Year	micrograms per cubic meter
1990	16.66158736
1995	16.39220976
2000	17.97651855
2005	17.47346106
2010	16.76999522
2011	16.21392774
2012	16.98500632
2013	16.85065699
2014	17.44234007
2015	18.22650845
2016	17.77201952
2017	17.74186912
2018	17.73840139
2019	17.72938154