Accumulation of aerosols over the Indo-Gangetic plains and southern slopes of the Himalayas: distribution, properties and radiative effects during the 2009 pre-monsoon season

Abstract. We examine the distribution of aerosols and associated optical/radiative properties in the Gangetic-Himalayan region from simultaneous radiometric measurements over the Indo-Gangetic Plains (IGP) and the foothill/southern slopes of the Himalayas during the 2009 pre-monsoon season. Enhanced dust transport extending from the Southwest Asian arid regions into the IGP, results in seasonal mean (April–June) aerosol optical depths of over 0.6 – highest over Southern Asia. The influence of dust loading is greater over the Western IGP as suggested by pronounced coarse mode peak in aerosol size distribution and spectral single scattering albedo (SSA). Transported dust in the IGP, driven by prevailing westerly airmass, is found to be more absorbing (SSA550 nm<0.9) than the near-desert region in Northwestern (NW) India suggesting mixing with carbonaceous aerosols in the IGP. On the contrary, significantly reduced dust transport is observed over eastern IGP and foothill/elevated Himalayan slopes in Nepal where strongly absorbing haze is prevalent, as indicated by lower SSA (0.85–0.9 at 440–1020 nm), suggesting presence of more absorbing aerosols compared to IGP. Additionally, our observations show a distinct diurnal pattern of aerosols with characteristic large afternoon peak, from foothill to elevated mountain locations, associated with increased upslope transport of pollutants – that likely represent large-scale lifting of absorbing aerosols along the elevated slopes during pre-monsoon season. In terms of radiative impact of aerosols, over the source region of NW India, diurnal mean reduction in solar radiation fluxes was estimated to be 19–23 Wm−2 at surface (12–15% of the surface solar insolation). Furthermore, based on limited observations of aerosol optical properties during the pre-monsoon period and comparison of our radiative forcing estimates with published literature, there exists a general spatial heterogeneity in the regional aerosol forcing, associated with the absorbing aerosol distribution over northern India, with both diurnal mean surface forcing and forcing efficiency over the IGP exceeding that over Northwestern India. Finally, the role of the seasonal progressive buildup of aerosol loading and water vapor is investigated in the observed net aerosol radiative effect over Northwestern India. The radiative impact of water vapor is found to amplify the net regional aerosol radiative forcing suggesting that the two exert forcing in tandem leading to enhanced surface cooling. It is suggested that water vapor contribution should be taken into account while assessing aerosol forcing impact for this region and other seasonally similar environments.

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Accumulation of aerosols over the Indo-Gangetic plains and southern slopes of the Himalayas: distribution, properties and radiative effects during the 2009 pre-monsoon Season

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-11-15697-2011 ◽

2011 ◽

Vol 11 (5) ◽

pp. 15697-15743 ◽

Cited By ~ 4

Author(s):

R. Gautam ◽

N. C. Hsu ◽

S. C. Tsay ◽

K. M. Lau ◽

B. Holben ◽

...

Keyword(s):

