Year	micrograms per cubic meter
1990	17.40916781
1995	16.37593902
2000	14.85608116
2005	13.33739937
2010	12.42896791
2011	12.99522944
2012	11.43181815
2013	11.42279633
2014	10.88658588
2015	10.37250919
2016	10.21445169
2017	10.35539774
2018	10.17141578
2019	10.09284402