Mechanical properties of size-dependent nanowires are important in nano-electro-mechanical systems (NEMS), and have attracted much research interest. It is desperate to characterize the size effect of nanowires at atmosphere directly to broaden its practical application instead of in high vacuum situation as reported previously. In this study, we systematically studied the Young's modulus of vertical ZnO nanowires at atmosphere. The diameters range from 48nm to 287 nm, with resonance method by non-contact atomic force microscopy (AFM). The values of Young's modulus at atmosphere present extremely strong increase tendency with the diameter decrease of nanowire by stronger surface atomic bonds, comparing with that in vacuum. A core-shell model for nanowires is proposed to explore the Young's modulus enhancement at atmosphere, which is correlated with atoms of oxygen occurred near nanowire surface. The new model is more accurate to analyze the mechanical behavior of nanowires at atmosphere comparing with the model in vacuum. Furthermore, it is possible to use this characterization method to measure the size-related elastic properties of similar wire-sharp nanomaterials at atmosphere and estimate the corresponding mechanical behavior. The study of size-dependent Young's modulus in ZnO nanowires at atmosphere will improve the understanding of mechanical properties of nanomaterials as well as providing guidance for applications in NEMS, nanogenerator, biosensor and other related areas.