Water Vapor ◽

Radiative Forcing ◽

Monsoon Season ◽

Radiative Properties ◽

Aerosol Size Distribution ◽

Surface Cooling ◽

Gangetic Plains ◽

Dust Transport ◽

Aerosol Forcing ◽

Radiative Impact

Abstract. We examine the distribution of aerosols and associated optical/radiative properties in the Gangetic-Himalayan region from simultaneous radiometric measurements over the Indo-Gangetic Plains (IGP) and the foothill/slopes of the Himalayas during the 2009 pre-monsoon season. Enhanced dust transport extending from the Southwest Asian arid regions into the IGP, results in seasonal mean (April–June) aerosol optical depths of over 0.6 – highest over southern Asia. The influence of dust loading is greater over the western IGP as suggested by pronounced coarse mode peak in aerosol size distribution and spectral single scattering albedo (SSA). The transported dust in the IGP, driven by prevailing westerly airmass, is found to be more absorbing (SSA550 nm ~0.89) than the near-desert region in NW India (SSA550 nm ~0.91) suggesting mixing with carbonaceous aerosols in the IGP. On the contrary, significantly reduced dust transport is observed over eastern IGP and foothill/elevated slopes in Nepal where strongly absorbing haze is prevalent, associated with upslope transport of pollution, as indicated by low values of SSA (0.85–0.9 for the wavelength range of 440–1020 nm), suggesting presence of more absorbing aerosols compared to IGP. Assessment of the radiative impact of aerosols over NW India suggests diurnal mean reduction in solar radiation fluxes of 19–23 Wm−2 at surface (12–15 % of the surface solar insolation). Based on limited observations of aerosol optical properties during the pre-monsoon period and comparison of our radiative forcing estimates with published literature, there exists spatial heterogeneity in the regional aerosol forcing, associated with the absorbing aerosol distribution over northern India, with both diurnal mean surface forcing and forcing efficiency over the IGP exceeding that over NW India. Additionally, the role of the seasonal progressive buildup of aerosol loading and water vapor is investigated in the observed net aerosol forcing over NW India. The radiative impact of water vapor is found to amplify the net regional aerosol radiative forcing suggesting that the two exert forcing in tandem leading to enhanced surface cooling. It is suggested that water vapor contribution should be taken into account while assessing aerosol forcing impact for this region and other seasonally similar environments.

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Increased absorption by coarse aerosol particles over the Gangetic–Himalayan region

Atmospheric Chemistry and Physics ◽

10.5194/acp-14-1159-2014 ◽

2014 ◽

Vol 14 (3) ◽

pp. 1159-1165 ◽

Cited By ~ 11

Author(s):

V. S. Manoharan ◽

R. Kotamarthi ◽

Y. Feng ◽

M. P. Cadeddu

Keyword(s):

Atmospheric Aerosols ◽

Radiative Forcing ◽

Climate Models ◽

Monsoon Season ◽

Regional Scale ◽

Chemical Properties ◽

Aerosol Forcing ◽

Post Monsoon Season ◽

Absorbing Aerosols ◽

Coarse Aerosol

Abstract. Each atmospheric aerosol type has distinctive light-absorption characteristics related to its physical/chemical properties. Climate models treat black carbon as the main light-absorbing component of carbonaceous atmospheric aerosols, while absorption by some organic aerosols is also considered, particularly at ultraviolet wavelengths. Most absorbing aerosols are assumed to be < 1 μm in diameter (sub-micron). Here we present results from a recent field study in India, primarily during the post-monsoon season (October–November), suggesting the presence of absorbing aerosols sized 1–10 μm. Absorption due to super-micron-sized particles was nearly 30% greater than that due to smaller particles. Periods of increased absorption by larger particles ranged from a week to a month. Radiative forcing calculations under clear-sky conditions show that super-micron particles account for nearly 44% of the total aerosol forcing. The origin of the large aerosols is unknown, but meteorological conditions indicate that they are of local origin. Such economic and habitation conditions exist throughout much of the developing world. Hence, large absorbing particles could be an important component of the regional-scale atmospheric energy balance.

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Increased absorption by giant aerosol particles over the Gangetic–Himalayan region

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-19837-2013 ◽

2013 ◽

Vol 13 (7) ◽

pp. 19837-19852

Author(s):

V. S. Manoharan ◽

R. Kotamarthi ◽

Y. Feng ◽

M. P. Cadeddu

Keyword(s):

