Year	micrograms per cubic meter
1990	30.06889972
1995	32.11741075
2000	35.37684627
2005	34.00185152
2010	31.94215239
2011	31.47118632
2012	32.49110136
2013	30.3495654
2014	31.87523927
2015	33.44000481
2016	32.79009631
2017	32.55274407
2018	32.66295035
2019	32.83308539