Measurements Of Trace Species Research Articles

Dual-laser cavity ring-down spectroscopy for real-time, long-term measurement of trace moisture in gas

Cavity ring-down spectroscopy (CRDS) is a highly sensitive technique that enables the measurement of trace species in gases at or lower than the nmol · mol−1 (ppb) level in amount-of-substance fraction (mole fraction). In particular, CRDS is a very powerful tool for trace-moisture measurement. In real-time, long-term measurement of trace moisture using CRDS, the accuracy is limited by undesired changes in ring-down time over a period of time (background drift). In this study, we introduced a dual-diode laser system in CRDS for trace-moisture measurements to compensate for the background drift. The frequency of the first laser was tuned to the on-peak position of a water absorption line to measure trace moisture, while that of the second laser was tuned to an off-peak position to track the background drift. The two laser diodes were alternately used to measure the ring-down times at on- and off-peak positions by rapidly switching the two laser beams. The effectiveness of dual-laser CRDS for real-time, long-term measurements of trace moisture was demonstrated by experiments based on a primary trace-moisture standard in the range of 8 nmol · mol−1–630 nmol · mol−1. The use of the primary trace-moisture standard in the experiments guaranteed the metrological traceability of the measurement results to the international system of units (SI), making the measurement results reported in this study highly reliable. The expanded uncertainty obtained using dual-laser CRDS at 8 nmol · mol−1 was more than four times better than that of the primary trace-moisture standard, implying the potential of a trace-moisture standard based on dual-laser CRDS whose uncertainty in a low range is better than that of other generator-based trace-moisture standards.

Analysis of inorganic cations and amino acids in high salinity samples by capillary electrophoresis and conductivity detection: Implications for in-situ exploration of ocean worlds.

With growing interest in exploring ocean worlds, such as Europa and Enceladus, there is a fundamental need to develop liquid-based analytical techniques capable of handling high salinity samples while performing both bulk and trace species measurements. In this context, CE with capacitively coupled contactless conductivity detection (CE-C4 D) has tremendous potential. One of its advantages is that this combination allows the detection of a wide number of charged species (both organic and inorganic) without the need of derivatization. Amino acids are an example of organic targets that are powerful biosignatures in the search for life beyond Earth. Simultaneous information on the inorganic cations in a sample helps with assessing the habitability of an extraterrestrial environment, as well as providing sample context for any measurements of trace amino acids. In this work, we present a series of flight-compatible methods capable of simultaneously measuring inorganic cations and amino acids in samples of varying salinity by CE-C4 D. Regardless of the sample total salinity, 5.0 M acetic acid was selected as the optimum BGE. The methods were evaluated by analyzing natural samples of low and high salinity from Hot Creek Gorge, Mono Lake, and Santa Monica beach. Prospects for mission implementation are also discussed.

Comparison of the detection characteristics of trace species using laser-induced breakdown spectroscopy and laser breakdown time-of-flight mass spectrometry.

The rapid and precise element measurement of trace species, such as mercury, iodine, strontium, cesium, etc. is imperative for various applications, especially for industrial needs. The elements mercury and iodine were measured by two detection methods for comparison of the corresponding detection features. A laser beam was focused to induce plasma. Emission and ion signals were detected using laser-induced breakdown spectroscopy (LIBS) and laser breakdown time-of-flight mass spectrometry (LB-TOFMS). Multi-photon ionization and electron impact ionization in the plasma generation process can be controlled by the pressure and pulse width. The effect of electron impact ionization on continuum emission, coexisting molecular and atomic emissions became weakened in low pressure condition. When the pressure was less than 1 Pa, the plasma was induced by laser dissociation and multi-photon ionization in LB-TOFMS. According to the experimental results, the detection limits of mercury and iodine in N2 were 3.5 ppb and 60 ppb using low pressure LIBS. The mercury and iodine detection limits using LB-TOFMS were 1.2 ppb and 9.0 ppb, which were enhanced due to different detection features. The detection systems of LIBS and LB-TOFMS can be selected depending on the condition of each application.

