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

www.nature.com/scientificreports OPEN Aerosol‑induced atmospheric heating rate decreases over South and East Asia as a result of changing content and composition S. Ramachandran1,2*, Maheswar Rupakheti2 & Mark G. Lawrence2,3 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 twentyfirst 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 stateof-the-art climate models to ensure accurate quantification of the complex and evolving impact of aerosols on the regional climate over Asia. Anthropogenic aerosol emissions over Asia are c hanging1,2. This region is extremely sensitive to climate change owing to a dependency on the strong seasonal variation in monsoon and the very high population densities, including a large poor populace1. The associated risks involve freshwater availability, climate extremes and large 1 Physical Research Laboratory, Ahmedabad, India. 2Institute for Advanced Sustainability Studies, Potsdam, Germany. 3Institute for Environmental Sciences and Geography, University of Potsdam, Potsdam, Germany. *email: Scientific Reports | (2020) 10:20091 | https://doi.org/10.1038/s41598-020-76936-z 1 Vol.:(0123456789) www.nature.com/scientificreports/ societal costs1. Thus understanding and quantifying the climate impacts of Asian aerosols from local to regional to global scales is crucial3,4. Model simulations and satellite observations show that the geographical distributions of emissions of trace gases and aerosol particles that can alter the atmospheric energy balance have changed in the last decade due to economic growth and air pollution r egulations2. This is especially the case over Asia, where a dipole in aerosol pollution has been noted between South and East A sia1, with a qualitative difference in the trend of emissions: over South Asia, the emissions have been increasing, while over East Asia they have been decreasing. The climate over Asia is governed by a variety of land–ocean-atmospheric processes, varying across spatial and temporal scales, making it a challenging task to simulate and predict the climate implications of such emerging patterns of aerosol forcing1. Atmospheric aerosols contribute to climate change by both heating and cooling the atmosphere, and cooling the surface. The net global radiative effect due to aerosol heating and cooling is negative and has offset a substantial portion of the warming due to greenhouse gases5. However, there exists at least a factor of three uncertainty in the aerosol radiative forcing e stimates5. One of the main factors causing this is the uncertainty in the atmospheric solar heating by black carbon (BC). Models grossly underestimate aerosol absorption in many regions, especially over Asia6. East Asia, in particular the North China Plain (NCP), and South Asia, notably the Indo-Gangetic Plain (IGP) are global air pollution hotspots. Previous studies on aerosol properties and emissions over Asia including the IGP and the NCP regions were based on (i) in-situ data at one or only a few locations for a limited time period or only a select set of aerosol parameters7–10, (ii) inventories of aerosol emissions11, (iii) satellite data on AOD for a limited period, e.g., between 2000 and 2 00912, and S O2 emissions between 2005 and 201513, and (iv) multi-model simulations of radiative forcing e stimates14. Recently, spatial distribution of microphysical and optical properties of aerosols and radiative forcing using the China Aerosol Remote Sensing Network was reported15. The climate impacts due to aerosols, including the impacts on the hydrological cycle and the Asian monsoon have been analyzed based on model simulated aerosol characteristics and observed trends3,16. However, as mentioned earlier, models grossly underestimate aerosol absorption in many r egions6,17. Thus, the trends in aerosol impact on climate and climate change including the changes in precipitation and hydrological cycle based on model simulated aerosol characteristics over these regions may not be very accurate given the uncertainties and limitations of models and satellite data, and the lack of observation-based trend analysis. 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 chan (...truncated)