Year	micrograms per cubic meter
1990	43.78464053773118
1995	45.87853398933528
2000	46.41971507297696
2005	45.93232297512862
2010	49.931601924807524
2011	47.338381543094336
2012	47.371146327101286
2013	48.244364748557935
2014	46.4429235593264
2015	48.40197161187743
2016	45.62863429220092
2017	45.772241299471744
2018	45.94033948118196
2019	45.60940217220565