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Accurate estimation of black carbon (BC) from the widely used optical attenuation technique is important for the reliable assessment of their climatic impact. The optical instruments use Mass Absorption Cross-section (MAC) for converting light attenuation records to BC mass concentrations and Aethalometer is a widely used optical instrument for BC estimation. Several studies have shown large variability in MAC values. It is thus necessary to examine the accuracy and consistency of MAC values obtained using Aethalometer over distinct geographic locations and seasons. In the present study, MAC values are derived using simultaneous observations (2014–2017) from an EC-OC analyzer and an Aethalometer (AE-42) over a high altitude central Himalayan site at Nainital (29.4oN, 79.5oE, 1958 a.m.s.l). The observations reveal that the annual mean value of MAC (5.03 ± 0.03 m2g− 1 at 880nm) is significantly lower than the constant value used by the manufacturer (16.6 m2g− 1 at 880nm). The estimated MAC values also showed significant seasonal variation, spanning over a range from 3.7 to 6.6 m2g− 1. It is found that the seasonal variability of elemental carbon (EC), air mass variation and meteorological parameters play an important role in the changes in MAC values over this region. Multi-wavelength determination of MAC shows the contribution of absorption by species other than EC at shorter wavelengths. MAC does not show a clear diurnal variation, unlike EC and absorption coefficient. The slope of EC vs. corrected equivalent black carbon (eBC) showed a significant improvement during all seasons when compared with uncorrected eBC. This lends credibility to the fact that the use of site-specific MAC leads to more reliable estimates of eBC over the central Himalayan region. It is found that, instead of using the site specific MAC value, had we used the one supplied by the instrument, we would have underestimated the radiative forcing by about 7.8Wm− 2 which amounts to a reduction by 24 %.

Continuous measurements of the mass concentrations of composite (MT) and Black Carbon (MB) aerosols were carried out on-board an Oceanographic Research Vessel (ORV) Sagar Kanya (cruise number – SK345) during its cruise (from 16 January to 13 February, 2018) over the southeastern Arabian Sea and equatorial Indian Ocean as a part of the Integrated Campaign for Aerosols, gases and Radiation Budget (ICARB-2018). In general, mass concentrations of aerosols during the cruise period of northern hemispheric winter were found to be high over the southeastern Arabian Sea (MT > 50 μg m−3; MB > 3 μg m−3), which gradually reduced to low values over the far oceanic regions of equatorial Indian Ocean (MT

Particle morphology and elemental compositions are among the crucial parameters of aerosols required for accurate understanding of the climatic effect of aerosols in the earth-atmosphere system; yet their vertical distributions and region specific properties are poorly characterised due to sparse in-situ measurements. This is the first study to classify and quantify the vertical distributions of the morphological characteristics and elemental composition of aerosols based on single particle as well as bulk chemical analysis over seven geographically diverse regions of northern and central parts of India during spring (April-May, 2013), carried out as a part of Regional Aerosol Warming Experiment (RAWEX). Significant regional distinctiveness in shapes (non-sphericity), sizes and elemental compositions of the airborne particles were conspicuous, having dominance of highly irregular granular aggregates over the north Indian sites. The non-spherical coarse mode particles dominated the lower free tropospheric regions (> 2 km) of the Indo-Gangetic Plains (IGP). These particles could be responsible for enhanced spring time aerosol absorption in the elevated region of the atmosphere. Elemental compositions of the single particle analysis indicate that the free tropospheric layer over the IGP and central India is enriched with Na and Ca compounds mixed with Fe or Al (soil particles), indicating long range transport of crustal aerosols. This finding is very well supported by the bulk particle analysis indicating abundance of Ca(2+)in the free troposphere with low contribution of ssNa(+). Particles with irregular rough surfaces having dominance of SiO(2)were observed over all the study sites. The percentage share of spherical (either smooth or rough) particles to the total morphological characteristics of the particles was found to be highly subdued (

