To reduce the uncertainty in climatic impacts induced by black carbon (BC) from global and regional aerosol–climate model simulations, it is a foremost requirement to improve the prediction of modelled BC distribution, specifically over the regions where the atmosphere is loaded with a large amount of BC, e.g. the Indo-Gangetic Plain (IGP) in the Indian subcontinent.
Here we examine the wintertime direct radiative perturbation due to BC with an efficiently modelled BC distribution over the IGP in a high-resolution (0.1

Black carbon (BC) is released into the atmosphere from the
incomplete combustion of carbon-based fuels

Though consensus is still to be achieved on BC DRF, nevertheless, the global atmospheric absorption attributable to BC was found to be too low in models and had to be enhanced by a factor of 3 to converge with observation-based estimates

To assess BC aerosol absorption accurately and
reduce the uncertainty in the
BC DRF as estimated from global and regional aerosol–climate models, it is therefore a foremost requirement to improve the prediction of atmospheric BC estimates in models, specifically
over regions where the atmosphere is loaded with a large amount of BC, e.g. the Indo-Gangetic Plain (IGP)
in the Indian subcontinent

However, it is also noted from the evaluation of BC concentration estimated
from the free-running aerosol simulations
using the Laboratoire de Météorologie Dynamique atmospheric
general circulation model (LMDZT-GCM) that simulated BC,
which is underestimated by a significant factor at stations close to emission sources (such as that over mainland India), exhibits a relatively lower discrepancy with
observed BC concentrations over the Indian
oceanic regions

The simulated atmospheric BC burden with atmospheric chemical transport models is related to the BC emission strength as input and simulated atmospheric residence time of BC

In this study, we examine
the wintertime direct radiative effects of BC over the
IGP by evaluating the efficacy of simulated atmospheric BC burden in a high-resolution (0.1

The specific objectives of this study are therefore the following:

characterise the model efficiency from five simulations through a detailed validation and statistical analysis of simulated BC concentration with respect to ground-based measurements at stations over the IGP and identify the regional hotspots;

utilise the multi-simulations to quantify the degree of variance in estimated BC concentration attributed to emissions corresponding to areas types (e.g. megacity, urban, semi-urban, low-polluted) and temporal distribution (e.g. daytime and evening hours);

evaluate the spatial features of BC-AOD from five simulations and analyse the association between simulated BC concentration and BC-AOD with BC emission strength; and

examine the spatial distribution of wintertime radiative perturbation due to BC aerosols over the IGP compared to the atmosphere considered without BC aerosols.

High-resolution BC transport simulations are carried out with a state-of-the-art Eulerian chemical transport model (CTM), CHIMERE.
The CHIMERE (model version 2014b) configuration in the present study is forced externally
by the Weather Research and Forecasting (WRF V3.7) model as a meteorological driver in offline mode, meaning that the meteorology is pre-calculated with WRF then read in CHIMERE.
Further, to compute the radiative perturbations due to BC, an offline coupling is executed again, forcing the WRF model with aerosol optical properties computed from CHIMERE output (refer to Sect. 2.3), thereby implying the need to incorporate interactions between the two models using a WRF–CHIMERE online coupled modelling system
for computing aerosol–radiation–cloud interactions

CHIMERE is a regional chemical transport model designed to model 10 gaseous species and aerosols. For chemistry, the gaseous mechanism MELCHIOR2 is used

The WRF model is a state-of-the-art numerical weather forecast and atmospheric simulation system designed for both research and operational applications.
The initial
and boundary meteorological conditions for WRF simulations are obtained from Global Forecast System (GFS)
National Center for Environmental Prediction FINAL operational global
analysis data (NCEP-FNL,

In the present study, five simulations are carried out subjected to the
same model processes with CHIMERE but implementing different BC emission inventories.
The BC inventories include recently estimated India-based
(i) Constrained

A BC emission inventory based on the
bottom-up approach is generally
compiled using information on activity data and generalised
emission factors (see the references for bottom-up emissions, Table

Besides BC emissions, emissions of OC, SO

Experimental set-up for the simulation of BC with CHIMERE.

Spatial distribution of

The spatial distribution of WRF-simulated surface temperature
over the IGP is compared with the available gridded distribution of observed temperature
from the Climatic Research Unit (CRU)

To compare the simulated BC surface concentration with observations,
the measured BC
surface concentration is obtained at stations over the
IGP from available studies (refer to Table

The model-estimated and measured BC concentrations are compared corresponding to daytime (10:00–16:00 LT)
and all-day (24-hourly) winter monthly mean values. This comparison is made because
measured BC concentrations are found to exhibit
a strong diurnal variability, with a relatively lower value
during daytime hours than during the late evening to
early morning hours; this is attributed to prevailing wintertime
meteorological conditions

Bias in
simulated estimates (

Statistical analyses are carried out
corresponding to daytime and all-day winter monthly mean
to evaluate the normalised mean bias (NMB, Eq.

