Readers may have noticed that the daily average baseline value for May 13, which was originally reported on May 14 as “Too Variable”, was subsequently changed on May 15 to a value of 400.17 parts per million (ppm). The reason for this change can be understood based on how baseline data and daily average baseline values are defined. We define baseline data to consist of periods of at least five consecutive hourly average points whose within-hour standard deviations are all less than 0.15 ppm and that agree within 0.22 ppm of each other. Data with such high stability are expected to be representative of the larger, well mixed, background air mass and largely free of any contribution from local pollution, vegetation or volcanic activity.
When data from May 13 was retrieved early in the morning of May 14, no periods for May 13 that satisfied these conditions were found. However, when data for May 14 was subsequently retrieved, a baseline period spanning from late in the evening of May 13 through about 5 a.m. on May 14 was identified and classified as baseline. The portion this baseline period that occurred on May 13 averaged 400.17 ppm. Since there was no other baseline air observed on May 13, the 400.17 ppm value was assigned as the daily average baseline for that date.