Year	micrograms per cubic meter
1990	20.37743795
1995	20.07166871
2000	20.71409122
2005	19.15743208
2010	19.06202926
2011	19.73341964
2012	17.37070897
2013	16.69353
2014	15.72743666
2015	16.04385798
2016	14.46980163
2017	14.43912017
2018	14.51537033
2019	14.2538714