Year	micrograms per cubic meter
1990	14.01506177
1995	12.74119843
2000	13.6642286
2005	12.34932779
2010	11.49190106
2011	12.44203689
2012	11.17997141
2013	10.3607232
2014	9.830912644
2015	10.53568076
2016	9.576497124
2017	9.749324875
2018	9.769206907
2019	9.728788477