Year	micrograms per cubic meter
1990	23.84569363
1995	23.65460191
2000	24.03724388
2005	24.43023197
2010	26.14248485
2011	26.54697541
2012	26.62372992
2013	25.67039823
2014	25.16656494
2015	26.15619973
2016	25.20036677
2017	24.88065732
2018	24.91056457
2019	24.71873626