Year	micrograms per cubic meter
1990	66.34276906
1995	59.40534304
2000	67.41490186
2005	61.29110011
2010	59.4274705
2011	58.86976922
2012	61.32736378
2013	56.76664192
2014	56.35775552
2015	69.51058018
2016	65.0564255
2017	62.3153609
2018	63.83099556
2019	64.47088106