Further, the BC-AOD estimated in the present study (refer to Sect. 2.3) is compared with aerosol absorption optical depth (AAOD) from Aerosol Robotic Network (AERONET; level: 2) measurements
over the IGP

A correlation study is also carried out
between the variance of emissions and simulated BC concentrations or
simulated BC-AOD from the simulations to examine the sensitivity of the simulated BC concentration or BC-AOD to the variation in emission magnitude.

Observational data used for model validation from available studies at identified locations over the study domain.

The BC-AOD is estimated
with OPTical properties SIMulation (OPTSIM)

For radiative transfer calculations, estimates from the Constrained simulation (which is determined to be the most efficient to simulate the BC distribution, as discussed later) and the Smog simulation (from India-based BC emissions as a representative bottomup simulation) are only considered. For estimating the radiative effect due to BC aerosols, simulation of aerosol optical properties (AOD, single-scattering albedo – SSA, and the Ångström exponent – AE) is conducted with OPTSIM for three different cases considering (i) the atmosphere including BC (with BC, BCaero), (ii) the atmosphere without BC (without BC, wBC), and (iii) the atmosphere with no aerosol (without aerosol, wAero). The three-dimensional aerosol species concentration as an input to OPTSIM is derived for each of the three cases from CHIMERE corresponding to the simulations (Constrained and Smog simulations).

Aerosol radiative transfer calculations are done
in WRF-Solar at a temporal resolution of 1 h and horizontal grid resolution of 0.1

Simulations for radiative flux with WRF-Solar are performed for each of the three cases, as mentioned above,
using respective simulated optical properties as input for each case.
The shortwave (SW) radiative flux (at 550 nm) for clear-sky conditions is estimated at the top (TOA) and bottom (SUR) layer of the
atmosphere for the atmosphere with BC and without BC.
This is done by subtracting the respective flux at TOA and SUR due to wAero from the flux due to wBC and
BCaero, respectively. The direct radiative perturbations or the direct radiative effects (DREs) due to BC aerosols at TOA (DRE

The WRF-simulated winter monthly mean distribution of the horizontal wind speed, vertical wind velocity, and PBLH over the IGP are presented in Fig.

Spatial distribution of the WRF-simulated

A high load of BC aerosols over the IGP as obtained (discussed later)
in the present
study is inferred due to confinement of pollution near the surface within the shallow boundary layer height in winter due to low vertical mixing and weak dispersion of atmospheric pollutants. This creates stagnant weather under the
prevailing meteorological conditions, with low temperature, weak wind speed, the downdraft of the air mass, and a narrow PBLH (as presented above). In addition, the Himalayan mountains northward further inhibit the dispersion of aerosol pollutants and favour their confinement over the IGP. This inference is also in corroboration with observational studies at stations over the IGP

We compare the spatial distribution of monthly mean temperature from WRF
simulations (Fig.

The WRF-simulated RH at both
stations (Fig.

Thus, the WRF-simulated winter monthly mean of the meteorological parameters, including their temporal trend, conforms well with the observations. However, it is required to reduce the discrepancy, specifically in the simulated magnitude of temperature during midnight to early morning hours. A better temporally resolved meteorological boundary condition in WRF (compared to 6-hourly from NCEP in the present study), aided by data assimilation at a fine temporal resolution (e.g. 1-hourly) using diurnal meteorological observations for India-based stations, would potentially lead to more accurate simulation of the observed magnitude of the diurnal distribution of meteorological parameters; an assessment in this regard is required in a future study.

The spatial distribution of the winter monthly mean BC surface concentration from five simulations over
the IGP is shown in Fig.

The simulated spatial pattern in the Constrained simulation is consistent with observations (Fig.

The mean and standard deviation of the simulated BC concentration from five simulations at stations under study are provided in Table

Hourly distribution of winter monthly mean BC concentration (

On comparing the temporal distribution of the simulated BC concentration from the Constrained simulation with that of the measured concentration, it is seen that the pattern of simulated diurnal variability (shown for selected stations; refer to Fig.

The bias in the simulated hourly distribution of the winter monthly mean
BC concentration (refer to Fig.

We also provide an animation showing a representation of the transboundary movement of BC pollutants over the IGP in the “Video supplement” (

The megacity of Delhi is surrounded by landmass on all sides and, as visualised from the animation, is influenced by the transport of pollutants from nearby regions (e.g. Punjab–Haryana) towards Delhi. In contrast, the megacity of Kolkata is a coastal location, and the atmospheric BC concentrations are also affected by the prevailing land–sea breeze activity there

The correlation coefficient (

Further, to statistically evaluate the simulated BC concentration from
each of the five simulations with respect to observations, we define the performance of the simulation considering the best, moderate, and poor efficiency based on their relative frequency to maintain the percentage bias in the all-day (daytime) mean simulated BC concentration as about

To evaluate the columnar distribution of wintertime BC aerosols over the IGP, the spatial distribution of the monthly mean BC-AOD at 550 nm from simulations is presented in
Fig.