Atmospheric Aerosols ◽

Radiative Forcing ◽

Climate Models ◽

Monsoon Season ◽

Regional Scale ◽

Chemical Properties ◽

Aerosol Forcing ◽

Post Monsoon Season ◽

Absorbing Aerosols ◽

Sky Conditions

Abstract. Each atmospheric aerosol type has distinctive light-absorption characteristics related to its physical/chemical properties. Climate models treat black carbon as the light-absorbing component of all carbonaceous atmospheric aerosols. Most absorbing aerosols are assumed to be <1 μm in diameter (sub-micron). Here we present results from a recent field study in India, primarily during the post-monsoon season (October–November), suggesting the presence of absorbing aerosols sized 1–10 μm. Absorption due to super-micron-sized particles was nearly 30% greater than that due to smaller particles. Periods of increased absorption by larger particles ranged from a week to a month. Radiative forcing calculations under clear-sky conditions show that super-micron particles account for nearly 44% of the total aerosol-forcing. The origin of the large aerosols is unknown, but meteorological conditions indicate that they are of local origin. Such economic and habitation conditions exist throughout much of the developing world. Hence, large absorbing particles could be an important component of the regional-scale atmospheric-energy balance.

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Influence of future air pollution mitigation strategies on total aerosol radiative forcing

Atmospheric Chemistry and Physics ◽

10.5194/acp-8-6405-2008 ◽

2008 ◽

Vol 8 (21) ◽

pp. 6405-6437 ◽

Cited By ~ 33

Author(s):

S. Kloster ◽

F. Dentener ◽

J. Feichter ◽

F. Raes ◽

J. van Aardenne ◽

...

Keyword(s):

Air Pollution ◽

Radiative Forcing ◽

Mitigation Strategies ◽

Radiation Budget ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Pollution Mitigation ◽

Aerosol Forcing ◽

Future Air Pollution ◽

Aerosol Radiative

Abstract. We apply different aerosol and aerosol precursor emission scenarios reflecting possible future control strategies for air pollution in the ECHAM5-HAM model, and simulate the resulting effect on the Earth's radiation budget. We use two opposing future mitigation strategies for the year 2030: one in which emission reduction legislation decided in countries throughout the world are effectively implemented (current legislation; CLE 2030) and one in which all technical options for emission reductions are being implemented independent of their cost (maximum feasible reduction; MFR 2030). We consider the direct, semi-direct and indirect radiative effects of aerosols. The total anthropogenic aerosol radiative forcing defined as the difference in the top-of-the-atmosphere radiation between 2000 and pre-industrial times amounts to −2.00 W/m2. In the future this negative global annual mean aerosol radiative forcing will only slightly change (+0.02 W/m2) under the "current legislation" scenario. Regionally, the effects are much larger: e.g. over Eastern Europe radiative forcing would increase by +1.50 W/m2 because of successful aerosol reduction policies, whereas over South Asia it would decrease by −1.10 W/m2 because of further growth of emissions. A "maximum feasible reduction" of aerosols and their precursors would lead to an increase of the global annual mean aerosol radiative forcing by +1.13 W/m2. Hence, in the latter case, the present day negative anthropogenic aerosol forcing could be more than halved by 2030 because of aerosol reduction policies and climate change thereafter will be to a larger extent be controlled by greenhouse gas emissions. We combined these two opposing future mitigation strategies for a number of experiments focusing on different sectors and regions. In addition, we performed sensitivity studies to estimate the importance of future changes in oxidant concentrations and the importance of the aerosol microphysical coupling within the range of expected future changes. For changes in oxidant concentrations caused by future air pollution mitigation, we do not find a significant effect for the global annual mean radiative aerosol forcing. In the extreme case of only abating SO2 or carbonaceous emissions to a maximum feasible extent, we find deviations from additivity for the radiative forcing over anthropogenic source regions up to 10% compared to an experiment abating both at the same time.

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MACv2-SP: a parameterization of anthropogenic aerosol optical properties and an associated Twomey effect for use in CMIP6

Geoscientific Model Development ◽

10.5194/gmd-10-433-2017 ◽

2017 ◽

Vol 10 (1) ◽

pp. 433-452 ◽

Cited By ~ 57

Author(s):

Bjorn Stevens ◽

Stephanie Fiedler ◽

Stefan Kinne ◽

Karsten Peters ◽

Sebastian Rast ◽

...

