Year	micrograms per cubic meter
1990	7.593652633
1995	7.475606975
2000	7.356627454
2005	6.904730947
2010	6.785235613
2011	6.707494797
2012	6.995491287
2013	6.964302296
2014	6.880050681
2015	6.848476723
2016	6.815366192
2017	6.673638956
2018	6.738086439
2019	6.745497859