Year	micrograms per cubic meter
1990	26.00047701
1995	26.07060417
2000	26.78376487
2005	25.96758518
2010	28.91206034
2011	27.60700271
2012	27.12238118
2013	27.81978844
2014	27.78269564
2015	28.61166279
2016	27.7790227
2017	26.28925045
2018	26.54401982
2019	26.03180162