Year	micrograms per cubic meter
1990	9.290175111
1995	9.074368401
2000	7.819470772
2005	7.53353071
2010	7.184061682
2011	6.859106867
2012	6.884723871
2013	7.840641715
2014	7.432922337
2015	10.04977671
2016	8.340218624
2017	7.745768475
2018	8.252069387
2019	8.221834725