An intercomparison of instrumentation for tropospheric measurements of dimethyl sulfide: Aircraft results for concentrations at the parts-per-trillion level

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In particular, research has focused on the production rate of DMS as a function of biological and meteorological conditions, the conversion of DMS to other sulfur compounds, and the mechanisms by which reaction products are transported throughout the global troposphere.Several techniques have been used for the measurement of tropospheric levels of DMS.The question arises as to the validity of DMS measurements by these techniques, especially at the low tropospheric concentrations which are usually in the parts-per-trillion (pptv) range.
As part of the NASA Tropospheric Chemistry Program, a series of field intercomparisons have been initiated to evaluate the state-of-the-art capability for measuring key tropospheric species [McNeal et al., 1983;Hoell et al., 1984; Gregory et al., 1985, Beck et al., 1987].These intercomparisons, designated as Chemical Instrumentation Test and Evaluation (CITE), are conducted as part of NASA's Global Tropospheric Experiment (GTE).The primary objective of the first intercomparison, GTE/CITE 1, was the evaluation of the capability for measurements o f background levels of carbon monoxide (CO), nitric oxide (NO), and the hydroxyl radical (OH) [Hoell et al., 1984[Hoell et al., , 1985a, b], b].CITE 2 extended the intercomparisons to the other major nitrogen gases, namely, nitrogen dioxide (NO2), nitric acid (HNO3), and peroxyacetyl nitrate (PAN) [Gregory et al., 1990a, b, c, d].The objectives of CITE 3 were to evaluate instrumentation for making reliable aircraft measurements of the major sulfur gases and to determine in a predominantly marine environment the abundance and distribution of the major sulfur gases over a wide range of atmospheric conditions.This paper reports the results from CITE 3 during which Gas chromatograph-mass spectrometer.Gas chromatography for separation of DMS from the sample stream followed by mass spectrometric quantitative analyses is the basic detection principle of the measurement.Sulfur gases in the incoming air stream are preconcentrated in a Teflon,  Gas chromatograph/fluorination-electron capture.Sulfur compounds in the incoming air sample are separated using gas chromatography and then are fluorinated with F 2 (200 ppmv) using a heated Ag catalyst.The fluorination product, presumably SF 6, is then measured using an electron capture detector.The F 2 stream is generated using a permeation source, and excess F 2 is removed by conversion to HF by reaction with H 2 on a heated Pd catalyst.The Pd catalyst also destroys any response from halocarbons, making the system sulfur specific.Cryogenic preconcentration is required (typically 1 min during CITE 3) followed by a 4-min period for separation and analysis.DMS analysis requires an oxidant scrubber (glass fiber filter impregnated with NaOH).During CITE 3, the system was configured to measure COS and CS 2 as well as DMS.Since the oxidant scrubber (for DMS) interferes with the other sulfur gas measurements, separate samples were collected for the DMS measurement.Typically, the sample sequence was a DMS analysis followed by an COS/CS 2 analysis.As a result, separate DMS measurements occurred about every 10 min.Gas chromatograph-flame photometric.Samples are preconcentrated on a thermoelectrically cooled polymer (Tenax) and thermally desorbed to a packed column (Chromosil 330) with detection using a flame photometric detector.The sample is preconcentrated for a period of 10 min followed by desorption and analyses.The system is automated with two sample channels to provide contiguous 10-min measurements by alternating the sample collection and analysis procedures between the two channels.Oxidants are removed by passing the sample stream through a cooled aqueous KI solution (neutral) prior to preconcentration.Residual water is removed by trapping at -20øC.Precision of the measurements is estimated at 10 and 5% for mixing ratios in the range of 20 to 100 pptv and 100 to 500 pptv, respectively.Accuracy (primary standard) is of the order of 5%.In-flight calibrations (liquid standards) were performed  As part of the data protocol for the ground standards test, the investigator or NIST had the option to declare a test invalid (when submitting the data) and request a retest.For the flight intercomparisons, data protocol required all measurements to be reported.Along with the submitted data, the investigator provided a comment code as to the quality of the data.

