  Spectroscopic observations of exoplanets are leading to unprecedented
constraints on their atmospheric compositions. However, molecular abundances
derived from spectra are degenerate with the absorption cross sections which
form critical input data in atmospheric models. Therefore, it is important to
quantify the uncertainties in molecular cross sections to reliably estimate the
uncertainties in derived molecular abundances. However, converting line lists
into cross sections via line broadening involves a series of prescriptions for
which the uncertainties are not well understood. We investigate and quantify
the effects of various factors involved in line broadening in exoplanetary
atmospheres - the profile evaluation width, pressure versus thermal broadening,
broadening agent, spectral resolution, and completeness of broadening
parameters - on molecular absorption cross sections. We use H$_2$O as a case
study as it has the most complete absorption line data. For low resolution
spectra (R$\lesssim$100) for representative temperatures and pressures (T
$\sim$ 500K-3000K, P$\lesssim$1 atm) of H$_2$-rich exoplanetary atmospheres we
find the median difference in cross sections ($\delta$) introduced by various
aspects of pressure broadening to be $\lesssim$1\%. For medium resolutions
(R$\lesssim$5000), including those attainable with JWST, we find that $\delta$
can be up to 40\%. For high resolutions (R$\sim$10$^5$) $\delta$ can be
$\gtrsim$100\%, reaching $\gtrsim$1000\% for low temperatures (T$\lesssim$500K)
and high pressures (P$\gtrsim$1 atm). The effect is higher still for self
broadening. We generate a homogeneous database of absorption cross sections of
molecules of relevance to exoplanetary atmospheres for which high temperature
line lists are available, particularly H$_2$O, CO, CH$_4$, CO$_2$, HCN, and
NH$_3$.
