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. Improving the accuracy of regional aerosol climate impact assessmentcalls for improvement in the accuracy of regional aerosol radiative effect (ARE)estimation. One of the most important means of achieving this is touse spatially homogeneous and temporally continuous datasets of criticalaerosol properties, such as spectral aerosol optical depth (AOD) andsingle scattering albedo (SSA), which are the most important parametersfor estimating aerosol radiative effects. However, observations do notprovide the above; the space-borne observations though provide wide spatialcoverage, are temporal snapshots and suffer from possible sensordegradation over extended periods. On the other hand, the ground-basedmeasurements provide more accurate and temporally continuous data butare spatially near-point observations. Realizing the need for spatiallyhomogeneous and temporally continuous datasets on one hand andthe near non-existence of such data over the south Asian region (which isone of the regions where aerosols show large heterogeneity in most oftheir properties), construction of accurate gridded aerosol products bysynthesizing the long-term space-borne and ground-based data has beentaken up as an important objective of the South West Asian AerosolMonsoon Interactions (SWAAMI), a joint Indo-UK field campaign, aimingat characterizing aerosol–monsoon links and their variabilities over the Indianregion. In Part 1 of this two-part paper, we present spatially homogeneousgridded datasets of AOD and absorption aerosoloptical depth (AAOD), generated for the first time over this region. These data products are developed bymerging the highly accurate aerosol measurements from the dense networks of 44(for AOD) and 34 (for AAOD) ground-based observatories ofAerosol Radiative Forcing over India NETwork (ARFINET) and AErosol ROboticNETwork (AERONET) spread across the Indian region, with satellite-retrieved AODand AAOD, following statistical assimilation schemes.The satellite data used for AOD assimilation include AODs retrieved fromMODerate Imaging Spectroradiometer (MODIS) and Multiangle ImagingSpectroRadiometer (MISR) over the same domain. For AAOD, theground-based black carbon (BC) mass concentration measurements fromthe network of 34 ARFINET observatories and satellite-based (Kalpana-1,INSAT-3A) infrared (IR) radiance measurements are blended with griddedAAODs (500 nm, monthly mean) derived from Ozone Monitoring Instrument(OMI)-retrieved AAODs (at 354 and 388 nm). The details of the assimilation methodsand the gridded datasets generated are presented inthis paper. The merged gridded AOD and AAOD products thus generatedare validated against the data from independent ground-based observatories,which were not used for the assimilation process but are representative ofdifferent subregions of the complex domain. This validation exercise revealed thatthe independent ground-based measurements are better confirmed by merged datasets than therespective satellite products. As ensured by assimilation techniques employed,the uncertainties in merged AODs and AAODsare significantly less than those in corresponding satellite products. Thesemerged products also all exhibit important large-scale spatial and temporalfeatures which are already reported for this region. Nonetheless, the merged AODsand 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 thestudy 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 ofregional aerosol radiative effects (direct),the results of which would be presented in a companion paper, Part 2 of this two-partpaper.

atmospheric chemistry and physics

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