Continuous measurements of Black Carbon (BC) mass concentration using an advanced multi wavelength Aethalometer (AE-33) during January–December 2016 were carrried out, for the first time, at the premises of KL University, Vijayawada (16.44°N, 80.62°E, 30 m amsl), a semi-urban site in the southeast India. The estimates of the percentage contribution of Biomass Burning (BB%) aerosols to total BC concentration is utilised to understand the contribution of anthropogenic activities over the observational site. In this context, the diurnal and seasonal variation of BC mass concentrations and BB (%) contributions have been investigated and the possible sources are examined. A significant diurnal variation in BC concentration is observed with two prominent peaks, one occuring in the morning hours (around 0800–0900 IST) and the other occuring in the evening hours (around 2000 IST). Annually, the maximum BC concentration is observed during November (6.88 ± 1.56 μg m−3) and minimum during August (1.7 ± 0.52 μg m−3) over this location. On a seasonal scale, mean BC mass concentration is highest during winter (DJF, 4.68 ± 1.32 μg m−3) followed by post-monsoon (SON, 4.52 ± 2.36 μg m−3), summer (MAM, 2.44 ± 0.8 μg m−3) and lowest during monsoon (JJA, 1.83 ± 0.61 μg m−3). The higher BB (%) contribution is observed during summer (25.21 ± 7.02%), while the contribution is minimum in monsoon season (16.83 ± 4.15%). Examination of the spectral properties of aerosol absorption in terms of Angstrom exponent (α) indicates that the major contributor of BC over this location is biomass burning (α ~ 1.66 ± 0.06). This is supported by the higher number of fire counts (obtained from Moderate Resolution Imaging Spectroradiometer-MODIS onboard Terra satellite) in and around the study region during winter and summer revealing the higher contribution of BB aerosols. The concentration weighted trajectories (CWT) analysis (based on 5-day airmass back trajectories arriving the receptor site at an altitude of 500 m AGL) reveals that synoptic airmasses arriving from the continental regions located northeast of the observational sites mainly influence the BC properties over Vijayawada.

The spectral estimates of Aerosol Optical Depth (AOD, τ) were made by operating a Microtops-II sun photometer in the spectral range 0.380–0.870 μm over Tripura in north-eastern India and analyzed to infer the aerosol types and source characteristics in different seasons. The Angstrom exponent (α) derived from spectral AOD in different wavelength (λ) range and subsequent second order derivative of Angstrom exponent (i.e., α′) and their curvature analysis in the ln τ versus ln λ relationship has revealed the crucial information related to the dominance of different aerosol types and their characteristics in different seasons. The average AOD (mean ± standard deviation) at 0.5 μm in winter, pre-monsoon, monsoon and post-monsoon seasons are observed to be 0.70 ± 0.28, 0.74 ± 0.18, 0.55 ± 0.20, 0.44 ± 0.19 respectively; while the corresponding seasonal mean values of α (over 0.380–0.870 μm spectral range) are found to be 1.09 ± 0.17, 0.92 ± 0.24, 0.51 ± 0.27, 0.89 ± 0.38 respectively. Examination of the Angstrom exponents derived from satellite retrieved AOD by Moderate Resolution Imaging Spectroradiometer (MODIS) instrument operating onboard Terra satellite, along with MODIS fine mode aerosol fraction and aerosol types indicates broad seasonal features of aerosol size spectrum over the study region similar to those observed from ground-based measurements. The estimation of the values of α at different spectral ranges indicate that winter season is mainly influenced by the fine-mode aerosols having fine-mode fraction (FMF) ∼ 0.7; whereas coarse-mode aerosols dominate in the monsoon season having FMF ∼ 0.3. The pre-monsoon and post-monsoon seasons exhibit the presence of mixed type of aerosols, with slightly greater fraction of fine-mode aerosol in pre-monsoon. Curvature analyses of Angstrom exponent put insight into the consistency of observed features of seasonal aerosol types.

Long-term monitoring of atmospheric aerosols and their interaction with radiation, cloud, and cryosphere over the Arctic and the Antarctic are very important for the global climate change related issues. In this regard, for conducting aerosol measurements, India has extended the concerted efforts to the Svalbard region of the Norwegian Arctic (Himadri, 78°55′N 11°56′E, 8 m a.s.l.) in the northern hemisphere and the Larsemann Hills of coastal Antarctic (Bharati, 69°24.4′S 76°11.7′E, 40 m a.s.l.) in the southern hemisphere. In the present study, we have examined the role of black carbon (BC) deposition in darkening the polar snow in different sunlit seasons and estimated the scavenging ratio of BC over both the poles from simultaneous measurements of atmospheric and snow deposited BC concentrations. The study reveals distinct spatio-temporal variability of BC in polar snow, even though the concentrations are, in general, low (

