The weathering characteristic of diesel exhaust particulate (DEP) matter is spectroscopically obtained by an atmospheric study under realistic, ambient conditions. DEP subjected to weathering by exposure to ambient atmosphere (air, humidity, solar radiation, and temperature variation) for seven weeks shows systematic changes in C(1s) x-ray absorption spectra. Quantitative analysis suggests that volatile polycyclic aromatic hydrocarbons are released from the DEP by desorption, paralleled or followed by decomposition and reaction of the amorphous part of the solid, non-volatile carbon and potential other volatile species. The observed spectral changes are rationalized in a simple two phase model and parameterized, combining desorption of volatiles and oxidation of carbon matrix. The weathering characteristics of soot is generalized toward biomass burning products and identified as a vector quantity to be implemented for climate modelling.