Year	micrograms per cubic meter
1990	22.33728424784404
1995	22.460154855631306
2000	24.082564433001874
2005	20.584919519541163
2010	19.850358977243587
2011	19.33998699679443
2012	20.73154168529948
2013	20.319226608887657
2014	20.198962318566092
2015	21.940367650595366
2016	20.74309356144688
2017	20.306489127861763
2018	20.200158598732298
2019	19.83844379598149