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Due to limited resources each aircraft profile of greenhouse gases is limited to one sample at each altitude which means that there are no duplicate samples to assess the precision of each measurement. While sampling and measurement errors can often be identified by the lack of pressure in a flask; low flow rates at the time of sampling or analysis or an acknowledged error at the time of sampling or analysis, there are often less definitive indicators of sampling or measurement errors that need to be noted. Based on our experience with the data we have created a second column flag that effectively scores each data point based on four factors; taken by themselves these factors may not indicate a sampling or measurement error, but when taken together may suggest errors. The four factors can be listed from least to most important.
- Other carbon cycle gases were outliers – The least important indicator is based on the idea that because other carbon cycle gases (CO2, CO, CH4 , SF 6, N2 O and H 2- CCG) are outside the normal variability observed for that site it is possible that the gas of interest (GOI) could be questionable. If this is the case we assign the GOI a 1 to the second column flag to indicate that other CCG gases from that same flask are “outliers”.
Gas of interest is an outlier – If the GOI is more than 3 standard deviations away from a function that fits the trend and the estimated seasonal cycle, an “O” (outlier) is added to the third column. If no other gases from that flask are flagged as outliers a 2 is assigned to the second column flag.
Aircraft cabin contaminant concentrations are high – With more than an order of magnitude increase in concentration of Benzene, C 2H 2(Acetylene), and Halon-1211 (which collectively we will call HATS gases) in an aircraft cabin, these gases have become very important indicators of leaks in the sampling systems (PCP, 12-pack, inlet etc.). If any of these HATS gases have been flagged with a second column "N", indicating a suspected, but not confirmed, leak, we assign a 3 to the second column flag in the gas of interest.
Suspect sampling error – The sampling log and first column "N" or "L" flags in HATS or other CCG gases (indicating a sampling problem, often due to a leak in the system) are believed to be a good indication that all trace gas measurements from that flask or set of flasks from a 12-pack are not valid. These samples are given a 6 in the second column suggesting that they should also be considered for a first column flag if they do not have one already.
Using these four factors a flagging system has been developed to indicate the potential for error in each sample. A numerical second column flag is automatically generated by software based on the criteria described above and summarized in the table below. A sample with a value of 6 or higher is re-examined individually to determine whether a first column flag is appropriate. If it is determined that that sample has been contaminated, a first column flag of "S" is assigned.
|1||CCG outlier (3-sigma)|
|2||Gas of Interest (GOI) outlier (3-sigma)|
|3||HATS 2nd column "N" or "L" flag; suspect (but not definite) sampling problem|
|4||1 & 3: outlier in both HATS and CCG but not GOI|
|5||(1 or 3) & 2: outlier in GOI and another gas|
|6||Suspect sample: either from a log, or from a HATS or CCG gas 1st column flag "N" or "L" meaning there is a sampling problem|
|7||2 & 6 (outlier in GOI and suspect sample)|
|8||4 & 6|
|9||5 & 6 or "N" or "L" in GOI first column, indicating certain leak already determined to affect GOI.|
This flagging system has been applied to the second column flags for each CCGG gas from 2004 to present.
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