year	pollutant_name	index
1990	NOx	100.0
1991	NOx	98.7
1992	NOx	99.8
1993	NOx	98.4
1994	NOx	95.1
1995	NOx	93.7
1996	NOx	93.9
1997	NOx	92.9
1998	NOx	92.6
1999	NOx	90.8
2000	NOx	86.5
2001	NOx	84.8
2002	NOx	82.0
2003	NOx	82.0
2004	NOx	82.7
2005	NOx	80.9
2006	NOx	81.3
2007	NOx	80.7
2008	NOx	76.8
2009	NOx	71.5
2010	NOx	68.9
2011	NOx	67.9
2012	NOx	64.2
2013	NOx	60.9
2014	NOx	59.9
2015	NOx	59.4