Year	micrograms per cubic meter
1990	10.63552833
1995	10.76816606
2000	10.90395976
2005	10.91209119
2010	10.89494195
2011	10.6046872
2012	10.65130025
2013	10.65422797
2014	10.6865204
2015	10.40546856
2016	10.7168388
2017	10.723832
2018	10.60572192
2019	10.58622123