Funded by the National Oceanic and Atmospheric Administration's
Environmental Services Data and Information Management Program ( ESDIM).
|Barrow, Alaska||Niwot Ridge, Colorado|
|Mauna Loa, Hawaii||Cape Matatula, American Samoa||South Pole, Antarctica
The Five RITS, Ground-based, In-situ Monitoring Sites
Much work has been done to upgrade the RITS (Radiatively Important Trace Species) dataset. Three-channel RITS system chromatographs, all of which have now been replaced by newer and more capable four-channel CATS (Chromatograph for Atmospheric Trace Species) systems, were operational over a period of 15 years from 1986 to 2001. RITS systems were installed at four baseline observatories and a fifth site at Niwot Ridge, Colorado (Table 1) to give measurements of N2O, CFC-12, CFC-11, CFC-113, CH3CCl3, and CCl4 with dual-channel redundancy for N2O and CFC-11. The top sample injection rate at all five field sites was 1 injection every 30 minutes for a combined maximum analysis burden of ~13,000 chromatographic peaks per week.
Especially during the early years of operation, RITS chromatography and the hardware, software, and analysis procedures used to record and manipulate the data evolved over time. Consequently, several file formats were used for storing both the raw data (~2.5 million saved chromatograms) and the database of chromatographic-peak analysis outputs (areas and heights). Long term storage media for these data files included several computer hard drives, 48 DC600 tape cartridges, 17 magneto optical disks, and several hundred floppy disks.
It is also worth noting that early chromatogram analysis and quality control measures were significantly constrained by limitations in processing power. The labor requirements involved in simply keeping up with the new data coming in and remotely troubleshooting the inevitable equipment problems arising at the field sites limited the time available for revisiting earlier data reductions. Accordingly, the computation of atmospheric concentrations from the areas/heights measurements was largely performed in a piecewise fashion on an annual basis.
The primary purpose of this upgrade effort, which was funded by a grant from the Environmental Services Data and Information Management Program (ESDIM), was to implement an enhanced system of quality control procedures and graphical techniques in order to re-examine the RITS data in its entirety while concurrently standardizing the chromatogram files and the areas/heights database files to common formats for renewed storage on CDROM. Particular emphasis was given to identifying and recovering data inadvertently lost or degraded during the original reduction.
The Raw Data
The initial phase of the upgrade effort involved the standardization and inventory of the RITS raw data. Chromatograms (examples) were converted to a common format and exposed to a thorough series of consistency checks prior to storage renewal on CDROM.
Chromatogram recording errors involving the timestamp and/or source-label of the sample injection often occurred for a variety of reasons. The format-standardizing program checked for time folds -- regions of overlapping chromatograms due to an improper system clock setting -- and other inconsistencies between the internal (file header) and external (filename) descriptors. Sample-source labeling errors were detected graphically by plotting ratios of processed peak response measures for nearby environmental and calibration sample injections. Cross-channel inconsistencies were detected by passing the chromatograms through an inventory program that recorded the station, timestamp, sample source, and channel of each chromatogram found within a 30- minute time slot. Inconsistencies were found in ~1 % of the chromatograms processed. These were corrected and reanalyzed to recover the data.
The Areas/Heights Database
The outputs generated during original chromatogram analyses were assembled in record-oriented binary or text format database files for later retrieval during the computation of atmospheric mixing ratios. Each of the original database files was structured in accordance with one of several multiple-injection sampling cycles. Data records were designed to accommodate a full cycle of injections to which a single timestamp was assigned. The details of the sampling cycle and the form of the timestamp both changed over time.
Newly-developed graphical displays of the original database found significant data degradation and loss that occurred during chromatogram processing due to the limitations of the analysis software. Problems included the misidentification of analysis peaks (Figure 3), peaks that were missed altogether because of an excessively-constrained analysis method (Figure 4), and analytical instabilities associated with an insufficiently-constrained analysis method (Figure 5). These problems ultimately resulted from the inability of the analysis software to focus all of its limited resources on one peak at a time. This was addressed by modifying the software to give it this ability and then reanalyzing the affected peaks.
