  The ice phase in clouds contributes largely to uncertainties in global
climate models partly due to a lack of atmospheric observations. At moderate
supercooling ice nucleating particles (INP) and ice particles (IP) are present
in small concentrations and a large volume of air is necessary for observation.
Here, we report on initial observations of IP in supercooled fog with a new
setup. We use a 0.3 m wide, vertical curtain of light and a camera pointing
perpendicularly at it to record light scattered by IP formed in radiation fog
near the ground at temperatures between -3 {\deg}C and -9 {\deg}C. Deposition
rates of IP were several times larger than expected from number concentrations
of INP found on $PM_{10}$-filters at a nearby air quality monitoring station.
The discrepancy might be explained by secondary ice formation through the
fragmentation of freezing water droplets, the entrainment of INP from above the
fog layer through settling, the loss or deactivation of INP on
$PM_{10}$-filters prior to analysis, or radiative cooling of INP below the
temperature of the surrounding air. In summary, radiation fog constitutes an
easily accessible form of a supercooled cloud, in which observations can be
made for long enough to quantify the deposition rate of rare IP produced in a
natural environment.