Ground reaching solar radiation flux was simulated using a 1-dimensional radiative transfer (SBDART) and a 3-dimensional regional climate (RegCM 4.4) model and their seasonality against simultaneous surface measurements carried out using a CNR4 net Radiometer over a sub-Himalayan foothill site of south-east Asia was assessed for the period from March 2013–January 2015. The model simulated incoming fluxes showed a very good correlation with the measured values with correlation coefficient R2 ~0.97. The mean bias errors between these two varied from −40 W m−2 to +7 W m−2 with an overestimation of 2–3% by SBDART and an underestimation of 2–9% by RegCM. Collocated measurements of the optical parameters of aerosols indicated a reduction in atmospheric transmission path by ~20% due to aerosol load in the atmosphere when compared with the aerosol free atmospheric condition. Estimation of aerosol radiative forcing efficiency (ARFE) indicated that the presence of black carbon (BC, 10–15%) led to a surface dimming by −26.14 W m−2 τ−1 and a potential atmospheric forcing of +43.04 W m−2 τ−1. BC alone is responsible for >70% influence with a major role in building up of forcing efficiency of +55.69 W m−2 τ−1 (composite) in the atmosphere. On the other hand, the scattering due to aerosols enhance the outgoing radiation at the top of the atmosphere (ARFETOA ~−12.60 W m−2 ω−1), the absence of which would have resulted in ARFETOA of ~+16.91 W m−2 τ−1 (due to BC alone). As a result, ~3/4 of the radiation absorption in the atmosphere is ascribed to the presence of BC. This translated to an atmospheric heating rate of ~1.0 K day−1, with ~0.3 K day−1 heating over the elevated regions (2–4 km) of the atmosphere, especially during pre-monsoon season. Comparison of the satellite (MODIS) derived and ground based estimates of surface albedo showed seasonal difference in their magnitudes (R2 ~0.98 during retreating monsoon and winter; ~0.65 during pre-monsoon and monsoon), indicating that the reliability of the satellite data for aerosol radiative forcing estimation is more during the retreating and winter seasons.

Continuous long-term monitoring of black carbon (BC) mass concentration and aerosol light scattering coefficient (σSCA), supplemented by number size distribution and chemical composition, are utilized in this study to understand the temporal changes in aerosol properties, associated source processes and radiative effects at Ny-Alesund (79 oN) in the Svalbard Archipelago. A statistically significant decreasing trend in BC (- 24.7 ng m−3 decade−1) is observed during spring of 2010–2019. In contrast, σSCA depicted a general increasing trend (5.2 Mm−1 decade−1) during 2011–2016. BC and σSCA were higher during winter and spring. Aerosol single scattering albedo was highest in May ~ 0.95 (during spring) and lowest in September ~ 0.87 (during summer). Fractional share of BC to total aerosol mass was higher in winter and summer. Anthropogenic SO42− and NO3− (after ssNa+) species dominated the summer, when total number and mass concentrations of aerosols were at their minimum. Elemental Carbon (EC) and Organic Carbon (OC) showed higher concentrations in spring with EC-to-OC ratio ~ 0.08 - 0.22. The columnar AOD varied between 0.01 and 0.20 (annual mean ~ 0.09), resulting in aerosol radiative forcing (in the top of the atmosphere) ~ 0.15 - 2.69 Wm-2 in the month of April (during spring). Potential source contribution function (PSCF) revealed the dominant source areas to be over Europe and Russia in terms of contributing to the seasonal high BC mass concentrations at Ny-Alesund. Our study has also revealed an unusual impact of biomass burning aerosols (advected from the Alaska wildfire) during July 2015.

Aircraft measurements of the vertical profiles of aerosol total number concentrations and size distributions (in the size range of 0.5–20 μm) were made over seven geographically diverse locations of the Indian mainland during two contrasting seasons, winter (December 2012) and spring (April–May 2013), as a part of the regional aerosol warming experiment (RAWEX). Our observations revealed an increase in the vertical extent of aerosol loading during spring having a significant enhancement in coarse mode aerosols in the lower free-troposphere (FT) over western and central parts of India and the Indo-Gangetic plains (IGP). The particulate depolarisation ratio (PDR) derived from the Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) over the same region showed the presence of dust (including polluted dust) at higher altitudes in spring. Concurrent and collocated measurements of aerosol scattering and absorption properties aboard the aircraft revealed that the FT enhancement in coarse mode aerosol loading during spring is associated with a decrease in single scattering albedo and an increase in columnar absorption aerosol optical depth. This confirms that the elevated layers of coarse mode aerosols seen during spring are absorbing in nature, especially over the IGP. The presence of such coarse-mode absorbing aerosols plays a crucial role in governing the radiation balance over the IGP in spring through the diabatic heating of the upper atmosphere.

