Year	micrograms per cubic meter
1990	26.55055955
1995	25.70409521
2000	22.23019169
2005	21.12988842
2010	19.19737442
2011	20.9420236
2012	18.59905794
2013	18.4258514
2014	18.05674056
2015	17.92089482
2016	16.52950459
2017	16.70789043
2018	17.08366574
2019	16.96940443