Year	micrograms per cubic meter
1990	59.46452558
1995	55.03201712
2000	60.26263791
2005	59.44666881
2010	57.53023063
2011	58.78662177
2012	59.8616705
2013	55.79298325
2014	53.37526383
2015	63.96919397
2016	59.85638516
2017	59.52255694
2018	59.20016634
2019	59.2825236