A complex mixture of inorganic and organic substances comprising mainly soot (elemental carbon), nitrates, carbonates and a variety of non, or partially, combusted hydrocarbon components of the fuel and lubricating oil. The quantity of these emissions depends on the quality of engine maintenance with PM emissions increasing as the level of maintenance decreases. Wet cleaning of the exhaust can remove over 60 percent of particulates, but creates the problem of disposing of the acid water. See also Exhaust gas emissions from ships.