Year	micrograms per cubic meter
1990	9.930088179
1995	9.93145734
2000	8.413852372
2005	7.845021263
2010	7.093306446
2011	6.862490575
2012	6.830379818
2013	6.487238908
2014	6.882271843
2015	7.007919722
2016	7.455581683
2017	7.195360215
2018	7.154130038
2019	7.069562328