Theoretical calculations of N₂-broadened half-widths of ν₅ transitions of HNO₃

A number of satellite instruments are measuring nitric acid, HNO₃, in the Earth's atmosphere. In order to do retrievals of temperature and concentration profiles, the spectral parameters for many thousands of HNO₃ transitions must be known. Currently the HITRAN database uses a constant estimated value for the air-broadened half-width of HNO₃. To help improve the line shape parameters, complex Robert-Bonamy calculations were made to determine N₂-broadened half-widths for some 5000 transitions of HNO₃ in the ν₅ band. The intermolecular potential is a sum of electrostatic terms (dipole-quadrupole and quadrupole-quadrupole) and the atom-atom potential expanded to eighth order. The trajectory parameters were adjusted to yield better agreement with measurement. Velocity integrated calculations were made at seven temperatures in order to determine the temperature dependence of the half-widths. The half-width data are compared with available rotation band measurements. The average percent difference between the measured and calculated half-widths is -2.38 for N₂-broadening and -0.65 for air-broadening. The temperature, vibrational, and rotational state dependence of the half-width are investigated.