Year	micrograms per cubic meter
1990	21.917064712302153
1995	20.879246582213035
2000	19.586271756130785
2005	18.191598966407035
2010	18.640311465061966
2011	19.046703334182542
2012	17.831417132202137
2013	17.655979465996005
2014	17.04504202239463
2015	16.864347947828634
2016	15.905689186416959
2017	15.809277953551343
2018	15.873462439244863
2019	15.770952493454825