I’ve dealt with this exact problem, and the key is to treat city grime like a mix of oily film, soot, and gritty dust. Start by moving the furniture to a spot where you can work—preferably on grass or a tarp, not bare concrete, so runoff doesn’t stain. If you can’t move it, protect nearby walls and plants. First, dust or rinse off loose soot. Don’t wipe dry dirt around, because city grime often contains grit that can scratch finishes. A hose on low pressure works; for delicate pieces, use a bucket and a soft cloth.
Mix a mild cleaning solution: a few drops of dish soap in a bucket of warm water. For oily pollution film, add a splash of white vinegar. For stubborn black soot or bird droppings, make a paste with baking soda and water, apply gently, let it sit for a few minutes, then rinse. Avoid bleach and harsh abrasives unless the manufacturer says it’s safe.
Wash from top to bottom with a soft-bristle brush or microfiber cloth. Pay attention to crevices, armrests, and under the seat, where pollution collects. Rinse well—soap residue actually attracts more dirt. If the furniture is metal, dry it immediately with a towel to prevent water spots and rust. For wood, use as little water as possible, then let it dry completely and re-oil or seal it. For plastic or resin, a vinegar-and-water spray is usually enough. For wicker or rattan, use a soft brush and avoid soaking; compressed air can help with deep grooves.
Cushions need separate care. Vacuum both sides, then spot-clean with mild detergent and water. If the covers are removable, check the label and wash them according to instructions. Let everything dry in the sun, but bring cushions inside before they get damp from evening air.
Once clean, add a layer of protection. A car wax or specialized outdoor furniture protectant can make future pollution easier to wipe away. For metal, use a rust-inhibiting spray on joints. For wood, a fresh coat of outdoor oil or sealant helps. Then try to cover furniture during long stretches of bad air or rain, and wipe it down every week or two with a damp cloth. In a polluted city, a quick rinse once a week is much easier than a deep clean once a season.