Indian Institute of Toxicology Research report cites addition of 1,28,217 vehicles on capital roads as main reason.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/charbagh-most-polluted-lucknow-0
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/indian-express-lucknow
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/uttar-pradesh
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/lucknow-t
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/vehicular-pollution
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-standards
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/oxides-nitrogen-nox
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/health-effects
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-quality-and-health