The diffusiophoresis of a spherical colloidal particle normal to an air-water interface subject to a uniform electrolyte concentration gradient is investigated theoretically. The governing electrokinetic equations are solved numerically with a pseudo-spectral method based on Chebyshev polynomials. Key parameters such as the distance between the particle and the interface, the double-layer thickness, and the surface potential of the particle are examined to analyze their respective effect on the diffusiophoretic velocity of the particle. Distinctive features pertinent to an air-water interface are investigated in particular as compared with a solid metal surface. It is found in this study, among other things, that the diffusiophoretic velocity of a particle moving toward an air-water interface is always less than that toward a planar metal surface when the diffusivities of cations and anions are identical. This can be explained nicely by the classic image-charge analogue in electrostatics, where the former (air-water interface) stands for an image-charge of the same sign, while the latter (planar metal surface) stands for an image-charge of the opposite sign. In the case of distinct diffusivities of cations and anions, the situation is much more complicated. No such simple and convenient analogue is observed.