Year	micrograms per cubic meter
1990	7.963653174
1995	6.826810749
2000	6.446695124
2005	6.693596917
2010	6.393486339
2011	7.345177681
2012	7.610637069
2013	7.616318907
2014	7.745818643
2015	7.926244379
2016	7.649925898
2017	7.348749548
2018	7.683014194
2019	7.680643971