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What Causes Observed Fog Trends: Air Quality or Climate Change?

Category: Aerosol and Atmospheric Chemistry

Volume: 16 | Issue: 5 | Pages: 1131-1142
DOI: 10.4209/aaqr.2015.05.0353
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Otto Klemm 1, Neng­Huei Lin2

  • 1 Universität Münster, Institut für Landschaftsökologie, Klimatologie, Heisenbergstr. 2, 48149 Münster, Germany
  • 2 Department of Atmospheric Sciences, National Central University, No. 300, Jhongda Rd., Jhongli 320, Taiwan

Highlights

Literature analysis shows a decrease of fog trends at hundreds of stations worldwide.
Fog may occur at sub-saturation conditions (humidity < 100%) in polluted air masses.
Both climate change and improvement of air quality may lead to a reduction of fog.
An increase of temperature by 0.1°C lead to the same decrease of fog as reduction of aerosols by 10%.


Abstract

Fog is a situation when the visual range, which is the horizontal visibility, is reduced to less than 1000 m near the Earth’s surface by the presence of cloud droplets. Fog trend analyses are reported in the literature for hundreds of stations worldwide, the majority of which showing a considerable reduction of fog. Although fog is often associated with conditions at which cloud condensation nuclei had been activated at rH (relative humidity) > 100% and rapid growth had lead to the formation of fog droplets, this study focusses on urban air masses and conditions when rH is just below 100%. Mie scattering analysis shows that fog can form under such conditions and the reduction of the visual range is mainly caused by submicron aerosol particles which grow to diameters around 1 µm through deliquescence. The liquid water content itself is poorly correlated with the visual range. Assuming equilibrium conditions, both an increase of the air temperature and a reduction of the aerosol particle concentration lead to reductions of fog. In our example case, the increment for a temperature increase by 0.1°C had about the same effect as the reduction of aerosol concentrations by 10%. Care must be taken in projecting this result to actual conditions because the system is non-linear. However, physical evidence is presented which confirms that both climate change and an improvement of air quality are mechanisms that can contribute to the reduction of fog.

Keywords

Fog trends Air quality Air pollution Visibility Visual range


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