Change Log and Notes
19 November 2008
We corrected an error in the data posted for the previous October on 3 November 2008. The assigned CO2 values for the reference gas mixtures in use at our Pt. Barrow Observatory, Alaska, had mistakenly been used in calculating CO2 mole fraction values for the Mauna Loa data, resulting in a value for October that was ~1.5 ppm too high. The error became apparent because of an abrupt shift in the comparison between measurements of flask air samples taken at Mauna Loa, but analyzed in Boulder, Colorado and the continuous analyzer operating at Mauna Loa. There is always a delay of several weeks between the collection and analysis of the flask air samples.
3 September 2008
On 24 July 2008 the middle reference gas cylinder was switched out, but the new reference value was not entered in the database at that time. The CO2 mole fraction in the new cylinder is higher than the old cylinder by 1.15 ppm, but the quadratic fit describing the analyzer response in volts to the CO2 mole fraction, did not "know" that the reference gas had a higher value. We saw the error two weeks later, leading to an upward correction of the July value of 0.78 ppm. The error was also visible in the jump exhibited by the target gas measurements on 24 July. For more detail, we recommend reading how we measure background CO2 levels.
4 August 2008
The CO2 analyzer was down during most of July. Measurements resumed on 22 July 2008, so that there were only 10 days of data from 22 through 31 July. Every year, photosynthesis by plants removes CO2 from the atmosphere during the growing season. At Mauna Loa the rate of decrease is highest in July and August. Therefore, the direct monthly average for July 2008, based only on the last 10 days, was lower than what could be expected for a full monthly average by about 0.7 ppm. On 3 August 2008 the direct monthly mean for July was posted to the CO2 trends web page. We had written a graphing program which corrected the directly calculated monthly mean back to the middle of the month, taking the multi-year average rate of CO2 drawdown during July into account. The program also makes the same corrections for all other months in our CO2 record when there are missing days. Note that these corrections are retroactive. However, they were not posted to the web on 3 August 2008.
A second change was made to the program at the same time. The average seasonal cycle for any given number of years is determined by a curve fit to the years included in the average. We used to define the average seasonal as the sum of four sine and four cosine functions, with periodicity of one, two, three and four cycles per year, respectively. In the revision, we increased that to six sine and cosine functions with periodicities of one through six cycles per year because the latter was able to capture better the temporary slowdown of the seasonal rate of CO2 increase that often occurs during February.
The revisions for all months, including July 2008, were posted to the web on 4 August. On average the corrections are small; one standard deviation of the revisions is 0.06 ppm.