What type of harmful pollution is emitted in great amounts by engines that use leaded gasoline?

Engines that use leaded gasoline emit significant amounts of particulate matter. This form of pollution consists of tiny solid or liquid particles suspended in the air. When leaded gasoline combusts, it releases not only lead but also other particulate pollutants that can lead to health problems such as respiratory issues and cardiovascular diseases. Particulate matter can vary in size and composition, but in the case of leaded gasoline, the lead compounds contribute specifically to the overall particulate pollution.

The other options, like carbon monoxide, nitrogen oxides, and hydrocarbons, are also emissions from engines but are primarily associated with the combustion of fuels rather than specifically tied to the use of leaded gasoline. This makes particulate matter the correct answer for this question regarding the pollution type most notably generated by leaded gasoline engines.