Keyword(s):

Optical Properties ◽

Radiative Forcing ◽

Industrial Emissions ◽

Max Planck ◽

Aerosol Optical Properties ◽

Anthropogenic Aerosol ◽

Aerosol Radiative Forcing ◽

Clear Sky ◽

Aerosol Forcing ◽

Aerosol Radiative

Abstract. A simple plume implementation of the second version (v2) of the Max Planck Institute Aerosol Climatology, MACv2-SP, is described. MACv2-SP provides a prescription of anthropogenic aerosol optical properties and an associated Twomey effect. It was created to provide a harmonized description of post-1850 anthropogenic aerosol radiative forcing for climate modeling studies. MACv2-SP has been designed to be easy to implement, change and use, and thereby enable studies exploring the climatic effects of different patterns of aerosol radiative forcing, including a Twomey effect. MACv2-SP is formulated in terms of nine spatial plumes associated with different major anthropogenic source regions. The shape of the plumes is fit to the Max Planck Institute Aerosol Climatology, version 2, whose present-day (2005) distribution is anchored by surface-based observations. Two types of plumes are considered: one predominantly associated with biomass burning, the other with industrial emissions. These differ in the prescription of their annual cycle and in their optical properties, thereby implicitly accounting for different contributions of absorbing aerosol to the different plumes. A Twomey effect for each plume is prescribed as a change in the host model's background cloud-droplet population density using relationships derived from satellite data. Year-to-year variations in the amplitude of the plumes over the historical period (1850–2016) are derived by scaling the plumes with associated national emission sources of SO2 and NH3. Experiments using MACv2-SP are performed with the Max Planck Institute Earth System Model. The globally and annually averaged instantaneous and effective aerosol radiative forcings are estimated to be −0.6 and −0.5 W m−2, respectively. Forcing from aerosol–cloud interactions (the Twomey effect) offsets the reduction of clear-sky forcing by clouds, so that the net effect of clouds on the aerosol forcing is small; hence, the clear-sky forcing, which is more readily measurable, provides a good estimate of the total aerosol forcing.

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Observations and Modeling of the Surface Aerosol Radiative Forcing during UAE2

Journal of the Atmospheric Sciences ◽

10.1175/2007jas2555.1 ◽

2008 ◽

Vol 65 (9) ◽

pp. 2877-2891 ◽

Cited By ~ 21

Author(s):

K. M. Markowicz ◽

P. J. Flatau ◽

J. Remiszewska ◽

M. Witek ◽

E. A. Reid ◽

...

Keyword(s):

United Arab Emirates ◽

Radiative Forcing ◽

Sea Breeze ◽

Single Scattering ◽

Land Breeze ◽

Gulf Region ◽

Diurnal Variability ◽

Aerosol Radiative Forcing ◽

Aerosol Forcing ◽

Aerosol Radiative

Abstract Aerosol radiative forcing in the Persian Gulf region is derived from data collected during the United Arab Emirates (UAE) Unified Aerosol Experiment (UAE2). This campaign took place in August and September of 2004. The land–sea-breeze circulation modulates the diurnal variability of the aerosol properties and aerosol radiative forcing at the surface. Larger aerosol radiative forcing is observed during the land breeze in comparison to the sea breeze. The aerosol optical properties change as the onshore wind brings slightly cleaner air. The mean diurnal value of the surface aerosol forcing during the UAE2 campaign is about −20 W m−2, which corresponds to large aerosol optical thickness (0.45 at 500 nm). The aerosol forcing efficiency [i.e., broadband shortwave forcing per unit optical depth at 550 nm, W m−2 (τ500)−1] is −53 W m−2 (τ500)−1 and the average single scattering albedo is 0.93 at 550 nm.

