While no one factor may be termed as the biggest contributor to air pollution in Delhi, the morning after Diwali last year saw a massive spike in levels of air pollutants.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/diwali-set-bring-back-delhis-toxic-date-pollution
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/zee-news
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/diwali
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/festival
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/pm-25
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/haryana
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/punjab
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/crops
[12] http://admin.indiaenvironmentportal.org.in/category/thesaurus/agriculture