Year	micrograms per cubic meter
1990	9.35853974
1995	9.362753867
2000	9.983340643
2005	10.02699513
2010	9.71516679
2011	9.446098507
2012	9.279395317
2013	9.377626462
2014	9.02903083
2015	9.019559314
2016	9.075500701
2017	9.153255835
2018	9.066089264
2019	9.044039796