Year	micrograms per cubic meter
1990	43.62075514563677
1995	41.61587145223656
2000	44.92774829407127
2005	43.45801261133037
2010	44.126605813716196
2011	44.55827548763902
2012	45.88545885839135
2013	43.54857013385366
2014	42.210669910359016
2015	48.41014360435935
2016	45.76946194648514
2017	44.96649777856368
2018	45.30626463199142
2019	45.40284142485899