Year	micrograms per cubic meter
1990	11.76053243
1995	10.68419393
2000	10.09080673
2005	8.168736987
2010	8.099763965
2011	9.050737279
2012	7.555663741
2013	7.27897533
2014	7.454135917
2015	7.00932544
2016	6.594523915
2017	6.652507161
2018	6.695799295
2019	6.642383374