Year	micrograms per cubic meter
1990	28.66082231
1995	27.29682483
2000	28.13389466
2005	28.31636067
2010	28.62971853
2011	28.8143365
2012	29.11115709
2013	29.20983831
2014	28.23624771
2015	29.1040242
2016	29.17977414
2017	28.0424247
2018	28.50920176
2019	28.4208104