Year	micrograms per cubic meter
1990	10.81210616
1995	10.87235353
2000	11.04364215
2005	11.55631281
2010	11.10471801
2011	10.70804506
2012	10.78258779
2013	10.86783061
2014	11.14080727
2015	11.01634581
2016	11.31508398
2017	10.90094994
2018	11.0444457
2019	11.01483371