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The large-scale convection during the Asian summer monsoon plays an important role in the rapid transport of boundary layer aerosols into the Asian summer monsoon anticyclone. Here, using the state-of-the-art ECHAM6–HAMMOZ aerosol-chemistry-climate model, we show that these aerosols are further transported to the Arctic along isentropic surfaces by the Brewer-Dobson-Circulation (BDC) during the monsoon season. Our model simulations show that East and South Asian anthropogenic emissions contribute significantly to the aerosol transported to the Arctic, which causes a higher negative net aerosol radiative forcing at the surface (dimming) of −0.09 ± 0.02 Wm−2 and −0.07 ± 0.02 Wm−2, respectively. Over the Arctic, the East Asian anthropogenic aerosols that include large amounts of sulfate cause a seasonal mean net radiative forcing at the top of the atmosphere (TOA) of −0.003 ± 0.001Wm−2 and a surface cooling of −0.56 K while the black carbon dominated aerosol from South Asia shows a positive TOA forcing of +0.004 ± 0.001Wm−2 with an only minor surface cooling of −0.043 K. Overall, the long-range transport of South Asian aerosols results in a notably warming throughout the atmospheric column but minimal temperature response at the Arctic surface. Conversely, East Asian aerosols cool the troposphere and heat the lower stratosphere in the Arctic. The Asian aerosol thus plays an ambivalent role, with the East Asian sources in particular having the potential to counteract the rapid rise in Arctic temperatures and the associated melting of snow and ice. [ABSTRACT FROM AUTHOR]

Anthropogenic aerosols play a major role for the Earth-Atmosphere system by influencing the Earth's radiative budget and climate. The effect of the perturbation induced by changes in anthropogenic aerosols on the Earth's energy balance is quantified in terms of the effective radiative forcing (ERF) which is the recommended metric for perturbations affecting the Earth's top-of-atmosphere energy budget since it is a better way to link this perturbation to subsequent global mean surface temperature change. In this work, the present-day ERF of anthropogenic aerosols is quantified using simulations from Earth system models (ESMs) participating in the Coupled Model Intercomparison Project Phase 6 (CMIP6). The ERFs of individual aerosol species, such as sulphates, organic carbon (OC), and black carbon (BC) are calculated along with the ERF due to all anthropogenic aerosols and the transient ERF over the historical period (1850–2014). Additionally, ERF is analyzed into three components: (a) ERFARI, representing aerosol-radiation interactions, (b) ERFACI, accounting for aerosol-cloud interactions, and (c) ERFALB, which is mainly due to the contribution of surface albedo changes caused by anthropogenic aerosols. Here, the total anthropogenic aerosol ERF (calculated using the piClim-aer experiment) is estimated to be -1.11 ± 0.26 W m-2, mostly due to the large contribution of ERFACI (-1.14 ± 0.33 W m-2), compared to ERFARI (-0.02 ± 0.20 W m-2) and ERFALB (0.05 ± 0.07 W m-2). The total ERF caused by sulphates (piClim-SO2) is estimated at -1.11 ± 0.31 W m-2, the OC ERF (piClim-OC) is -0.35 ± 0.21 W m-2, whereas the ERF exerted by BC (piClim-BC) is 0.19 ± 0.18 W m-2. On top of that, our analysis reveals that ERFACI clearly prevails over the largest part of the Earth except for the BC experiment where ERFARI prevails over land. By the end of the historical period (1995–2014), the global mean total aerosol ERF is estimated at -1.28 ± 0.37 W m-2 (calculated using the histSST experiment). We find that sulphates dominate both present-day and transient ERF spatial patterns at the top of the atmosphere, exerting a strongly negative ERF especially over industrialized regions of the Northern Hemisphere, such as North America, Europe, East and South Asia. Since the mid-1980s ERF has become less negative over Eastern North America and Western and Central Europe, while over East and South Asia there is a steady increase in ERF magnitude towards more negative values until 2014. [ABSTRACT FROM AUTHOR]

Long-range transport of black carbon from South Asia to the Tibetan plateau and its deposition on glaciers directly enhances glacier melt. Here we find South Asian black carbon also has an indirect effect on the plateau's glaciers shrinkage by acting to reduce the water supply over the southern Tibetan plateau. Black carbon enhances vertical convection and cloud condensation, which results in water vapor depletion over the Indian subcontinent that is the main moisture flux source for the southern Tibetan plateau. Increasing concentrations of black carbon causes a decrease in summer precipitation over the southern Tibetan plateau, resulting in 11.0% glacier deficit mass balance on average from 2007 to 2016; this loss rises to 22.1% in the Himalayas. The direct (accelerated melt) and indirect (mass supply decrease) effects of black carbon are driving the glacial mass decline of the so-called "Asian Water Tower". South Asian black carbon's pairing activities (direct and indirect effects) are driving glacial mass decline of the so-called "Water Tower of Asia" with the potential to threaten future water resources for the countries that rely upon its source. [ABSTRACT FROM AUTHOR]

Through a comprehensive investigation into the historical profiles of black carbon derived from ice cores, the spatial distributions of light-absorbing impurities in snowpit samples, and carbon isotopic compositions of black carbon in snowpit samples of the Third Pole, we have identified that due to barriers of the Himalayas and remove of wet deposition, local sources rather than those from seriously the polluted South Asia are main contributors of light-absorbing impurities in the inner part of the Third Pole. Therefore, reducing emissions from residents of the Third Pole themselves is a more effective way of protecting the glaciers of the inner Third Pole in terms of reducing concentrations of light-absorbing particles in the atmosphere and on glaciers. [Display omitted] • LAPs deposited on glaciers of the inner TP are primarily sourced from the TP itself. • The Himalayas serve as barrier of transport LAPs from South Asia into inner TP. • Atmosphere of the inner TP remains largely devoid of long-range transported LAPs. [ABSTRACT FROM AUTHOR]

