Year	micrograms per cubic meter
1990	21.32660649
1995	21.36466719
2000	21.56575577
2005	19.3594652
2010	20.12399348
2011	19.82404669
2012	20.21083792
2013	20.3861167
2014	20.63455256
2015	22.36411414
2016	21.74480107
2017	21.3879586
2018	21.87542393
2019	21.97072547