Year	micrograms per cubic meter
1990	25.2079681
1995	23.08216971
2000	21.8129049
2005	21.00241299
2010	22.09841962
2011	19.55318145
2012	18.88122818
2013	19.78007712
2014	20.21540632
2015	18.50294397
2016	16.71207883
2017	16.40896425
2018	16.50189248
2019	16.42351285