Year	micrograms per cubic meter
1990	67.8473854
1995	64.2017057
2000	67.03447171
2005	69.70727002
2010	77.14093136
2011	78.11293037
2012	86.99389723
2013	86.3556379
2014	94.81583099
2015	88.68734566
2016	91.49270402
2017	80.62596922
2018	82.68815279
2019	83.10319363