Year	micrograms per cubic meter
1990	52.98078092
1995	48.26150047
2000	50.92544334
2005	50.85365062
2010	57.24593137
2011	53.98373198
2012	53.54775973
2013	51.54630059
2014	47.58535292
2015	53.09642572
2016	50.08978666
2017	50.02941919
2018	49.14829956
2019	48.46825216