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

Abstract

Abstract. The study assesses the contribution of aerosols to the extinction of visible radiation in the mist-fog-mist cycle. Relative humidity is large in the mist-fog-mist cycle, and aerosols most efficient in interacting with visible radiation are hydrated and compose the accumulation mode. Measurements of the microphysical and optical properties of these hydrated aerosols with diameters larger than 0.4 µm were carried out near Paris, during November 2011, under ambient conditions. Eleven mist-fog-mist cycles were observed, with a cumulated fog duration of 96 h, and a cumulated mist-fogmist cycle duration of 240 h.In mist, aerosols grew by taking up water at relative humidities larger than 93 %, 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 their number concentration (N ha ) increased from approximately 160 to approximately 600 cm −3 . When fog formed, droplets became the strongest contributors to visible radiation extinction, and liquid water content (LWC) increased beyond 7 mg m −3 . Hydrated aerosols of the accumulation mode co-existed with droplets, as interstitial non-activated aerosols. Their size continued to increase, and some aerosols achieved diameters larger than 2.5 µm. The mean transition diameter between the aerosol accumulation mode and the small droplet mode was 4.0 ± 1.1 µm. N ha also increased on average by 60 % after fog formation. Consequently, the mean contribution to extinction in fog was 20 ± 15 % from hydrated aerosols smaller than 2.5 µm and 6 ± 7 % from larger aerosols. The standard deviation was large because of the large variability of N 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 a droplet component and an aerosol component, which can be approximated by 3.5 N ha (N ha in cm −3 and particle extinction coefficient in Mm −1 ). We observed an influence of the main formation process on N 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 by nocturnal radiative cooling (RAD) under cloud-free sky, the mean N ha was 600 ± 350 cm −3 . 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.Measurements showed that visibility in RAD fogs was smaller than in STL fogs due to three factors: (1) LWC was larger in RAD than in STL fogs, (2) droplets were smaller, (3) hydrated aerosols composing the accumulation mode were more numerous.

Experimental study of the aerosol impact on fog microphysics

“…During the winters of 2010 to 2013, specific instrumentation were deployed for the PreViBOSS project (Elias et al, 2012) to provide continuous observation of aerosol and fog microphysics. The experimental setup was already presented in Burnet et al (2012), Hammer et al (2014), Elias et al (2015) and Dupont et al (2016). The instruments used in this study are listed in Table 1.…”

“…The WELAS-2000 (hereafter referred to as WELAS) provides particle size spectrum between 0.4 and 40 µm in diameter according to the constructor. However, the detection efficiency decreases drastically below ∼ 1 µm (Heim et al, 2008;Elias et al, 2015), resulting in a strong underestimation of the concentration of the submicronic particles. Hammer et al (2014) choose to consider only data with diameters larger than 1.4 µm.…”

“…Currently, their definition mainly deals with the instrumentation limitation that generally begins at 2 µm in diameter to measure droplet size distribution. However, Hammer et al (2014) and Elias et al (2015) experimentally determined threshold diameter higher than 2 µm to characterize droplets. As the smallest hydrated particles are more numerous, considering a smallest threshold diameter could lead to a significant overestimation of the droplet concentration.…”

“…Burnet et al (2012) have shown that, in terms of droplet number concentration and effective diameter, the microphysical properties of fog events sampled at SIRTA present a large variability. Elias et al (2015) examined the data collected during November 2011 and assessed the contribution of hydrated aerosols that led to the extinction of the visible radiation in the mist-fog-mist cycle. Hammer et al (2014) focused on the activation properties of developed fog events observed during the winter of 2012/2013, but they did not provide activated particle concentrations values.…”

Aerosol-fog interaction and the transition to well-mixed radiation fog

“…We note here that we have excluded the lowest (1-2 µm) bin of the cloud-droplet probe from our analysis to avoid potential noise issues with large, dry aerosol affecting the counts. Because of the low supersaturation and therefore high activation diameter, hydrated aerosol particles within fog can grow larger than micrometer in size and can contribute up to 68% of total light-scattering during different fog periods (Hammer et al, 2014;Elias et al, 2015). However, most of these hydrated particles are still too small to considerably affect interaction with long-10 wave radiation after fog has developed.…”