Year	micrograms per cubic meter
1990	20.96666628
1995	20.22869552
2000	21.72911591
2005	23.06554667
2010	21.39452778
2011	23.87684453
2012	21.51418461
2013	23.46972436
2014	21.43012396
2015	21.89483769
2016	21.10660159
2017	19.79808272
2018	20.55766527
2019	20.43900913