Year	micrograms per cubic meter
1990	47.3546656
1995	40.89112012
2000	46.84866156
2005	40.2537776
2010	40.0552493
2011	41.37893343
2012	39.74657182
2013	39.40271655
2014	39.10904578
2015	47.84805559
2016	46.74295432
2017	42.80037217
2018	44.88882315
2019	45.28465901