Year	micrograms per cubic meter
1990	27.59605203
1995	28.62897455
2000	32.24000904
2005	32.57264648
2010	32.91817527
2011	31.56370096
2012	30.42396929
2013	29.38557188
2014	29.40059385
2015	32.08758908
2016	30.88602135
2017	34.10665652
2018	33.92957286
2019	35.05284088