Single scattering albedo (SSA) is a leading contributor
to the uncertainty in aerosol radiative impact assessments. Therefore
accurate information on aerosol absorption is required on a global scale. ...In
this study, we have applied a multi-satellite algorithm to retrieve SSA (550
nm) using the concept of critical optical depth. Global maps of SSA were
generated following this approach using spatially and temporally collocated
data from Clouds and the Earth's Radiant Energy System (CERES) and Moderate
Resolution Imaging Spectroradiometer (MODIS) sensors on board Terra and Aqua
satellites. Limited comparisons against airborne observations over India and
surrounding oceans were generally in agreement within ±0.03. Global
mean SSA estimated over land and ocean is 0.93 and 0.97, respectively.
Seasonal and spatial distribution of SSA over various regions are also
presented. Sensitivity analysis to various parameters indicate a mean
uncertainty around ±0.044 and shows maximum sensitivity to changes in
surface albedo. The global maps of SSA, thus derived with improved accuracy,
provide important input to climate models for assessing the climatic impact
of aerosols on regional and global scales.
Measurements of the vertical profiles of the optical properties (namely the extinction
coefficient and scattering and absorption coefficients respectively
σext ∕ σscat ∕ σabs)
of aerosols have been ...made across the Indo-Gangetic Plain (IGP) using an
instrumented aircraft operated from three base stations – Jodhpur (JDR),
representing the semi-arid western IGP; Varanasi (VNS), the central IGP
characterized by significant anthropogenic activities; and the industrialized
coastal location in the eastern end of the IGP (Bhubaneswar, BBR) – just
prior to the onset of the Indian summer monsoon. The vertical profiles
depicted region-specific absorption characteristics, while the scattering
characteristics remained fairly uniform across the region, leading to a
west–east gradient in the vertical structure of single-scattering
albedo (SSA). Integrated from near the ground to 3 km, the highest
absorption coefficient and hence the lowest 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, arising from a change in the aerosol type
from a predominantly natural (dust and sea salt) type in the western IGP to a
highly anthropogenic type (industrial emissions, fossil fuel and biomass
combustion) in the eastern IGP, with the central IGP exhibiting a mixture of
both. Aerosol-induced short-wave radiative forcing, estimated using
altitude-resolved SSA information, revealed significant atmospheric warming
in the central IGP, while a top-of-atmosphere cooling is seen, in general, in
the IGP. Atmospheric heating rate profiles, estimated using altitude-resolved
SSA and column-averaged SSA, revealed considerable underestimation in the
latter case, emphasizing the importance and necessity of having
altitude-resolved SSA information as against a single value for the entire
column.
Over the Indian region, aerosol absorption is considered to have a potential impact on the regional climate, monsoon and hydrological cycle. Black carbon (BC) is the dominant absorbing aerosol, whose ...absorption potential is determined mainly by its microphysical properties, including its concentration, size and mixing state with other aerosol components. The Indo-Gangetic Plain (IGP) is one of the regional aerosol hot spots with diverse sources, both natural and anthropogenic, but still the information on the mixing state of the IGP aerosols, especially BC, is limited and a significant source of uncertainty in understanding their climatic implications. In this context, we present the results from intensive measurements of refractory BC (rBC) carried out over Bhubaneswar, an urban site in the eastern coast of India, which experiences contrasting air masses (the IGP outflow or coastal/marine air masses) in different seasons. This study helps to elucidate the microphysical characteristics of BC over this region and delineates the IGP outflow from the other air masses. The observations were carried out as part of South West Asian Aerosol Monsoon Interactions (SWAAMI) collaborative field experiment during July 2016–May 2017, using a single-particle soot photometer (SP2) that uses a laser-induced incandescence technique to measure the mass and mixing state of individual BC particles and an aerosol chemical speciation monitor (ACSM) to infer the possible coating material. Results highlighted the distinctiveness in aerosol microphysical properties in the IGP air masses. BC mass concentration was highest during winter (December–February) (∼1.94±1.58 µg m-3), when the prevailing air masses were mostly of IGP origin, followed by post-monsoon (October–November) (mean∼1.34±1.40 µg m-3). The mass median diameter (MMD) of the BC mass size distributions was in the range 0.190–0.195 µm, suggesting mixed sources of BC, and, further, higher values (∼ 1.3–1.8) of bulk relative coating thickness (RCT) (ratio of optical and core diameters) were seen, indicating a significant fraction of highly coated BC aerosols in the IGP outflow. During the pre-monsoon (March–May), when marine/coastal air masses prevailed, BC mass concentration was lowest (∼0.82±0.84 µg m-3), and larger BC cores (MMD > 0.210 µm) were seen, suggesting distinct source processes, while RCT was ∼ 1.2–1.3, which may translate into higher extent of absolute coating on BC cores, which may have crucial regional climate implications. During the summer monsoon (July–September), BC size distributions were dominated by smaller cores (MMD ≤ 0.185 µm), with the lowest coating indicating fresher BC, likely from fossil fuel sources. A clear diurnal variation pattern of BC and RCT was noticed in all the seasons, and daytime peak in RCT suggested enhanced coating on BC due to the condensable coating material originating from photochemistry. Examination of submicrometre aerosol chemical composition highlighted that the IGP outflow was dominated by organics (47 %–49 %), and marine/coastal air masses contained higher amounts of sulfate (41 %–47 %), while ammonium and nitrate were seen in minor amounts, with significant concentrations only during the IGP air mass periods. The diurnal pattern of sulfate resembled that of the RCT of rBC particles, whereas organic mass showed a pattern similar to that of the rBC mass concentration. Seasonally, the coating on BC showed a negative association with the mass concentration of sulfate during the pre-monsoon season and with organics during the post-monsoon season. These are the first experimental data on the mixing state of BC from a long time series over the Indian region and include new information on black carbon in the IGP outflow region. These data help in improving the understanding of regional BC microphysical characteristics and their climate implications.
