Year	micrograms per cubic meter
1990	40.238707618855294
1995	37.81453522049992
2000	41.29526923521434
2005	39.45704665329121
2010	40.169179642403
2011	41.646259543406764
2012	42.54228032855977
2013	40.245545187476395
2014	39.447625238084996
2015	44.306160008978054
2016	42.48193975559694
2017	40.912047636944166
2018	40.91945011616812
2019	40.673469658457286