Year	micrograms per cubic meter
1990	55.01450539
1995	50.50935562
2000	53.0035338
2005	50.87115318
2010	51.07018507
2011	51.94689259
2012	52.85547204
2013	45.97257249
2014	48.19905641
2015	56.21359737
2016	53.58936352
2017	53.48751596
2018	52.44233437
2019	52.51253174