Advanced Laser Retroreflectors for Astrophysics and Space Science

We developed advances laser retroreflectors for solar system exploration, geodesy and for precision test of General Relativity (GR) and new gravitational physics: a micro-reflector array (INRRI, Instrument for landing-Roving laser Retroreflectors Investigations), a midsize reflector array for the European Earth Observation (EO) program, Copernicus (CORA, COpernicus laser Retroreflector Array), a large, single-retroreflector (MoonLIGHT, Moon Laser Instrumentation for General relativity High accuracy Tests). These laser retroreflectors will be fully characterized at the SCF_Lab (Satellite/lunar/GNSS laser ranging/altimetry Cube/microsat Characterization Facilities Laboratory), a unique and dedicated infrastructure of INFN-LNF (www.lnf.infn.it/esperimenti/etrusco/). Our research program foresees several activities: 1) Developing and characterizing the mentioned laser retroreflector devices to determine landing accuracy, rover positioning during exploration and planetary/Moon’s surface georeferencing. These devices will be passive, laser wavelength- independent, long-lived reference point. INRRI will enable the performance of full-column measurement of trace species in the Mars atmosphere by future space-borne lidars. These measurements will be complementary to highly localized measurements made by gas sampling techniques on the Rover or by laser back-scattering lidar techniques on future orbiters and/or from the surface. INRRI will also support laser and quantum communications, carried out among future Mars Orbiters and Mars Rovers. This will be possible also because the INRRI laser retroreflectors will be metal back-coated and, therefore, will not change the photon polarization. The added value of INRRI is its low mass, compact size, zero maintenance and its usefulness for any future laser altimetry, ranging, communications, atmospheric lidar capable Mars orbiter, for virtually decades after the end of the Mars surface mission, like the Apollo and Lunokhod lunar laser retroreflectors. MoonLIGHT and INRRI are proposed for landings on the Moon (two Google Lunar X Prize Missions, namely Moon Express; Russia’s Luna-27 mission, as well as others under consideration/negotia- tion, also with the help of ASI, ESA and other partnerships); 2) Precision tests of GR with LLR to MoonLIGHT reflectors. Development of new fundamental gravity physics models and study of experimental constraints to these models use also laser ranging and laser reflectors throughout the solar system: extension of general relativity to include Spacetime Torsion, Non-Minimal Coupling between matter and curvature (so-called “ ” theories, or NMC gravity); 3) Extension of program to: Mars, Phobos and Deimos, Jupiter and Saturn icy/rocky moons, Near Earth Asteroids.

CO/H2ABUNDANCE RATIO ≈ 10–4IN A PROTOPLANETARY DISK

The relative abundances of atomic and molecular species in planet-forming disks around young stars provide important constraints on photochemical disk models and provide a baseline for calculating disk masses from measurements of trace species. A knowledge of absolute abundances, those relative to molecular hydrogen (H2), are challenging because of the weak rovibrational transition ladder of H$_{2}$ and the inability to spatially resolve different emission components within the circumstellar environment. To address both of these issues, we present new contemporaneous measurements of CO and H2 absorption through the "warm molecular layer" of the protoplanetary disk around the Classical T Tauri Star RW Aurigae A. We use a newly commissioned observing mode of the Hubble Space Telescope-Cosmic Origins Spectrograph to detect warm H2 absorption in this region for the first time. An analysis of the emission and absorption spectrum of RW Aur shows components from the accretion region near the stellar photosphere, the molecular disk, and several outflow components. The warm H2 and CO absorption lines are consistent with a disk origin. We model the 1092-1117A spectrum of RW Aur to derive log10 N(H2)~=~19.90$^{+0.33}_{-0.22}$ at T$_{rot}$(H2) ~=~440~+/-~39 K. The CO $A$~--~$X$ bands observed from 1410-1520A are best fit by log10 N(CO)~=~16.1~$^{+0.3}_{-0.5}$ at T$_{rot}$(CO) ~=~200$^{+650}_{-125}$ K. Combining direct measurements of the HI, H2, and CO column densities, we find a molecular fraction in the warm disk surface of $f_{H2}$~>=~0.47 and derive a molecular abundance ratio of CO/H2~=~1.6$^{+4.7}_{-1.3}$~x~10$^{-4}$, both consistent with canonical interstellar dense cloud values.

Investigating enhanced thorium ionization in TIMS using Re/Pt porous ion emitters

Thermal ionization mass spectrometry (TIMS) is a widely used, benchmark method in actinide isotopic analyses efforts relevant to various nuclear and geological fields. Despite significant previous use and inherent advantages, however, poor sample ionization continues to hamper the use of TIMS in the measurement of trace species; actinide ionization efficiencies frequently fall below 0.1 % using traditional instrument sources. These low efficiencies seriously limit the ability to measure several highly refractory metals (e.g. U and Th) that may provide key signatures data in non-proliferation, safeguards and forensics efforts. Herein, a relatively new TIMS ion source strategy, employing porous ion emitters (PIEs) atop traditional filament assemblies, is investigated for the first time as a straightforward means of enhancing the ionization of Th, arguably a worst case scenario for TIMS-based actinide measurements. These sources yielded up to 410 % greater Th sample utilization, relative to previously published values and in-house measurements collected using traditional methods. Accompanying scanning electron microscopy investigations provide preliminary insight into the mechanisms of PIE functioning and explore the impacts of extended heating on the constructed source’s structure and composition.

