Sherry O. Farwell

Sampling and Analysis

Publisher Summary This chapter discusses sampling and analysis. In atmospheric analyses, the mass or concentration of the desired analyte is not measured directly. Instead, a numerical value of a physical or chemical property for the analyte, which can then be correlated with either mass or concentration, is obtained. The relationship between the measured physical or chemical property ( X i ) and the quantity ( q i ) for analyte species ( i ) is given by an analytical calibration function X i = f i ( q i ). If X i is determined for a series of standards of different known masses or concentrations, these results establish the analytical calibration curve, that is, the relationship between the analytical signal and the analyte mass or concentration. The choice of an appropriate calibration technique is affected by the analytical method, potential interferences in the sample matrix, the number of samples to be analyzed, the degree of accuracy necessary for the datas intended use, and the relative magnitude of the error contributed by the measurement process compared to combined errors from sampling, sample preparation, and data reduction. Knowledgeable interpretation of trace atmospheric measurement data is an important factor in a good-quality assurance program, and greater recognition of this principle would prevent considerable confusion.

Biogenic Sulfur Gas Emissions from Soils in Eastern and Southeastern United States

Data are presented for the first systematic measurements of biogenic sulfur gas flux from the major soil orders within the eastern and southeastern United States. Sulfur flux samples were collected and analyzed on-site during the fall of 1977, spring and summer of 1978 and summer of 1979. A total of 27 sampling locales in 17 states were examined. Eight additional sites were visited in 1980. At some locales, two to four soils were examined, providing an even broader sampling of the soil orders. Three of the locales were revisited two or three times during the course of the study to establish the influence of seasonal climatology upon the measured emission rates and chemical composition of the sulfur flux mixtures. The sulfur gas enhancement of sulfur-free sweep air passing through dynamic emission flux chambers placed over selected sampling areas was determined by combined cryogenic enrichment sampling and wall-coated, open tubular, capillary column, cryogenic gas chromatography (WCOT/GC) using a sulfur se...

Measurement of biogenic sulfur emissions from soils and vegetation using dynamic enclosure methods: Total sulfur gas emissions via MFC/FD/FPD determinations

Metal foil collection/flash desorption/flame photometric detection (MFC/FD/FPD) was one of the analytical methods used to measure emissions of gaseous, sulfur-containing compounds from several terrestrial natural sources during a cooperative field program in the summer of 1985. Nonspeciated, total sulfur gas emissions were determined by using the MFC/FD/FPD technique in combination with a Nafion Perma-Pure drying device to sample air from three designs of dynamic enclosure chambers. These enclosures were placed over various soil orders and vegetation in the vicinity of field sites in Iowa and Ohio previously examined during the 1977–80 SURE study of biogenic sulfur fluxes. Because of the sensitivity and detection characteristics of the MFC/FD/FPD technique, it was possible to obtain measurements on enclosure air samples that were collected for relatively short time periods,. e.g., 1 to 5 min. The magnitudes of these time-resolved, total sulfur gas emissions are correlated exponentially with internal enclosure air temperatures. Potential errors and uncertainties associated with this application of the MFC/FD/FPD methodology are assessed.The total sulfur gas flux values obtained from this study and the SURE program are compared. Unquantified sources of error in the current two parameter extrapolation model used to calculate regional and global terrestrial source strengths of biogenic sulfur emissions are also summarized and are shown to prevent a reliable estimate of overall uncertainty limits in the resultant inventory.

Sulfur compounds volatilized from soils at different moisture contents

Abstract Releases of volatile sulfur-containing compounds from a tidal marsh soil, an agricultural soil, and a forest soil at four moisture contents and under both aerobic and anaerobic conditions were measured. A dynamic, cryogenic collection procedure with subsequent separation and detection by glass-capillary gas chromatography was used. The largest emission fluxes, predominantly hydrogen sulfide and carbon disulfide, were from the tidal marsh soil samples dried to moisture contents typical of a well-drained aerobic soil. Smaller amounts of carbonyl sulfide, methyl mercaptan, dimethylsulfide, and dimethyldisulfide were also detected. The only significant S emissions from the agricultural soil were dimethylsulfide and carbon disulfide from water-saturated samples. No gaseous S compounds were detected from the forest soil.

Generation and evaluation of test gas mixtures for the Gas‐Phase Sulfur Intercomparison Experiment

The automated sulfur gas dilution system (ASGDS) was used during the Gas-Phase Sulfur Intercomparison Experiment (GASIE) to generate and analyze test gas mixtures containing pptv levels of SO2 in the presence of various suites of potential measurement interferent gases. These test gas mixtures were provided in a 100 standard liters per minute air matrix manifold, allowing simultaneous sampling by all seven intercomparison participants. The test gas mixtures were homogeneous within the 90-min test periods of the synthetic air phases of GASIE, allowing rigorous intercomparison of any two or more data points obtained during a given test period. A number of delivered SO2 concentrations were successfully replicated throughout the experiment, permitting the collection of a sufficient amount of data for statistical evaluation. The SO2 concentrations measured by the ASGDS quality control instrumentation agreed with values obtained by the intercomparison participants but were different from the set point values.

