Year	micrograms per cubic meter
1990	58.31124322
1995	56.04283361
2000	56.42631336
2005	52.66763856
2010	52.80501579
2011	53.95598891
2012	56.80371059
2013	47.569479
2014	49.37871651
2015	57.5086131
2016	54.0903092
2017	57.49587976
2018	54.03820301
2019	54.06829369