Year	micrograms per cubic meter
1990	43.998054411800254
1995	45.81837422019058
2000	46.844842996318334
2005	46.64263334829699
2010	50.467763493132594
2011	48.495551843284396
2012	48.78800283482122
2013	49.228876761449534
2014	46.7933214692633
2015	49.56535059397257
2016	46.410067986428224
2017	46.95441384075118
2018	46.93174153504668
2019	46.60359077741153