Year	micrograms per cubic meter
1990	59.23854675709703
1995	55.315623869577124
2000	58.850826582161794
2005	58.23669232280731
2010	61.542084605359435
2011	63.098294421866235
2012	67.47663205859695
2013	68.3338734385221
2014	71.38028250300137
2015	69.08339012887778
2016	70.15268859673797
2017	63.706760771969485
2018	64.4160477246487
2019	64.43563313511784