Black carbon is always co - emitted with each other particles and gases, some of which have a cooling effect on the climate .

Solid pollutants are particulate matter or particles that are suspended in the air. They can be emitted directly into the atmosphere or formed through chemical reactions. These particles can have a significant impact on air quality and human health.

Carbon, carbon dioxide, sulphur dioxide, and carbon monoxide are all air pollutants, but only carbon is a solid pollutant.

Carbon as a solid pollutant:
- Carbon is a solid element that exists in various forms, including graphite and diamond.
- It can be emitted into the atmosphere as particulate matter, commonly referred to as black carbon or soot.
- Sources of carbon emissions include the combustion of fossil fuels, biomass burning, and industrial processes.
- Carbon particles are very small and can remain suspended in the air for long periods, contributing to air pollution.
- These particles can have detrimental effects on human health when inhaled, as they can penetrate deep into the respiratory system and cause respiratory and cardiovascular problems.
- Carbon particles also have an impact on climate change, as they can absorb and scatter sunlight, leading to warming of the atmosphere.
- Efforts to reduce carbon emissions and control solid carbon pollutants include implementing stricter emission standards, promoting renewable energy sources, and improving industrial processes.

Other air pollutants:
- Carbon dioxide (CO2) is a greenhouse gas and is primarily emitted through the combustion of fossil fuels. It is a gas at room temperature and not considered a solid pollutant.
- Sulphur dioxide (SO2) is a gas that is emitted from burning fossil fuels that contain sulphur, such as coal and oil. It can react with other compounds in the atmosphere to form solid particles, but as a gas, it is not considered a solid pollutant.
- Carbon monoxide (CO) is a gas that is produced by incomplete combustion of carbon-containing fuels. It is not considered a solid pollutant.

In conclusion, carbon is the only option listed that is a solid pollutant. Carbon dioxide, sulphur dioxide, and carbon monoxide are gases, although they can contribute to air pollution in various ways.