Year	micrograms per cubic meter
1990	29.45036561
1995	28.26313943
2000	28.35304869
2005	23.55652816
2010	25.0816067
2011	25.94720366
2012	23.4265212
2013	24.2848162
2014	23.85881131
2015	22.92043409
2016	22.72386043
2017	22.0407325
2018	22.11926649
2019	22.01072747