Year	micrograms per cubic meter
1990	36.05044768
1995	38.34889454
2000	37.00236184
2005	31.30170584
2010	32.29140696
2011	41.43852028
2012	39.02130032
2013	39.4845918
2014	37.5033592
2015	35.48586783
2016	35.1167798
2017	35.06871307
2018	34.78486605
2019	34.78609557