Diagnosis
This is caused by excessive airflow over a shallow bed of charcoal. The ash drawer is open too far, creating a high-velocity draft that picks up fine ash particles.
How to Fix It
For low and slow cooking (225-275°F), use the following vent protocol. Bottom Ash Drawer: open only 1/2 inch to 1 inch. Top Kontrol Tower: set to the first or second open line. This reduces intake velocity while maintaining sufficient draft for temperature. Also, ensure your firebox is at least 3/4 full with lump charcoal to provide a deep, consistent coal bed.
If That Didn't Work
Your charcoal may be producing excess fines. Switch to a different brand of lump charcoal. Some brands are notoriously dusty. Also, avoid aggressively stirring or poking the coal bed during a cook, as this creates new ash.
Heads Up
A small amount of ash particulate is normal, especially in windy conditions. If it is excessive, the above fixes are required. Do not try to block the bottom vent opening, as this will smother the fire and cause temperature spikes later.