FTIR studies of the nucleation and composition properties of terrestrial and Martian clouds

For application to the remote sensing of cirrus and polar stratospheric clouds, the optical constants of ice in the near infrared (7000–4000 cm−1) at tropospheric temperatures were calculated from FTIR transmission spectra. At each temperature, a series of ice films ranging from 200 to 1100 μm was condensed. The optical constants were calculated using an iterative Kramers-Kronig program based on the optical design of our chamber.; Secondly, during the 2000 SOLVE Arctic campaign, the composition of polar stratospheric clouds (PSCs) was probed by solar IR extinction, a direct application of laboratory determined optical constants. High resolution FTIR occultation spectra from aboard the DC-8 were analyzed to identify the stratospheric aerosol. Absorption spectra were produced by ratioing spectra recorded on days in which PSCs were present to spectra recorded on clear days, in spectral windows with little or no gas phase absorption. The resulting broad absorption features were fit to spectra calculated from a Mie extinction model using the optical constants of PSC particles of different compositions. Spectra recorded on January 27, 2000 clearly show the absorption features representative of nitric-acid trihydrate (NAT). This is the first direct field confirmation of NAT outside of mountain lee wave-induced PSCs.; Lastly, greenhouse warming due to the infrared scattering by CO ₂ clouds may have allowed for liquid water on early Mars and may even be responsible for having maintained the early Earth's surface temperature above freezing. The warming potential of these clouds depends on their particle size, which is determined by the nucleation and growth conditions. I studied the nucleation and growth of CO₂ on water ice under Martian atmospheric conditions, and found that a critical saturation, S = 1.34, is required for nucleation, corresponding to a contact parameter between ice and CO₂ of m = 0.95. After nucleation occurs, growth of CO₂ is rapid and proceeds without a surface kinetic barrier. These results suggest particles larger than those being currently suggested for the present and past Martian atmospheres. Using this information in a microphysical model described in a companion paper, we found that CO ₂ clouds are best described as “snow”, having a small number of large particles.