Aerosols are a suspension of solid or liquid particles in a gas. They can influence the climate directly through scattering or absorption of solar radiation, or indirectly through their ability to act as cloud condensation nuclei. Tropospheric aerosols are comprised from an organic as well as inorganic phase with the organic phase being a major constituent. Organic aerosol composition is affected by both chemical and physical atmospheric processes in the gas and particle phase. Gas to particle partitioning is dependent on species volatility.