Year	micrograms per cubic meter
1990	37.76866855
1995	38.06961746
2000	39.70649376
2005	38.35606228
2010	41.37853622
2011	42.55901964
2012	41.48378839
2013	41.23888443
2014	39.85297971
2015	40.89116772
2016	39.84131218
2017	38.9645715
2018	38.60762301
2019	38.01508315