"tendency_of_X" means derivative of X with respect to time. The construction "atmosphere_moles_of_X" means the total number of moles of X in the entire atmosphere, i.e. summed over the atmospheric column and over the entire globe. The chemical formula for propene is C3H6. Propene is a member of the group of hydrocarbons known as alkenes. There are standard names for the alkene group as well as for some of the individual species.