Concurrent measurements of the altitude profiles of cloud condensation nuclei (CCN) concentration, as a function of supersaturation (ranging from 0.2 % to 1.0 %), and aerosol optical properties (scattering and absorption coefficients) were carried out aboard an instrumented aircraft across the Indo-Gangetic Plain (IGP) covering coastal, urban and arid environments, just prior to the onset of the Indian summer monsoon (ISM) of 2016, under the aegis of the SWAAMI - RAWEX campaign. In general, the CCN concentration has been highest in the Central IGP, decreasing spatially from east to west above the planetary boundary layer (PBL), which is ~ 1.5 km for the IGP during pre-monsoon. Despite of this, the CCN activation efficiency at 0.4 % supersaturation has been, interestingly, the highest over the eastern IGP (~ 72 %), followed by the west (~ 61 %), and has been the least over the central IGP (~ 24 %) within the PBL. In general, higher activation efficiency is noticed above the PBL than below it. The Central IGP showed remarkably low CCN activation efficiency at all the heights, which appears to be associated with high black carbon (BC) mass concentration there, indicating the role of anthropogenic sources in suppressing the CCN efficiency. First ever CCN measurements over the western IGP, encompassing "The Great Indian desert", show high CCN efficiency, ~ 61 % at 0.4 % supersaturation, indicating hygroscopic nature of the dust. The vertical structure of CCN properties is found to be airmass-dependent; with higher activation efficiency even over the central IGP during the prevalence of marine airmass. Precipitation episodes seem to reduce the CCN activation efficiency below cloud level. An empirical relation has emerged between the CCN concentration and the scattering aerosol index (AI), which would facilitate prediction of CCN from aerosol optical properties.

The plausible impact of light-absorbing aerosols on snow darkening and subsequent retreating of glaciers is a global climatic concern. In this study, we present the characteristics of multi-layer distribution and effects of Black Carbon (BC) and inorganic ions in the snowpacks of Khardung (KG) and Phuche (PG) glaciers (>5 km a.s.l.) in the western Himalayas. We observed significant vertical heterogeneity of BC in the snowpacks of KG (~42–428 ng g−1) and PG (~59 and 299 ng g−1), with higher concentrations in aged snow. Similar to BC, ions in the multi-layer snowpack also depicted prominent vertical heterogeneity with strong crustal influence (as indicated by abundant nssCa2+) in the aged snow layers of KG, which also possess a higher snow-melt rate as compared to PG. Among the other inorganic ions, the vertical profiles of nssSO42− and NO3− indicated elution and refreezing effects. The computation of the effective snow albedo for different snow-darkening and snow-physical processes vindicated the need of considering the multilayer model for the accurate quantification of effects of heterogeneous distributions of light absorbing aerosols (LAA) in the snowpacks. Following this, the multi-layer simulations of snow-albedo in the SNICAR model demonstrated the change in snow albedo by 2.5–9.0% for the amount of LAA observed in our study. This resulted in snow albedo forcing of 49.2 Wm-2 for PG, 30.8 Wm-2 for KG1 and 29.6 Wm-2 for KG2 for the typical snow-physical properties in the study region. Comprehensive data sets comprising physical, morphological and chemical properties of aerosols and snow are imperative to predict aerosol-induced snow darkening and the associated anomalous melting of snow/glacier over the Himalayan region.

Northeast India (NEI) is located within the boundary of the great Himalayas in the north and the Bay of Bengal (BoB) in the southwest, experiences the mixed influence of the westerly dust advection from the Indian desert, anthropogenic aerosols from the highly polluted Indo-Gangetic Plains (IGP) and marine aerosols from BoB. The present study deals with the estimation and characterization of aerosol radiative forcing over a semi-continental site Tripura, which is a strategic location in the western part of NEI having close proximity to the outflow of the IGP. Continuous long term measurements of aerosol black carbon (BC) mass concentrations and columnar aerosol optical depth (AOD) are used for the estimation of aerosol radiative forcing in each monthly time scale. The study revealed that the surface forcing due to aerosols was higher during both winter and pre-monsoon seasons, having comparable values of 32 W/m 2 and 33.45 W/m 2 respectively. The atmospheric forcing was also higher during these months due to increased columnar aerosol loadings (higher AOD ~ 0.71) shared by abundant BC concentrations (SSA ~ 0.7); while atmospheric forcing decreased in monsoon due to reduced magnitude of BC (SSA ~ 0.94 in July) as well as columnar AOD. The top of the atmosphere (TOA) forcing is positive in pre-monsoon and monsoon months with the highest positive value of 3.78 W/m 2 in June 2012. The results are discussed in light of seasonal source impact and transport pathways from adjacent regions.

