Downwelling Far-Infrared Emission Spectra Measured by FIRST at Cerro Toco, Chile and Table Mountain Facility, CA

The Far-Infrared Spectroscopy of the Troposphere (FIRST) instrument is a fourier transform spectrometer developed to measure the far-infrared portion of the Earth's longwave spectrum. Specifically, the purpose is measurement of the important, yet under-investigated 100 to 1000 cm-1 band. Presented here are measurements made by FIRST at two successful deployments in a ground-based configuration to measure downwelling longwave radiation. The initial deployment was to Cerro Toco, Chile. FIRST operated from August to October, 2009 as part of the Radiative Heating in Underexplored Bands Campaign (RHUBC-II) campaign. After recalibration, FIRST was deployed to the Table Mountain Facility from August to October, 2012. Dry days for both campaigns were chosen for analysis ? 09/24/2009 and 10/19/2012. Also available from both deployments are coincident radiosonde temperature and water vapor vertical profiles used as inputs to radiance calculations performed by two platforms, the AER Line-by-Line Radiative Transform Model (using the AER line database), and a research platform, MRTA utilizing HITRAN 2012. Thus, we have three streams of data for the closure studies, and results of the comparisons are presented showing good agreement in the optically thick regions of 200-400 cm-1 and 600-800 cm-1. Differences between measured and coincident calculated spectra between 400 and 600 cm-1 are less than 10 mW/m^2/sr/cm-1. Uncertainties accounted for in this study include uncertainty from multiple detectors, water vapor, temperature, line strength and width as presented in HITRAN 2012, and due to internal heating caused by black bodies. The residuals fall within the root-sum-square envelope of the uncertainties.