7 June 2007

Dear Colleagues,

  We would like to inform you that we are revising our values for OH and HO_2 
for the NASA missions TRACE-P, INTEX-A, and INTEX-B. The revised numbers are 
factor of 1.64 higher.

  The reason for the revision is a problem with the ATHOS absolute calibration. 
In February, we purchased a NIST-calibrated Si photodiode and compared it to the 
photomultiplier tube (PMT) that we have used as our primary standard for fifteen 
years. This PMT had been calibrated every five years in laboratories with standards 
tied to NIST, the last time in late 2001.  This comparison showed that the PMT 
calibration was different from the NIST-calibrated Si photodiode by a factor of 
about 1.9. We reported this number in the INTEX-B Science Workshop in March.

  Since that time, we have done repeated calibrations with two other methods: 
measuring the production of ozone from oxygen photolysis and the production of 
NO from nitrous oxide photolysis. In addition, we were able to work with NIST in 
Gaithersburg to have our PMT calibrated independently there. This last check, which 
we believe was essential, took some time to schedule, thus delaying our decision to 
change our calibration and this notification.

  All of our four calibration methods indicate that the PMT calibration is incorrect, 
but they differ in the exact value by ±20%. Each technique has its strengths and its 
weaknesses. Since we do not know which calibration method is most accurate, we have 
simply averaged the results of the four of them. The average change is a factor of 1.64.

  We are sorry for the problems this incorrect calibration may have caused you. However, 
this change has cleared up a major inconsistency between the ATHOS-measured OH and modeled 
OH values in the free troposphere for TRACE-P, INTEX-A, and INTEX-B. At the same time, it 
has exacerbated inconsistencies between measurements and models for HO_2 at high altitude 
in INTEX-A and for OH over forests in INTEX-A. We can now focus our attention on understanding 
these two discrepancies.


Sincerely,

Bill Brune and the ATHOS team