Year	micrograms per cubic meter
1990	35.7738673
1995	35.15792827
2000	33.01938236
2005	31.3759518
2010	31.39235052
2011	33.88729136
2012	30.48810995
2013	28.69870346
2014	28.20153736
2015	27.89252544
2016	25.34308507
2017	26.41632295
2018	25.88760434
2019	25.35417205