  The remote sensing of abundance and properties of HCl -- the main atmospheric
reservoir of Cl atoms which directly participate in ozone depletion -- are
important for monitoring the partitioning of chlorine between "ozone-depleting"
and "reservoir" species. Such remote studies require knowledge of the shapes of
molecular resonances of HCl, which are perturbed by collisions with the
molecules of the surrounding air. In this work, we report the first fully
quantum calculations of collisional perturbations of the shape of a pure
rotational line in H$^{35}$Cl perturbed by an air-relevant molecule (as the
first model system we choose the R(0) line in HCl perturbed by O$_2$). The
calculations are performed on our new highly-accurate
HCl($X^1\Sigma^+$)-O$_2$($X^3\Sigma^-_g$) potential energy surface. In addition
to pressure broadening and shift, we determine also their speed dependencies
and the complex Dicke parameter. This gives important input to the community
discussion on the physical meaning of the complex Dicke parameter and its
relevance for atmospheric spectra (previously, the complex Dicke parameter for
such systems was mainly determined from phenomenological fits to experimental
spectra and the physical meaning of its value in that context is questionable).
We also calculate the temperature dependence of the line-shape parameters and
obtain agreement with the available experimental data. We estimate the total
combined uncertainties of our calculations at 2% relative RMSE residuals in the
simulated line shape at 296~K. This result constitutes an important step
towards computational population of spectroscopic databases with accurate ab
initio line-shape parameters for molecular systems of terrestrial atmospheric
importance.
