The pathways of other atmospheric constituents are also affected. Principal among these effects for NO x emissions is the reduction in the atmospheric lifetime and concentration of methane, another greenhouse gas. The effects on ultraviolet fluxes at the Earth s surface are discussed in Chapter 5 . A similarly complex system of atmospheric processes and effects exists for particles. There are many types of particles, each with its own complex physics and chemistry.
Aerosols resulting from human activities include sulfate aerosols and soot from fossil fuel burning.
Carbonaceous aerosols are produced from biomass burning and fossil fuel burning. These particles are capable of seeding contrails and cirrus clouds, thus potentially changing the total cloud cover in the upper troposphere. The climate impact of clouds is a balance of their capabilities to reflect sunlight back to space and to trap outgoing infrared radiation from the Earth s surface. For high clouds, the latter effect is larger, and increased cirrus coverage would result in a warming tendency. Particles are also involved in the chemical balance of the atmosphere.