Year	micrograms per cubic meter
1990	17.93831381
1995	17.36049931
2000	19.58669508
2005	18.63563931
2010	18.05516481
2011	17.73545429
2012	18.54325277
2013	18.71624801
2014	19.17668414
2015	19.88014557
2016	19.14929749
2017	19.12974699
2018	19.14097132
2019	19.03230796