Year	micrograms per cubic meter
1990	18.45590948320461
1995	17.5288626229811
2000	16.69877247452197
2005	15.765003423676353
2010	15.598051725285188
2011	15.927525289417035
2012	15.329630024893396
2013	15.189135739957647
2014	14.619582271941665
2015	14.810933070301372
2016	13.87223840197935
2017	14.129486021468546
2018	14.154104664499744
2019	14.081954916245419