Gas chromatograph GC
Detailed results of the flight intercomparisons (first release of results to the DMS investigators) were discussed during a data workshop convened approximately 6 months after the field mission.After the workshop, only minor changes to the DMS data base were made by the investigators.None of the changes were significant in affecting the intercomparison results.Data changes made after the workshop are given below.
Premission estimates of lower detection limits of many of the instruments were based upon laboratory results; accordingly, all investigators were given an opportunity to reevaluate (based on workshop discussions) the stated lower detection limits for their techniques.Most investigators revised (lowered slightly) the detection limits for their techniques.

The data of Table 1 reflect the revised values.
After the workshop, the Max Planck investigator (GW-Na and GW-COT techniques) requested that all data from flight 6 be "commented" as questionable.During this flight the oxidant scrubbers were exposed beyond the rated capacity.In preworkshop submittal of the data from flight 6, some data had already been "commented" as such.These flight 6 data are not included in the analyses.Intercomparison analyses were performed including and excluding these data.While excluding the data does not improve the level of agreement among the techniques, exclusion does result in a slight improvement in the statistical quality of the results.

Standards Intercomparison
The  The intercomparison data used in the analyses are the data measured during the IC periods and constructed by defining a "simultaneous" or "overlapped" measurement as one having some overlap between any portion of the sample period reported by the investigators.The instrument or measurement having the longest integration time set the overlap period, and as such, only a single measurement from that instrument is used for the overlap period.Where more than one value of DMS is reported by any one of the remaining instruments during the defined overlap period, the arithmetic average of those measurements is used as the intercomparison value.Using this procedure, several data bases were constructed by considering different combinations of measurement overlap (i.e., overlapping periods including data from all six techniques, combinations of five techniques, combinations of four).The term "data base" implies the ensemble of overlapped data periods constructed for a given combination of instruments and includes time periods from all 16 intercomparison flights.While numerous overlapping data bases have been constructed and analyzed, most of the results presented are from the prime data base (overlap periods involving data from all six techniques).
Results from all of the data bases constructed were found to be similar.
While the structured IC period approach tended to maximize the temporal overlap among the various measurements, overlap among the measurements was by no means near 100%.For example, overlap between the gold wool techniques and the other techniques was only about 20 to 30%.By the nature of the sampling frequency of the various gold wool instruments, overlap among these measurements was higher and often approached 80%. Figure 1

Screening Analyses
The data base was examined to evaluate measurements that were not representative of the overall results, to identify data categories (i.e., subsets) under which intercomparison results should be stated independently, and to identify outlier events for which the DMS measurements should not be intercompared.In particular, the overlapped data base was evaluated to identify the influence of (1) the degree of temporal overlap (i.e., the ratio of common sample time of a measurement to the total duration of the overlap period), (2) data reported during periods in which significant ambient variations of DMS were occurring, (3) the altitude at which the measurements were made, (4) systematic day-by-day variability, (5) the nature and type of air mass (e.g., total sulfur, water vapor, or ozone content of the air), and (6) the distribution of DMS mixing ratios.In performing these analyses, numerous data correlations, regressions, confidence intervals, etc., were examined.Pertinent observations and conclusions from these analyses are given below.Four of the overlap periods of the data base included some form of data reported by one or more of the techniques as below the detection limit of a instrument (1 to 5 pptv).Each of the periods were examined to check the consistency of all DMS data reported during these overlap periods.All data during these periods are consistent.In particular, all DMS concentrations reported during these time periods are only a few pptv.In order to include these data in the analyses, data reported as lower detection limit are assigned a value of the lower detection limit; i.e., datareported as below 2 pptv are assigned a value of 2 pptv.
The fourth overlap period (noted earlier) excluded from analysis was due to the gas chromatograph-mass spectrometer data (data point circled in Plate 2a).While there is no evidence to suggest that the measurement is in error, the levels of agreement between the gas chromatograph-mass spectrometer system and the other techniques are not typical of those observed during the other 49 overlapping data periods.Thus, for this reason alone, it has been excluded from the analyses as not representative.No other special cases, outliers, or abnormalities were identified in the prime data base.

