Year	micrograms per cubic meter
1990	26.463374462661523
1995	26.425052367638855
2000	26.07066730932077
2005	24.478307620794602
2010	26.016238086371015
2011	27.830410773853206
2012	27.021024766352582
2013	27.197612687681087
2014	26.5738869330828
2015	26.17225113855482
2016	25.13227411115776
2017	24.617003623412625
2018	24.69392861516624
2019	24.53458565212339