Year	micrograms per cubic meter
1990	7.222283054
1995	6.861635874
2000	6.862278769
2005	6.649553492
2010	6.595638323
2011	6.809186658
2012	6.191914657
2013	6.091801055
2014	5.939603293
2015	5.833587216
2016	5.436588878
2017	5.709265082
2018	5.690458273
2019	5.698616135