Another problem was discovered to be related to the coarse time-resolution of the original database files. The grouping of an entire sampling cycle into a single data record with a single timestamp led to spurious timestamp modification and data loss by overwriting after interruptions to the normal sampling cycle. This was addressed by restructuring the areas/heights database to include timestamps for every sample injection. This was accomplished by initializing the restructured database with timestamps and sample source identifiers from the chromatogram inventory and employing an algorithm that matched the peak analysis outputs stored in the original database with the corresponding initialized data records in the new database. Although this problem was relatively minor, restructuring the database offered several important additional advantages.
First, the restructured database is compatible with all structural variants found among the original database files. Thus, all of the data associated with a given analysis peak was able to be collected into a single file without regard to the details of the sampling cycle.
Second, upon scanning the new database in search of overwritten samples (i.e. initialized records not corresponding to any peak analysis outputs in the original database) -- which typically numbered on the order of a thousand per station -- tens of thousands more good quality samples were discovered to have been overlooked during the original reduction. All overwritten and overlooked chromatograms were fetched and analyzed to fill in the gaps.
Finally, storage space was added to each data record to facilitate the flagging (i.e. filtering) of individual injections for equipment problems. Because a single calibration sample of poor quality can adversely affect several individual calculations of a compound's atmospheric mixing ratio, marking these samples prior to final reduction is a powerful way to enhance the overall quality of the dataset. A combination of graphical and statistical methods was used to scan the entire restructured areas/heights database and flag well-known chromatography problems (Figure 6).
RITS systems were calibrated using tertiary ("working") gas standards that were shipped to the field sites in stainless steel cylinders. The working standards, or "caltanks", were filled with atmosphere in the Colorado Rockies at or above 3000 meters of altitude. These gases were referenced in the laboratory against calibration scales derived from a series of gravimetrically-developed, primary gas standards. Gravimetrically-developed calibration scales are periodically updated each time a new gravimetric standard is developed, and each new scale differs often negligibly but sometimes significantly from its predecessors. For each of the RITS compounds, multiple gravimetric scales were used to calibrate the many working standards shipped to field sites over the 15-year history of measurements. The primary issue remaining involves the need for decisions about how best to reconcile instances when changing calibration scales or changing caltanks produced significant shifts in the measured atmospheric signals of RITS compounds.
RITS systems typically alternated sampling from two separate atmospheric-intake lines and two caltanks. Efforts were made to use caltank pairs with gas concentration very close to that normally found in relatively clean troposhperic air but with enough separation to give a reasonably stable two-point local estimate of the resonse curve for the ECD. The success of this approach is sensitively dependent upon the relative response characteristics of the three independent sample streams (i.e. the two calibration streams and the combined atmospheric stream).
Figure 7 is a comparison of three linear approximations to a hypothetical ECD response curve. In this idealized example, the two-point CAL1 through CAL2 calibration is the most accurate of the three shown. However, the accuracy of any of these calibrations is sensitive to the closeness of the air concentration to the calibration concentration(s) involved in the computation. Also, in situations where the calibration concentrations are poorly separated, the statistical noise in the calibration response measurements can lead to chaotic fluctuations in the two-point linear approximation to the response curve. The extent to which this problem will affect the atmospheric concentration estimate increases rapidly with increasing distance between the atmospheric concentration and both calibration concentrations. The one-point calibrations through zero are comparatively immune to this noise-amplifying interplay between the normal statistical noise of the calibration response measurements and a relatively small separation between calibration concentrations (Figure 8).
For this reason, and also for those time periods when only one available caltank was installed on a RITS system, we have sought to improve upon the accuracy of the one-point calibration through the usage of a statistically-developed approximation to the normalized response curve for the ECD. The method looks at comparisons between the calibration response ratio and the calibration concentration ratio over the history of RITS measurements of a given compound at a given field site. Links to materials that describe the calibration method in more detail are provided below:
Last modified January 2005 by David Nance.