Year	micrograms per cubic meter
1990	27.96905139
1995	26.48486237
2000	27.53958991
2005	25.50601519
2010	24.06241788
2011	28.6901789
2012	25.51904301
2013	28.04343945
2014	25.60894093
2015	26.72226603
2016	24.75521888
2017	22.74896563
2018	23.36239296
2019	22.87512366