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Radiative Impacts of the 2011 Abrupt Drops in Water Vapor and Ozone in the Tropical Tropopause Layer

Journal of Climate ◽

10.1175/jcli-d-15-0167.1 ◽

2016 ◽

Vol 29 (2) ◽

pp. 595-612 ◽

Cited By ~ 19

Author(s):

Daniel M. Gilford ◽

Susan Solomon ◽

Robert W. Portmann

Keyword(s):

Water Vapor ◽

Radiative Forcing ◽

Tropical Tropopause Layer ◽

Stratospheric Water Vapor ◽

Tropical Tropopause ◽

Radiative Impacts ◽

Radiative Impact ◽

Rf Values ◽

Mean Structure ◽

Large Water

Abstract An abrupt drop in tropical tropopause layer (TTL) water vapor, similar to that observed in 2000, recently occurred in 2011, and was concurrent with reductions in TTL temperature and ozone. Previous studies have indicated that such large water vapor variability can have significant radiative impacts. This study uses Aura Microwave Limb Sounder observations, the Stratospheric Water Vapor and Ozone Satellite Homogenized dataset, and two radiative transfer models to examine the radiative effects of the observed changes in TTL water vapor and ozone on TTL temperatures and global radiative forcing (RF). The analyses herein suggest that quasi-isentropic poleward propagation of TTL water vapor reductions results in a zonal-mean structure with “wings” of extratropical water vapor reductions, which account for about half of the 2011 abrupt drop global radiative impact. RF values associated with the mean water vapor concentrations differences between 2012/13 and 2010/11 are between −0.01 and −0.09 W m−2, depending upon the altitude above which perturbations are considered. TTL water vapor and ozone variability during this period jointly lead to a transient radiative cooling of ~0.25–0.5 K in layers below the tropopause. The 2011 abrupt drop also prolonged the reduction in stratospheric water vapor that followed the 2000 abrupt drop, providing a longer-term radiative forcing of climate. Water vapor concentrations from 2005 to 2013 are lower than those from 1990 to 1999, resulting in a RF between these periods of about −0.045 W m−2, approximately 12% as large as, but of opposite sign to, the concurrent estimated CO2 forcing.

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Intercomparison of shortwave radiative transfer schemes in global aerosol modeling: results from the AeroCom Radiative Transfer Experiment

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-12-32631-2012 ◽

2012 ◽

Vol 12 (12) ◽

pp. 32631-32706 ◽

Cited By ~ 3

Author(s):

C. A. Randles ◽

S. Kinne ◽

G. Myhre ◽

M. Schulz ◽

P. Stier ◽

...

Keyword(s):

Radiative Transfer ◽

Multiple Scattering ◽

Radiative Forcing ◽

Model Performance ◽

Aerosol Radiative Forcing ◽

Aerosol Forcing ◽

Aerosol Absorption ◽

Relative Standard ◽

Aerosol Modeling ◽

Aerosol Radiative

Abstract. In this study we examine the performance of 31 global model radiative transfer schemes in cloud-free conditions with prescribed gaseous absorbers and no aerosols (Rayleigh atmosphere), with prescribed scattering-only aerosols, and with more absorbing aerosols. Results are compared to benchmark results from high-resolution, multi-angular line-by-line radiation models. For purely scattering aerosols, model bias relative to the line-by-line models in the top-of-the atmosphere aerosol radiative forcing ranges from roughly −10 to 20%, with over- and underestimates of radiative cooling at higher and lower sun elevation, respectively. Inter-model diversity (relative standard deviation) increases from ~10 to 15% as sun elevation increases. Inter-model diversity in atmospheric and surface forcing decreases with increased aerosol absorption, indicating that the treatment of multiple-scattering is more variable than aerosol absorption in the models considered. Aerosol radiative forcing results from multi-stream models are generally in better agreement with the line-by-line results than the simpler two-stream schemes. Considering radiative fluxes, model performance is generally the same or slightly better than results from previous radiation scheme intercomparisons. However, the inter-model diversity in aerosol radiative forcing remains large, primarily as a result of the treatment of multiple-scattering. Results indicate that global models that estimate aerosol radiative forcing with two-stream radiation schemes may be subject to persistent biases introduced by these schemes, particularly for regional aerosol forcing.