The Asian Tropopause Aerosols Layer (ATAL) refers to an accumulation of aerosols in the upper troposphere and lower stratosphere during boreal summer over Asia, which has a fundamental impact on the monsoon system and climate change. In this study, we primarily analyze the seasonal to sub-seasonal variations of the ATAL and the factors potentially influencing those variations based on MERRA2 reanalysis. The ability of the reanalysis to reproduce the ATAL is well validated by CALIPSO observations from May to October 2016. The results reveal that the ATAL has a synchronous spatiotemporal pattern with the development and movement of the Asian Summer Monsoon. Significant enhancement of ATAL intensity is found during the prevailing monsoon period of July–August, with two maxima centered over South Asia and the Arabian Peninsula. Owing to the fluctuations of deep convection, the ATAL shows an episodic variation on a timescale of 7–12 days. Attribution analysis indicates that deep convection dominates the variability of the ATAL with a contribution of 62.7%, followed by a contribution of 36.6% from surface pollutants. The impact of precipitation is limited. The ATAL further shows a clear diurnal variation: the peak of ATAL intensity occurs from 17:30 to 23:30 local time (LT), when the deep convection becomes strongest; the minimum ATAL intensity occurs around 8:30 LT owing to the weakened deep convection and photochemical reactions in clouds. The aerosol components of the ATAL show different spatiotemporal patterns and imply that black carbon and organic carbon come mainly from India, whereas sulfate comes mainly from China during the prevailing monsoon period. [Display omitted] [ABSTRACT FROM AUTHOR]

The Pan‐Third Pole contains the largest number of glaciers outside the polar region that plays a crucial role in atmospheric circulation and the hydrological cycle. However, this pristine region has undergone rapid change through complex interactions including the black carbon (BC) enhanced warming effect and glacier melting. Study shows, Weather Research and Forecasting coupled with Chemistry (WRF‐Chem) simulation is able to capture distinctive seasonal variability of BC. The result from our sensitivity experiments revealed that South Asia (SA; 60.7%) and East Asia (EA; 32.9%) contributed more toward the Tibetan Plateau (TP). Our analysis on aerosol‐boundary feedback interaction revealed BC expand planetary boundary layer height by 5.0% and 4.8% over SA and EA, respectively, which facilitates BC dispersion and transportation. Whereas, we also found that under the influence of different wind regimes the significant BC transport flux aloft over the TP and the upper troposphere and lower stratosphere. Additionally, mountain‐valley channel and synoptic and local meteorological processes also facilitated BC transport to the TP. This study also evaluated the effect of BC on direct radiative forcing and calculated subsequent temperature changes. A strong dimming effect of BC corroborated with the following negative surface temperature changes. However, enhanced BC concentration during winter and spring caused the increase in temperature over the TP. Here, the WRF‐Chem model, synergy on aerosol‐boundary feedback, BC transport flux, and source‐receptor methods confirmed the significant BC contribution and transportation, and notable BC‐induced warming over TP. Such trans‐Himalayan BC transport and associated warming could grim glacier melt and water availability in the region. Key Points: Trans‐Himalaya long‐range transport is evident with significant black carbon (BC) transport flux aloft above Himalayas and Tibetan Plateau and free troposphere from South Asia (SA) and East Asia (EA) regionSynergy on aerosol‐boundary feedback interaction manifested that BC expand planetary boundary layer height by 5.01% and 4.80% over SA and EA, respectivelyElevated BC caused notable warming over Tibetan Plateau that could pose a threat in the cryosphere over that region [ABSTRACT FROM AUTHOR]

In 1995, a multimillion-dollar experiment – the Indian Ocean Experiment – discovered a dark mass of polluting air hovering above the Indian subcontinent. This mass of air was termed a cloud and found to be composed of a high amount of black carbon that was judged to be the second biggest threat to climate change after carbon-dioxide. In this article, an attempt is made to trace the life of black carbon by documenting its changing forms since the experiment. It emerges that the changing forms allow for the movement of air – smoke from traditional cookstoves and vehicular diesel emissions in India lead to the formation of the cloud – and reveal how an ethnography of air can be undertaken. [ABSTRACT FROM AUTHOR]

Regional climatic implications of aerosol black carbon (BC), which has a wide variety of anthropogenic sources in large abundance, are well recognized over South Asia. Significant uncertainties remain in its quantification due to a lack of sufficient information on the microphysical properties (its concentration, size, and mixing state with other aerosol components) that determine the absorption potential of BC. In particular, the information on the mixing state of BC is extremely sparse over this region. In this study, the first observations of the size distribution and mixing state of individual refractory black carbon (rBC) particles in the South Asian outflow to the south-eastern Arabian Sea and the northern and equatorial Indian Ocean regions are presented based on measurements using a single particle soot photometer (SP2) aboard the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB-2018) ship during winter 2018 (16 January to 13 February). The results revealed significant spatial heterogeneity of BC characteristics. The highest rBC mass concentrations (∼938±293 ngm-3) with the highest relative coating thickness (RCT; the ratio of BC core to its coating diameters) of ∼2.16±0.19 are found over the south-east Arabian Sea (SEAS) region, which is in the proximity of the continental outflow. As we move to farther oceanic regions, though the mass concentrations decreased by nearly half (∼546±80 ngm-3), BC still remained thickly coated (RCT∼2.05±0.07). The air over the remote equatorial Indian Ocean, which received considerable marine air masses compared to the other regions, showed the lowest rBC mass concentrations (∼206±114 ngm-3) with a moderately thick coating (RCT∼1.73±0.16). Even over oceanic regions far from the landmass, regions that received the outflow from the more industrialized east coast/the Bay of Bengal had a thicker coating (∼104 nm) compared to regions that received outflow from the west coast and/or peninsular India (∼86 nm). Although different regions of the ocean depicted contrasting concentrations and mixing state parameters due to the varied extent and nature of the continental outflow as well as the atmospheric lifetime of air masses, the modal parameters of rBC mass–size distributions (mean mass median diameters ∼ 0.19–0.20 µm) were similar over all regions. The mean fraction of BC-containing particles (FBC) varied in the range of 0.08–0.12 (suggesting significant amounts of non-BC particles), whereas the bulk mixing ratio of coating mass to rBC mass was highest (8.31±2.40) over the outflow regions compared to the remote ocean (4.24±1.45), highlighting the role of outflow in providing condensable material for coatings on rBC. These parameters, along with the information on the size-resolved mixing state of BC cores, throw light on the role of sources and secondary processing of their complex mixtures for coatings on BC under highly polluted conditions. Examination of the non-refractory sub-micrometre aerosol chemical composition obtained using the aerosol chemical speciation monitor (ACSM) suggested that the overall aerosol system was sulfate-dominated over the far-oceanic regions. In contrast, organics were equally prominent adjacent to the coastal landmass. An association between the BC mixing state and aerosol chemical composition suggested that sulfate was the probable dominant coating material on rBC cores. [ABSTRACT FROM AUTHOR]