Performance Characterization and Optimization of the AgNO3-Filter/FMA Fluorimetric Method for Atmospheric H2S Measurements

This sensitive, albeit precarious, method for measuring ppb-ppt (V/V) concentrations of H/sub 2/S was examined for various sources of potential error within the procedure. Filter preparation, filter storage, filter extraction, fluorimetric reagent stabilities, matrix differences between standards and samples, and possible interferences from other sulfur-containing compounds were separately studied for their effects on the analytical performance of the method. The overall method showed no interference from SO/sub 2/, CS/sub 2/, COS, CH/sub 3/SH, CH/sub 3/SCH/sub 3/, and SO/sub 4//sup -2/. To minimize bias and obtain a reliable estimate of precision, the method should be calibrated with H/sub 2/S standards rather than liquid bisulfide standards. The measurement precision is a function of the quantity of H/sub 2/S collected as Ag/sub 2/S and/or AgSH on the impregnated filters. Because of the methods linear dynamic range, sufficient air should be sampled to achieve filter loadings of 15 to 35 ng S/filter. A quality control method based on fluorescein mercuric acetate (FMA) is presented that ensures data quality while reducing the otherwise frequent need for fluorimetric calibration.

Airborne measurements of total sulfur gases during NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 3

A metal foil collection/flash desorption/flame photometric detection (MFC/FD/FPD) technique was used by investigators from the University of Idaho (UI) to measure ambient total sulfur gas concentrations from an aircraft platform during the NASA Global Tropospheric Experiment/Chemical Instrumentation Test and Evaluation 3 (GTE/CITE 3) program. The MFC/FD/FPD technique allowed rapid quantitation of tropospheric background air masses using sample integration times of 1–3 min with little or no gap between measurements. The rapid and continual sampling nature of this technique yielded data covering approximately 75% of the entire CITE 3 programs air track. Ambient air measurement data obtained during northern hemisphere (NH) flights often exhibited relatively high total sulfur gas values (up to 19 ppb) and an extremely high degree of sample heterogeneity, especially in coastal locations. Data from southern hemisphere (SH) flights typically exhibited relatively low total sulfur gas concentrations and a low degree of sample heterogeneity. A bimodal interhemispheric total sulfur gas gradient was observed using data obtained during transit flights between the two CITE 3 program ground bases. Comparisons were made of UI total sulfur gas measurements with composite sulfur gas values generated using speciated sulfur gas measurements from other CITE 3 participants. Only a relatively small number of overlap periods for comparison were obtained from all the available CITE 3 data because of large differences in measurement integration times and lack of synchronization of sample start/stop times for the various investigators. These effects were compounded with extreme sample heterogeneity in the NH and the speed at which the aircraft traversed the air masses being sampled. Despite these constraints, sufficient overlapping data were available for the comparative evaluations. Comparison of NH UI total with composite sulfur gas values showed excellent correlation and linear curve fit, indicating substantial qualitative agreement. Simple linear regression of total on composite sulfur gas data yielded a slope of 1.9 for coastal NH regions and 1.2 for marine NH regions. The marine NH slope is not statistically different from one, indicating substantial quantitative agreement between UI total and composite sulfur gas values in these regions. However, a significant difference was observed when these same data were treated with a paired t test. SH data exhibited no significant correlation or linear regression slope. A paired t test showed a statistically significant difference when all SH flights were used. However, data from three SH flights that were classified into a unique group using discriminant analysis showed no significant difference between UI total and composite sulfur gas values when analyzed with the paired t test.

A Modified Microcomputer-Controlled Proportioning Valve Instrument for Programmable Dilution of Gases

Abstract A commercial VG-400 gas dilution apparatus from Microsensor Systems, Inc. was modified with an updated microcomputer, revised software, electronic mass flow controllers, and a needle valve pressure restrictor in order to enhance its performance. The instruments maximum linear dilution factor has been extended to 330,000 from the previous practical limit of 100,000 while its dilution accuracy, precision, resolution, and stability have been increased. The modification also decreased the fluctuations in the instruments output concentrations to ≤ 1.5%.

Gas-Phase sulfur intercomparison Experiment 2: Analysis and conclusions

A diffusion denuder, total sulfur chemiluminescence detector instrument for the measurement of SO2 was tested as the second part of the Gas-Phase Sulfur Intercomparison Experiment (GASIE 2). The SO2 at mixing ratios between 27 and 182 parts per trillion by volume (pptv) was provided by a dynamic dilution apparatus. The data were kept blind from the other party and analyzed by two independent referees. The following was concluded: (1) The independent calibrations of each system are within a few percent. (2) The precision on any one day is better than day-to-day variability. (3) Runs in dry air show a small but significant nonzero intercept in correlation plots. (4) No effects of adding NO2 + O3 or CO2 + CH4 + CO + dimethylsulfide are distinguishable. (5) A small but significant effect due to added H2O is evident in both slope and intercept, but the source could not be discerned. (6) On any one day the systems can distinguish among 0, 20, and 40 pptv, but because of day-to-day variability, they can only distinguish among 0, 30, and 60 pptv on different days.

The Emission of Sulfur and Nitrogen to the Remote Atmosphere Working-Group Report

Sulfur and nitrogen compounds are emitted to the remote atmosphere from many varied biological and geological sources in the form of their diverse chemical species. Since the atmospheric fate of the emitted compounds depends on their chemical reactivities with other molecules in the atmosphere, we considered the emissions of sulfur and nitrogen in terms of chemical species rather than in terms of elements. To represent accurately the temporal and spatial variabilities of the various sources, they must be understood in terms of processes rather than as “black boxes” so that emission measurements can be placed into a biogeographical and ecological framework.