Year	micrograms per cubic meter
1990	34.45562826
1995	34.74639363
2000	36.18281228
2005	33.64156521
2010	33.17087207
2011	35.29849847
2012	37.10546174
2013	35.06835865
2014	34.92429065
2015	35.79366221
2016	35.48687315
2017	32.8625076
2018	33.76642862
2019	33.27977863