Year	micrograms per cubic meter
1990	16.49244468
1995	16.58836145
2000	16.00051716
2005	15.31761524
2010	17.53479836
2011	16.1746136
2012	14.08691429
2013	13.32212417
2014	13.75861976
2015	13.4950264
2016	12.8402408
2017	12.50133169
2018	13.0359469
2019	12.9910498