Long-term change in aerosol characteristics over Indo-Gangetic Basin: How significant is the impact of emerging anthropogenic activities?
Graphical abstract
Introduction
Atmospheric aerosols play crucial role in changing our climate system, which are known to perturb the radiative balance of the overall Earth's climate system through direct and indirect effects (IPCC, 2013). Aerosols possess direct radiative effects through scattering and absorption of solar and terrestrial radiation. However, through indirect effect, they alter cloud microphysical properties and thus the cloud lifetime, and can cause the overall precipitation processes (Ramanathan et al., 2001; Satheesh and Moorthy, 2005; Rosenfeld et al., 2008, Rosenfeld et al., 2014; Srivastava et al., 2019). The various aerosol characteristics such as physical, chemical and optical properties have significant heterogeneity in their spatio-temporal variability and thus to their direct and indirect radiative effects (Moorthy et al., 2009; Tiwari et al., 2013, Tiwari et al., 2015; Paulot et al., 2018). However, the uncertainties in their effects on the atmosphere and climate are reported to be still high and the understanding level is comparatively less than that of the greenhouse gases (IPCC, 2013). The enhanced aerosol burden associated with the emerging anthropogenic activities has been estimated to be masked approximately one third of the continental warming attributed due to the greenhouse gases (IPCC, 2013; Storelvmo et al., 2016), which was estimated to have significant implications to the regional as well as global climate (Bollasina et al., 2011; Srivastava et al., 2012; Ginoux et al., 2012; Myhre et al., 2017). A global climatology of Moderate resolution Imaging Spectroradiometer (MODIS)-derived aerosol optical depth (AOD) during the period from 2000 to 2006 showed the highest annual mean over South Asia when compared to the rest of the world (Remer et al., 2008). This was largely associated with the observed increase in the emissions of anthropogenic aerosols over South Asia due to rapid urbanization, industrialization and population growth in the past decades (Ramanathan et al., 2005; Gautam et al., 2009; Ramachandran et al., 2012; Moorthy et al., 2013; Srivastava, 2017; Sahu et al., 2021). A significant rising trend in anthropogenic aerosols was also observed over the Indian subcontinent using the multi-angle imaging spectroradiometer (MISR)-derived AOD data during 2000–2010 (Dey and Di Girolamo, 2011).
The Indian subcontinent, especially the Indo-Gangetic Basin (IGB) in the northern part of India has been identified as a regional aerosol hotspot, having variety of emission sources with their distinct seasonal characteristics (Srivastava et al., 2012; Tiwari et al., 2015; Moorthy et al., 2016; Singh et al., 2017a, Singh et al., 2017b). The high aerosol loading over the region is commonly attributed to the various anthropogenic activities, which are largely associated with the formation of secondary aerosols and the natural aerosol fluxes from long-range transport (Srivastava et al., 2012; Hooda et al., 2016; Kishore et al., 2019; Srivastava et al., 2021). The region also suffers from the dense fog/haze and severe smog conditions during the winter/post-monsoon period, which could possibly be due to the intense burning of agriculture residue and other solid waste materials along with the use of fossil-fuels in various sectors. This may cause significant reduction in air quality of the region (Kaskaoutis et al., 2014; Sharma et al., 2017; Kishore et al., 2019; Beig et al., 2020; Kanawade et al., 2020; Ojha et al., 2020), and may consequently have increase risk to the human health (Ghude et al., 2016; Chowdhury et al., 2020). Though, the total aerosol loading is higher over the IGB compared to the rest part of India, the number of hazy days was observed to increase at relatively higher rate over the Central India (~2.6 days/year) as compared to the IGB (~1.7 days/year) region, which are largely associated with the relative increase in biomass burning emissions over the Central India region (Thomas et al., 2019). On the other hand, during the pre-monsoon/summer season, the region is highly impacted due to the transported dust aerosols from the adjoining Desert regions (Gautam et al., 2010; Srivastava et al., 2011, Srivastava et al., 2014; Dumka et al., 2019; Kumar et al., 2019). Furthermore, the atmospheric processes occurring over the region have been shown to affect the atmospheric composition, chemistry and climate over the large regional as well as global scales (Lelieveld et al., 2018 and references therein). Thus, the region not only imparts local and regional climate implications but can also considered to be one of the global hotspots of enhanced aerosol loading, and driving effect of aerosol burden beyond local and regional scale (Moorthy et al., 2013; Srivastava, 2017; Thomas et al., 2019; Xia et al., 2021). There are couple of long-term studies on aerosol characterizations over the IGB in the recent past (Jethva et al., 2005; Dey and Di Girolamo, 2011; Kaskaoutis et al., 2012; Ramachandran et al., 2012; Lodhi et al., 2013; Kumar et al., 2018), but most of these studies are focused on the variability of a single aerosol parameter (e.g., AOD). Thus, a comprehensive understanding of the size of aerosol particles, nature (absorbing and/or scattering), and aerosol composition is lacking based upon a long time-series data in different environments.