There is ample evidence that Black Carbon (BC) is harmful to the Arctic. BC can darken the otherwise highly reflective surfaces of snow and ice and increase atmospheric and ice surface temperatures. Because of the importance of BC to the Arctic, this work was designed to resolve the most significant source regions of Arctic BC as measured by monitoring stations in the Arctic. Using a bottom‐up approach, it is shown for the first time that there is one particular BC transport pathway from lower latitudes into the Arctic that registers at all but one of the Arctic surface monitoring stations included in this study. Through this pathway, pollutants are transported from the Indo‐Gangetic plane over Central Asia into the high Arctic in as little as 7 days. The measurement sites and BC pathways in this study are shown to be well representative of the Arctic as a whole. Plain Language Summary: Snow and sea ice are two of the most reflective surfaces occurring naturally on planet Earth. Light absorbing aerosols, such as wind blown black carbon, that stick to these pristine surfaces can make them less reflective, thus converting more sunlight into heat. The added heat leads to increased surface temperatures and is detrimental to the Arctic climate. There are few sources of light absorbing aerosols in the Arctic compared to the vast amounts of black carbon aerosols that are emitted closer to the equator. This research highlights an important pathway for light absorbing aerosols to enter the Arctic. Light absorbing aerosols that enter the Arctic through this pathway can then subsequently spread out throughout the Arctic. This pathway is shown to transport light absorbing airborne pollutants from South Asia, over Central Asia, and into the high Arctic in quantities that well exceed the background levels, thus highlighting the importance of this newly found pathway. Key Points: Black Carbon aerosols can travel from South Asia over Central Asia and to the Arctic in 7 daysBlack Carbon transported via this pathway measures well above the station average at almost all Arctic stationsThe identified pathway is significant for Black Carbon concentrations throughout the Arctic [ABSTRACT FROM AUTHOR]

Atmospheric black carbon (BC) in the Tibetan Plateau (TP) can largely impact regional and global climate. Still, studies on the inter-annual variation in atmospheric BC over the TP and associated variation in BC sources and controlling factors are rather limited. In this study, we characterize the variations in atmospheric BC over the TP surface layer through analysis of 20-year (1995–2014) simulations from a global chemical transport model, GEOS-Chem. The results show that surface BC concentrations over the TP vary largely in space and by season, reflecting complicated interplays of BC sources from different origins. Of all areas in the TP, surface BC concentrations are highest over the eastern and southern TP, where surface BC is susceptible to BC transport from East Asia and South Asia, respectively. Applying a backward-trajectory method that combines BC concentrations from GEOS-Chem and trajectories from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model, we assess the contributions of worldwide source regions to surface BC in the TP. We estimate that on the 20-year average, 77 % of surface BC in the TP comes from South Asia (43 %) and East Asia (35 %). Regarding seasonal variation in non-local influences, South Asia and East Asia are dominant source regions in winter and summer, respectively, in terms of the amount of BC imported. However, in terms of affected areas in the TP, South Asia is the dominant contributor throughout the year. Inter-annually, surface BC over the TP is largely modulated by atmospheric transport of BC from non-local regions year-round and by biomass burning in South Asia, mostly in spring. We find that the extremely strong biomass burning in South Asia in the spring of 1999 greatly enhanced surface BC concentrations in the TP (31 % relative to the climatology). We find that the strength of the Asian monsoon correlates significantly with the inter-annual variation in the amount of BC transported to the TP from non-local regions. In summer, a stronger East Asian summer monsoon and a stronger South Asian summer monsoon tend to, respectively, lead to more BC transport from central China and north-eastern South Asia to the TP. In winter, BC transport from central China is enhanced in years with a strong East Asian winter monsoon or a strong Siberian High. A stronger Siberian High can also bring more BC from northern South Asia to the TP. This study underscores the impacts of atmospheric transport and biomass burning on the inter-annual variation in surface BC over the TP. It reveals a close connection between the Asian monsoon and atmospheric transport of BC from non-local regions to the TP. [ABSTRACT FROM AUTHOR]

The contribution of individual aerosol species and greenhouse gases to precipitation changes during the South Asian summer monsoon is uncertain. Mechanisms driving responses to anthropogenic forcings need further characterization. We use an atmosphere‐only climate model to simulate the fast response of the summer monsoon to different anthropogenic aerosol types and to anthropogenic greenhouse gases. Without normalization, sulfate is the largest driver of precipitation change between 1850 and 2000, followed by black carbon and greenhouse gases. Normalized by radiative forcing, the most effective driver is black carbon. The precipitation and moisture budget responses to combinations of aerosol species perturbed together scale as a linear superposition of their individual responses. We use both a circulation‐based and moisture budget‐based argument to identify mechanisms of aerosol and greenhouse gas induced changes to precipitation and find that in all cases the dynamic contribution is the dominant driver to precipitation change in the monsoon region. Plain Language Summary: Small particles suspended in the atmosphere and emitted by human activities, known as atmospheric aerosols, contribute to changes in precipitation in South Asia, yet the exact importance of individual components is not well understood. Further, the impact of human‐caused greenhouse gas emissions is also uncertain. We use a computer model to simulate the monsoon and find that aerosols are the largest driver of precipitation change compared to greenhouse gases. We also breakdown the precipitation changes by their components and find that changes in atmospheric vertical and horizontal motion of air dominate over increases in moisture for explaining the total rainfall changes. These results help further understand the impact of climate change on precipitation in South Asia, which is important to the lives of billions of people. Key Points: Per unit radiative forcing, black carbon is the most effective driver of precipitation change during the South Asian summer monsoonAnomalous sinking and rising explain modeled precipitation responses to anthropogenic greenhouse gases and aerosolsThe dynamic component dominates the mean column‐integrated moisture flux convergence over the thermodynamic component [ABSTRACT FROM AUTHOR]

