Enhanced extinction of visible radiation due to hydrated aerosols in mist and fog
Atmospheric Chemistry and Physics Discussions
The study assesses the contribution of aerosols to the extinction of visible radiation in the mist-fog-mist cycle. Measurements of the microphysical and optical properties of hydrated aerosols with diameters larger than 400 nm, composing the accumulation mode, which are the most efficient to interact with visible radiation, were carried out near Paris, during November 2011, in ambient conditions. Eleven mist-fog-mist cycles were observed, with cumulated fog duration of 95 h, and cumulated
... and cumulated mist-fog-mist duration of 240 h. <br><br> In mist, aerosols grew up by taking up water at relative humidities larger than 93%, causing a visibility decrease below 5 km. While visibility decreased down to few km, the mean size of the hydrated aerosols increased, and their number concentration (<i>N</i><sub>ha</sub>) increased from approximately 160 to approximately 600 cm<sup>−3</sup>. When fog formed, droplets became the strongest contributors to visible radiation extinction, and liquid water content (LWC) increased beyond 7 mg m<sup>−3</sup>. Hydrated aerosols of the accumulation mode co-existed with droplets, as interstitial non-activated aerosols. Their size continued to increase, and a significant proportion of aerosols achieved diameters larger than 2.5 μm. The mean transition diameter between the accumulation mode and the small droplet mode was 4.0 ± 1.1 μm. Moreover <i>N</i><sub>ha</sub> increased on average by 60% after fog formation. Consequently the mean aerosol contribution to extinction in fog was 20 ± 15% for diameter smaller than 2.5 μm and 6 ± 7% beyond. The standard deviation is large because of the large variability of <i>N</i><sub>ha</sub> in fog, which could be smaller than in mist or three times larger. <br><br> The particle extinction coefficient in fog can be computed as the sum of a droplet component and an aerosol component, which can be approximated by 3.5 <i>N</i><sub>ha</sub> (<i>N</i><sub>ha</sub> in cm<sup>−3</sup> and particle extinction coefficient in Mm<sup>−1</sup>). We observed an influence of the main formation process on <i>N</i><sub>ha</sub>, but not on the contribution to fog extinction by aerosols. Indeed in fogs formed by stratus lowering (STL), the mean <i>N</i><sub>ha</sub> was 360 ± 140 cm<sup>−3</sup>, close to the value observed in mist, while in fogs formed by nocturnal radiative cooling under cloud-free sky (RAD), the mean <i>N</i><sub>ha</sub> was 600 ± 350 cm<sup>−3</sup>. But because visibility (extinction) in fog was also lower (larger) in RAD than in STL fogs, the contribution by aerosols to extinction depended little on the fog formation process. Similarly, the proportion of hydrated aerosols over all aerosols (dry and hydrated) did not depend on the fog formation process. <br><br> Measurements show that visibility in RAD fogs was smaller than in STL fogs because: (1) LWC was larger in RAD than in STL fogs, (2) droplets were smaller, (3) as already said, hydrated aerosols composing the accumulation mode were more numerous.