ACE-FTS observations of pyrogenic trace species in boreal biomass burning plumes during BORTAS

Abstract. To further our understanding of the effects of biomass burning emissions on atmospheric composition, the BORTAS campaign (BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites) was conducted on 12 July to 3 August 2011 during the boreal forest fire season in Canada. The simultaneous aerial, ground and satellite measurement campaign sought to record instances of boreal biomass burning to measure the tropospheric volume mixing ratios (VMRs) of short- and long-lived trace molecular species from biomass burning emissions. The goal was to investigate the connection between the composition and the distribution of these pyrogenic outflows and their resulting perturbation to atmospheric chemistry, with particular focus on oxidant species to determine the overall impact on the oxidizing capacity of the free troposphere. Measurements of pyrogenic trace species in boreal biomass burning plumes were made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) onboard the Canadian Space Agency (CSA) SCISAT-1 satellite during the BORTAS campaign. Even though biomass burning emissions are typically confined to the boundary layer, outflows are often injected into the upper troposphere by isolated convection and fire-related convective processes, thus allowing space-borne instruments to measure these pyrogenic outflows. An extensive set of 14 molecules – CH3OH, C2H2, C2H6, C3H6O, CO, HCN, HCOOH, HNO3, H2CO, NO, NO2, OCS, O3, and PAN – have been analysed. Included in this analysis is the calculation of age-dependent sets of enhancement ratios for each of the species originating from fires in North America (Canada, Alaska) and Siberia for a period of up to 7 days. Ratio values for the shorter lived primary pyrogenic species decrease over time primarily due to oxidation by the OH radical as the plume ages and values for longer lived species such as HCN and C2H6 remain relatively unchanged. Increasing negative values are observed for the oxidant species, including O3, indicating a destruction process in the plume as it ages such that concentrations of the oxidant species have dropped below their off-plume values. Results from previous campaigns have indicated that values for the molar ratios of ΔO3 /ΔO obtained from the measurements of the pyrogenic outflow from boreal fires are highly variable and range from negative to positive, irrespective of plume age. This variability has been attributed to pollution effects where the pyrogenic outflows have mixed with either local urban NOx emissions or pyrogenic emissions from the long-range transport of older plumes, thus affecting the production of O3 within the plumes. The results from this study have identified another potential cause of the variability in O3 concentrations observed in the measurements of biomass burning emissions, where evidence of stratosphere–troposphere exchange due to the pyroconvective updrafts from fires has been identified. Perturbations caused by the lofted emissions in these fire-aided convective processes may result in the intrusion of stratospheric air masses into the free troposphere and subsequent mixing of stratospheric O3 into the pyrogenic outflows causing fluctuations in observed ΔO3/ΔCO molar ratios.

Robust extraction of baseline signal of atmospheric trace species using local regression

Abstract. The identification of atmospheric trace species measurements that are representative of well-mixed background air masses is required for monitoring atmospheric composition change at background sites. We present a statistical method based on robust local regression that is well suited for the selection of background measurements and the estimation of associated baseline curves. The bootstrap technique is applied to calculate the uncertainty in the resulting baseline curve. The non-parametric nature of the proposed approach makes it a very flexible data filtering method. Application to carbon monoxide (CO) measured from 1996 to 2009 at the high-alpine site Jungfraujoch (Switzerland, 3580 m a.s.l.), and to measurements of 1,1-difluoroethane (HFC-152a) from Jungfraujoch (2000 to 2009) and Mace Head (Ireland, 1995 to 2009) demonstrates the feasibility and usefulness of the proposed approach. The determined average annual change of CO at Jungfraujoch for the 1996 to 2009 period as estimated from filtered annual mean CO concentrations is −2.2 ± 1.1 ppb yr−1. For comparison, the linear trend of unfiltered CO measurements at Jungfraujoch for this time period is −2.9 ± 1.3 ppb yr−1.

