Calculations of atmospheric and molecular absorption spectra of gases using the HITRAN database. The user specifies the concentrations of molecules, total pressure, temperature, optical pathlength and other parameters which are used to model the gas mixture optical transmission spectra as a function of wavelength. Automatic HITRAN database downloads (from HITRAN.org) or manual placement of the database onto the portable device. Temperature, total pressure, wavelengths and molecular concentrations may be specified. Optional usage of SensorTag and Cell phone sensors for empirical readings of total pressure, temperature and H2O vapor concentration through humidity measurement (humidity measurement is currently only available through SensorTag) to provide input for line-by-line calculations instead of manual entry and generate absorption spectra based on measured ambient conditions. Built in atmospheric models. Voigt, Lorentzian and Doppler lineshapes. Instrument functions. Sharing calculation results (images and data) from within the application. Absorption, Transmission or Optical Depth spectra. Currently limited to horizontal path mode. Additional information is available in the following publication, http://dx.doi.org/10.1007/s12145-017-0288-4 (a view only version of this publication is available at http://rdcu.be/piDB).