Aerosol radiative forcing can influence climate both locally and far outside the emission region. Here we investigate black carbon (BC) aerosols emitted in four major emission areas and evaluate the importance of emission location and magnitude as well as the concept of the absolute regional temperature-change potentials (ARTP). We perform simulations with a climate model (NorESM) with a fully coupled ocean and with fixed sea surface temperatures. BC emissions for year 2000 are increased by factors of 10 and 20 in South Asia, North America, and Europe, respectively, and by 5 and 10 in East Asia (due to higher emissions there). The perturbed simulations and a reference simulation are run for 100 years with three ensemble members each. We find strikingly similar regional surface temperature responses and geographical patterns per unit BC emission in Europe and North America but somewhat lower temperature sensitivities for East Asian emissions. BC emitted in South Asia shows a different geographical pattern in surface temperatures, by changing the Indian monsoon and cooling the surface. We find that the ARTP approach rather accurately reproduces the fully coupled temperature response of NorESM. Choosing the highest emission rate results in lower surface temperature change per emission unit compared to the lowest rate, but the difference is generally not statistically significant except for the Arctic. An advantage of high-perturbation simulations is the clearer emergence of regional signals. Our results show that the linearity of normalized temperature effects of BC is fairly well preserved despite the relatively large perturbations but that regional temperature coefficients calculated from high perturbations may be a conservative estimate. Regardless of emission region, BC causes a northward shift of the ITCZ, and this shift is apparent both with a fully coupled ocean and with fixed sea surface temperatures. For these regional BC emission perturbations, we find that the effective radiative forcing is not a good measure of the climate response. A limitation of this study is the uncertainties in BC–cloud interactions and the amount of BC absorption, both of which are model dependent. [ABSTRACT FROM AUTHOR]

Black carbon (BC) and dust impart significant effects on the South Asian monsoon (SAM), which is responsible for ∼80 % of the region's annual precipitation. This study implements a variable-resolution (VR) version of the Community Earth System Model (CESM) to quantify two radiative effects of absorbing BC and dust on the SAM. Specifically, this study focuses on the snow darkening effect (SDE), as well as how these aerosols interact with incoming and outgoing radiation to facilitate an atmospheric response (i.e., aerosol–radiation interactions, ARIs). By running sensitivity experiments, the individual effects of SDE and ARI are quantified, and a theoretical framework is applied to assess these aerosols' impacts on the SAM. It is found that ARIs of absorbing aerosols warm the atmospheric column in a belt coincident with the May–June averaged location of the subtropical jet, bringing forth anomalous upper-tropospheric (lower-tropospheric) anticyclogenesis (cyclogenesis) and divergence (convergence). This anomalous arrangement in the mass fields brings forth enhanced rising vertical motion across South Asia and a stronger westerly low-level jet, the latter of which furnishes the Indian subcontinent with enhanced Arabian Gulf moisture. Precipitation increases of 2 mm d -1 or more (a 60 % increase in June) result across much of northern India from May through August, with larger anomalies (+5 to +10 mm d -1) in the western Indian mountains and southern Tibetan Plateau (TP) mountain ranges due to orographic and anabatic enhancement. Across the Tibetan Plateau foothills, SDE by BC aerosols drives large precipitation anomalies of > 6 mm d -1 (a 21 %–26 % increase in May and June), comparable to ARI of absorbing aerosols from April through August. Runoff changes accompany BC SDE-induced snow changes across Tibet, while runoff changes across India result predominantly from dust ARI. Finally, there are large differences in the simulated SDE between the VR and traditional 1 ∘ simulations, the latter of which simulates a much stronger SDE and more effectively modifies the regional circulation. [ABSTRACT FROM AUTHOR]

Improving the accuracy of regional aerosol climate impact assessment calls for improvement in the accuracy of regional aerosol radiative effect (ARE) estimation. One of the most important means of achieving this is to use spatially homogeneous and temporally continuous datasets of critical aerosol properties, such as spectral aerosol optical depth (AOD) and single scattering albedo (SSA), which are the most important parameters for estimating aerosol radiative effects. However, observations do not provide the above; the space-borne observations though provide wide spatial coverage, are temporal snapshots and suffer from possible sensor degradation over extended periods. On the other hand, the ground-based measurements provide more accurate and temporally continuous data but are spatially near-point observations. Realizing the need for spatially homogeneous and temporally continuous datasets on one hand and the near non-existence of such data over the south Asian region (which is one of the regions where aerosols show large heterogeneity in most of their properties), construction of accurate gridded aerosol products by synthesizing the long-term space-borne and ground-based data has been taken up as an important objective of the South West Asian Aerosol Monsoon Interactions (SWAAMI), a joint Indo-UK field campaign, aiming at characterizing aerosol–monsoon links and their variabilities over the Indian region. In Part 1 of this two-part paper, we present spatially homogeneous gridded datasets of AOD and absorption aerosol optical depth (AAOD), generated for the first time over this region. These data products are developed by merging the highly accurate aerosol measurements from the dense networks of 44 (for AOD) and 34 (for AAOD) ground-based observatories of Aerosol Radiative Forcing over India NETwork (ARFINET) and AErosol RObotic NETwork (AERONET) spread across the Indian region, with satellite-retrieved AOD and AAOD, following statistical assimilation schemes. The satellite data used for AOD assimilation include AODs retrieved from MODerate Imaging Spectroradiometer (MODIS) and Multiangle Imaging SpectroRadiometer (MISR) over the same domain. For AAOD, the ground-based black carbon (BC) mass concentration measurements from the network of 34 ARFINET observatories and satellite-based (Kalpana-1, INSAT-3A) infrared (IR) radiance measurements are blended with gridded AAODs (500 nm, monthly mean) derived from Ozone Monitoring Instrument (OMI)-retrieved AAODs (at 354 and 388 nm). The details of the assimilation methods and the gridded datasets generated are presented in this paper. The merged gridded AOD and AAOD products thus generated are validated against the data from independent ground-based observatories, which were not used for the assimilation process but are representative of different subregions of the complex domain. This validation exercise revealed that the independent ground-based measurements are better confirmed by merged datasets than the respective satellite products. As ensured by assimilation techniques employed, the uncertainties in merged AODs and AAODs are significantly less than those in corresponding satellite products. These merged products also all exhibit important large-scale spatial and temporal features which are already reported for this region. Nonetheless, the merged AODs and AAODs are significantly different in magnitude from the respective satellite products. On the background of above-mentioned quality enhancements demonstrated by merged products, we have employed them for deriving the columnar SSA and analysed its spatiotemporal characteristics. The columnar SSA thus derived has demonstrated distinct seasonal variation over various representative subregions of the study domain. The uncertainties in the derived SSA are observed to be substantially less than those in OMI SSA. On the backdrop of these benefits, the merged datasets are employed for the estimation of regional aerosol radiative effects (direct), the results of which would be presented in a companion paper, Part 2 of this two-part paper. [ABSTRACT FROM AUTHOR]

