Year	micrograms per cubic meter
1990	16.80969086
1995	16.96833987
2000	17.36915156
2005	16.44163266
2010	15.43740141
2011	15.61181668
2012	16.42049683
2013	15.70876377
2014	16.18216874
2015	16.20392015
2016	15.86345443
2017	15.39542797
2018	15.72864111
2019	15.59065792