Explain why sour substances are effective in cleaning Copper Vessels?

Copper metal slowly reacts with water, carbon dioxide, and oxygen present in the air to form basic copper carbonate which is green in colour. Its layer slowly gets deposited on the surface of the metal.                                                               

As lemon is acidic in nature, hence when we rub the tarnished copper vessel with lemon, the basic copper oxide or copper carbonate reacts with the acid present in lemon to form a salt which is washed away with water.