Year	micrograms per cubic meter
1990	24.29823522
1995	23.18355421
2000	25.08789463
2005	24.53315863
2010	25.76200797
2011	27.1748805
2012	27.1977447
2013	26.47693677
2014	26.83022559
2015	27.06821686
2016	26.05141291
2017	23.9485906
2018	23.47253554
2019	22.34665143