Year	micrograms per cubic meter
1990	30.04312773
1995	33.01822468
2000	31.57183418
2005	32.92590812
2010	36.06128484
2011	33.87448361
2012	32.78220108
2013	35.79313049
2014	34.50384906
2015	35.3216485
2016	31.47268397
2017	32.14535358
2018	31.89103296
2019	31.30254529