  To understand the role that planet formation history has on the observable
atmospheric carbon-to-oxygen ratio (C/O) we have produced a population of
astrochemically evolving protoplanetary disks. Based on the parameters used in
a pre-computed population of growing planets their combination allows us to
trace the molecular abundances of the gas that is being collected into
planetary atmospheres. We include atmospheric pollution of incoming (icy)
planetesimals as well as the effect of refractory carbon erosion noted to exist
in our own solar system. We find that the carbon and oxygen content of
Neptune-mass planets are determined primarily through solid accretion and
result in more oxygen-rich (by roughly two orders of magnitude) atmospheres
than Hot-Jupiters, whose C/O are primarily determined by gas accretion.
Generally we find a `main-sequence' between the fraction of planetary mass
accreted through solid accretion and the resulting atmospheric C/O - with
planets of higher solid accretion fraction having lower C/O. Hot-Jupiters whose
atmospheres have been chemically characterized agree well with our population
of planets, and our results suggest that Hot Jupiter formation typically begins
near the water ice line. Lower mass hot-Neptunes are observed to be much more
carbon-rich (with 0.33 $\lesssim$ C/O $\lesssim$ 1) than is found in our models
(C/O $\sim 10^{-2}$), and suggest that some form of chemical processing may
affect their observed C/O over the few Gyrs between formation and observation.
Our population reproduces the general mass-metallicity trend of the solar
system and qualitatively reproduces the C/O-metallicity anti-correlation that
has been inferred for the population of characterized exoplanetary atmospheres.
