Diesel smoking is usually caused by an incomplete burn of the air/fuel mixture.
It could be caused by lower than normal engine idle speed, dirty injectors not atomizing the fuel during injection, carbon buildup on the tops of the pistons, incorrect injector timing, low fuel pressure, or too long of an injector pulse.
It's hard to say which one it is. About the only one YOU can check is the idle speed when the A/C turns on.
Does the engine seem to drop a little and "lug".
There should be an idle adjust screw for the throttle plate that is electrically activated when the A/C is turned on and it should be adjustable with a small screw that opens the throttle a little bit.