Year	micrograms per cubic meter
1990	17.282226804551335
1995	16.272659481334777
2000	15.289875264801838
2005	14.107209848887727
2010	13.302896002120736
2011	13.662905688031742
2012	12.871409088590338
2013	12.748312495023075
2014	12.343112303137369
2015	12.250790168175604
2016	11.470348997818686
2017	11.627662004385037
2018	11.650292031078761
2019	11.547761490984845