Multi-year measurements of spectral properties of aerosol absorption are examined over four geographically distinct locations of northeastern India. Results indicated significant spatio-temporal variation in aerosol absorption coefficients (σabs) with highest values in winter and lowest in monsoon. The western parts of the region, close to the outflow of Indo-Gangetic Plains, showed higher values of σabs and black carbon (BC) concentration - mostly associated with fossil fuel combustion. But, the eastern parts showed higher contributions from biomass burning aerosols, as much as 20-25% to the total aerosol absorption, conspicuously during pre-monsoon season. This is attributed to a large number of burning activities over the Southeast Asian region, as depicted from MODIS fire count maps, whose spatial extent and magnitude peaks during March/ April. The nearly consistent high values of Aerosol Index (AI) and layer height from OMI indicates the presence of absorbing aerosols in the upper atmosphere. The observed seasonality has been captured fairly well by GOCART as well as WRF-Chem model simulations. The ratio of column integrated optical depths due to particulate organic matter (POM) and BC from GOCART showed good coincidence with satellite based observations, indicating the increased vertical dispersion of absorbing aerosols, probably by the additional local convection due to higher fire radiative power caused by the intense biomass burning activities. The WRF-Chem though underperformed by different magnitude in winter, the values are closer or overestimated near the burnt areas. Atmospheric forcing due to BC was highest (~30 Wm-2) over the western part associated with the fossil fuel combustion.

The particle number concentrations, size distributions, and new particle formation (NPF) events were investigated in the South Asian outflow over the southeastern Arabian Sea (SEAS), the northern (NIO) and equatorial Indian Ocean (EIO) during the winter of 2018 based on the extensive measurements of particle number size distributions in size range 10–414 nm in the marine atmospheric boundary layer (MABL) as part of the Integrated Campaign for Aerosols, gases, and Radiation Budget (ICARB - 2018) experiment. The total particle concentrations were higher (as high as 106 cm−3; mean ~8857 ± 13425 cm−3) over the SEAS in the proximity to the continental outflow, which gradually decreased to reach the lowest values over the remote EIO (~3206 ± 4406 cm−3). Several NPF events with conspicuous bimodal size distributions and 3–10 times enhancement in nucleation particle (

With a view to understanding the vertical distribution of aerosols and estimating the radiative impacts of elevated aerosols in the lower free troposphere, extensive profiling of the vertical variation of the optical properties, namely the extinction/ scattering and absorption coefficients (respectively $\sigma _{\mathrm{ext}}/ \sigma _{\mathrm{sca}}\mathrm {t}/ \sigma _{\mathrm{abs}})$ have been carried out from three base stations in the Indo-Gangetic Plain (IGP) using an instrumented aircraft, prior to onset of the Indian Summer Monsoon. These stations represented the semiarid western IGP (Jodhpur, JDR), the anthropogenically affected central IGP (Varanasi, VNS), and the industrialized coastal location in the eastern end of the IGP, close to the northern Bay of Bengal (Bhubaneswar, BBR). The vertical profiles of the optical properties differed significantly across these locations and this resulted in a regionally significant heating rate gradient. While the integrated (ground to 3 km altitude $) \sigma _{\mathrm{scat}}$ remained quite comparable across the IGP, the highest $\sigma _{\mathrm{abs}}$ and hence the lowest single scattering albedo (SSA) occurred in the central IGP (Varanasi). Size distribution, inferred from the spectral variation of the scattering coefficient, showed a gradual shift from coarse particle dominance in the western IGP to strong accumulation dominance in the eastern coast, with the central IGP coming in-between. Source speciation of aerosol, using spectral aerosol properties, revealed aerosol system in the west IGP is predominantly natural (dust and sea-salt) and that in the east IGP is highly anthropogenic type (industrial emissions, fossil fuel and biomass combustion).The central IGP exhibited a mixture of both.

