Chances are you are running a bit lean -- this means higher combustion temps and hence a little more NOx. I'd say you needed to clean the EGR, but you (thankfully) don't have one.
Check for vac leaks -- at this age, ALL the rubber hoses on the engine are suspect, plan on replacing all of them, including the crankcase ventilation restriction. Don't overlook the idle control valve hoses.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!