Year	micrograms per cubic meter
1990	33.06140005
1995	32.26028861
2000	28.40976142
2005	25.70762668
2010	25.15673695
2011	27.80248173
2012	26.06008923
2013	24.01188607
2014	23.74483028
2015	23.35826543
2016	21.21821429
2017	21.91565485
2018	21.67742225
2019	21.25685256