There are a number of additional gases and particles, resulting in part from human sources, that produce radiative forcing of the Earth’s climate but are not included under the Framework Convention or the Montreal Protocol. In general, these gases are short-lived, they have only indirect climate effects, or there is a fair amount of uncertainty about their climatic impacts. They can be broken down into three general classes:
Ozone, both tropospheric and stratospheric
Criteria pollutants that are indirect greenhouse gases
Aerosols, including sulphates and black soot
There are also a number of compounds (carbon monoxide, nitrogen oxides, and volatile organic compounds) that are indirect greenhouse gases. These gases are regulated in the US pursuant to the Clean Air Act, and they are often referred to as ‘criteria pollutants.’ They are emitted primarily as byproducts of combustion (both of fossil fuels and of biomass), and they influence climate indirectly through the formation of ozone and their effects on the lifetime of methane emissions in the atmosphere. Carbon monoxide, through its effects on hydroxyl radicals, can help promote the abundance of methane in the atmosphere, as well as increase ozone formation. Some IPCC model calculations indicate that 100 t of carbon monoxide emissions is equivalent to the emissions of about five t of methane.
Nitrogen oxides, including NO and NO2, influence the climate through their impacts on other greenhouse gases. NOx not only promote ozone formation, they also impact (negatively) methane and HFC concentrations in the atmosphere. The deposition of NOx could also reduce atmospheric CO2 concentrations by fertilising the biosphere.
One of the primary precursors of sulphates is SO2, which is emitted largely as a by-product from the combustion of sulphur-containing fossil fuels, particularly coal. SO2 reacts in the air to form sulphate compounds. The major source of anthropogenic black soot and organic carbon aerosols is the burning of fossil fuels, primarily coal and diesel fuels. Biomass-burning aerosols are formed by the incomplete combustion of forest products. The IPCC estimates the direct radiative forcing for aerosols as follows: sulphates, -0.4 watts per m2; black soot, +0.2 watts per m2; fossil fuel organic carbon, -0.1 watts per m2; and biomass-burning aerosols, -0.2 watts per m2. Although the indirect climate effects of aerosols are uncertain, some preliminary evidence points to an indirect cooling effect due to cloud formation.
From the NRG Expert Report The Global Coal Industry Historical, Present, and Future Generation Statistics to 2025.