Year	micrograms per cubic meter
1990	33.34687337
1995	32.25544382
2000	27.85155025
2005	26.83820327
2010	27.29182037
2011	27.07180922
2012	24.60328947
2013	23.69389027
2014	23.91940281
2015	22.76911952
2016	21.81786779
2017	22.779038
2018	22.74705532
2019	22.6420051