alone could account for >50 % of the total warming (aerosols + greenhouse gases) of the lower atmosphere and surface over this region. We demonstrate that the current state-of-the-art models used in climate assessments significantly underestimate aerosol-induced heating, efficiency and warming over the Hindu Kush - Himalaya - Tibetan Plateau (HKHTP) region, indicating a need for a more realistic representation of aerosol properties, especially of black carbon and other aerosols. The significant, regionally coherent aerosol induced warming that we observe in the high altitudes of the region, is a significant factor contributing to increasingair temperature, observed accelerated retreat of the glaciers, and changes in the hydrological cycle and precipitation patterns over this region. Thus, aerosols are heating up the Himalayan climate, and will remain a key factor driving climate change over the region.

Dust transport and spatial distribution are poorly represented in current global climate models (GCMs) including the Community Atmosphere Model version 5 (CAM5). Particularly, models lack explicit representation of super-coarse dust, which may have important implications for dust radiative forcing and impacts on biogeochemistry. A nine-mode version of the modal aerosol model (MAM9) has been developed to address these issues. In this new aerosol scheme, four dust modes have been designed to treat dust particles of sizes up to 20 mu m. The MAM9-simulated results are compared with those from the default four-mode version of MAM (MAM4) and also with the in situ surface measurements of dust concentration and deposition flux, satellite-retrieved dust extinction profile, and in situ vertical measurements of dust concentrations from the NASA Atmosphere Tomography Mission (ATom). Overall, MAM9 improves the dust representation in remote regions while maintaining reasonably good results near the dust source regions. In addition, MAM9 reduces the fine dust burden and increases the coarse dust burden globally. The increased coarse dust burden has slightly increased the dust direct radiative effect by 0.01 W m(-2) while it enhanced dust indirect radiative effect by 0.36 W m(-2), globally.

Recent satellite observations of atmospheric aerosol loading over Asia indicate a dipole pattern in the aerosol optical depth (AOD) with a substantial decrease in AOD over East Asia and persistent increase in AOD over South Asia, the two global hotspots of aerosol emissions. Aerosol emissions over Asia are also changing rapidly. However, the evolution of physical, optical and chemical columnar aerosol characteristics, and their radiative effects over time, and the resultant impacts of such evolving trends on climate and other associated risks are not yet properly quantified, and used in climate impact assessments. In order to do so, we closely examine, in addition to satellite observations, for the first time, high-quality, ca. two-decade long ground-based observations since 2001 of aerosols and their radiative effects from several locations in the Indo-Gangetic Plain (IGP) in South Asia and the North China Plain (NCP) in East Asia. A clear divergence in the trends in AODs is evident between the IGP and the NCP. The single scattering albedo (SSA) is increasing, and the absorption AOD due to carbonaceous aerosols (AAOD(CA)) is decreasing over both regions, confirming that aerosols are becoming more scattering in nature. The trends in observed aerosol content (AOD) and composition (SSA) are statistically significant over Kanpur in the IGP and Beijing in the NCP, two locations with longest ground-based records. The aerosol radiative forcing of atmosphere (ARF(ATM)) and resultant atmospheric heating rate (HR) are decreasing over both regions. However, current regionally coherent and high annual HR of 0.5-1.0 K day(-1) has severe implications to climate, hydrological cycle, and cryosphere over Asia and beyond. These results based on high-quality observations over a large spatial domain are of great significance and are crucial for modelling and quantifying aerosol-climate interactions. (C) 2021 The Author(s). Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

