Year	micrograms per cubic meter
1990	30.13244912979104
1995	29.12162845017496
2000	31.359177280949265
2005	30.318434751291264
2010	31.974952575774186
2011	33.20570775485128
2012	33.833868024013924
2013	33.1190504668925
2014	31.77571569026762
2015	33.495671325434635
2016	32.473834450653854
2017	31.457435607515748
2018	31.374556026367465
2019	30.879069347927498