Year micrograms per cubic meter 1990 47.44695547 1995 46.98602472 2000 50.90455589 2005 42.85174163 2010 34.11058979 2011 46.89453877 2012 52.29133025 2013 42.89778424 2014 39.18447447 2015 53.84034644 2016 47.92595122 2017 52.61609912 2018 50.05668918 2019 51.11721004