Year	micrograms per cubic meter
1990	43.67823825
1995	43.53369703
2000	43.64834394
2005	43.18399077
2010	43.5280748
2011	45.90353609
2012	47.64094019
2013	44.3465132
2014	41.09563287
2015	45.90634891
2016	43.22089056
2017	43.82369016
2018	43.81418539
2019	43.66702552