Scott Hannon, 19 July 2007

Columns in the text file are:

1: latitude [degrees]
2: longitude [degrees]
3: time [seconds since 00z 1 Jan 1993]
4: SO2 column [Dobson units]
5: nlevs_cloud [cloud level 1-100]
6: nlevs [surface level 1-100]
7: lbad [0=false; 1=true; cloud/SO2 retrieval failed]

See "ql7.txt" to translate AIRS layers to pressure.
The altitude info in this file is only a rough estimate
as the altitude depends on the profile and latitude.
"nlevs" refers to a layer boundary level, ie there
are 100 AIRS layers defined by 101 level boundaries.
For example, layer 50 is bounded by level 50 and 51.

All SO2 retrievals assumed the SO2 was located in AIRS
layers 41 thru 61 (ie between level 41 and 62).  The
profile is the nominal AFGL SO2 profile multiplied by
a single scale factor over all these layers.

AIRS does not have complete global coverage in the
tropics.  That is, there are gaps in the spatial
coverage between adjacent orbits at low latitudes.
  
nlevs_cloud is an estimate of where (vertically) the
cloud top/surface appears to be located. It was
derived assuming 100% uniform cloud cover in the
AIRS FOV, and thus will not be accurate whenever that
assumption is not true (ie often).  But even when
innaccurate it is still a useful metric to estimate
the cloud impact on the SO2 retrieval.  Whenever
nlevs_cloud is less than ~60, the AIRS SO2 retrieval
was significantly degraded by clouds, and the SO2
relibility gets worse as nlevs_cloud decreases.  If
nlevs_cloud is below ~50 the SO2 is highly suspect.

The lbad flag indicates failed cloud/SO2 retrievals.

Scott.