Extinction of light during the fog life cycle: A result from the Parisfog experiment

Data set acquired by five particle-dedicated instalments set up on the SIRTA experimental site during the ParisFog field campaign are exploited to document microphysical properties of particles contributing to extinction of visible radiation in variable situations. The case study is a 48-hour period when atmospheric conditions are highly variable: relative humidity changes between 50 and 100%, visibility ranges between 35000 and 65 m, the site is either downwind Paris area either under maritime influence. A dense and homogeneous fog formed by radiative cooling during the 18-19 February night. In 7 hours, visibility decreases from 26 000 m to 65 m, because of transported pollution (factor3 in visibility reduction), aerosol hydration (factor 20) and aerosol activation (factor 6). According to Mie theory, extinction in clear-sky polluted and unpolluted regimes is due equally to Aitken and accumulation modes. Extinction in haze is due to hydrated aerosols distributed, in the accumulation mode, for diameter smaller than 2 μm. Hydrated. aerosols of the accumulation mode still contribute to 20-30% extinction in the fog. Measurements show that fog droplets, with diameter included between 2 and 10 |im, contribute to 40% extinction during the first hours of the fog.