Sable Island Station:

Fog Chemistry

(September 2003)

The fog chemistry program, which began on Sable Island in 2003, examines the transport and composition of atmospheric toxins carried in fog. 

Fogs are generally less than 600 m thick, and are formed in, and sweep, the ground layer of the atmosphere where gases and aerosol are most concentrated.  All atmospheric water droplets (in rain, cloud and fog) are formed when moisture condenses on particles in the air.  These "condensation nuclei" are aerosol (e.g. soot, dust, bug fragments etc) and are both soluble and nonsoluble.  Because fog droplets are approximately 100 times smaller than raindrops, they can be highly concentrated with respect to the chemicals (i.e. the condensation nuclei and soluble gases) they carry.  For example, the acidity in "acid-fog" can be far higher than in "acid-rain", and fog-derived inputs can add substantially to the burden of acid deposition caused by precipitation. 

Air-borne toxins - including mercury, heavy metals, organochlorines - occur in air masses moving across the Scotian Shelf region from continental North America.  Elemental mercury, for example, emitted from coal-fired utilities, can be carried in the atmosphere for thousands of kilometers.  Such toxins can be concentrated and carried in fog.  The fog chemistry monitoring program on Sable Island is conducted in association with the Meteorological Service of Canada.

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