Year	micrograms per cubic meter
1990	49.97818193
1995	43.14937091
2000	29.95434417
2005	27.56278704
2010	30.99186302
2011	27.93127669
2012	27.74731988
2013	26.09198709
2014	27.64739605
2015	28.74197769
2016	27.56633411
2017	27.1847545
2018	27.01261068
2019	26.83286559