Year	micrograms per cubic meter
1990	48.88419264
1995	49.09456138
2000	50.76470176
2005	51.55204637
2010	53.28667651
2011	58.40555752
2012	58.90513577
2013	63.28657095
2014	60.26956062
2015	56.20412426
2016	51.58657201
2017	49.92778443
2018	49.27568382
2019	47.67263439