Standards
Test Results DMS standards intercomparisons showed that within the stated uncertainties (about 10% for NIST and 10 to 20% for each instrument), all the measurements agreed with the NIST standards.
In the case of retest for the Max Planck investigator, both the retest data and resubmitted (prior to retest) data for the first test agreed with NIST values (+19% resubmitted, +6% retest).Agreement between the measurements from the various instruments and the NIST values ranges from about-3% to +19% with a tendency for most measurements to be high relative to NIST.At the 95% confidence level, no statistically significant biases exist between any investigator's measurement and NIST or between pairs of investigators' measurements.The regressions were repeated where both X and Y were treated as dependent variables (i.e., equally likely to be in error), and results were not significantly different than those given in Figure 4. Table 3 summarizes the regression results.
Results from Figure 4 and Table 3 suggest good agreement between measurements from the individual techniques and the average reported DMS as well as among the techniques themselves.Regression coefficients are all above 0.9, and slope and intercept biases are within about 6% (one exception) and a few pptv, respectively.Considering the estimated accuracy and precision of the techniques (Table 1), one concludes that each instrument is measuring DMS to within their stated uncertainties.The tendency of the gas chromatograph-fluorination (FLUOR) to report slightly higher values of DMS was observed in all data bases.For example, the data of Figure 5    The DMS data were also analyzed by averaging the instrument data over longer time periods.For these analyses, the earlier defined official intercomparison periods were designated as the overlap periods and the average DMS calculated for each instrument (arithmetic average of all data reported) .Table 6, in the format of Table 4, summarizes the results for the three mixing ratio categories of interest.In some cases, data are not reported by all instruments for all the IC periods; thus the column headings show a range for the number of samples (n) in each category.Asterisked data indicate the statistically significant 95% confidence intervals.The results of Tables 4 and 6

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• -so • -50 .integration period; required support facilities of power, weight, space, personnel, etc.; and financial resources) than the uncertainty associated with the DMS measurement.The CITE 3 data can also be used to provide a conservative estimate of the uncertainty associated with an airborne DMS measurement.Assuming that no bias exists among the six instruments, an ambient DMS measurement is estimated to be accurate to about 23% (mixing ratios >50 pptv) and 4 to 5 pptv (mixing ratios <25 pptv).A similar estimate using results from the groundlevel standards tests of the instruments installed on the aircraft (more controlled sampling conditions) gave an uncertainty of the order of 12% for mixing ratios of 100 to 200 pptv.
three gold wool techniques differ mainly in the type of oxidant scrubber used and minor operational procedures.As noted in the table, two of the techniques provided measurements of other sulfur gases and participated in the intercomparisons conducted for other sulfur gases.The notations in the last two columns of Table I are used in the paper to identify the instrumentation.A brief description of each instrument and its operation is given below.Detailed descriptions of the instruments are found in the is removed, capped, and stored for postflight analysis.Generally, DMS samples were analyzed within 24 hours of collection.Analyses consisted of thermal desorption of the sulfur from the gold wool and analyses by gas chromatography using a flame photometric detector.Sample collection times for CITE 3 were typically 5 to 10 min for boundary layer samples and 10 to 20 min for samples taken above the boundary layer.Replacement of gold wool tubes in the sample line requires only a few seconds; thus, for most CITE 3 flights, near-contiguous samples were taken for the duration of a scheduled "official" intercomparison period.(As discussed below, time periods of 30-to 60-min duration were scheduled during the flights to obtain the intercomparison data.)Precision estimates for the gold wool method are 10% for mixing ratios in the range of 20 to 500 pptv.Accuracy (primary standard) is about 20%.Inflight calibrations were not performed for the gold wool absorpsubmittal of all DMS standards data, the final results from the standards tests were discussed with the DMS investigators during the field activities.Only a qualitative assessment of the progress of the flight intercomparison tests was provided to the investigators while in the field.Final flight data were submitted to the project office 3 months after completion of the field missions.These data were not adjusted (i.e., normalized to NIST) based on the results from the standards test.
standards intercomparison was performed by having each instrument sample from the output of a mobile DMS reference source provided by NIST.The values and uncertainties (about 10%) of the NIST gas mixtures were based on the values for the standard and subsequent dilution parameters.The output from the NIST system was sampled with the same sampling system (inlet, flow rates, etc.) used during the aircraft flights.Standards tests were conducted while at Wallops from August 7 to 24, 1989.A single mixing ratio, different for each investigator and in the range of 100 to 200 pptv, was provided to each instrument as installed aboard the aircraft.Each investigator was required to provide at least three separate measurements of the NIST standard.The average of the three values is compared to the NIST value to arrive at a level of agreement between NIST and the instrument, and the standard deviation on the average provides an estimate of precision.For the standards tests, no distinction was made between the Na2CO 3 and cotton scrubber data reported by Max Planck.The gas standards test was repeated for the Max Planck investigator.Max Planck's permeation tubes were damaged during shipment to Wallops, and the investigator noted concern as to the condition of his primary calibration devices (DMS and H2S ).Results from the first test showed that agreement (35% difference) between the Max Planck measurement and NIST was not typical in comparison to the other investigators' results.Thus, the project requested a retest to verify the initial results.Prior to the retest, the investigator resubmitted (not knowing the magnitude or direction of the discrepancy with NIST) his earlier data.Conclusions from the standards tests are discussed below.Aircraft Flights Twenty-one flights were conducted as part of the CITE 3 program.The first three were test flights based at the Wallops Flight Center (WFC), Virginia.DMS data obtained during these flights were designated "a priori" by the project as test data and not intercomparison data.Due to a security threat associated with worldwide NASA aircraft operations, the last two flights (ferry from Brazil to Wallops Island) were also designated as nonintercomparison flights.The remaining 16 flights including the ferry flights between WFC and Natal, were intercomparison data flights.DMS measurements were made by all techniques on each of the 16 flights with the exception of the gas chromatograph/fluorination-electron capture technique (flights 9 and 10).Flights were predominantly over water off the coast of either the eastern United States or Natal.Flights from WFC sampled the marine mixed layer and free troposphere at various distances from the continent (marine and continental flow).Natal flights were generally northeast from Natal over the tropical Atlantic Ocean.Three night flights were flown from Natal.Flight altitudes ranged from 150 to 5000 m above sea level.