The present study focuses on long-term assessment of multiple aerosol parameters, obtained from different satellite measurements over the IGB to understand the impact of emerging anthropogenic activities. The study was conducted during the period from 2007 to 2017 at the two urban megacities: Lucknow, which represents one of the fast-developing cities over the central IGB, and Delhi, one of the most polluted cities in the world, situated at the northwest IGB. Interestingly, the measurement period was chosen based on large scale development activities in the region and that might attribute to high anthropogenic emissions load.
Section snippets
Sites description
The measurement sites (Lucknow and Delhi), marked over the IGB (Fig. 1) are in urban setting with a semi-arid climate, and the analyses of climatology and variation of aerosol characteristics is carried out for these two highly urbanized cities. The climatic conditions of both the sites are extreme in nature with very hot summers (~45 °C) and cold winters with temperatures as low as 3 °C. Lucknow is located at 26.8°N latitude, 80.9°E longitude and approx. 128 m above mean sea level in the
MODIS data
NASA's earth observing system (EOS) launched the Terra and Aqua satellites to monitor the Earth, terrestrial and oceanic characteristics. The Moderate Resolution Imaging Spectrometer (MODIS) is a key sensor carried by Terra and the Aqua satellites to observe aerosol and clouds with high accuracy from space, which has been in operation since February 2000 (Xiong et al., 2009). The MODIS observes the upwelling radiation at the top of the atmosphere in 36 channels from 0.41 μm to 14.2 μm, which is
Spatio-temporal distribution of decadal mean AOD during 2007–2017
Fig. 1 shows spatial distribution of decadal monthly mean AOD over India obtained from MODIS during the period from 2007 to 2017. The IGB reveals a massive aerosol burden, with the AOD > 0.5 throughout the year. The persistence of exceptionally high AOD (>0.8) was observed over the western parts of the IGB during the pre-monsoon (due to enhanced dust impacts) and post-monsoon months (due to enhanced agriculture burning impacts) and also in the eastern parts of the IGB during the winter month.
Conclusions
The decadal analyses of long-term measurements of satellite-derived columnar AOD, AE, SSA and UV-AI have been carried out at Lucknow, in the central and at Delhi, in the north-west IGB during the period from 2007 to 2017. The high decadal mean AODs (~0.7), with nearly similar increasing trend of ~20% was observed at both the stations, indicating high aerosol loading across the IGB, which shows significant monthly, seasonal and annual variability. The high decadal mean AE (>1.0), with an
Author statement
Sunil Kumar: Formal analysis; Investigation; Methodology; Software; Validation; Visualization; Writing - original draft; Writing - review & editing. Amarendra Singh: Investigation; Methodology; Writing - review & editing. Atul K. Srivastava: Conceptualization; Formal analysis; Investigation; Methodology; Supervision; Writing - original draft; Writing - review & editing. Saroj K. Sahu: Investigation; Resources; Writing - review & editing. Rakesh K. Hooda: Investigation; Resources; Writing -
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Authors express their sincere gratitude to the Director, IITM, Pune for his encouragements and supports throughout the study. SK and AS sincerely thank to the Director, IITM for providing the necessary infrastructure facilities at IITM (Delhi Branch). Authors also acknowledge the use of aerosol products obtained from MODIS and OMI in this study. Authors are grateful to the two anonymous reviewers for their constructive comments and suggestions to improve the manuscript.
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