Year	micrograms per cubic meter
1990	9.88604398
1995	9.588367156
2000	7.97776548
2005	7.633029514
2010	7.189279108
2011	7.040822394
2012	6.985875583
2013	7.690390513
2014	7.349006477
2015	9.546252635
2016	8.374308489
2017	7.839653053
2018	8.192170775
2019	8.151978907