ACE-FTS measurements of trace species in the characterization of biomass burning plumes

Abstract. To further our understanding of the effects of biomass burning emissions on atmospheric composition, we report measurements of trace species in biomass burning plumes made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) instrument on the SCISAT-1 satellite. An extensive set of 15 molecules, C2H2, C2H6, CH3OH, CH4, CO, H2CO, HCN, HCOOH, HNO3, NO, NO2, N2O5, O3, OCS and SF6 are used in our analysis. Even though most biomass burning smoke is typically confined to the boundary layer, some of these emissions are injected directly into the free troposphere via fire-related convective processes and transported away from the emission source. Further knowledge of the aging of biomass burning emissions in the free troposphere is needed. Tracer-tracer correlations are made between known pyrogenic species in these plumes in an effort to characterize them and follow their chemical evolution. Criteria such as age and type of biomass material burned are considered.

Sensitive Method for the Kinetic Measurement of Trace Species in Liquids Using Cavity Enhanced Absorption Spectroscopy with Broad Bandwidth Supercontinuum Radiation

A novel spectrometer for the rapid and sensitive detection of liquid phase analytes at trace concentrations is presented. Broad bandwidth supercontinuum radiation was coupled into a linear optical cavity incorporating an intracavity liquid-sample cuvette. Cavity enhanced absorption spectra of trace species covering more than 300 nm were acquired on time scales of milliseconds. Single shot acquisition times of 10-50 ms are demonstrated here. The effective absorption path length exceeds 2 m in sample volumes measuring 2.7 mL. A key feature of the instrument is that it can be calibrated using cavity ring-down spectroscopy without the requirement of changing the optical alignment. The sensitivity of the instrument is exemplified by measurements of trace concentrations of dye molecules and nickel sulfate dissolved in water. A minimum detectable absorption coefficient of 9.1 x 10(-7) cm(-1) Hz(-1/2) at 550 nm was obtained. The capability to capture broad bandwidth absorption spectra on short time scales permits kinetic studies of liquid phase reactions. We demonstrate this by recording the oscillatory behavior of a Belousov-Zhabotinsky reaction.

IASI measurements of reactive trace species in biomass burning plumes

Abstract. This work presents observations of a series of short-lived species in biomass burning plumes from the Infrared Atmospheric Sounding Interferometer (IASI), launched onboard the MetOp-A platform in October 2006. The strong fires that have occurred in the Mediterranean Basin – and particularly Greece – in August 2007, and those in Southern Siberia and Eastern Mongolia in the early spring of 2008 are selected to support the analyses. We show that the IASI infrared spectra in these fire plumes contain distinctive signatures of ammonia (NH3), ethene (C2H4), methanol (CH3OH) and formic acid (HCOOH) in the atmospheric window between 800 and 1200 cm−1, with some noticeable differences between the plumes. Peroxyacetyl nitrate (CH3COOONO2, abbreviated as PAN) was also observed with good confidence in some plumes and a tentative assignment of a broadband absorption spectral feature to acetic acid (CH3COOH) is made. For several of these species these are the first reported measurements made from space in nadir geometry. The IASI measurements are analyzed for plume height and concentration distributions of NH3, C2H4 and CH3OH. The Greek fires are studied in greater detail for the days associated with the largest emissions. In addition to providing information on the spatial extent of the plume, the IASI retrievals allow an estimate of the total mass emissions for NH3, C2H4 and CH3OH. Enhancement ratios are calculated for the latter relative to carbon monoxide (CO), giving insight in the chemical processes occurring during the transport, the first day after the emission.

An Empirical Approach to Estimating Detection Limits Using Collocated Data

Measurements of trace species generally become less certain as concentrations decrease. Data analysts need guidance on the ranges in which particular measurements are meaningful. This guidance is normally stated in the form of detection limits. The International Union for Pure and Applied Chemistry (IUPAC) has defined several parameters to characterize measurement detection limits (Currie, L. A. Pure Appl. Chem. 1995, 67, 1699). The published guidelines envision an ability to prepare reference materials with concentrations close to the detection limits using the same methods as for normal samples. For multianalyte methods such as X-ray fluorescence (XRF), multiple reference materials may be required for each analyte to characterize the effects of interferences. The creation and characterization of such complex reference materials atthe detection limits of modern XRF systems represents a considerable technical challenge. This paper describes an observational approach to estimating the detection limits defined by IUPAC. Our empirical approach takes advantage of collocated (duplicate) measurements that are routinely collected by the Interagency Monitoring of Protected Visual Environments (IMPROVE) network and Speciation Trends Network (STN). The analysis is successfully demonstrated by deriving detection limits at the measurement system level for six elements measured on PM2.5 samples by XRF in both networks. The two networks' detection limits are found to be similar in terms of loading (areal density, ng cm(-2)) on the filters as measured by the XRF instruments despite many differences in sample collection, handling, and analysis. IMPROVE detection limits are an order of magnitude lowerthan STN's in terms of atmospheric concentrations (ng m(-3)), because IMPROVE uses smaller fitters and higher flow rates which lead to more concentrated sample deposits.

