J. Rosinski and F. Parungo
National Center for Atmospheric Research, Boulder, Colorado
(Manuscript received 1 November 1965)
Silver iodide particles deposited on vegetation may photolyse [chemically decompose] and combine with natural terpenes from tree oils to form compounds which either themselves become aerosols, or become attached to aerosol particles.
In either case, the new compounds may become active centers acting as freezing nuclei.
Silver iodide particles may persist for several months when deposited on coniferous trees, and may release variable doses of such freezing nuclei during that time.
Although the concentrations of ice nuclei so produced are probably too small to influence precipitation, they may nevertheless contaminate large areas and thus may be significant for long-range research programs concerned with measuring natural concentrations of freezing nuclei.
Some of the silver iodide particles released into the atmosphere during a seeding program are deposited in the vicinity of the seeding sites.
Although these small particles have negligible settling velocity, they can be deposited on vegetation and on the ground by turbulence and Brownian motion.
They are also returned directly to earth by precipitation, either when they have served as freezing nuclei, or by scavenging from the atmosphere.
Rain may deposit some of this entrapped silver iodide on vegetation because the surface of the silver iodide particle is usually hydrophobic.
Silver iodide particles are also left on vegetation by sublimation or melting of snow.
These deposited particles evidently may return to meterological activity.
L.O. Grant (1963) inspected ice nuclei data from Clima and Berthoud Pass, Colorado, and Mount Washington, N.H. and noted that ice nuclei concentrations on non-seeded days during seeded intervals were substantially higher than during non-seeded intervals.
A similar increase in the background of freezing nuclei was found by K. Bigg (private communication) in Tasmania, after seeding with silver iodide.
This suggests a carry-over or residual effect from seeding with silver iodide.
Such an effect would have considerable bearing on the design of weather modification experiments, and on the planning, operation, and analysis of commercial seeding efforts.
The appearance of high freezing-nuclei concentrations after seeding with silver iodide cannot be explained by re-entrainment of particles from vegetation or ground (Rosinski and Ngamoto, 1965).
Even if this process were possible, silver iodide particles could not serve as freezing nuclei over prolonged periods because of their deactivation in time by ultraviolet radiation.
It was suggested, therefore, that during photolysis of silver iodide particles deposited on vegetation, some iodine compounds are formed with resinous material present on leaves, and that their vapors form particles by condensation or are attached to aerosol particles present in the air (Rosinski and Parungo, 1965).
These particles would subsequently serve as freezing nuclei.
Conclusions and comments
Silver iodide deposited on vegetation can increase the air population of freezing nuclei by combining with natural terpenes (tree oils) during photolysis.
Natural oils of pine and fir may form iodine compounds which condense to form aerosol particles directly.
In forested areas where pine or fir does not predominate, natural tree oils may form different terpene-iodine compounds, which do not condense to form aerosol particles directly, but which may attach preferentially to existing aerosol particles.
The attached compounds may become active centers acting as freezing nuclei.
Silver iodide may persist on vegetation, and especially on coniferous trees for several months, and during that time seeded and adjacent areas may receive variable doses of freezing nuclei.
Image Credits in order of appearance:
Walker River, Colorado- Silver Iodide Ground Based Seeding Generator: Nevada Cloud Seeding Program
Water droplets on blades of grass: wikipedia.com