Year	micrograms per cubic meter
1990	25.58181863
1995	25.08759254
2000	21.3395071
2005	20.28580736
2010	20.010989
2011	20.74074009
2012	18.42206445
2013	17.48896034
2014	17.52501676
2015	17.54829004
2016	15.73414047
2017	16.57651016
2018	16.63089611
2019	16.49434933