Numerical estimation of the effects of condensation and coagulation on visibility using the moment method

An algorithm in which both aerosol dynamics and overall particle extinction coefficient are calculated is developed. The change of overall extinction coefficient and resultant visibility due to coagulation and condensation processes in the atmospheric aerosol is simulated for the aerosol size range of 0.005–2.5μm in diameter. The moment method is used under the assumption that aerosol retains log-normal size distributions with varying geometric mean diameters and geometric standard deviations. The single particle extinction efficiency is approximated with a six order polynomial for the nuclei and accumulation mode particles, respectively. Three refractive indices which represent typical ambient aerosol conditions, i.e., hazy, urban, and clean area (Garcia Níeto, 2002, Aerosol Science and Technology, 36, 814–827) are considered. Subsequently, overall particle extinction coefficient and visibility from Koschmieder (1925, Atmosphere, 12, 171–181) for polydisperse aerosol are obtained. The simulation results show that coagulation enhances the visibility and condensation can enhance or reduce the visibility depending on the particle size and refractive index of accumulation mode particles for the simulated particles. The degree of the change is the highest for the hazy condition.