South Asian megacities are strong sources of regional air pollution. Delhi is a key hotspot of health- and climate-impacting black carbon (BC) emissions, affecting environmental sustainability in densely populated northern India. Effective mitigation of BC impact is hampered by highly uncertain emission source estimates. Here, we use dual-carbon isotope fingerprints (δ13C/∆14C) of BC to constrain the seasonal source variability in Delhi. These measurements show that lower BC concentrations in summer are predominantly from fossil fuel sources (~83%). However, large-scale open burning of post-harvest crop residue/wood in nearby rural regions is contributing to severe haze pollution in Delhi during winter and autumn (~42 ± 17%). Hence, the common conception that megacities affect their surroundings is here amended or seasonally reversed. Therefore, to combat the severe air pollution problems in Delhi and the environmental quality of northern India, current urban efforts need to be complemented with countryside regional mitigation.

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Data availability

The observational data that support the findings of this study are available in the Bolin Centre Database (http://bolin.su.se/data/) and from the corresponding author upon request.

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This work was funded by the Swedish Research Council (FORMAS grant no. 214-2009-970), the Swedish Energy Agency (STEM grant no. 35450-2), and the Swedish Research Council VR (Distinguished Professor Grant no. 2017-01601). The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the provision of the HYSPLIT transport and dispersion model and/or READY website (http://www.ready.noaa.gov) used in this study.

Author information


  1. Dept of Environmental Science and Analytical Chemistry, Bolin Centre for Climate Research, Stockholm University, Stockholm, Sweden

    • Srinivas Bikkina
    • , August Andersson
    • , Elena N. Kirillova
    • , Henry Holmstrand
    •  & Örjan Gustafsson
  2. Indian Institute of Tropical Meteorology, Regional Centre, New Delhi, India

    • Suresh Tiwari
    • , A. K. Srivastava
    •  & D. S. Bisht


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O.G. and A.A. collaborated with S.T. for the Delhi year-round campaign. S.B. and E.K. performed the clean sample preparation of BC isolates for the dual-carbon isotopic composition. S.B., A.A. and O.G. wrote the paper with input from all co-authors. A.A. performed the error propagations and MCMC simulations.

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The authors declare no competing interests.

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Correspondence to Örjan Gustafsson.

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