Year	micrograms per cubic meter
1990	17.25015977
1995	15.39446635
2000	14.54206031
2005	13.09783363
2010	13.29038466
2011	13.56725749
2012	10.82992224
2013	10.4485287
2014	11.03144858
2015	10.16512436
2016	9.286433949
2017	10.28076623
2018	9.868317849
2019	9.785959755