Year	micrograms per cubic meter
1990	30.06183701
1995	27.40849476
2000	30.87949696
2005	31.47408068
2010	34.02327199
2011	34.91967839
2012	35.57819849
2013	35.36549106
2014	32.99059157
2015	34.42531071
2016	33.61502933
2017	33.94832602
2018	33.85373372
2019	33.7807199