Validating the reported random errors of ACE-FTS measurements through analysis of tropical variability

Measurements produced by the Atmospheric Chemistry Experiment - Fourier Transform Spectrometer (ACE-FTS) have been used extensively in recent studies of the chemistry and dynamics of the stratosphere and upper troposphere. To date, validation of the ACE-FTS data set has focused primarily on the determination of systematic bias between the ACE-FTS retrieved trace gas profiles and those from numerous other instruments, and have shown that in general ACE-FTS retrieved profiles show good agreement with other space-based missions. There has, however, been no validation of the reported errors of the ACE-FTS measurements, which would be desirable in order to gain confidence in the scientific use and interpretation of measurements in small sample sizes. In order to validate the reported random errors (RREs) of the ACE-FTS measurements, we apply a technique used in past validation studies, which involves comparing the RREs with the variability of measurements in the tropical stratosphere, where natural variability is weak. We introduce a method of quantifying the scatter of tropical measurements which aims to minimize the effects of short-term atmospheric variability, while maintaining large enough sample sizes that the results can be taken as representative of the full data set. Using this simple technique, RREs for a number of chemical species are successfully validated over certain altitude regions, especially when robust statistics are used to eliminate the effect of outliers. Simulated chemical fields from the Canadian Middle Atmosphere Model are used to show that for some species, stratospheric tropical measurement scatter is dominated by natural variability, not random error. This result, while not strictly validating the RREs for these species, nonetheless adds confidence in the scientific utility of single profile measurements.