Salt/seawater is a possibility. The proposed method is called marine cloud brightening.

https://en.wikipedia.org/wiki/Marine_cloud_brightening

Isn't Sulfur ok if it is in upper atmosphere, but in lower atmosphere causes acid rain???

So we could seed the upper atmosphere by shooting rockets into it (as in book Termination Shock), and thus bypass the layer where the sulfur was being added by pollution and causing acid rain.

calcium carbonate is often considered a safer alternative