Year	micrograms per cubic meter
1990	16.75082661
1995	16.11801169
2000	17.96339577
2005	18.192241
2010	18.10693704
2011	19.56315996
2012	18.87125053
2013	19.18037888
2014	18.28090193
2015	18.5648497
2016	17.90242599
2017	17.43892166
2018	17.52546596
2019	17.42174135