Year	micrograms per cubic meter
1990	21.67806892
1995	18.90217376
2000	18.79918179
2005	19.62900732
2010	21.29437515
2011	19.5062242
2012	20.88473743
2013	19.99272951
2014	20.83237429
2015	20.91779236
2016	20.71374205
2017	19.21356831
2018	20.05559153
2019	19.97447395