European surface ozone in the extreme summer 2003

Measurements of ozone and other trace species in the European EMEP network in 2003 are presented. The European summer of 2003 was exceptionally warm, and the surface ozone data for central Europe show the highest values since the end of the 1980s. The 95th percentiles of daily maximum hourly ozone concentrations in 2003 were higher than the corresponding parameter measured in any previous year at many sites in France, Germany, Switzerland and Austria. In this paper we argue that a number of positive feedbacks between the weather conditions and ozone contributed to the elevated surface ozone. First, we calculated an extended residence time of air parcels in the atmospheric boundary layer for several sites in central Europe. Second, we show that it is likely that extensive forest fires on the Iberian Peninsula, resulting from the drought and heat, contributed to the peak ozone values in north Europe in August. Third, regional‐scale model calculations indicate that enhanced levels of biogenic isoprene could have contributed up to 20% of the peak ozone concentrations. Measurements indicate elevated concentrations of isoprene compared to previous years. Sensitivity runs with a global chemical transport model showed that a reduction in the surface dry deposition due to drought and the elevated air temperature both could have contributed significantly to the enhanced ozone concentrations. Because of climate change, such heat waves may occur more frequently in the future and may gradually overshadow the effect of reduced emissions from anthropogenic sources of VOC and NOx in controlling surface ozone.

Temperature dependent air-broadened linewidths of ozone rotational transitions

Ozone resonant features in the rotational band are useful for composition measurements in the stratosphere and upper troposphere. However, the density of ozone features in the long wavelengths often decreases precision of other trace species measurements, especially when insufficient spectral parameters exist. This work presents systematic studies of temperature dependent air-broadened linewidth parameters for ozone. Eighteen new experimental measurements are presented and compared with 57 calculated parameters that cover spectral regions currently probed by the Earth Observing System—Microwave Limb Sounder. Comparisons with previous measurements and similar broadening studies in the infrared wavelengths are made.

Technical Note: Intercomparison of ILAS-II version 2 and 1.4 trace species with MIPAS-B measurements

Abstract. The Improved Limb Atmospheric Spectrometer (ILAS)-II sensor aboard the Japanese ADEOS-II satellite was launched into its sun-synchronous orbit on 14 December 2002 and performed solar occultation measurements of trace species, aerosols, temperature, and pressure in the polar stratosphere until 25 October 2003. Vertical trace gas profiles obtained with the balloon version of the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS-B) provide one of the sparse data sets for validating ILAS-II version 2 and 1.4 data. The MIPAS-B limb emission spectra were collected on 20 March 2003 over Kiruna (Sweden, 68° N) at virtually the same location that has been sounded by ILAS-II about 5.5 h prior to the sampling of MIPAS-B. The intercomparison of the new ILAS-II version 2 (Northern Hemispheric sunrise) data to MIPAS-B vertical trace gas profiles shows a good to excellent agreement within the combined error limits for the species O3, N2O, CH4, H2O (above 21 km), HNO3, ClONO2, and CFC-11 (CCl3F) in the compared altitude range between 16 and 31 km such that these data appear to be very useful for scientific analysis. With regard to the previous version 1.4 ILAS-II data, significant improvements in the consistency with MIPAS-B are obvious especially for the species CH4 and H2O, but also for O3, HNO3, ClONO2, NO2, and N2O5. However, comparing gases like NO2, N2O5, and CFC-12 (CCl2F2) exhibits only poor agreement with MIPAS-B such that these species cannot be assumed to be validated at the present time.

A comparison of gas- and liquid-loaded sorbent standards for the calibration of measurements of volatile organic compounds

The monitoring of volatile organic compounds (VOCs) at environmental levels is commonly carried out in the field using Perkin-Elmer-type sorbent-based passive or active samplers. Measurements of important trace species, such as benzene, toluene and xylenes, are often calibrated using thermal desorption standards that are liquid loaded onto the sorbent. This method is normally adopted over the method of gas loading of standards, on the grounds of ease of application and cost. We report a comparison between sorbent standards prepared by both methods using the sorbent Carbopack X and show that under the conditions used here the two show good agreement with each other. We also present work using liquid-loaded standards on the sorbents Tenax and Chromosorb-106, prepared with the solvents cyclohexane and methanol. The results indicate that, to obtain 100% recovery of the determinands from the desorbed samplers, issues such as type of solvent and sorbent combination together with purge flow should be considered.

