Year	micrograms per cubic meter
1990	32.12429552
1995	31.47305054
2000	31.07417312
2005	31.92346789
2010	35.39548583
2011	39.93109313
2012	40.53562091
2013	38.4153887
2014	38.00214063
2015	37.38765045
2016	34.24036582
2017	34.90719855
2018	34.03041055
2019	33.65637939