Year	micrograms per cubic meter
1990	39.28774194515005
1995	37.50066482484495
2000	40.3521808266706
2005	39.06200703020769
2010	40.33653694617554
2011	41.876020840652984
2012	42.85654223147112
2013	41.59763510672968
2014	40.19792034789172
2015	43.84840130833364
2016	41.85201388220168
2017	40.757026282182586
2018	40.811028578175005
2019	40.507573796384136