Abstract Black carbon (BC) mitigation can reduce adverse environmental impacts on climate, air quality, human health, and water resource availability. To facilitate the identification of mitigation priorities, we use a state-of-the-science global chemistry-climate coupled model (AM3), with additional tagged BC tracers representing regional (East Asia, South Asia, Europe and North America) and sectoral (land transport, residential, industry) anthropogenic BC emissions to identify sources with the largest impacts on air quality, human health and glacial deposition. We find that within each tagged region, domestic emissions dominate BC surface concentrations and associated premature mortality (generally over 90%), as well as BC deposition on glaciers (∼40–95% across glaciers). BC emissions occurring within each tagged source region contribute roughly 1–2 orders of magnitude more to their domestic BC concentrations, premature mortality, and BC deposition on regional glaciers than that caused by the same quantity of BC emitted from foreign regions. At the sectoral level, the South Asian residential sector contributes ∼60% of BC associated premature mortality in South Asia and ∼40–60% of total BC deposited on southern Tibetan glaciers. Our findings imply that BC mitigation within a source region, particularly from East and South Asian residential sectors, will bring the largest reductions in BC associated air pollution, premature mortality, and glacial deposition. Highlights • We simulate BC impacts with a state-of-the-science global chemistry-climate model. • We evaluate the global BC health and deposition impacts based on an integrated assessment. • We identify regions and sectors with the highest BC mitigation priorities. • We build source-receptor matrixes for BC on human health and glacier deposition. [ABSTRACT FROM AUTHOR]

Abstract As an important part of the anthropogenic aerosol, Black Carbon (BC) aerosols in the atmospheric environment have strong impacts on climate change. Recently, most remote sensing studies on aerosol components detection are limited to the inversion of aerosol optical properties, integration of chemistry models or in situ observations. In this paper, an algorithm based on Effective Medium Approximations (EMA) and statistically optimized aerosol inversion algorithm was integrated for retrieving the surface mass concentration of BC aerosols from satellite signals. The sensitivity analyses for the developed forward model proved that the volume fraction of vertical BC is sensitive to the satellite observations and significantly improved especially over bright surface targets or under polluted atmospheric conditions. By updating the forward model and retrieved parameters of the statistically optimized inversion algorithm, three cases of high aerosol loading days were retrieved from Polarization and Anisotropy of Reflectance for Atmospheric Sciences Coupled with Observations from a LiDAR (PARASOL) measurements, which shows a significant ability of BC aerosol detection. Additionally, the validation and closure studies of BC concentration retrievals also indicates an encouraging consistency with correlation (R) of 0.71, mean bias of 3.55, and root-mean-square error (RMSE) of 3.75 when compared against the in-situ observations over South Asia. The accuracy of the retrievals also demonstrates different trends under different levels of aerosol loadings, which shows a higher accuracy in biomass burning seasons (R = 0.75, RMSE = 4.04, Bias = 3.27) while exaggerates the results in the case of clear conditions (R = 0.47, RMSE = 4.83, Bias = 4.00). Finally, the uncertainties of three assumptions, including proposing uniform vertical profile for BC, neglecting light-absorbing aerosols and using spherical EMA models were discussed in our manuscript. The maximum standard deviations caused by these uncertainties on low BC aerosol volume fractions (f BC < 1%) are 0.8%, 0.35% and 0.2% while these deviations will change to 0.25%, 0.05% and 1.5% respectively under higher BC fractions (f BC > 5%). This conclusion confirmed that the proposed algorithm for BC surface concentration retrieval extends the application of satellite remote sensing in monitoring the extreme biomass burning pollution. Highlights • Satellite based BC surface concentration retrieval algorithm was developed. • Detailed sensitivities of integral forward modules of EMA and RTM were proposed. • Global high aerosol loads cases and closure validation studies of BC were tested. • Detailed uncertainties of expected assumptions in retrieval were analyzed. [ABSTRACT FROM AUTHOR]

Black carbon (BC) particles over the Qinghai-Tibet Plateau (Q-TP), whether suspended in the air or deposited on snow, can affect snow melt and accelerate glacier retreat. Since BC over the Q-TP may originate from different regions, it is necessary to understand the transport mechanism of BC and then provide concentrations and spatial-temporal distributions of BC over Q-TP, in order to study its radiative and climatic effects. In this study, Weather Research and Forecasting model with Chemistry (WRF-Chem), with MIX emission inventory (a mosaic Asian anthropogenic emission inventory for the MICS-Asia and the HTAP projects), was used to study the BC transport from South Asia, one of the major BC source regions, to Q-TP in April (pre-monsoon season), July (summer monsoon season) and December (winter monsoon season). The model's ability to simulate the BC transport from South Asia to the Q-TP was firstly evaluate using various reanalysis data, including 2 m temperature, precipitation, 10 m wind speed and direction and BC concentration at surface. The simulated meteorological fields and seasonal distribution and variation of BC are generally in good agreement with reanalysis data. The model showed good performance in simulating transport of BC as well. Our study showed that during pre-monsoon season, BC aerosols accumulated in the foothills of the Himalayas and were transported to southern Q-TP and even Southeast Asia via dry and wet deposition and westerly winds below 200hpa. Although the cyclonic airflow, which was favorable to transport, prevailed in South Asia, slight BC aerosols would be transported to Q-TP through wet deposition due to the efficient wet scavenging by abundant precipitation during summer monsoon season. During winter monsoon season, the prevailing westerly winds at 500hpa would transport BC aerosols from South Asia to Q-TP by dry deposition. A sensitivity simulation using PKU emission inventory (Peking University's global BC emission inventory) was then conducted to investigate the impact of different emission inventory on the modeling results. It is shown that the simulations using the different BC emission inventories overall did not have significant impacts on spatial distributions of surface concentration and column burden during three studies seasons. However, the simulated BC concentrations and burden based on PKU were overall higher than that based on MIX, that is, main differences were found in the southern part of Q-TP (i.e. adjacent to South Asia), especially during pre-monsoon and winter monsoon seasons. The differences of mean BC surface concentration ranging from 0.1 to 1 μg/m3, and burden ranging from 0.1 to 1 mg/m2 due to both higher dry and wet deposition (0.1–2 μg/m2/s for dry deposition flux, and 0.1–3 μg/m2/s for wet deposition flux) during pre-monsoon. During winter monsoon seasons, relatively large differences (0–3 μg/m3 for concentration, and 0–2 mg/m2 for burden, and 0.1–3 μg/m2/s for dry deposition flux) were found in southern Q-TP. • By using different emission inventories, MIX and PKU, WRF-Chem model is used to simulate transport of BC. • The simulations using the different BC emission inventories may not have significant impacts on spatial distributions of surface concentration and column burden. • The major differences of BC by using MIX and PKU are located in South Asia and Southeast Asia while relatively small over Q-TP. • The transmission of BC is mainly influenced by large-scale atmospheric circulation. [ABSTRACT FROM AUTHOR]

