Year	micrograms per cubic meter
1990	36.6967399
1995	35.47223745
2000	37.1754755
2005	34.34349626
2010	35.45526085
2011	36.03479593
2012	34.93156522
2013	37.16684022
2014	34.1318172
2015	39.73197591
2016	39.0009896
2017	35.36985071
2018	36.93641776
2019	36.74675483