ACE version 1.0
September 11, 2004
update: January 20, 2005

An overview of the ACE mission is presented in the following paper:

Bernath, P.F et al., Atmospheric Chemistry Experiment (ACE):
mission overview, Geophys. Res. Lett., submitted (2005)

A description of the ACE-FTS retrieval process can be found in the
following paper:

Boone, C.D. et al., Retrievals for the Atmospheric Chemistry
Experiment Fourier Transform Spectrometer, Geophys. Res. Lett.,
submitted (2005)

Pre-prints of the papers can be found on the following Web site:

http://www.ace.uwaterloo.ca/data

Over the course of a year, the altitude spacing for ACE-FTS 
measurements varies from ~2 to ~6 km, depending on the angle of
the satellite orbit plane to the sun.  The standard output files
interpolate the results for all occultations onto a standard 
altitude grid with 1 km spacing.  This does not imply that the 
altitude resolution of the ACE-FTS is 1 km.


Some issues to be aware of with the ACE data

     In the VMR results, an entry of -999 indicates that no
retrieval was performed at that altitude.  At high altitudes, above
the highest measurement used in the analysis for a given molecule,
I include a VERY rough estimate of the molecule's VMR (it is a 
constant times the a priori value, with the same constant used for
all altitudes above the highest analysed measurement).  These data
are flagged by the uncertainties being set to -888.  Do not trust 
these results too far above the highest analyzed measurement.

     Pressure and temperature values were retrieved down to no 
lower than 12 km (the columun labelled T_Fit indicates whether 
temperature was retrieved at that altitude: T for True and F for 
False).  Below 12 km, temperature and pressure were fixed to data 
from the Canadian Meteorological Center.

     High altitude results (above about 95 km) should be viewed with
skepticism.  The temperature profiles above this altitude require 
further work.  Above 70 km for a relatively small percentage of
occultations also gave dubious results (there were problems in the
retrieval of CO2 vmr during the P/T retrieval process).  However,
the majority of the time, there were no known issues for the
retrievals between 70 and ~95 km.

     Processing problems were encountered when measurements were 
separated by less than 1 km (i.e., less than the altitude grid 
spacing), occasionally leading to large spikes in the retrieved
temperature at low altitudes.  If a result looks suspicious between
10 and 20 km, check the temperature profile for the presence of a 
spike or a large fluctuation in this altitude region.

     No provision was made for indentifying occultations with
significant ice contamination on the FTS detectors.  Therefore, some
occultations (particularly earlier ones, e.g., before March 2004) 
could experience a deterioration of results at low altitudes, some 
molecules worse than others.

     Uncertainties provided for the VMR results are statistical
errors from the fitting process (1-sigma), and do not include 
systematic contributions.  A more detailed error budget will be
determined later.

The molecule NO sometimes has extremely low absorption through the 
mesosphere (increasing for both higher and lower altitudes).  For such
occultations, the retrieved NO profile through the mesosphere will look
quite ugly.  The results are to be ignored when this happens.

For occultations that cut out above 10-17 km (due to clouds), the
bottom-most measurement often gives results that are clearly out 
(presumably from the clouds affecting the measurement just before the 
suntracker loses lock).  Simply ignore the bottom point if it looks 
inconsistent.

For molecules with significant interferences (e.g., N2O5 and SF6), the 
VMR for the highest analyzed measurement is sometimes suspiciously high.
If you see a sharp increase in the highest retrieved points, don't trust 
it.  It appears that N2O5 suffers from significant interference due to a
CFC.  This interference will be taken into account in future processing.
For SF6, I was trying to push the retrievals to overly high altitudes, 
leading to poor results, and the retrieval altitude range for the 
molecule has since been decreased.

The results for HCFC-22 are very noisy.  The reasons for this are being
investigated.

The retrievals are not optimized for low altitudes (below 10-12 km).  For
this round of processing, don't rely too much on the results below 10 km.

For version 1.0 processing, columns in the output files were reserved for
two molecules that were never retrieved: HCN and CF4.


Chris Boone
cboone@uwaterloo.ca