Year	micrograms per cubic meter
1990	33.66732041
1995	30.76381966
2000	34.15202995
2005	34.46843181
2010	30.47158597
2011	32.12056277
2012	29.38329504
2013	30.76797737
2014	28.00675623
2015	36.14480676
2016	34.73052829
2017	29.99540715
2018	31.12162978
2019	31.05885758