  We present a method to mitigate the atmospheric effects (residual atmospheric
lines) in single-dish radio spectroscopy caused by the elevation difference
between the target and reference positions. The method is developed as a script
using the Atmospheric Transmission at Microwaves (ATM) library built into the
Common Astronomy Software Applications (CASA) package. We apply the method to
the data taken with the Total Power Array of the Atacama Large
Millimeter/submillimeter Array. The intensities of the residual atmospheric
(mostly O3) lines are suppressed by, typically, an order of magnitude for the
tested cases. The parameters for the ATM model can be optimized to minimize the
residual line and, for a specific O3 line at 231.28 GHz, a seasonal dependence
of a best-fitting model parameter is demonstrated. The method will be provided
as a task within the CASA package in the near future. The atmospheric removal
method we developed can be used by any radio/millimeter/submillimeter
observatory to improve the quality of its spectroscopic measurements.
