Year	micrograms per cubic meter
1990	26.1230785
1995	25.05722042
2000	26.61195132
2005	26.10704001
2010	27.02852733
2011	28.11089482
2012	28.3606157
2013	27.28505272
2014	27.34918603
2015	28.26273741
2016	27.40906413
2017	26.18112194
2018	26.34770053
2019	25.92546019