Year	micrograms per cubic meter
1990	24.95127354
1995	24.26881701
2000	24.22215497
2005	23.59097074
2010	22.85407832
2011	25.45681317
2012	25.79816942
2013	27.53881372
2014	28.1810993
2015	26.97657307
2016	27.80560684
2017	26.88210754
2018	27.43635201
2019	27.44033926