Crop residue burning in Punjab and Haryana is often blamed for worsening air quality in the Delhi National Capital Region.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/crop-residue-burning-north-affecting-rest-india-too-study
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/business-line-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/crops
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/agriculture
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/punjab
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/haryana
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/nasa
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/black-carbon
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-standards