Year micrograms per cubic meter 1990 21.32660649 1995 21.36466719 2000 21.56575577 2005 19.3594652 2010 20.12399348 2011 19.82404669 2012 20.21083792 2013 20.3861167 2014 20.63455256 2015 22.36411414 2016 21.74480107 2017 21.3879586 2018 21.87542393 2019 21.97072547