Year	micrograms per cubic meter
1990	46.176043437823495
1995	47.49364631618954
2000	49.011025225197045
2005	48.896804127787014
2010	52.460051343261895
2011	50.30443200575211
2012	50.99863012652164
2013	51.14639652704409
2014	48.20921607179042
2015	51.651490660590504
2016	48.32636241169374
2017	49.33543212340643
2018	49.36326699310497
2019	49.2271393538395