Year	micrograms per cubic meter
1990	64.25884741184302
1995	59.62849025690164
2000	66.12367917640022
2005	61.16196249720693
2010	58.30997494420971
2011	59.964204450319464
2012	61.98744568728303
2013	54.88501055193149
2014	54.86031083307528
2015	68.84337381686481
2016	64.26926868299013
2017	62.75581355199265
2018	63.02200226943841
2019	63.35737266532711