"Content" indicates a quantity per unit area. The "atmosphere content" of a quantity refers to the vertical integral from the surface to the top of the atmosphere. For the content between specified levels in the atmosphere, standard names including content_of_atmosphere_layer are used. Aromatic compounds in organic chemistry are compounds that contain at least one benzene ring of six carbon atoms joined by alternating single and double covalent bonds. The simplest aromatic compound is benzene itself. In standard names "aromatic_compounds" is the term used to describe the group of aromatic chemical species that are represented within a given model. The list of individual species that are included in a quantity having a group chemical standard name can vary between models. Where possible, the data variable should be accompanied by a complete description of the species represented, for example, by using a comment attribute. Standard names exist for some individual aromatic species, e.g. benzene and xylene.