Parkers are quoting the MY2009 figure for the 1.9JTDm 120 bhp 8 valve, which is now only available as the Eco version with a substantial reduction in the CO2 levels. (It is 140 g/km for the SW) . The 2008 version is a dirtier 157/159.
The 1.9 JTDm 150 has had the CO2 levels reduced for the 2009 version from 157 to 150 g/km for the saloon and from 159 to 153 for the SW. The 2008 saloon version remains 157 g/km
These figures are all from the Alfa Romeo 159 Drivers Manual and the August 2009 technical booklet.
The manual also gives figures for the 2009 2.4 Q4 as 192 g/km saloon and 194 g/km for the SW - some variation from Parkers.
The Qtronics for 2009 are as follows
1.9 saloon 180 g/km 1.9 SW 182 g/km
2.4 saloon 208 g/km 2.4 SW 210 g/km
The 2.0JTDm was not of course available in 2008 which is the year that the OP was looking at.
One thing that Parkers show up very graphically is how much the CO2 is affected by having Q4 or Qtronic.
To put all of these in perspective, the 3.2 JTS Q4 measures 270 g/km