Year	micrograms per cubic meter
1990	17.48988591
1995	18.79731478
2000	18.48744599
2005	17.96776089
2010	18.80666676
2011	18.49284254
2012	16.81350235
2013	16.29507954
2014	16.41203304
2015	17.13237684
2016	14.41331917
2017	15.1441739
2018	15.58861663
2019	15.57380293