Fig. 1 .
Fig. 1.Typical overlapping data period showing temporal overlap and sampling schedule of DMS measurements.DMS measurements during these periods were used in the intercomparison analyses.
Figures 2 and 3 illustrate the general characteristics of the prime data set of overlapping data periods which include data from all six techniques.Figure 2 i• a histogram of the DMS mixing ratios for the 49-sample data base.The abscissa of the histogram is the average DMS mixing ratio (avg) for an overlap period calculated as the arithmetic Fig 2. Histogram of average DMS concentration for overlap periods constructed by considering sampling overlap among all six techniques.

Figure
Fig. 3. -Histogram of the time duration of an overlap period for the data base constructed by considering sampling overlap among all six techniques.

ForFig. 5 .
Fig. 5. -Scatter plot of DMS flight data: results from overlapping data periods from the CITE 3 "official" intercomparison periods.Data based constructed by considering only overlapping data between the gas chromatograph-mass spectrometer and gas chromatograph-fluorination techniques.Linear regression resuits of slope, intercept, +1(• values on the slope and intercept, and correlation coefficient are also shown. Fig. 6. -Delta difference plots of DMS flight data: results from overlapping data (all six instruments) periods from the CITE 3 "official" intercomparison periods.Broken lines represent a +10% uncertainty around avg. Data base is the same as that of Figure 3. (a) Gas chromatograph-mass spectrometer instrument.(b) Gas chromatograph-fluorination instrument.(c) Gas chromatograph-flame photometric instrument.(d) Gold wool absorption collection with KOH oxidant scrubber instrument.(e) Gold wool absorption collection with Na2CO 3 oxidant scrubber instrument.(f) Gold wool absorption collection with cotton oxidant scrubber instrument Fig. 6. (continued)

TABLE 3 . Summary of Linear Regression Results (y =mX + b) Slope Intercept Correlation
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