Year	micrograms per cubic meter
1990	37.47397925
1995	39.69390311
2000	39.6695789
2005	43.8599594
2010	48.07737731
2011	40.00846258
2012	37.51566204
2013	43.22205102
2014	39.66495649
2015	39.74162327
2016	40.85182106
2017	37.83073851
2018	38.94887339
2019	38.59119845