Aerosol radiative properties using recently available high-quality columnar aerosol data collected at several AERONET sites in South Asia, with a focus on pollution outflow from continental South Asia observed over Hanimaadhoo in Maldives, a small island in northern Indian Ocean are quantified. The seasonal mean aerosol optical depth (AOD) over Hanimaadhoo is >= 0.3 (except ca. 0.2 during monsoon season), and single scattering albedo (SSA) is > 0.90 in all seasons. Fine mode aerosols contribute dominantly to AOD. SSA decreases as a function of wavelength due to influence of continental aerosols, except during the monsoon season when its spectral trend reverses due to increase in dust. Carbonaceous aerosols dominate (>90%) contribution to absorption AOD (AAOD). Black carbon (BC) and brown carbon (BrC) contribute >75% and -25 Wm(-2), > -20 Wm(-2) and similar to+20 Wm(-2), respectively. Aerosol loading and atmospheric heating have increased over this background site over the last decade. A regional-scale analysis of aerosol properties and radiative effects across and surrounding the Indo-Gangetic Plain (IGP) shows that AOD is >= 0.3 over entire region, and aerosols reduce seasonally 30-50 Wm(-2) of solar radiation reaching the surface, contributing significantly to solar dimming effect. The atmospheric solar heating rate due to aerosols (HR) is >= 1 K day(-1) across IGP. These high ARFs, ARFE(SFC) and HR, and increasing trends have significant implications to climate and hydrological cycle over South Asia and beyond.

This study reports comprehensive analysis of seasonal and inter-annual variations of aerosol properties (optical, physical and chemical) and radiative effects over Pokhara Valley in the foothills of central Himalayas in Nepal utilizing the high-quality multi-year columnar aerosol data observed recently from January 2010 to December 2017. This paper focusses on the seasonal and inter-annual variations of chemical (composition), and absorption properties of aerosols and their radiative effects. The single scattering albedo (SSA) either decreases as a function of wavelength or remains independent of wavelength. The seasonal mean aerosol absorption optical depth (AAOD) exhibits a behavior opposite to that of SSA. Carbonaceous aerosols (CA) dominate (>= 60%) aerosol absorption during the whole year. Black carbon (BC) alone contributes >60% to AAOD(C)(A) while brown carbon (BrC) shares the rest. The absorbing aerosol types are determined to be BC, and mixed (BC and dust) only. Dust as absorbing aerosol type is absent over the Himalayan foothills. The ARF(SFC) is >= -50 Wm(-2) except in monsoon almost every year. The ARF(ATM) is >= 50 Wm(-2) during winter and pre-monsoon in all the years. ARFE(SFC), ARFE(TOA) and ARFE(ATM) follow a similar pattern as that of ARF. High values of ARFE at SFC, TOA and ATM (except during monsoon when values are slightly lower) suggest that aerosols are efficient in significantly modulating the incoming solar flux throughout the year. The annual average aerosol-induced atmospheric heating rate (HR) over Pokhara is nearly 1 K day(-1) every year during 8-year observation, and is highest in 2015 (similar to 2.5 K day(-1)). The HR is about 1 K day(-1) or more over all the locations in IGP during the year. These quantitative results can be used as inputs in global/regional climate models to assess the climate impact of aerosols, including on regional temperature, hydrological cycle and melting of glaciers and snowflelds in the region. (C) 2020 Elsevier Ltd. All rights reserved.

Systematic measurements of mass concentration and chemical composition of aerosols have been carried out at Kharagpur in the Indo-Gangetic Plains during winter to identify the major sources over the region and to examine the changes in aerosol characteristics during haze events. Aerosol concentration is significantly large at the site, more than two-fold that of the National Ambient Air Quality Standards for residential areas in India. The main sources of aerosols over the region are anthropogenic activities and mineral dust. Species like SO42-, NO3-, NH4+, BC, CI-, etc. are associated with anthropogenic sources, while Al, Fe, Ca, Na, Ti, Mn, etc. originate mainly from crustal sources. Though the site is only similar to 100 km away from the Bay of Bengal, oceanic contribution is insignificant (similar to 2%), mainly due to prevailing north/northeasterly winds. A mean chemical composition evolved for the location shows that the aerosol system is composed of 17% mineral dust, 18% water-soluble components, 6% black carbon and 23% particulate organic matter along with a residual fraction of 36%. This residual fraction is attributed to organic aerosols of natural or secondary origin and water content of aerosols. An uncertainty of the order of 6-45% is involved in these estimations. Nevertheless, this mean chemical composition can act as a realistic input chemical model in the estimation of aerosol radiative forcing for this region. Analysis indicates that anthropogenic influence can be comparable to or exceeds natural aerosols at the location. The total aerosol mass concentration as well as that of anthropogenic species revealed enhancement on hazy days. (C) 2011 Elsevier Ltd. All rights reserved.