Year	micrograms per cubic meter
1990	16.46789079
1995	16.41713972
2000	17.57244296
2005	17.37458137
2010	17.12974601
2011	17.91529648
2012	17.34428608
2013	17.73411712
2014	17.0517918
2015	17.9161831
2016	17.31528735
2017	16.63714372
2018	17.18012088
2019	17.15567571