Year	micrograms per cubic meter
1990	61.29463947
1995	56.70963764
2000	60.98426266
2005	59.6884795
2010	62.42665398
2011	58.97191431
2012	63.43015874
2013	52.78016724
2014	56.00353617
2015	65.82960573
2016	60.61509832
2017	61.60379119
2018	60.53457655
2019	60.62414043