Aerosols over the Tibetan Plateau (TP) play an important role in the regional and global climate change and ecological environment. The Second Tibetan Plateau Scientific Expedition and Research Program (STEP) carried out observation experiment at the Nam Co Station for Multisphere Observation and Research, Chinese Academy of Sciences (Nam Co Station, 30°46′N, 90°59′E, 4730 m) in the central TP from August 5 to September 11, 2020. This paper investigated the aerosol optical properties and its potential sources over Nam Co region. The main results are as follow: the average aerosol optical depth was 0.025 during the observation period; the average black carbon (BC) concentration and aerosol number concentration was 37 ng/m3 and 129 N/cm3, respectively; the diurnal variation and vertical distribution of aerosol were affected by the local meteorological elements. Frequent southerly winds during the summer monsoon led to a certain contribution of South Asia to the BC concentration in Nam Co region. And particles from the Taklimakan Desert led to dramatic increasing of aerosol number concentration over Nam Co region. The aerosol optical characteristics and influencing factors over the central TP were analyzed, which is beneficial to further study the influence of aerosols over the TP on regional and global weather and climate. • Optical properties of aerosol over Nam Co station were observed by the STEP of China. • Nam Co region was generally influenced by BC from South Asia, local pollution emissions and north particles. • The average of AOD, depolarization ratio and BC over Nam Co station was 0.026, 0.033 and 37 ng/m3, respectively. [ABSTRACT FROM AUTHOR]

Refractory Black carbon (rBC) emitted from the combustion of biomass and fossil fuels plays an important role in the climate system. In this study, we established a record of the rBC concentration spanning 1932–2013 from an ice core retrieved from the Chongce ice cap of the West Kunlun Mountains in the western Tibetan Plateau. The record showed an increasing trend since the 1980s. The mean concentration of rBC was 2.66 ng g−1 before 1980 and 5.33 ng g−1 since 1980. The significant increase since the 1980s was very different from similar records from European ice cores. An analysis of atmospheric circulation and backward trajectories suggested that former USSR, the Middle East, and South Asia were the most likely source regions for the rBC deposited at the Chongce ice cap. This conclusion was also supported by the historical emission data in these regions. Phase analysis indicated that high rBC concentrations were closely associated with drought-induced biomass burning in the rBC source regions at the decadal timescale. These findings suggested that anthropogenic emissions controlled the long-term rBC trend, while the peak phases were caused from an increase of biomass burning. •Anthropogenic activity and climate change play a critical role in the rBC variations over the northwestern Tibetan Plateau. • Anthropogenic emissions since 1960s contributed more to rBC deposition. • Droughts-induced biomass burning were responsible for high rBC concentration phases. [ABSTRACT FROM AUTHOR]

Black carbon (BC), by the combustion of fossil fuels and biomass, has profound effects on climate change and glacier retreat in industrial eras. In the present study, we report refractory BC (rBC) in an ice core spanning 1850–2014, retrieved from the Hariqin Glacier of the Tanggula Mountains in the central Tibetan Plateau, measured using a single particle soot photometer (SP2). The rBC concentration shows a three-fold increase since the 1950s. The mean rBC concentration was 0.71 ± 0.52 ng mL−1 during 1850s–1940s and 2.11 ± 1.60 ng mL−1 during 1950s–2010s. The substantial increase in rBC since the 1950s is consistent with rBC ice core records from the Tibetan Plateau and Eastern Europe. According to the predominant atmospheric circulation patterns over the glacier and timing of changes in regional emissions, the post-1950 amplification of rBC concentration in the central Tibetan Plateau most likely reflects increases in emissions in Eastern Europe, former USSR, the Middle East, and South Asia. Despite the low-level background rBC concentrations in the ice cores from the Tibetan Plateau, the present study highlights a remarkable increase in anthropogenic BC emissions in recent decades and the consequent influence on glaciers in the Tibetan Plateau. Image 1 • BC deposition (1850–2014) in a Central Tibetan glacier has tripled since the 1950s. • BC emission from Indian monsoon domain mainly contributes to the increase. • BC deposition in Tibetan glaciers is at the lowest level in Northern Hemisphere. [ABSTRACT FROM AUTHOR]

The short term effect of multiple air pollutants e.g. aerosols (black carbon, BC; PM 2.5 and PM 10) and trace gases (NO 2 , SO 2, and O 3) on all-cause mortality was systematically investigated in a typical urban pollution hotspot over central Indo-Gangetic Plain (IGP). To our knowledge, this would be the first report of mortality estimates for exposure to BC aerosols and multiple trace gases over South Asia. Daily all-cause mortality and ambient air quality were analyzed from 2009 to 2016 following a semiparametric quasi-Poisson regression model adjusting mean temperature (Tmean), relative humidity (RH), and long term time trend (Time) as potential confounders. Single pollutant model clearly established the significant impact of BC aerosols (against 10-unit increase in pollutant; 4.95%, 95% CI: 2.16–7.74), NO 2 (2.38%, 95% CI: 0.88–3.87%) and PM 2.5 exposure (1.06%, 95% CI: 0.45–1.66%) on mortality. The inclusion of co-pollutants in the multi-pollutant model increased the individual mortality risks for BC aerosols (7.3%). Mortality estimates were further stratified considering different effect modifiers viz. sex, age, place of death, and season. Almost in all the cases statistically insignificant differences in effect modification were noted for all the pollutants except PM 10. We also explored a distributed lag nonlinear model to estimate the lag effect and all the pollutants showed significant lag up to 3 days while BC showed lag effect up to 5 days. The exposure-response curves for individual air pollutants were mostly linear, while a considerable increase in mortality was noted for an exposure >15 μg m−3 for BC aerosols and >60 μg m−3 for PM 2.5. The effect estimates of air pollutants during haze and no-haze days were also defined. During haze days, mortality rose to 6.11% and 3.06% for each 10-unit increase in BC and NO 2 exposure, respectively. Significant effect of BC aerosol exposure on human mortality was established which reaffirms its inclusion as a potential health regulator for epidemiological studies. Image 1 • Mortality estimate of black carbon aerosols and multiple trace gases are reported. • Mortality estimate was particularly high for BC followed by NO 2 levels. • All the pollutants showed significant lag effect between lag 0–1 and lag 0–6 days. • Synergistic effect was noted when BC, Res.PM 2.5 and NO 2 were combined. • Mortality estimates of BC aerosols, PM 2.5 and NO 2 were higher during hazy days. [ABSTRACT FROM AUTHOR]

