Year	micrograms per cubic meter
1990	19.22167236
1995	19.24067287
2000	19.15179877
2005	17.52865528
2010	17.6150034
2011	16.88759795
2012	17.78294307
2013	17.79640821
2014	17.49910192
2015	19.2530028
2016	18.53218593
2017	18.1758119
2018	18.56540278
2019	18.60844325