Radiative effects of absorbing aerosols over northeastern India: Observations and model simulations

Abstract

Multiyear measurements of spectral properties of aerosol absorption are examined over four geographically distinct locations of northeastern India. Results indicated significant spatiotemporal variation in aerosol absorption coefficients (σabs) with highest values in winter and lowest in monsoon. The western parts of the region, close to the outflow of Indo‐Gangetic Plains, showed higher values of σabs and black carbon (BC) concentration—mostly associated with fossil fuel combustion. But, the eastern parts showed higher contributions from biomass‐burning aerosols, as much as 20–25% to the total aerosol absorption, conspicuously during premonsoon season. This is attributed to a large number of burning activities over the Southeast Asian region, as depicted from Moderate Resolution Imaging Spectroradiometer fire count maps, whose spatial extent and magnitude peaks during March/April. The nearly consistent high values of aerosol index (AI) and layer height from Ozone Monitoring Instrument indicate the presence of absorbing aerosols in the upper atmosphere. The observed seasonality has been captured fairly well by Goddard Chemistry Aerosol Radiation and Transport (GOCART) as well as Weather Research and Forecasting–Chemistry (WRF‐Chem) model simulations. The ratio of column‐integrated optical depths due to particulate organic matter and BC from GOCART showed good coincidence with satellite‐based observations, indicating the increased vertical dispersion of absorbing aerosols, probably by the additional local convection due to higher fire radiative power caused by the intense biomass‐burning activities. In the WRF‐Chem though underperformed by different magnitude in winter, the values are closer or overestimated near the burnt areas. Atmospheric forcing due to BC was highest (~30 Wm−2) over the western part associated with the fossil fuel combustion.

Spatial Variability and Radiative Impact of Aerosol along the Brahmaputra River Valley in India: Results from a Campaign

“…The TOA RF also is positive for all locations except the GHY and JRH, which are the two major urban locations along the BRV. Significant contribution of absorbing aerosols to lower atmospheric warming over different parts of NE region of India has also been reported by Gogoi et al [62]. Pathak et al [63] have reported that atmospheric forcing due to composite aerosols over Imphal (a valley location south of DMD in NE region of India) was 25. .…”

Aerosols characteristics, trends and their climatic implications over Northeast India and adjoining South Asia

“…Like the other parts of South Asia (Babu et al, ; Moorthy et al, ), a strong seasonality and distinct spatial features of aerosol distribution are present in this region (Figure ). The observed prominent longitudinal gradient in aerosol distribution with higher AOD values in the western part, during all the seasons is attributed to variability of source types and proximity to the western influences through transport (Pathak et al, 2014, ; Gogoi et al, ). As expected AOD 550 decreases with increase in altitude during all the seasons (e.g.…”

“…Earlier studies have identified biomass‐burning (BB) as a major source of aerosols in this region, apart from the locally emitted pollutants (e.g. vehicular exhaust, oil/gas fields, coal mines and biofuels emissions, biogenic emissions from forest) (Pathak et al, ; Gogoi et al, ). However, at DAC (Salam et al, ) and AGA/DHB, soil dust is the major contributor to total aerosols apart from the local and transported anthropogenic aerosols.…”

“…Further, unlike rest of India, NER and adjoining areas in the Himalayan foothills experience substantial orographic precipitation (Dhar and Farooqui, ; Prokop and Walanus, ; Deka et al, ). In order to better depict the regional aerosol‐radiation‐cloud picture of NER and adjoining areas, multiple satellite and model data sets have been utilized (discussed in Section 2.2) as ground‐based observations reported earlier are only from four locations under the Indian Space Research Organization (ISRO)'s Aerosol Radiative Forcing over India NETwork (ARFINET) (Pathak et al, ; Gogoi et al, ).…”

Satellite-based study of physico-optical properties of aerosols over a westernmost location of Brahmaputra valley

“…AOD exhibits an east-west asymmetry within the North-East region with high values in the western locations (Agartala and Shillong) compared to the eastern locations (Dibrugarh and Imphal) because locations in the western North-East Indian region influenced by both natural (sea salt and soil dust) and anthropogenic aerosols produced locally or transported from IGPs and Bangladesh [12] . The western parts of the North-East region, close to the out flow of IGPs, showed higher values of Black Carbon concentration as well as absorption coefficient which are mostly associated with fossil fuel combustion [13] . By using satellite observations, very recently it has been reported that there is a slightly increasing trend in AOD over seven locations in North-East India for the period July 2002 to December 2013 [14] .…”