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Four things I would consider.
PCV valve partially clogged, easy to clean it, it sticks out of the valve cover with a hose attached. Drivers side I think. Pull it out and clean it thoroughly. Easy job, takes a few minutes. mcrews had an excellent post on how to check crankcase ventilation, put a balloon over the dipstick tube and see if the crankcase pressure inflates the balloon fairly rapidly. If so suspect PCV issues.
EGR passageways partially plugged, not enough to activate a Check engine light but enough to increase emissions.
Carbon in engine.
Cat partially contaminated due to frequent starts and shut downs.
Basically the frequency of starts and shut downs is just about the same as the worst case scenario as far as short haul short distance types of driving considered severe operating conditions in the owners manual.
If your car has an EGR system then the frequent on-off cycles mean a much greater potential for EGR passageways getting clogged with carbon build up. Low EGR flow is a principal cause of high NO emissions and will cause the timing to be retarded which will contribute to incomplete combustion.
High crankcase pressures (PCV partially plugged) will cause unburned fuel and crankcase vapors to be pushed up past the rings into the combustion chamber where they could contribute to incomplete combustion.
If it has an EGR system then I would look at the tube from the exhaust manifold to the EGR valve itself. If it is clogged, even partially then clean the tube and the EGR valve.
To clean the carbon out of the engine and the Cat, I would try an additive like techron or seafoam in the fuel and (I know you won't like this) take it out and run the crap out of it for a long time, something like 100 miles. It will kill your mileage, but might wake up the Cat and blow the carbon out of the pistons and cylinder head. Full bore acceleration runs whenever possible, wind her arse out.
If it is not to costly then have the emissions checked again to see what that has done.
regards
Mech
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