In response to a recent reader comment about wide fluctuations in CO2 readings at Mauna Loa, Ralph Keeling and Stephen Walker of the Scripps CO2 Group gave the following answer:

It’s fairly normal, especially in the summer time, to see strong afternoon “dips” in the CO2 concentration.  The dips are generally caused by upslope winds that that are depleted in CO2 by photosynthesis at lower elevations.  In the last few days have had very large swings of this sort showing the island influences can be strong.  We do our best to exclude these island influences from the record by retaining only data when the concentration is stable for many hours.  How unusual are the large recent swings?  The magnitudes of the swings can vary significantly from year to year, with daily dips greater than 10ppm seen in some years.  So the recent oscillations observed in a single day of about 6ppm are well within the range of normal variability.