As per a 2015 IIT-Kanpur study on air pollution, road dust (32%) is the largest contributor to air pollution, followed by vehicular pollution (20%).
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/delhi-govt-proposes-aerial-sprinkling-water-keep-dust-bay
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-new-delhi
[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/indian-institute-technology-iit
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environment-pollution-control-authority-epca
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi-pollution-control-committee-dpcc
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/diwali
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/festival