With a view to improving the performance of WRF-Chem over the Indian region in simulating BC (black carbon) mass concentrations as well as its short-term variations, especially on a diurnal scale, a ...region-specific diurnal variation scheme has been introduced in the model emissions and the performance of the modified simulations has been evaluated against high-resolution measurements carried out over eight ARFI (Aerosol Radiative Forcing over India) network observatories spread across India for distinct seasons: pre-monsoon (represented by May), post-monsoon (represented by October) and winter (represented by December). In addition to an overall improvement in the simulated concentrations and their temporal variations, we have also found that the effects of prescribing diurnally varying emissions on the simulated near-surface concentrations largely depend on the boundary layer turbulence. The effects are perceived quickly (within about 2–3 h) during the evening–early morning hours when the atmospheric boundary layer is shallow and convective mixing is weak, while they are delayed, taking as much as about 5–6 h, during periods when the boundary layer is deep and convective mixing is strong. This information would also serve as an important input for agencies concerned with urban planning and pollution mitigation. Despite these improvements in the near-surface concentrations, the simulated columnar aerosol optical depth (AOD) still remains largely underestimated vis-à-vis the satellite-retrieved products. These modifications will serve as a guideline for further model-improvement initiatives at a regional scale.
Concurrent measurements of the altitude profiles of the concentration of cloud
condensation nuclei (CCN), 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) just prior to the
onset of the Indian summer monsoon (ISM) of 2016. The experiment was
conducted under the aegis of the combined South-West Asian Aerosol–Monsoon Interactions and Regional
Aerosol Warming Experiment (SWAAMI–RAWEX) campaign. The measurements
covered coastal, urban and arid environments. In general, the CCN
concentration was 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 period. Despite this, the CCN
activation efficiency at 0.4 % supersaturation was, interestingly,
the highest over the eastern IGP (∼72 %), followed by that
in the west (∼61 %), and it was 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 altitudes, which
appears to be associated with high black carbon (BC) mass concentration
there, indicating the role of anthropogenic sources in suppressing the CCN
efficiency. These first-ever CCN measurements over the western IGP, encompassing
“the Great Indian Desert” also known as “the Thar Desert”, showed high CCN
efficiency, ∼61 % at 0.4 % supersaturation, indicating the
hygroscopic nature of the dust. The vertical structure of CCN properties is
found to be air mass dependent, with higher activation efficiency even over
the central IGP during the prevalence of marine air mass. Wet scavenging
associated with precipitation episodes seems to have reduced 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 the prediction of CCN from aerosol optical properties.
Tropical Rainfall Measuring Mission (TRMM) precipitation radar measurements are used to examine the variation in vertical structure of precipitation with sea surface temperature (SST) over the ...Arabian Sea (AS) and Bay of Bengal (BOB). The variation in reflectivity and precipitation echo top with SST is remarkable over the AS but small over the BOB. The reflectivity increases with SST (from 26 to 31 ∘C) by ∼1 and 4 dBZ above and below 6 km, respectively, over the AS, while its variation is <0.5 dBZ over the BOB. The transition from shallow storms at lower SSTs (≤27 ∘C) to deeper storms at higher SSTs is strongly associated with the decrease in stability and mid-tropospheric wind shear over the AS. In contrary, the storms are deeper at all SSTs over the BOB due to weaker stability and mid-tropospheric wind shear. At lower SSTs, the observed high aerosol optical depth (AOD) and low total column water (TCW) over AS results in the small cloud effective radius (CER) and weaker reflectivity. As SST increases, AOD decreases and TCW increases, leading to a large CER and high reflectivity. The changes in these parameters with SST are marginal over the BOB and hence the CER and reflectivity. The predominance of collision–coalescence process below the bright band is responsible for the observed negative slopes in the reflectivity over both the seas. The observed variations in reflectivity originate at the cloud formation stage over both the seas, and these variations are magnified during the descent of hydrometeors to the ground.
