Year	micrograms per cubic meter
1990	15.62276872
1995	15.5412485
2000	14.16360006
2005	13.29202423
2010	13.7363052
2011	14.18995612
2012	13.35883878
2013	14.70187086
2014	13.77937715
2015	14.58721171
2016	14.11129971
2017	13.61076539
2018	13.63250289
2019	13.51118918