Transboundary emission contribution to PM<sub>2.5</sub> concentrations in Indian cities
Raj M. Lal, Ajay Singh Nagpure, Abhinav Anand, Sujit Maji, Kushal Tibrewal, Ganesh Gupta, Chandra Venkataraman, Kangkang Tong
Abstract
Abstract Targeting urban air quality improvements in India, the National Clean Air Program (NCAP) was launched in 2019 to reduce PM 2.5 concentrations by 20%–30% in 122 initial cities over a 7 year period (2017–2024). However, considering the regional nature of air pollution nationwide and the significant emission load from rural areas, a potentially large fraction of urban PM 2.5 might originate from emissions outside of a city’s boundary, i.e. transboundary emissions. Here, we couple top-down (STILT-PM 2.5 ) and bottom-up (WRF-Chem) modeling approaches with a new, nationwide, monthly-resolved, and fine-scale (5 km × 5 km) anthropogenic emission inventory to assess the impact of transboundary emissions to urban PM 2.5 concentrations across 143 cities (122 NCAP and 67 million plus -cities with population >1 million, among which 46 cities are also NCAP cities). We find that, on average, ∼85% (STILT-PM 2.5 : 82% [95% CI: 80–85%; IQR: 77%–94%] & WRF-Chem: 89% [95% CI: 87%–91%; IQR: 88%–96%]) of urban PM 2.5 across the 143 cities originates from transboundary emissions, with domestic biomass burning (32%), energy generation (16%) and industry (15%) being the leading average emission sources to the transboundary contribution. In addition, 107 of the 122 NCAP cities from both modeling approaches have annual transboundary PM 2.5 contributions exceeding 80%, indicating that an entire mitigation of within-boundary emissions alone in these cities will not achieve the most conservative targets outlined as part of NCAP. Our findings underscore the need for multi-scale, regional action planning and implementation to achieve PM 2.5 -air quality targets throughout India.