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Link between local scale BC emissions in the Indo-Gangetic Plains and large scale atmospheric solar absorption

Atmospheric Chemistry and Physics ◽

10.5194/acp-12-1173-2012 ◽

2012 ◽

Vol 12 (2) ◽

pp. 1173-1187 ◽

Cited By ~ 35

Author(s):

P. S. Praveen ◽

T. Ahmed ◽

A. Kar ◽

I. H. Rehman ◽

V. Ramanathan

Keyword(s):

Optical Properties ◽

Radiative Forcing ◽

Large Scale ◽

Positive Impact ◽

Monsoon Season ◽

Regional Scale ◽

Local Scale ◽

Dry Season ◽

Aerosol Optical Properties ◽

Gangetic Plains

Abstract. Project Surya has documented indoor and outdoor concentrations of black carbon (BC) from traditional biomass burning cook stoves in a rural village located in the Indo-Gangetic Plains (IGP) region of N. India from November 2009–September 2010. In this paper, we systematically document the link between local scale aerosol properties and column averaged regional aerosol optical properties and atmospheric radiative forcing. We document observations from the first phase of Project Surya and estimate the source dependent (biomass and fossil fuels) aerosol optical properties from local to regional scale. Data were collected using surface based observations of BC, organic carbon (OC), aerosol light absorption, scattering coefficient at the Surya village (SVI_1) located in IGP region and integrated with satellite and AERONET observations at the regional scale (IGP). The daily mean BC concentrations at SVI_1 showed a large increase of BC during the dry season (December to February) with values reaching 35 μg m−3. Space based LIDAR data revealed how the biomass smoke was trapped within the first kilometer during the dry season and extended to above 5 km during the pre-monsoon season. As a result, during the dry season, the variance in the daily mean single scattering albedo (SSA), the ratio of scattering to extinction coefficient, and column aerosol optical properties at the local IGP site correlated (with slopes in the range of 0.85 to 1.06 and R2>0.4) well with the "IGP_AERONET" (mean of six AERONET sites). The statistically significant correlation suggested that in-situ observations can be used to derive spatial mean forcing, at least for the dry season. The atmospheric forcing due to BC and OC exceeded 20 Wm−2 during all months from November to May, supporting the deduction that elimination of cook stove smoke emissions through clean cooking technologies will likely have a major positive impact not only on human health but also on regional climate.

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Ocean mediation of tropospheric response to reflecting and absorbing aerosols

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-5827-2015 ◽

2015 ◽

Vol 15 (10) ◽

pp. 5827-5833 ◽

Cited By ~ 26

Author(s):

Y. Xu ◽

S.-P. Xie

Keyword(s):

Radiative Forcing ◽

Hadley Circulation ◽

Temperature Structure ◽

Co2 Response ◽

Mountain Glaciers ◽

Aerosol Forcing ◽

Deep Temperature ◽

Absorbing Aerosols ◽

The Stability ◽

The Tropics

Abstract. Radiative forcing by reflecting (e.g., sulfate, SO4) and absorbing (e.g., black carbon, BC) aerosols is distinct: the former cools the planet by reducing solar radiation at the top of the atmosphere and the surface, without largely affecting the atmospheric column, while the latter heats the atmosphere directly. Despite the fundamental difference in forcing, here we show that the structure of the tropospheric response is remarkably similar between the two types of aerosols, featuring a deep vertical structure of temperature change (of opposite sign) at the Northern Hemisphere (NH) mid-latitudes. The deep temperature structure is anchored by the slow response of the ocean, as a large meridional sea surface temperature (SST) gradient drives an anomalous inter-hemispheric Hadley circulation in the tropics and induces atmospheric eddy adjustments at the NH mid-latitudes. The tropospheric warming in response to projected future decline in reflecting aerosols poses additional threats to the stability of mountain glaciers in the NH. Additionally, robust tropospheric response is unique to aerosol forcing and absent in the CO2 response, which can be exploited for climate change attribution.

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