Abstract
Abstract. Several ambient air quality records corroborate the severe and persistent degradation of air quality over northern India during the winter months, with evidence of a continued, increasing trend of pollution across the Indo-Gangetic Plain (IGP) over the past decade. A combination of atmospheric dynamics and uncertain emissions, including the post-monsoon agricultural stubble burning, make it challenging to resolve the role of each individual factor. Here we demonstrate the potential use of an atmospheric transport model, the Weather Research and Forecasting model coupled with chemistry (WRF–Chem) to identify and quantify the role of transport mechanisms and emissions on the occurrence of the pollution events. The investigation is based on the use of carbon monoxide (CO) observations from the TROPOspheric Monitoring Instrument (TROPOMI) on board the Sentinel-5 Precursor satellite and the surface measurement network, as well as the WRF–Chem simulations, to investigate the factors contributing to CO enhancement over India during November 2018. We show that the simulated column-averaged dry air mole fraction (XCO) is largely consistent with TROPOMI observations, with a spatial correlation coefficient of 0.87. The surface-level CO concentrations show larger sensitivities to boundary layer dynamics, wind speed, and diverging source regions, leading to a complex concentration pattern and reducing the observation-model agreement with a correlation coefficient ranging from 0.41 to 0.60 for measurement locations across the IGP. We find that daily satellite observations can provide a first-order inference of the CO transport pathways during the enhanced burning period, and this transport pattern is reproduced well in the model. By using the observations and employing the model at a comparable resolution, we confirm the significant role of atmospheric dynamics and residential, industrial, and commercial emissions in the production of the exorbitant level of air pollutants in northern India. We find that biomass burning plays only a minimal role in both column and surface enhancements of CO, except for the state of Punjab during the high pollution episodes. While the model reproduces observations reasonably well, a better understanding of the factors controlling the model uncertainties is essential for relating the observed concentrations to the underlying emissions. Overall, our study emphasizes the importance of undertaking rigorous policy measures, mainly focusing on reducing residential, commercial, and industrial emissions in addition to actions already underway in the agricultural sectors.
Highlights
Biomass burning (BB) has been recognized as the secondlargest source of radiatively and chemically active trace gases and aerosols in the global atmosphere, which has significant implications for climatic change and human health (Andreae, 2001; Bond, 2004; Crutzen and Andreae, 1990; Guenther et al, 2006; Kaiser et al, 2012; van der Werf et al, 2017)
Central (CI) and southern regions (SI) of India add about 33 % towards the pre-monsoon fire carbon monoxide (CO) emissions, while northern India (NI) shows fewer emissions during the whole year
Our findings suggest that the enhanced CO levels during pollution episodes over Delhi and the greater part of IndoGangetic Plain (IGP) are affected by biomass burning
Summary
Biomass burning (BB) has been recognized as the secondlargest source of radiatively and chemically active trace gases (e.g. carbon monoxide – CO; carbon dioxide – CO2; and sulfur dioxide – SO2) and aerosols (e.g. particulate matter – PM10 and PM2.5) in the global atmosphere, which has significant implications for climatic change and human health (Andreae, 2001; Bond, 2004; Crutzen and Andreae, 1990; Guenther et al, 2006; Kaiser et al, 2012; van der Werf et al, 2017). As per the previous studies, the primary crop residues generated in India are rice straw (112 Mt), wheat straw (109.9 Mt), rice husk (22.4 Mt), sugarcane tops (97.8 Mt), and bagasse (101.3 Mt), the major part of which is burnt in the open air (Lasko and Vadrevu, 2018). Most of these burning activities are found over the northern part of India along the foothills of the Himalayas known as the Indo-Gangetic Plains (hereafter called the IGP). Based on VIIRS (Visible Infrared Imaging Radiometer Suite) thermal anomalies, a recent study has estimated burnt crop residues of 20.4 Mt and 9.6 Mt in Punjab and Haryana for the agricultural year 2017– 2018 in which most of the residue burnt (>90 %) at the field was from rice and wheat crops (Singh et al, 2020)
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