Year	micrograms per cubic meter
1990	24.203409819825087
1995	23.472886389072933
2000	22.269479089579914
2005	21.025770580996944
2010	22.1904740082303
2011	22.71091263574556
2012	21.95015111139116
2013	21.971740629865085
2014	21.69410859562658
2015	21.02573957759922
2016	20.118642149256786
2017	19.76793305717308
2018	19.862865549170653
2019	19.773854476329014