# AIRS Spectral Response Functions

## 1 Introduction

The following describes the AIRS spectral response functions (SRFs) for the AIRS Level 1C radiance data that is available at the the GES DISC.. Sample SRFs for the AIRS L1b data are available from L. Strow (strow@umbc.edu). However, the AIRS L1b radiances experience small, but complicated, wavenumber drifts that vary in time. This means that the actual SRFs for the AIRS L1b change with time. We recommend that AIRS L1C radiance data be used instead of AIRS L1b.

The AIRS L1c SRF file is available at http://asl.umbc.edu/pub/airs/srf/airs_l1c_srfs.nc.

## 2 The SRF File Format

The AIRS L1C spectral response functions for each channel are available in netcdf4 format. SRFs are provided for each of the AIRS L1C 2645 channels. Note that 2314 of these channels correspond to actual AIRS Level 1b channels, the rest are "fill" channels that are estimates of the radiances at the fill channel centers. See the AIRS Level 1C documentation for more details.

Each channel in the SRF file has a distinct spectral response functions, currently tabulated at 471 points. The SRF file contains the following variables:

freq      2645 x 1       SRF center wavenumbers
fwgrid    471 x 1        SRF tabulation points
srfval    2645 x 471     SRF values
width     2645 x 1       SRF widths

freq
The frequency of the nominal center points of the SRFs, i.e. the wavenumber for fwgrid = 0, see equation 1, below. These nominal center values are from an intermediate stage of the SRF modeling, and may not correspond exactly to peaks, integral centers, or midpoints of the line through the full width at half max of the tabulated SRFs.
fwgrid
The scaled frequency grid for tabulation of the SRFs, in units of the full width at half max. The grid extends far enough into the wings of the SRF for accurate radiative transfer calculations. fwgrid is not tabulated at a regular spacing; points are spaced closer together where the SRFs have greater variability.
srfval
The SRFs peak-normalized to unity
width
The full widths at half max of the SRFs, in wavenumbers

The fwgrid vector can be converted to a frequency grid in wavenumbers via

 freqgrid = ( fwgrid ×width ) + freq (1)

The resulting freqgrid variable will not be at a regular spacing; if desired, the srfval can be linearly interpolated to a finer, regular grid. Do not extrapolate the SRF beyond the endpoints of fwgrid.