Scattering And Absorption Characteristics Of Aerosols Over Indo-Gangetic Basin: Implications To Radiative Forcing

Credit: Wikimedia Commons

Atmospheric aerosols, both scattering and absorbing types, arising from different natural and anthropogenic emission sources are known to influence the Earth’s radiation budget through direct and indirect effects, which in turn affect the regional hydrological cycle and Earth’s climate system. In spite of its significant contribution towards radiative and climatic effects, these aerosol particles can also substantially affect the regional air quality and human health.

Many Indian cities, especially those over the Indo-Gangetic Basin (IGB) in north India, are among the most polluted cities in the world. There has been significant focus on aerosol characterizations over the region due to massive and diverse aerosol burden from multiple emissions, unique topography, regional meteorology, socioeconomic development, and human behavior.

New Delhi, an urban megacity located in the north-west part of the IGB, suffers from the intense pressure from urbanization, industrialization, and dense population. The entire region, including Delhi, experienced the enhancements in aerosol emissions mainly from anthropogenic sources, fossil-fuel, biofuel/biomass combustion, which along with the long-range transport of natural dust aerosols from surrounding Desert regions have led to severely turbid atmosphere (see Fig. 1). As a consequence, these aerosols can strongly modify the regional climate through radiative forcing and changes in cloud microphysics and monsoon processes.

The magnitude and sign of the aerosol radiative forcing are mainly determined by the scattering and absorption characteristics of aerosols, which play crucial role in radiative transfer model to address aerosol-cloud-climate interactions and the main research theme of a recent article published in an International Journal “Atmospheric Research” by Dr. Atul Kumar Srivastava, Senior Scientist from Indian Institute of Tropical Meteorology and his group.

Fig. 1. IGB region, showing unique topography surrounded by the variety of emission sources (Source: personal presentation of William K. M. Lau).

Based on different in-situ measurements, aerosol scattering and absorption characteristics were investigated simultaneously at an urban megacity Delhi during the period from October 2011 to September 2012 (see Fig. 2). Dr. Srivastava and his team made a first attempt to understand the possible implications of crucial aerosol optical parameters on direct aerosol forcing from this location. Results show that the magnitude of scattering coefficient (σsp ~710±615 Mm-1) was about ten times higher than the absorption coefficient (σabs ~67±40 Mm-1) during the entire study period.

Seasonally, both σsp and σabs were substantially higher (about two-fold) during the winter/post-monsoon periods as compared to the summer, which gave rise to single scattering albedo (SSA) by ~5%. A significant cooling (-61 Wm-2) was observed at the surface and warming (42 Wm–2) into the atmosphere (more pronounced during summer/monsoon season), which exerted an atmospheric heating rate of 1.18 K day−1. The observed large cooling at the surface and strong heating in the atmosphere can strongly affect the atmospheric dynamics, which can have significant impacts on regional climate and monsoon circulation systems.

Fig. 2. Monthly mean variability in (a) σsp, (b) σabs, (c) SSA and (d) atmospheric heating rate over Delhi during the study period.

These findings are described in the article entitled Scattering and absorption characteristics of aerosols at an urban megacity over IGB: Implications to radiative forcing, recently published in the journal Atmospheric Research. This work was conducted by A. K. Shrivastava, D. S. Bisht, N. Kishore, and S. Tiwari from Indian Institute of Tropical Meteorology, Sachchidananand Singh from the CSIR-National Physical Laboratory, and V. K. Soni and Siddhartha Singh from the India Meteorological Department.

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