Year	micrograms per cubic meter
1990	20.96419496
1995	20.17513191
2000	14.10382753
2005	13.30959933
2010	13.74643253
2011	12.28813159
2012	12.8234432
2013	12.45355352
2014	11.10396316
2015	10.19679349
2016	12.10828489
2017	11.16442084
2018	10.89985209
2019	10.94405416