The percentages of BC-AOD fraction and BC mass fraction from the Constrained simulation (Fig.

To gain insight into the degree of association of the simulated BC burden with the BC emission strength, we utilise the five simulations to evaluate the correlation coefficient between the variation in emission strength and that in simulated BC-AOD (Fig.

Further, the wintertime SW direct radiative perturbation due to BC aerosols over the IGP (Fig.

The uncertainty in estimated wintertime direct radiative perturbations
in the present study is inferred to be within 40 %. This estimation
is based on taking into account NMB in the simulated BC
concentration (as presented
in Sect.

In the present study, wintertime direct radiative perturbation due to black carbon (BC) aerosols was examined over the Indo-Gangetic Plain (IGP) by evaluating the efficacy of the fine grid-resolved (0.1

The WRF-simulated winter monthly mean of the meteorological parameters resembled (bias

A strong association of the winter monthly mean BC concentration between modelled and measured values for stations under study corresponding to each of the five simulations was noticed.
The efficacy to simulate the magnitude of the observed wintertime BC distribution was found to be moderate to poor for the bottomup simulation.
The Constrained simulation estimated high BC pollution over the IGP, with a wintertime all-day monthly mean BC surface concentration (BC-AOD) of 14–25

The BC-AOD fraction (10 %–16 %) from the Constrained simulation was noted to be about twice as large as the BC mass fraction (6 %–10 %) over most of the IGP region.
Five hotspots with a large BC load (surface concentration

Analysis of multi-simulations of BC transport in CHIMERE indicated that increased emissions in the megacities potentially amplify the accumulation of BC pollutants, specifically during the late evening to morning hours, raising concern for megacity commuters. The correlation between the variance in emissions and the simulated BC mass concentration and BC-AOD from the five simulations manifested in the sensitivity of the simulated BC concentration and BC-AOD primarily to the change in BC emission strength over most of the IGP (including the megacity of Kolkata). There is also sensitivity to the transport of BC aerosols as governed by model processes over the megacity of Delhi and the area around the megacities of Delhi and Kolkata.

The transboundary movement of the wintertime BC plume in the IGP was visualised to be spreading towards the north (Himalayan side) during afternoon hours (12:00–18:00 LT) as well as towards the south (central India) and from the upper northern IGP (e.g. Delhi) towards the lower eastern IGP (e.g. Kolkata) during evening until morning hours (18:00–06:00 LT).

Analysis of direct radiative perturbations due to BC aerosols showed that
wintertime BC aerosol over the IGP enhances
atmospheric warming by 2–3 times more and reduces surface
cooling by 10 %–20 % less than considering atmosphere-eliminating
BC aerosols.
The BC-induced net warming effect at the top of the
atmosphere (TOA) from the Constrained simulation was estimated as 10–17 W m

The present study showed that adequate BC emission strength and meteorological forcing in a state-of-the-art chemical transport model at a fine grid resolution led to the successful simulation of the wintertime BC distribution (surface concentration and BC-AOD) over the IGP, unlike previous studies (refer to Sect. 1). We believe this distribution provided a reasonably more accurate representation of the simulated wintertime direct radiative perturbations due to BC aerosols, including identifying the BC hotspots over the IGP. The wintertime radiative perturbation due to BC aerosols simulated in the present study is further utilised to evaluate the potential response to temperature, air quality, and regional climate over the IGP; the outcome from these evaluations will be presented in a future study. The present study is also further extended to evaluate the inter-seasonal BC distribution and associated radiative impacts over the Indian subcontinent, with implications for the south-west monsoon rainfall.

The data in this study are available from the corresponding author upon request (shubha@iitkgp.ac.in).

The animation is available at

SG conducted the BC transport simulations, radiative transfer simulations, evaluation, and validation of the model estimates, including the statistical analyses, as well as participating with SV in synthesising and analysing the results. SV planned and coordinated the study. SG and SV wrote the paper. JK and LM contributed to the writing and analysis of results. LM also advised on the technicality of the CHIMERE model configuration.

The authors declare that they have no conflict of interest.

Simulations were performed in a high-performance computing cluster developed at the Indian Institute of Technology Kharagpur (IIT-KGP) supported through the National Carbonaceous Aerosol Programme–Carbonaceous Aerosol Emissions: Source Apportionment and Climate Impacts (NCAP–COALESCE). Contributions of Rhitamvar Ray, the project engineer at IIT-KGP supported by the NCAP–COALESCE project, are duly acknowledged towards maintaining the computing cluster, handling simulations, data extraction, and preparing the BC animation in the “Video supplement”.

This research has been supported by the National Carbonaceous Aerosol Programme–Carbonaceous Aerosol Emissions: Source Apportionment and Climate Impacts (NCAP–COALESCE) from the Ministry of Environment, Forest, and Climate Change, Govt. of India (grant no. 14/10/2014-CC (Vo. II)), at the Indian Institute of Technology Kharagpur.

This paper was edited by Yafang Cheng and reviewed by five anonymous referees.