Year	micrograms per cubic meter
1990	22.14420675
1995	18.11365004
2000	18.36867682
2005	16.01396045
2010	15.07604341
2011	16.42814491
2012	17.53182291
2013	18.59998502
2014	17.73247308
2015	18.29501195
2016	17.63850257
2017	18.55801565
2018	18.49335298
2019	18.76588248