Organic aerosol alters cloud and fog properties through surface tension and solubility effects. This study characterizes the role of organic compounds in affecting fog droplet number concentration by initializing and comparing detailed particle microphysical simulations with two field campaigns in the Po Valley. The size distribution and chemical composition of aerosol were based on the measurements made in the Po Valley Fog Experiments in 1989 and 1998--1999. Two types of aerosol with different hygroscopicity were considered: the less hygroscopic particles, composed mainly of organic compounds, and the more hygroscopic particles, composed mainly of inorganic salts. The organic fraction of aerosol mass was explicitly modeled as a mixture of seven soluble compounds <Fuzzi et al., 2001> by employing a functional group-based thermodynamic model <Ming and Russell, 2002>. Condensable gases in the vapor phase included nitric acid, sulfuric acid, and ammonia. The maximum supersaturation in the simulation is 0.030% and is comparable to the calculation by Noone et al. <1992> inferred from measured residual particle fractions. The minimum activation diameters of the less and more hygroscopic particles are 0.49 ¿m and 0.40 ¿m, respectively. The predicted residual particle fractions are in agreement with measurements. The organic components of aerosol account for 34% of the droplet residual particle mass and change the average droplet number concentration by -10--6%, depending on the lowering of droplet surface tension and the interactions among dissolving ions. The hygroscopic growth of particles due to the presence of water-soluble organic compounds enhances the condensation of nitric acid and ammonia due to the increased surface area, resulting in a 9% increase in the average droplet number concentration. Assuming ideal behavior of aqueous solutions of water-soluble organic compounds overestimates the hygroscopic growth of particles and increases droplet numbers by 6%. The results are sensitive to microphysical processes such as condensation of soluble gases, which increases the average droplet number concentration by 26%. Wet deposition plays an important role in controlling liquid water content in this shallow fog.