Year	micrograms per cubic meter
1990	20.5580271
1995	18.63671331
2000	16.43916499
2005	15.08202344
2010	16.08934354
2011	15.57511307
2012	13.8086903
2013	13.57835046
2014	12.24528447
2015	12.72693664
2016	11.77992518
2017	11.88161987
2018	11.92460308
2019	11.80580716