there are multipe ways to convey dirt in the air. fog particle billboards or for godrays you can create trapezoid prism shapes or cone shapes and texture them with additive or modulate materials. light and dark. there are some cheats to get the soft edges. like vertex colored support edges or fresnel blended edges. you could also add local post postprocessing volumes to convey a lift or darkening inside of those “zones”. volume shadow maps can be baked too. that’s the low end tech.