Year	micrograms per cubic meter
1990	35.1484599
1995	36.88496227
2000	39.92874967
2005	36.4345178
2010	38.79164944
2011	41.56924447
2012	42.79371506
2013	42.01385372
2014	40.76668868
2015	43.15615417
2016	41.73858584
2017	38.52500218
2018	37.19901219
2019	35.21822646