Year	micrograms per cubic meter
1990	38.8456207
1995	37.31743369
2000	42.5053916
2005	39.76661984
2010	42.00889257
2011	45.33865903
2012	47.01194744
2013	46.26029567
2014	42.85562418
2015	44.89784611
2016	43.47619484
2017	43.66118465
2018	43.39312927
2019	43.16577438