Year	micrograms per cubic meter
1990	39.17526931272381
1995	38.99049538636297
2000	39.87137029547873
2005	40.18082390502743
2010	41.32554496855768
2011	44.2150128330242
2012	44.38374738378364
2013	47.16054400145001
2014	44.93047824867534
2015	42.19957210694204
2016	39.26319951205629
2017	38.05111144175268
2018	37.6926199880652
2019	36.7047435991034