Year	micrograms per cubic meter
1990	44.9983915
1995	41.77925302
2000	44.91584852
2005	42.13880224
2010	43.29440019
2011	43.39379676
2012	44.54206936
2013	44.70930865
2014	39.15059178
2015	43.06823697
2016	41.32723515
2017	43.85000214
2018	43.4782267
2019	44.07053957