  In this work we perform computer simulations of the ice surface in order to
elucidate the role of nitrogen in the crystal growth rates and crystal habits
of snow in the atmosphere. In pure water vapor at temperatures typical of ice
crystal formation in cirrus clouds, we find that basal and primary prismatic
facets exhibit a layer of premelted ice, with thickness in the subnanometer
range. For partial pressures of 1 bar, well above the expected values in the
troposphere, we find that only small amounts of nitrogen are adsorbed. The
adsorption takes place onto the premelted surface, and hardly any nitrogen
dissolves within the premelting film. The premelting film thickness does not
change either. We quantify the resulting change of the ice/vapor surface
tension to be in the hundredth of mN/m and find that the structure of the
pristine ice surface is not changed in a significant manner. We perform a
trajectory analysis of colliding water molecules, and find that the attachment
rates from direct ballistic collision are very close to unity irrespective of
the nitrogen pressure. Nitrogen is however at sufficient density to deflect a
fraction of trajectories with smaller distance than the mean free path. Our
results show explicitly that the reported differences in growth rates measured
in pure water vapor and a controlled nitrogen atmosphere are not related to a
significant disruption of the ice surface due to nitrogen adsorption. On the
contrary, we show clearly from our trajectory analysis that nitrogen slows down
the crystal growth rates due to collisions between water molecules with bulk
nitrogen gas. This clarifies the long standing controversy of the role of inert
gases on crystal growth rates and demonstrates their influence is solely
related to the diffusion limited flow of water vapor across the gas phase.
