Year	micrograms per cubic meter
1990	13.41568004
1995	12.89770264
2000	11.94563014
2005	10.91341302
2010	9.221851825
2011	9.26299254
2012	8.869988146
2013	8.64162897
2014	8.433002378
2015	8.163752791
2016	7.378939412
2017	7.74468451
2018	7.775115257
2019	7.663692516