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

Klemm O, Lin N-H

Zusammenfassung

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