In view of the climate impact of aerosol Black Carbon (BC) over snow covered regions (through enhanced absorption of radiation as well as snow-albedo forcing), and in view of the increasing anthropogenic presence and influence in the northern polar regions, continuous long term measurements of airborne BC have been undertaken from the Svalbard region of Norwegian Arctic (Ny-Alesund, 79°N, 12°E, 8 m a.s.l.). This study, employing data over a period of 4-years (2010–2013) have shown a consistent spring-time enhancement in BC concentrations, having a (climatological) seasonal mean value of ∼50.3 ± 19.5 ng m−3, nearly 3-times higher than the lowest BC concentrations in summer (∼19.5 ± 6.5 ng m−3). Spectral variation of absorbance indicates that long-range transported biomass burning aerosols contribute as high as 25% to the high BC concentrations in the Arctic atmosphere in spring. Concurrent estimates of BC concentrations in the Arctic snow (for an ensemble of snow samples collected over a period of time during spring) showed values ranging from 0.6 ppb to 4.1 ppb. These values have been used to estimate the BC scavenging ratio (SR). Our studies revealed a mean value of SR ∼98 ± 46, which varied over wide range from 40 to 184 for individual samples. In a broader perspective, the seasonal variations of atmospheric BC concentrations at the Arctic are similar to those seen at the high altitude Himalayas; even though the concentrations are much lower at Arctic. It is found that synoptic conditions mainly influence the high altitude Himalayas, while the influences of local anthropogenic influences are not negligible at the Arctic in modulating the seasonal variations of absorbing aerosols.

To characterize the vertical distribution of aerosols and its seasonality (especially the single scattering albedo, SSA) extensive profiling of aerosol scattering and absorption coefficients have been carried out using an instrumented aircraft from seven base stations spread across the Indian mainland during winter 2012 and spring/pre-monsoon 2013 under the Regional Aerosol Warming Experiment (RAWEX). Spatial variation of the vertical profiles of the asymmetry parameter, the wavelength exponent of the absorption coefficient and the single scattering albedo, derived from the measurements, are used to infer the source characteristics of winter and pre-monsoon aerosols as well as the seasonality of free tropospheric aerosols. The relatively high value of the wavelength exponent of absorption coefficient over most of the regions indicates the contribution from biomass burning and dust aerosols up to lower free tropospheric altitudes. A clear enhancement in aerosol loading and its absorbing nature is seen at lower free troposphere levels (above the planetary boundary layer) over the entire mainland during spring/pre-monsoon season compared to winter, whereas concentration of aerosols within the boundary layer showed a decrease from winter to spring. This could have significant implications on the aerosol heating structure over the Indian region and hence the regional climate.

Four years (2010–2014) of spectral aerosol optical depth (AOD) data from 4 Indian Space Research Organisation's ARFINET (Aerosol Radiative Forcing over India) stations (Shillong, Agartala, Imphal and Dibrugarh) in the North-Eastern Region (NER) of India (lying between 22–30°N and 89–98°E) are synthesized to evolve a regional aerosol representation, for the first time. Results show that the columnar AOD (an indicator of the column abundance of aerosols) is highest at Agartala (0.80 ± 0.24) in the west and lowest at Imphal (0.59 ± 0.23) in the east in the pre-monsoon season due to intense anthropogenic bio-mass burning in this region aided by long-range transport from the high aerosol laden regions of the Indo-Gangetic Plains (IGP), polluted Bangladesh and Bay of Bengal. In addition to local biogenic aerosols and pollutants emitted from brick kilns, oil/gas fields, household bio-fuel/fossil-fuel, vehicles, industries. Aerosol distribution and climatic impacts show a west to east gradient within the NER. For example, the climatological mean AODs are 0.67 ± 0.26, 0.52 ± 0.14, 0.40 ± 0.17 and 0.41 ± 0.23 respectively in Agartala, Shillong, Imphal and Dibrugarh which are geographically located from west to east within the NER. The average aerosol burden in NER ranks second highest with climatological mean AOD 0.49 ± 0.2 next to the Indo-Gangetic Plains where the climatological mean AOD is 0.64 ± 0.2 followed by the South and South-East Asia region. Elevated aerosol layers are observed over the eastern most stations Dibrugarh and Imphal, while at the western stations the concentrations are high near the surface. The climate implications of aerosols are evaluated in terms of aerosol radiative forcing (ARF) and consequent heating of the atmosphere in the region which follows AOD and exhibit high values in pre-monsoon season at all the locations except in Agartala. The highest ARF in the atmosphere occurs in the pre-monsoon season ranging from 48.6 Wm −2 in Agartala to 25.1 Wm −2 in Imphal. Winter radiative forcing follows that in pre-monsoon season at these locations. The heating rate is high at 1.2 K day −1 and 1.0 K day −1 over Shillong and Dibrugarh respectively in this season. However, Agartala experiences higher surface forcing (−56.5 Wm −2 ) and consequent larger heating of the atmosphere of 1.6 K day −1 in winter.

