Year	micrograms per cubic meter
1990	13.083159378037598
1995	12.586987175337887
2000	11.67571662173461
2005	10.726613073131315
2010	9.088965796966447
2011	9.071695296883119
2012	8.74321395841022
2013	8.504724099737746
2014	8.338496202052218
2015	8.07501324739453
2016	7.261033903784479
2017	7.697830626289607
2018	7.704080647913408
2019	7.605668882771223