Year	micrograms per cubic meter
1990	8.969294845
1995	9.811958766
2000	10.06469304
2005	10.27696685
2010	9.806587652
2011	9.763735686
2012	9.70650164
2013	9.738884647
2014	9.287738199
2015	9.097105532
2016	9.691195176
2017	9.564153332
2018	9.484560864
2019	9.468882294