Equilibrium temperature of water-ice aerosols in the high-latitude summer mesosphere

Previous models of the equilibrium temperature and existence regions of mesospheric aerosols have shown significant radiative heating of the aerosols and, consequently, a substantially reduced existence region. We have developed an iterative model that extends this previous work by incorporating a complete collisional energy transfer algorithm, including the effects of vertical winds and particle fall velocity, that is appropriate for the free molecular flow conditions found in the mesosphere. We have also updated the ice refractive index used in the model and accounted for the dependence of the radiative heating and collisional cooling terms on particle tempe;ature. Finally, a radiation model has been used to calculate the solar, terrestrial and atmospheric radiative inputs including the effects of multiple scattering and atmospheric absorption. As with the previous models, the particle temperature is calculated under steady-state conditions, assuming the background gas temperature remains constant and the aerosol does not change size, state or altitude. Under these conditions, the largest differences from previous models occur as a result of the updated ice index of refraction, particularly in the visible, which produces significantly less aerosol heating. These temperatures are combined with the observed properties of mesospheric aerosols to place limits on the water vapour mixing ratio, vertical-wind speeds, and maximum particle sizes. It is found that H2O mixing ratios of 10 ppmv and vertical winds of order 0.02 m s(-1) are consistent with observed particle distributions, and these lead to a radiative limit on the maximum particle radius of 250 nm.

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