  We present observations of Neptune's 1- and 3-mm spectrum from the Combined
Array for Research in Millimeter-wave Astronomy (CARMA). Radiative transfer
analysis of the CO (2-1) and (1-0) rotation lines was performed to constrain
the CO vertical abundance profile. We find that the data are well matched by a
CO mole fraction of 0.1^+0.2_-0.1 parts per million (ppm) in the troposphere,
and 1.1^+0.2_-0.3 ppm in the stratosphere. A flux of 0.5-20 times 10^8 CO
molecules cm-2 s-1 to the upper stratosphere is implied. Using the Zahnle et
al. (2003) estimate for cometary impact rates at Neptune, we calculate the CO
flux that could be formed from (sub)kilometer-sized comets; we find that if the
diffusion rate near the tropopause is small (200 cm2 s-1), these impacts could
produce a flux as high as 0.5^+0.8_-0.4 times 10^8 CO molecules cm-2 s-1. We
also revisit the calculation of Neptune's internal CO contribution using
revised calculations for the CO ->CH4 conversion timescale in the deep
atmosphere (Visscher et al. 2011). We find that an upwelled CO mole fraction of
0.1 ppm implies a global O/H enrichment of at least 400, and likely more than
650, times the protosolar value.
