Year	micrograms per cubic meter
1990	9.908308897
1995	9.334029522
2000	8.43278366
2005	8.36496091
2010	8.648386553
2011	7.749831914
2012	7.484387015
2013	7.541812181
2014	8.097020858
2015	6.563380324
2016	6.517719719
2017	5.683914429
2018	5.985799478
2019	5.893562381