Agricultural Fires and Air Quality Degradation in Northwestern India
In northwestern India, the double crop system alternates between the monsoon crop (kharif, predominantly rice) and winter crop (rabi, predominantly wheat). The short turnaround between harvest and sowing pressures farmers to quickly remove crop residue from the field to prepare for the next planting. In addition, the use of combine harvesters, which saves time and decreases labor costs, leaves behind abundant root-bound residues that are difficult to remove manually. The prevailing, cost-effective method is to burn the crop residues. However, crop residue burning releases a suite of pernicious gases and aerosols into the atmosphere that degrades both rural and urban air quality. Although crop residue burning is just one of many pollution sources, it is highly episodic and seasonal. Every year, the burning of rice residue from October to November contributes to thick haze over the region. New Delhi, a mega-city already suffering from heavy local pollution, is located downwind of these seasonal agricultural fires. During the post-monsoon, meteorology (e.g. slow winds and colder temperatures) help trap pollutants near the surface in north India, where the flat terrain is surrounded by the Himalayas.
We first used HYSPLIT atmospheric back trajectories to define Delhi's post-monsoon (October-November) and pre-monsoon (April-May) airsheds, or approximate regions where emissions can affect Delhi's local air pollution (Liu et al., 2018a). Focusing on the post-monsoon burning season, we then used the STILT model to estimate the PM2.5 enhancement in the Delhi National Capital Territory (Cusworth et al., 2018). We also developed a fusion method using MODIS (500 m x 500 m) and Landsat (30 m x 30 m) imagery to approximate burned area; however, we find that moderate-resolution sensors, such as MODIS, are unable to see many small fires, therefore underestimating overall fire activity (Liu et al., 2019). In a more recent approach, we used satellite and household survey data to build a new agricultural fire emissions inventory, SAGE-IGP, for north India (Liu et al., 2020b).
North India faces dual challenges of severe groundwater depletion and air pollution. To alleviate groundwater depletion in Punjab and Haryana, the Preservation of Sub-Soil Water Act of 2009 delayed rice sowing dates to align the timing closer to the monsoon onset. However, this delay in the monsoon rice growing season has led to a delay in the post-monsoon fire season of around two weeks from 2003-2016 (Liu et al., 2021a). Through atmospheric modeling of smoke PM2.5 driven by SAGE-IGP fire emissions, we find that the delay in the post-monsoon fire season has consistently exacerbated poor air quality in nearby downwind areas, such as in New Delhi (Liu et al., 2022). The unintended air pollution resulting from the cascading delays in the crop cycle and fire season suggests that future policies should consider groundwater depletion and smoke pollution as linked challenges.
Publications: Liu et al. (2018a, Atmos. Environ.), Cusworth et al. (2018, Environ. Res. Lett.), Liu et al. (2019, Environ. Res. Commun.), Liu et al. (2020b, Atmos. Environ. X), Liu et al. (2021a, Environ. Res. Lett.), Liu et al. (2022, J. Geophys. Res. Atmos.), Lan et al. (2022, Nat. Commun.)
Primary Collaborators: Miriam Marlier (UCLA), Ruth DeFries (Columbia), Alex Karambelas (Columbia) Dan Cusworth (Harvard), Loretta Mickley (Harvard), Ritesh Gautam (EDF), Manoj Singh (UPES), Meha Jain (U Michigan)
Pre-burn (Oct 12, 2014) and post-burn (Oct 20, 2014) in western Punjab, India (DigitalGlobe, Google Earth)
SAGE-IGP dry matter (DM) burned from agricultural fires in north India, averaged over 2003-2018 (Liu et al., 2020b).