Year	micrograms per cubic meter
1990	21.24364734
1995	20.38051606
2000	18.03449697
2005	17.53591329
2010	18.97825386
2011	22.31865641
2012	24.00908326
2013	22.85330812
2014	21.45655621
2015	20.51355436
2016	20.86881423
2017	20.10344809
2018	20.25210336
2019	20.29485447