Year	micrograms per cubic meter
1990	10.30763774
1995	10.69234948
2000	11.05677923
2005	10.89265862
2010	10.52119627
2011	10.37657887
2012	10.26183249
2013	10.70325232
2014	10.19444017
2015	11.72674947
2016	10.25635183
2017	10.1509403
2018	10.39904238
2019	10.38285056