Year	micrograms per cubic meter
1990	20.08741907
1995	19.57760506
2000	20.69946783
2005	19.00076003
2010	18.07248309
2011	18.92277826
2012	19.05070976
2013	19.19138531
2014	19.5657393
2015	20.0554292
2016	19.35410515
2017	18.38056914
2018	18.21883434
2019	17.72659724