Year	micrograms per cubic meter
1990	22.06752894
1995	21.27313532
2000	19.83799172
2005	18.68745983
2010	18.48995556
2011	19.50868234
2012	17.84168596
2013	16.48954454
2014	16.55872786
2015	16.09492405
2016	15.19734324
2017	15.71390854
2018	15.69941205
2019	15.67108952