Year	micrograms per cubic meter
1990	26.29438193
1995	21.92286815
2000	30.39747836
2005	26.5204689
2010	27.81831288
2011	30.33188409
2012	31.3100816
2013	30.10150263
2014	28.71473783
2015	28.41561243
2016	28.00473928
2017	29.95291683
2018	29.76843426
2019	30.36142727