Year	micrograms per cubic meter
1990	45.77724141039443
1995	44.325308779535426
2000	46.01934116805673
2005	46.10814318367296
2010	48.808980436327005
2011	50.787591523509796
2012	52.64652896261624
2013	54.98759061721181
2014	55.317909688169564
2015	52.27426315595789
2016	51.7920866086978
2017	48.0004768531515
2018	48.268956694522636
2019	47.839266846227176