This study used a regional climate-chemistry transport model, WRF-Chem v3.9.1, to evaluate the impact of South Asian biomass burning on black carbon (BC) over the Tibetan Plateau (TP) and its climatic effects for an entire year. The simulation, which was validated by comparing surface meteorological parameters and BC concentration against in-situ observations over South Asia and the TP, provided a perspective on the seasonal variations and regional spatial patterns of BC concentration. Using a sensitivity simulation where BC emissions from biomass burning were removed from South Asia, this study found South Asian biomass burning emissions contributed up to 90% of BC mass over the TP during the pre-monsoon season, specifically emissions from western India for the simulated year. The emissions led to reduced surface radiative forcing, causing the temperature to decrease accordingly. However, column cloud water was increased. This study suggested that the biomass burning emissions from South Asia have significant impact on atmospheric BC over the TP, especially during the pre-monsoon season. Therefore, reducing biomass burning emissions from South Asia is potentially important for alleviating the effects of BC on climatic and environmental conditions over the TP and surrounding regions. Image 1 • South Asian biomass burning notably affected black carbon over the Tibetan Plateau. • South Asian biomass burning reduced surface radiative forcing and temperature. • Black carbon as cloud condensation nuclei increased the cloud water mixing ratio. [ABSTRACT FROM AUTHOR]

• Study dynamic character of biomass burning aerosol (BBA) originated from South Asia. • POLDER/GRASP satellite aerosol data + MODIS fire product + HYSPLIT model. • The possible transport region is concluded from the available dataset. • The BBA evolution trend during aging process is quantitatively concluded. • Significant differences exist between BBAs over land and over ocean. The dynamic characteristics of biomass burning aerosol originated from South Asia are investigated in this research using nearly 9 years of POLDER/GRASP satellite aerosol dataset. The POLDER/GRASP remote sensing data can provide global, repeatable, various, and sufficient real-world aerosol information even in the remote ocean region, which can't be offered by the ground measurement, laboratory observation or model simulation. The MODIS thermal anomalies/fire dataset and HYSPLIT backward trajectory are applied to search the aerosol originated from South Asia biomass burning. The biomass burning aerosol originated from South Asia could transport to and influence the north part of Indian Ocean (including Bay of Bengal and Arabian Sea), the north part of Indo-China Peninsula, South China, and even far to the Pacific Ocean (including part of East China Sea and South China Sea). The chemical, physical and optical characteristics of biomass burning aerosol over land and over ocean show different features and evolution patterns. Such difference is caused by the different ambient environment and different mixed aerosol during the transport process (urban/industrial aerosol over land and sea salt over ocean). During the 48-hours aging process, the volume fraction of black carbon, AAOD and Angstrom Exponent decrease. Meanwhile, the aerosol sphere fraction and SSA increase. The biomass burning aerosol over land shows a more obvious evolution trend than that over ocean. The biomass burning aerosol over ocean generally have higher SSA and lower volume fraction of black carbon, aerosol sphere fraction, AAOD and Angstrom Exponent. The aerosol radiative forcing efficiency also varies between land and ocean, due to their different features of aerosol and surface properties. In general, a negative clear-sky aerosol radiative forcing efficiency (cooling effect) at the TOA is observed. The aerosol cooling effect at the TOA over ocean (−82 W/m2 on average) is much stronger than that over land (−36 W/m2 on average). During the 48-hours aging process, a significant enhancement of the negative radiative forcing efficiency at the TOA is found over land. Over ocean, the enhancement of the negative radiative forcing efficiency at the TOA is weaker. [ABSTRACT FROM AUTHOR]

• Contribution of BC, BrC and dust to aerosol absorption over Himalayas quantified. • SSA over Kathmandu is the lowest of all the locations in Himalayas. • SSA decreases with increasing elevation in the Himalayas. • BC dominates (≥75%) the aerosol absorption over the IGP and the Himalayas. • Aerosol absorption over South Asia is very high, compared to East Asia. This study, based on new and high quality in situ observations, quantifies for the first time, the individual contributions of light-absorbing aerosols (black carbon (BC), brown carbon (BrC) and dust) to aerosol absorption over the Indo-Gangetic Plain (IGP) and the Himalayan foothill region, a relatively poorly studied region with several sensitive ecosystems of global importance, as well as highly vulnerable populations. The annual and seasonal average single scattering albedo (SSA) over Kathmandu is the lowest of all the locations. The SSA over Kathmandu is < 0.89 during all seasons, which confirms the dominance of light-absorbing carbonaceous aerosols from local and regional sources over Kathmandu. It is observed here that the SSA decreases with increasing elevation, confirming the dominance of light absorbing carbonaceous aerosols at higher elevations. In contrast, the SSA over the IGP does not exhibit a pronounced spatial variation. BC dominates (≥75%) the aerosol absorption over the IGP and the Himalayan foothills throughout the year. Higher BC concentration at elevated locations in the Himalayas leads to lower SSA at elevated locations in the Himalayas. The contribution of dust to aerosol absorption is higher throughout the year over the IGP than over the Himalayan foothills. The aerosol absorption over South Asia is very high, exceeding available observations over East Asia, and also exceeds previous model estimates. This quantification will be valuable as observational constraints to help improve regional simulations of climate change, impacts on the glaciers and the hydrological cycle, and will help to direct the focus towards BC as the main contributor to aerosol-induced warming in the region. [ABSTRACT FROM AUTHOR]

