Year	micrograms per cubic meter
1990	22.03546704
1995	22.02975898
2000	22.09745927
2005	19.76708368
2010	19.93972371
2011	19.16245671
2012	20.0960981
2013	20.12125502
2014	20.19670971
2015	21.94444639
2016	21.08043284
2017	20.92639263
2018	21.26503022
2019	21.34389367