Year	micrograms per cubic meter
1990	15.0615212
1995	16.00643713
2000	16.37744579
2005	15.8964359
2010	15.1257082
2011	16.5159476
2012	16.05158537
2013	15.87951496
2014	15.1301922
2015	15.31346781
2016	15.98290713
2017	15.88743775
2018	15.60358619
2019	15.5639189