During the post-monsoon season, the air in northern India is particularly stagnant, meaning smoke particles do not vent into the atmosphere as they would during other times of the year.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/stubble-burning-doubles-delhi-pollution-harvard-study
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/agriculture
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/punjab
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/delhi
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-standards
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/who
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/nasa