  This paper presents Spitzer-IRS spectroscopy of the CO2 15.2 micron bending
mode toward a sample of 50 embedded low-mass stars in nearby star-forming
clouds, taken mostly from the ``Cores to Disks (c2d)'' Legacy program. The
average abundance of solid CO2 relative to water in low-mass protostellar
envelopes is 0.32 +/- 0.02, significantly higher than that found in quiescent
molecular clouds and in massive star forming regions. It is found that a
decomposition of all the observed CO2 bending mode profiles requires a minimum
of five unique components. Roughly 2/3 of the CO2 ice is found in a water-rich
environment, while most of the remaining 1/3 is found in a CO environment.
Ground-based observations of solid CO toward a large subset of the c2d sample
are used to further constrain the CO2:CO component and suggest a model in which
low-density clouds form the CO2:H2O component and higher density clouds form
the CO2:CO ice during and after the freeze-out of gas-phase CO. It is suggested
that the subsequent evolution of the CO2 and CO profiles toward low-mass
protostars, in particular the appearance of the splitting of the CO2 bending
mode due to pure, crystalline CO2, is first caused by distillation of the
CO2:CO component through evaporation of CO due to thermal processing to ~20-30
K in the inner regions of infalling envelopes. The formation of pure CO2 via
segregation from the H2O rich mantle may contribute to the band splitting at
higher levels of thermal processing (>50 K), but is harder to reconcile with
the physical structure of protostellar envelopes around low-luminosity objects.
