Aerosol-induced atmospheric heating rate decreases over South and East Asia as a result of changing content and composition

S Ramachandran,Mark G Lawrence,Maheswar Rupakheti

doi:10.1038/s41598-020-76936-z

S Ramachandran, Mark G Lawrence + Show 1 more

Open Access

https://doi.org/10.1038/s41598-020-76936-z

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Abstract

Aerosol emissions from human activities are extensive and changing rapidly over Asia. Model simulations and satellite observations indicate a dipole pattern in aerosol emissions and loading between South Asia and East Asia, two of the most heavily polluted regions of the world. We examine the previously unexplored diverging trends in the existing dipole pattern of aerosols between East and South Asia using the high quality, two-decade long ground-based time series of observations of aerosol properties from the Aerosol Robotic Network (AERONET), from satellites (Moderate Resolution Imaging Spectroradiometer (MODIS) and Ozone Monitoring Instrument (OMI)), and from model simulations (Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). The data cover the period since 2001 for Kanpur (South Asia) and Beijing (East Asia), two locations taken as being broadly representative of the respective regions. Since 2010 a dipole in aerosol optical depth (AOD) is maintained, but the trend is reversed—the decrease in AOD over Beijing (East Asia) is rapid since 2010, being 17% less in current decade compared to first decade of twenty-first century, while the AOD over South Asia increased by 12% during the same period. Furthermore, we find that the aerosol composition is also changing over time. The single scattering albedo (SSA), a measure of aerosol’s absorption capacity and related to aerosol composition, is slightly higher over Beijing than Kanpur, and has increased from 0.91 in 2002 to 0.93 in 2017 over Beijing and from 0.89 to 0.92 during the same period over Kanpur, confirming that aerosols in this region have on an average become more scattering in nature. These changes have led to a notable decrease in aerosol-induced atmospheric heating rate (HR) over both regions between the two decades, decreasing considerably more over East Asia (− 31%) than over South Asia (− 9%). The annual mean HR is lower now, it is still large (≥ 0.6 K per day), which has significant climate implications. The seasonal trends in AOD, SSA and HR are more pronounced than their respective annual trends over both regions. The seasonal trends are caused mainly by the increase/decrease in anthropogenic aerosol emissions (sulfate, black carbon and organic carbon) while the natural aerosols (dust and sea salt) did not change significantly over South and East Asia during the last two decades. The MERRA-2 model is able to simulate the observed trends in AODs well but not the magnitude, while it also did not simulate the SSA values or trends well. These robust findings based on observations of key aerosol parameters and previously unrecognized diverging trends over South and East Asia need to be accounted for in current state-of-the-art climate models to ensure accurate quantification of the complex and evolving impact of aerosols on the regional climate over Asia.

Highlights

Aerosol emissions from human activities are extensive and changing rapidly over Asia
In order to understand the trends in aerosol properties over two important regions, East Asia and South Asia, we conduct the first analysis of high quality time series observations over a period of two-decades of the key climate-relevant aerosol parameters over Kanpur in South Asia and Beijing in East Asia, along with satellite observations that give a larger regional perspective (Fig. 1)
The MODIS Terra version 6.1 data, i.e., aerosol optical depth (AOD) retrieved using the Combined Dark Target and Deep Blue algorithms for land and ocean, and the AOD retrieved using the Dark Target algorithm for land are used[19]. Both the MODIS datasets are used as the Dark Target product of MODIS tends to discard a significant fraction of pixels with heavy aerosol loads leading to underestimation of the total aerosol s cene[19]

Summary

Introduction

Aerosol emissions from human activities are extensive and changing rapidly over Asia. A complete, comprehensive and a more accurate analysis of aerosol characteristics, aerosol radiative effects and their trends over this region is crucial to develop, since the radiative forcing that results from the changing distribution patterns of aerosols is expected to be different from those observed in the late twentieth century[1] These changes may trigger large scale atmospheric responses which will have wide-ranging impacts on climate, clouds, chemistry and other atmospheric processes extending well beyond source r egions[1,18]. In order to understand the trends in aerosol properties over two important regions, East Asia and South Asia, we conduct the first analysis of high quality time series observations over a period of two-decades of the key climate-relevant aerosol parameters over Kanpur in South Asia and Beijing in East Asia, along with satellite observations that give a larger regional perspective (Fig. 1)

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