The absorption characteristics and source processes of aerosols are investigated at two nearby distinct altitude sites: Nainital, located over the central Himalayas (~1958 m amsl) and Pantnagar, in the adjacent foothill region (~231 m amsl) in the Indo-Gangetic Plain region (IGP); based on in-situ measurements and model (GEOS-Chem) simulations. The study reveals the significant influence of biomass burning sources over both the locations during spring, indicating the efficiency of the vertical transport of biomass burning aerosols during the peak of the fire activity period over the northern Indian region. On the other hand, the dominance of fossil fuel emission sources is seen during most part of the year at the mountain site, while biomass/biofuel sources are prevalent at the foothill site. Simulations of different aerosol components in the GEOS-Chem model have revealed that dust aerosols, in addition to carbonaceous aerosols from fossil fuel and biomass burning sources, significantly influence aerosol burden over this broad region covering both high-altitude site Nainital and adjacent foothill site Pantnagar in IGP. Examination of dominant aerosol types and their contribution to the columnar abundance of aerosols is performed. During spring, the contribution of dust aerosols is as high as 22%, even though inorganic aerosols (42%), organic carbon (29%) play a dominant role in modulating aerosol absorption characteristics in the column over this region. This study highlights the importance of absorbing aerosol, their types and quantification for better estimates of radiative forcing of aerosols over this region. This might also provide valuable information for the regional impact assessment of aerosols over the Himalayan region.

This paper presents first of its kind results on the aerosol mass concentrations and size distributions, their chemical properties and columnar aerosol optical depth (AOD) over a high-altitude background site of Lahaul and Spiti valley (32.4°N, 77.6°E, 4080 m a.s.l.) in the western Himalayas, obtained from collocated and concurrent measurements made during August (summer) to October (autumn) of the year 2017. The results depicted very low values of columnar AOD (mean ~ 0.07) with ~4% contribution of BC (mean BC ~ 168 ng m−3) to near surface composite aerosol mass. The total suspended particulate (TSP) matter showed significant variations between 5 μg m−3 and 40 μg m−3, having dominance of mineral dust components (~67%) during the entire study period. The ratio of organic carbon (OC) to elemental carbon (EC) varied over a wide range from 7.8 to 18.9, with a mean value of 12.5 ± 2.95. The higher fraction of water-soluble OC in the total OC component indicated the aged nature of the carbonaceous species. An enrichment factor estimated from the concentrations of trace elements also revealed the influence of anthropogenic source contribution. The regions located west and northwest of the measurement site, which includes the regional hot-spots of Indo-Gangetic Plains, west Asia and the Middle East are possible source regions to modulate the aerosols over the observational site through long range transport. These observations have far reaching implications in view of the role of aerosols on regional radiative balance and impact on snow/glacier coverage.

Aerosol absorption in the lower troposphere over continental India was assessed using extensive measurements of the vertical distribution of absorption coefficients aboard an instrumented aircraft. Measurements were made from seven base stations during winter (November-December 2012) and spring (April-May 2013), supplemented by the data from the networks of surface observatories. A definite enhancement in aerosol absorption has been observed in the lower free troposphere over the Indo-Gangetic Plain (IGP) during spring, along with a reduction near the surface. The regional mean aerosol absorption optical depth (AAOD) over IGP, which was derived from aircraft observations (integrated from the ground to 3 km), increased from 0.020 +/- 0.009 in winter to 0.048 +/- 0.01 in spring. The columnar AAOD depicted weak and distinctly different seasonal variations than that of surface level black carbon mass concentrations. This contrasting difference in the seasonality indicates the presence of elevated layers of absorbing aerosols during spring in association with the long-range transport and vertical convective lofting of aerosols.