Assessment of regional aerosol radiative effects under the SWAAMI campaign – Part 1: Quality-enhanced estimation of columnar aerosol extinction and absorption over the Indian subcontinent
Authors: Pathak Harshavardhana Sunil , Satheesh Sreedharan Krishnakumari , Nanjundiah Ravi Shankar , Moorthy Krishnaswamy Krishna , Lakshmivarahan Sivaramakrishnan , Babu Surendran Nair Suresh 
Published: September 2019

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