The OPC integral number, volume, and surface area data was
  recently resubmitted (1/12/98).  The data was unchanged; however
  the reported start, stop, and midpoint times have been changed.
  There was some confusion as to the OPC timing.  The major source of
  confusion arose due to the fact that the OPC can scan in an automated
  mode or in an operator controlled mode.  The latter was usually only
  done for vertical profiles except for a few exceptions and the length
  of the data scan can vary considerably.  In the automated mode, the
  operator can change how long the scan will last (usually 6 minutes,
  although sometimes different depending on the variability of the air
  mass).  Additionally, in the automated mode the OPC cycles between
  unheated, heated to 150 deg C, and heated to 300 deg C scans throughout
  the sampling period.  The updated times are now as accurate as possible
  and reflect the "true" sampling period.  Hopefully this update.txt file
  answered any questions, if not please contact Tony Clarke at
  <tclarke@soest.hawaii.edu>