Year	micrograms per cubic meter
1990	40.206013951022676
1995	39.87078902631341
2000	40.51928093171212
2005	40.710127451979865
2010	41.489320751329565
2011	43.77796993312069
2012	44.40460788232962
2013	47.34698640811431
2014	44.89793608731698
2015	42.37772102613125
2016	39.80224614524547
2017	38.094499209369616
2018	37.81071810509353
2019	36.838011199491085