Year	micrograms per cubic meter
1990	61.13442627
1995	58.5159852
2000	63.07217982
2005	66.59652745
2010	63.75377018
2011	65.33843793
2012	68.07534915
2013	64.28454239
2014	59.51273914
2015	66.36107967
2016	61.64582457
2017	61.67431284
2018	60.43815811
2019	59.21710593