Year	micrograms per cubic meter
1990	28.34054536
1995	26.79196053
2000	27.44127631
2005	29.35286761
2010	28.99049207
2011	24.76841347
2012	24.96952852
2013	28.06749912
2014	24.24631214
2015	24.05520361
2016	24.82502572
2017	22.01765339
2018	22.29493219
2019	22.1120864