Year	micrograms per cubic meter
1990	47.44695547
1995	46.98602472
2000	50.90455589
2005	42.85174163
2010	34.11058979
2011	46.89453877
2012	52.29133025
2013	42.89778424
2014	39.18447447
2015	53.84034644
2016	47.92595122
2017	52.61609912
2018	50.05668918
2019	51.11721004