Using satellite measurements and mesoscale modelling to understand the contribution to an extreme air pollution event in India

Abstract

Abstract. Several ambient air quality records corroborate severe and persistent degradation of air quality over North 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 CO observations from TROPOspheric Monitoring Instrument (TROPOMI), onboard the Sentinel 5-Precursor satellite, and the surface measurement network as well as 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 as well as residential, industrial and commercial emissions in the production of the exorbitant level of air pollutants in North India. We find that biomass burning plays only a minimal role in both column and surface enhancements of CO, except for in 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 to relate 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.