Year	micrograms per cubic meter
1990	71.4997561
1995	65.62043097
2000	70.47513018
2005	70.80607699
2010	76.67834222
2011	79.99719051
2012	86.38168408
2013	89.45226202
2014	95.24264397
2015	88.44512429
2016	92.23194665
2017	81.81164804
2018	83.04086375
2019	83.19931634