Enhanced extinction of visible radiation due to hydrated aerosols in mist and fog

摘要

The study assesses the contribution of aerosols to the extinction of visibleradiation in the mist–fog–mist cycle. Relative humidity is large in themist–fog–mist cycle, and aerosols most efficient in interacting with visibleradiation are hydrated and compose the accumulation mode. Measurements ofthe microphysical and optical properties of these hydrated aerosols withdiameters larger than 0.4 μm were carried out near Paris, during November2011, under ambient conditions. Eleven mist–fog–mist cycles were observed,with a cumulated fog duration of 96 h, and a cumulated mist–fog–mist cycleduration of 240 h.In mist, aerosols grew by taking up water at relative humidities larger than93%, causing a visibility decrease below 5 km. While visibility decreased down from 5 to a few kilometres, the mean size of the hydrated aerosols increased, and theirnumber concentration (N_ha) increased from approximately 160 toapproximately 600 cm?3. When fog formed, droplets became the strongestcontributors to visible radiation extinction, and liquid water content(LWC) increased beyond 7 mg m?3. Hydrated aerosols of the accumulation modeco-existed with droplets, as interstitial non-activated aerosols. Their sizecontinued to increase, and some aerosols achieved diameters larger than 2.5 μm. The mean transition diameter between the aerosol accumulation modeand the small droplet mode was 4.0 ± 1.1 μm. N_ha alsoincreased on average by 60 % after fog formation. Consequently, the meancontribution to extinction in fog was 20 ± 15% from hydratedaerosols smaller than 2.5 μm and 6 ± 7% from larger aerosols.The standard deviation was large because of the large variability ofN_ha in fog, which could be smaller than in mist or 3 times larger.The particle extinction coefficient in fog can be computed as the sum of adroplet component and an aerosol component, which can be approximated by 3.5N_ha (N_ha in cm?3 and particle extinction coefficient inMm?1. We observed an influence of the main formation process onN_ha, but not on the contribution to fog extinction by aerosols. Indeed,in fogs formed by stratus lowering (STL), the mean N_ha was 360 ± 140 cm?3, close to the value observed in mist, while in fogs formed bynocturnal radiative cooling (RAD) under cloud-free sky, the mean N_ha was600 ± 350 cm?3. But because visibility (extinction) in fog wasalso lower (larger) in RAD than in STL fogs, the contribution by aerosols toextinction depended little on the fog formation process. Similarly, theproportion of hydrated aerosols over all aerosols (dry and hydrated) did notdepend on the fog formation process.Measurements showed that visibility in RAD fogs was smaller than in STL fogsdue to three factors: (1) LWC was larger in RAD than in STL fogs, (2) dropletswere smaller, (3) hydrated aerosols composing the accumulation mode were morenumerous.