|September 29:||418.30 ppm|
|September 28:||418.20 ppm|
|September 27:||418.30 ppm|
|September 26:||418.65 ppm|
|September 25:||418.21 ppm|
The graph, updated daily, shows as individual points daily and hourly mean CO2 for the last 31 days. Daily average CO2 values are computed from selected hourly values that satisfy 'background' conditions, i.e. stability and persistence of CO2 concentrations (read below for more information). That means the daily averages shown on the plot are likely computed from a subset of the hourly averages. It also is possible that no daily average can be computed even though hourly averages exist, because all of the hourly averages for a day do not satisfy the background conditions. These daily average values may also change as a result of the selection process as additional data become available.
The daily means are based on hours during which CO2 was likely representative of “background” conditions, defined as times when the measurement is representative of air at mid-altitudes over the Pacific Ocean. That air has had several days time or more to mix, smoothing out most of the CO2 variability encountered elsewhere, making the measurements representative of CO2 over hundreds of km or more.
The selection process is designed to filter out any influence of nearby emissions, or removals, of CO2 such as caused by the vegetation on the island of Hawaii, and likewise emissions from the volcanic crater of Mauna Loa. We require low variability within each hour and between successive hourly averages, as well as a degree of persistence of the likely valid "background" hours between successive days. Thus, some of the hours that are initially designated as background, may still be flagged as unlikely to represent background conditions as information about the following days becomes available.
Also note that the dates and times in the plot are in UTC, which is 10 hours ahead of Hawaii local time. So a 0-24 hour UTC day runs from 2PM the previous day to 2PM current day local Hawaii time.
For details, see ”How we measure background CO2 levels at Mauna Loa”.