Year	micrograms per cubic meter
1990	19.280529231942463
1995	18.51334070162711
2000	17.649136514567736
2005	15.959305613671779
2010	15.49259358619439
2011	16.049999786328893
2012	15.072118173871775
2013	15.120508076642928
2014	14.646173759214074
2015	14.620156081559582
2016	13.909693263465112
2017	13.887284534781248
2018	13.948878967901699
2019	13.828595003735945