Fresh lemon juice is about 5-6% citric acid, while concentrate is about 3.5-4%, which is good enough. But cleaning in pure lemon juice is expensive no matter where you get it. You can use a smaller quantity as an additive to detergents as a brightener, but dilution will lower its working speed by raising pH.
There are surfactants that can help remove carbon, but it's not easy to break down. The citric acid solution soaks into carbon and it helps the ultrasonic lift it from underneath by making small gas bubbles. But if you add a surfactant, like a few drops of Kodak Photo-Flo, it will penetrate and lift better. A drop of dishwashing liquid also helps wetting and suspends dirt and carbon. I expect the car washing detergents without wax that don't allow water spots to form as they dry would be a good way to get a combination of the two in one package.
5% citric acid plus a little Dawn:
Before:
After:
