Year	micrograms per cubic meter
1990	66.63510423
1995	74.62389167
2000	69.84884709
2005	69.6572716
2010	78.51079086
2011	69.78315269
2012	69.70422324
2013	74.75519581
2014	72.02485383
2015	73.13098828
2016	66.74703838
2017	66.95918516
2018	68.47289608
2019	67.88817813