One of the two programs that monitor greenhouse gases said on Monday that it had revised a reading from last week suggesting that carbon dioxide in the air had surpassed the symbolic level of 400 parts per million.

The new reading by the National Oceanic and Atmospheric Administration is 399.89 parts per million for the 24 hours that ended at 8 p.m. Eastern Daylight Time on Thursday. However, a second monitoring program run by the Scripps Institution of Oceanography continues to show a level of 400.08 parts per million for the same period — a historic level indicative of the rapid rise in human-produced emissions.

Scientists said slight differences between the measurements taken by the two programs are not unusual. The readings are taken at the same place, but the programs use different machines and slightly different analytical protocols.

NOAA revised its figure for Thursday downward based on continuing computer analysis that filters out localized spikes in the carbon dioxide level, as well as other data problems, and a spokesman said that the Thursday figure might be revised again, in either direction.