The intriguing, but complex, association among sea surface temperature (SST) and different types of precipitating systems (deep and shallow) over the Bay of Bengal and the Arabian Sea has been ...elucidated utilizing 16 years of tropical rainfall measuring mission (TRMM) data. The characteristics and their convective forcing of these two seas are fundamentally different due to differences in SST. At a given SST, the occurrence of deeper (shallow) systems is higher (lower) over the Bay of Bengal than in the Arabian Sea. The occurrence of deeper systems increases with SST over both the seas till 30°C and decreases from 30 to 31°C. On the other hand, the occurrence of shallow systems is independent of SST over the Bay of Bengal while it decreases with rise in SST over the Arabian Sea. The prevalence of weaker convective available potential energy and drier mid‐troposphere over the Arabian Sea than over the Bay of Bengal is detrimental for cloud growth and is responsible for lower occurrence of deep systems and higher occurrence of shallow systems. The differences in mid‐tropospheric dynamics and moisture between the Arabian Sea and the Bay of the Bengal play a major role in the occurrence of deep and shallow systems.
The occurrence of deeper systems increases with SST, however, the rate of increase in the occurrence of deeper systems is quite different over these seas. On the other hand, the occurrence of shallow systems is independent of SST over the Bay of Bengal while it decreases with rise in SST over the Arabian Sea. The association of precipitating systems with SST is clear over the Arabian Sean and not over the Bay of Bengal.
Black carbon (BC) is known to have major impacts on both climate and human health and is therefore of global importance, particularly in regions close to large populations that have strong sources. ...The size-resolved mixing state of BC-containing particles was characterised using a single-particle soot photometer (SP2). The study focusses on the Indo-Gangetic Plain (IGP) during the pre-monsoon and monsoon seasons. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights during the pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons of 2016.
Vertical distributions (altitude profiles) of condensation nuclei (CN) and cloud condensation nuclei (CCN) and their spatial variations across the Indo-Gangetic Plain (IGP) have been investigated ...based on airborne measurements carried out during the SWAAMI field campaign (June to July 2016) capturing the contrasting phases of the Indian monsoon activity in 2016 just prior to its onset and during its active phase. Prior to the monsoon onset, high concentrations of CN and CCN prevailed across the IGP, and the profiles revealed frequent occurrence of elevated layers (in the altitude range 1–3 km). Highest concentrations and elevated peaks with high values occurred over the central IGP. The scenario changed dramatically during the active phase of the monsoon, when the CN and CCN concentrations dropped (CN by 20 % to 30 % and CCN by 6 % to 25 %) throughout the IGP with more pronounced changes at altitudes higher than 3 km where decreases as high as > 80 % were observed. These reductions have an east-to-west decreasing gradient, being most remarkable in the eastern IGP and very weak over the western IGP where the CN concentrations above 3 km increased during the monsoon. The activation ratios (ARs) showed contrasting features, increasing with increase in altitude, prior to the onset of monsoon, reversing the trend to decrease with increase in altitude during the active phase of the monsoon. The supersaturation spectrum became flatter during the active phase of the monsoon, indicating an increase in the hygroscopicity of aerosols following the mixing of surface-based emissions with the advected marine air mass.
Light-absorbing organic aerosols, known as brown carbon (BrC), counteract the overall cooling effect of aerosols on Earth's climate. The spatial and temporal dynamics of their light-absorbing ...properties are poorly constrained and unaccounted for in climate models, because of limited ambient observations. We combine carbon isotope forensics (δ
C) with measurements of light absorption in a conceptual aging model to constrain the loss of light absorptivity (i.e., bleaching) of water-soluble BrC (WS-BrC) aerosols in one of the world's largest BrC emission regions-South Asia. On this regional scale, we find that atmospheric photochemical oxidation reduces the light absorption of WS-BrC by ~84% during transport over 6000 km in the Indo-Gangetic Plain, with an ambient first-order bleaching rate of 0.20 ± 0.05 day
during over-ocean transit across Bay of Bengal to an Indian Ocean receptor site. This study facilitates dynamic parameterization of WS-BrC absorption properties, thereby constraining BrC climate impact over South Asia.
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