Year	micrograms per cubic meter
1990	62.52099761
1995	63.21986982
2000	63.66018386
2005	56.80914644
2010	56.68739592
2011	58.02145733
2012	64.50905978
2013	53.04187379
2014	55.40981308
2015	65.39361923
2016	59.47930794
2017	63.42037083
2018	60.0827093
2019	60.21139678