An
atmospheric condition resulting from the
mutual destruction of large quantities of airborne
nanotech robots.
In the novel The Diamond Age, author Neal Stephenson posits a future in which the world is divided into competing nation-tribes, or phyles. The most powerful of these phyles have extensive nanotechnology capabilities, and use nanotech in various ways to gain an edge over their competitors and neighbors. Most notably, they create various sorts of airborne nanotech robots -- known as mites -- to infiltrate one another's territory and perform espionage or sabotage. Naturally, the other phyles retaliate with antibody mites, which hunt out and disable the invading mites, turning them into inert black dust, or toner. And so it goes.
The phyles' populaces themselves rarely see this conflict in their own airspace. Rather, it plays itself out in the Leased Territories and no-man's-lands between phyle enclaves. The thetes (classless members of no phyle, who live in these no-man's-lands), on the other hand, intermittently find their air full of clashing mites, flashing lidar, and the resulting microscopic debris.