We measured black carbon (BC) with a seven-wavelength aethalometer (AE-31) at the Nam Co Lake (NCL), the hinterland of the Tibetan Plateau (TP) from May 2015 to April 2016. The daily average concentration of BC was 145 ± 85 ng m−3, increasing by 50% since 2006. The seasonal variation of BC shows higher concentrations in spring and summer and lower concentrations in autumn and winter, dominated by the adjacent sources and meteorological conditions. The diurnal variation of BC showed that its concentrations peaked at 9:00–16:00 (UTC + 8), significantly related to local human activities (e.g., animal-manure burning and nearby traffic due to the tourism industry). The concentration-weighted trajectory (CWT) analysis showed that the long-distance transport of BC from South Asia could also be a potential contributor to BC at the NCL, as well as the biomass burning by the surrounding residents. The analyses of the absorption coefficient and absorption Ångström exponent show the consistency of sourcing the BC at the NCL. We suggest here that urgent measures should be taken to protect the atmospheric environment at the NCL, considering the fast-increasing concentrations of BC as an indicator of fuel combustion. [ABSTRACT FROM AUTHOR]

This study uses Weather Research and Forecasting model, coupled with Chemistry (WRF-Chem), to understand black carbon (BC) vertical transport over South Asia during the active convection period. We analyze the climatology (1995–2014) of convective weather using TRMM lightning data. The results show both temporal and spatial variability within South Asia. The WRF-Chem model simulation for BC transport was conducted for an ENSO (El-Nino Southern Oscillation) moderate year - 2013. The model simulates the observed concentrations trend for most of the region except the central Himalayan cities. Our analysis indicates the dominant role of synoptic atmospheric dynamics and upper boundary conditions for the inverted seasonal trend of BC during the pre-monsoon season over these cities. The study presents time series and seasonal scale analysis of BC concentration over South Asia at various altitude levels. Our model simulations suggest that free troposphere (FT) BC concentration is higher in the monsoon season compared to the winter season at the same elevation. In addition, our results show the presence of a persistent BC plume from 500 hPa and above over central India with implications for the summer monsoon. The presence of an anti-cyclonic region during the monsoon season at the higher troposphere (500–300 hPa) contributes to this stable BC layer. Although individual high convection days (high helicity/CAPE) can easily transport a significant amount of BC from the surface to the FT, simulations do not show significant BC transported to the upper troposphere-lower stratosphere during these days. While satellite derived products provide some insights into model predictions, there is a need for in-situ measurements of BC concentration aloft over South Asia to further constrain model performance and improve understanding of BC transport to FT. • Presence of persistent Black Carbon (BC) plume from 500 hPa and above over central India during summer monsoon season. • On a daily scale, even with occurrence of strong CAPE/Helicity, BC transport to UT/LS is not seen in model simulations. • Advection of BC from IGP to central Himalaya cities need better parameterization for improvement in model performance. [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosols from incomplete combustion of biomass and fossil fuel exert asignificant influence on the climate system in South Asia, effecting in excess of 1.2 billionpeople. Models—seeking to advise mitigation policy—are constantly challenged inreproducing observations of seasonally varying BC concentrations in the densely populatedregions of South Asia e.g., over the Indo-Gangetic Plain (IGP). The systematicunderestimation of the absorption aerosol optical depth over South Asia by a factor of 2-3in climate models relative to observation-based estimates illustrates the currentlylarge uncertainties. This uncertainty may be related to several different factors,including inaccurate estimates of relative source contributions and emissions ofBC. As part of the South Asian Pollution Experiment 2016 (SAPOEX-16), we investigatedwintertime BC and its dual carbon-isotopic signature (δ13C/Δ14C) - diagnosed sources fromregional and large-scale background receptor sites — the Bangladesh Climate Observatory atBhola Island (BCOB), strategically located in the outflow region of the IGP to intercept theintegrated IGP emission signal and the remote Indian ocean-based Maldives ClimateObservatory at Hanimaadhoo Island (MCOH) to provide an integrated footprint of SouthAsia, respectively. Our results of "top-down" radiocarbon measurements of atmospheric BCfrom two regional-receptor sites show that fossil fuel combustion produces 55±3% (Δ14C:-440 ± 6 per mil) of BC emitted from South Asia. In contrast, the δ13C signalsfor BCOB-BC (∼ -27.2 ± 0.2) are more depleted relative to that of MCOH-BC(∼ −25 ± 0.2). Using a Bayesian statistical approach, we resolve the isotopicsignatures into three source classes namely biomass, coal and liquid fossil fuel.We find the liquid fossil fuel component of BC emissions is 49 ± 5% in the IGPversus the larger South Asian footprint of 23 ± 9%. Our observation is in starkcontrast to previous "Bottom-up" emission inventory-based modelling estimatessuggesting over ∼ 60% BC from biomass burning in the IGP. This finding brings outthe previously unknown sharp contrast in regional emission sources of BC fromanthropogenic combustion in South Asia. Observationally-constrained insights onBC sources allows for a more credible scientific underpinning of policy aimedat mitigating the acuteness and scale of ramifications due to BC in South Asia. [ABSTRACT FROM AUTHOR]

Aerosol radiative forcing can influence climate both locally and far outside the forcing region. Here we investigate the sensitivity of regional climate to Black Carbon aerosols emitted in four major emissions areas using a fully coupled climate model. We perform simulations where we increase Black Carbon emissions by a factor of 10 and 20 in South Asia, North America and Europe, respectively, and by a factor of 5 and 10 in East Asia (due to higher emissions here). Each perturbation has been run for 100 years with three ensemble members. We find strikingly similar regional surface temperature responses per unit Black Carbon emissions in Europe and North America, with similar but somewhat lower temperature sensitivities for Asian emissions. Choosing the highest emission factor (i.e. 20 instead of 10) results in lower surface temperature change per emission unit compared to the lowest factor, but the difference is generally not statistically significant. An advantage of high-perturbation simulations is the clearer emergence of regional signals. This shows that the linearity of normalized temperature effects of Black Carbon is fairly preserved, and that regional temperature coefficients calculated from high perturbations may be a conservative estimate. [ABSTRACT FROM AUTHOR]