Year	micrograms per cubic meter
1990	14.63356471
1995	14.47943507
2000	14.59147229
2005	14.56811401
2010	14.0825006
2011	14.6948008
2012	13.78803347
2013	13.15581696
2014	13.00059985
2015	11.69149461
2016	11.42284509
2017	11.66527298
2018	11.62699706
2019	11.65399121