Particulate carbon and fine dust particles cause browning of the marble.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/taj-pollutants-causing-discolouration-identified
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/hindu-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/uttar-pradesh
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/agra-t
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/monuments
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/micropollutant
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/environmental-science
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/organic-pollutants
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/air-pollution