  We present an investigation of chemical influences on the growth of ice
crystals from water vapor. In one set of experiments, we grew ice crystals in a
vapor diffusion chamber, observing crystal morphologies at temperatures from 0
C to -25 C in different background gases and with a variety of gaseous chemical
additives. In a second set of experiments, we measured ice crystal growth rates
at -5 C and -15 C in a free-fall flow chamber, using normal laboratory air and
ultra-clean nitrogen gas, both with and without chemical additives. Conclusions
from these experiments include:\ 1) In nitrogen gas at a pressure of one
atmosphere, no tested chemical additives at concentrations below 10 ppm
produced any observable changes in ice crystal growth morphologies; 2) Growth
in ultra-clean nitrogen gas was not significantly different from growth in
ordinary laboratory air; 3) Chemical additives affected plate-like dendritic
growth at -15 C more readily than growth at higher temperatures; 4) Chemical
additives tended to promote the growth of columnar crystals over plate-like
crystals; 5) Ice growth in air, nitrogen, helium, argon, hydrogen, and methane
gases at a pressure of one atmosphere yielded essentially the same
temperature-dependent crystal morphologies.
