Year	micrograms per cubic meter
1990	22.47989867
1995	24.92926693
2000	23.32743848
2005	23.26803051
2010	24.98858454
2011	24.70951301
2012	22.35572158
2013	23.00431254
2014	21.76878839
2015	22.51370748
2016	18.85915608
2017	20.20136227
2018	20.05327737
2019	19.75989688