Year	micrograms per cubic meter
1990	26.88206326
1995	25.31042819
2000	23.34531803
2005	21.75442599
2010	22.31721025
2011	18.94685133
2012	20.06768554
2013	20.01905868
2014	18.89054816
2015	19.92482789
2016	21.15210797
2017	18.55575927
2018	19.11325719
2019	19.37146134