Aerosol charging in the 6-15 nm size range by photoionization and diffusion mechanisms was studied numerically and experimentally. By comparing the charging rate of both mechanisms, their relative importance was determined as a function of nanoparticle size and charge. A photoionization expression derived from the Fowler-Nordheim equation was used and found to be applicable not only for metal particles (silver), but also for organic particles (sucrose). Photoemission yield enhancement from small nanoparticles was validated and the size dependent photoemission constant in the Fowler-Nordheim equation was determined by comparing numerical results to experimental data. For both silver and sucrose nanoparticles, the photoemission constant increased by a factor of 2 as the particle diameter decreased from 15 to 6 nm, consistent with theoretical results reported in the literature. The enhancement of the photoemission constant with decreasing particle size increases the relative importance of photocharging compared to diffusion charging. Using these size dependent photoemission constant values, the charging dynamics and capture of silver and sucrose nanoparticles by both photoionization and diffusion charging were examined numerically. Numerical results were in excellent agreement with the experimental data, and the charging and capture efficiency of both silver and sucrose in 6-15 nm size range decreased with decreasing particle size.