Biomass treatment by atmospheric pressure plasmas to enable the production of bioethanol is a big issue today. Discharges favor the degradation of lignin because it creates efficiently reactive species that interact strongly with the biomass. A surface wave discharge (SWD) in argon at atmospheric-pressure generated by a Surfatron device was studied by optical emission and mass spectrometries. The electron density ne, electron temperature Te and gas temperature Tg was determined as a function of applied power and gas flow rate. Te was estimated using a collisional-radiative model while ne was determined through the Stark broadening of Hβ line profile. The self-absorbing method was used to estimate the population of the metastable state Ar(1s5). Gas temperature was obtained from rotational spectra of OH. The profile of ne presented a maximum value under certain conditions, in contrast with typical ne profiles of SWDs, which are usually monotonically decreasing. A correlation between the ne and the metastable state Ar(1s5) was found in one of these cases, suggesting that stepwise ionization from metastable states play an important role. Employing mass-resolved ion-energy distribution measurements, it was possible to study the behavior of fourteen ionic species (Ar+, Ar2 +, ArH+, H2O+, H+(H2O), H+(H2O)2, O+, O2+, OH+, NO+, NO2+, N+, N2+, N4+) of the plasma in diffuse and contracted modes. The crucial role of three molecular ions (Ar2+, ArH+, H2O+) during the radial contraction and the relationship between power, gas flow and molecular ions production was pointed out. Modeling effort was done with the development of a self-consistent model of an SWD in Ar at atmospheric pressure. The model is based on the solution of the homogeneous electron Boltzmann equation, considering inelastic and superelastic collisions with the Ar(4si) states and electron-electron collisions, coupled with a system of rate balance equations describing the creation and destruction of the most important heavy particles, namely ground-state Ar(1S0) atoms, the four Ar(4si) states, Ar(4pj) states, treated here as a single effective level, the molecular dimmer Ar2*, as well as Ar+ and Ar2+ ions. The non-equilibrium plasma produced was used to treat a natural fiber (sugarcane bagasse) prior to hydrolysis in order to assist the bioethanol production. Natural fibers are usually made of cellulose, hemicellulose, lignin and pectin with a small quantity of watersoluble materials. Pellets of sugarcane bagasse subjected to plasma treatment were analyzed by Diffuse Reflectance Infrared - Fourier Transform Spectrometry. Results confirm that plasma promotes drastic attacks of lignin bonds leading to degradation of the lignin macromolecule.