Year	micrograms per cubic meter
1990	23.16909559889995
1995	23.205204067942166
2000	24.152505515044112
2005	24.477241221030575
2010	25.082887542201863
2011	25.804616789388504
2012	26.49532681013064
2013	25.46690824735289
2014	24.92948032745749
2015	26.31401039309602
2016	25.115925979320455
2017	25.076117909474124
2018	24.96181556305727
2019	24.816539599305912