Single photon ionization (SPI) via incoherent VUV-excimer light: robust and compact time-of-flight mass spectrometer for on-line, real-time process gas analysis.

Fast on-line detection of organic compounds from complex mixtures, such as industrial process gas streams, require selective and sensitive analytical methods. One feasible approach for this purpose is the use of mass spectrometry (MS) with a selective and soft (fragment-free) ionization technique, such as chemical ionization (CI) or photo ionization (PI). Single photon ionization (SPI) with vacuum ultraviolet (VUV) light is a particularly sof tionization technique, well-suited for detection of both aromatic and aliphatic species. Problematic, however, is the generation of the VUV light. In general, the vacuum ultraviolet (VUV) light sources for SPI-MS are based either on lasers (e.g., 118-nm radiation generated by frequency-tripling of the third harmonic of a Nd:YAG laser) or on conventional VUV lamps, such as deuterium lamps. Althoughthe laser-based techniques are very sophisticated and expensive, the conventional lamps have serious drawbacks regarding their optical parameters, such as low-output power, low spectral power density, and broad emission bands. In this work, a novel excimer VUV light source, in which an electron beam is used to form rare gas excimer species, is used. The excimer VUV light sourceproduces brilliant and intense VUV light. The novel VUV light source was coupled to a compact and mobile time-of-flight mass spectrometer (TOFMS). A special interface design, including optical (VUV optics) as well as electronic measures (e.g., pulsed ion extraction) was realized. The use of the excimer VUV lamp for SPI will allow the realization of very compact, rugged, and sensitive SPI-TOFMS devices, which preferably will be adapted for process analytical application or monitoring issues (e.g., chemical warfare detection). The excimer VUV-lamp technology delivers VUV light with a good beam quality and high-output power at low costs. Furthermore, it allows changing the emitted wavelength as well as the bandwidth of the excimer VUV lamp in t he 100-200-nm region by changing the gas filling. Consequently, SPI-TOFMS with an excimer light source is a fast detection technique that can be used for online monitoring, for example, in environmental studies or industrial manufacturing processes. In this paper, technology and characteristics of the new excimer light source, as well as the combination with the TOFMS, are presented. Furthermore, a first characterization of the SPI-TOFMS instrument, regarding analytical parameters such as detection limits and selectivity, is given. This includes a discussion of potential improvements that probably will be achievable within a future prototype genertation. Finally, first applications of the system for on-line measurement of organic trace species in a complex gas mixture (here, motorcycle exhaust gas) are presented.

Use of capillary gas chromatography with negative ion-chemical ionization mass spectrometry for the determination of perfluorocarbon tracers in the atmosphere.

A sensitive and selective technique for the quantitative measurement of atmospheric perfluorocarbon trace species at the sub part per quadrillion (10(-15)) levels is presented. The method utilizes advances in adsorbent enrichment techniques coupled with benchtop capillary gas chromatography and negative ion-chemical ionization mass spectrometry. The development and enhancement of sampling technology for tracer experiments is described, and the results from background measurements and a preliminary field experiment are presented. The overall precision of the analytical method with respect to the preferred tracer for these atmospheric transport studies, perfluoromethylcyclohexane, was +/-1.7%. The background concentrations of perfluorodimethylcyclobutane, perfluoromethylcyclopentane, and perfluoromethylcyclohexane at a remote coastal location (Mace Head, Ireland, 53 degrees N, 10 degrees W) were found to be 2.5 (+/-0.4), 6.8 (+/-1.0), and 5.2 fL L(-1) (+/-1.3), respectively. Background concentrations within an urban conurbation (Bristol, U.K.) were slightly greater at 3.0 (+/-1.5), 8.1 (+/-1.8), and 6.3 fL L(-1) (+/-1.1), respectively.

High-resolution Fourier-transform ultraviolet–visible spectrometer for the measurement of atmospheric trace species: application to OH

A compact, high-resolution Fourier-transform spectrometer for atmospheric near-ultraviolet spectroscopy has been installed at the Jet Propulsion Laboratory's Table Mountain Facility (34.4 degrees N, 117.7 degrees W, elevation 2290 m). This instrument is designed with an unapodized resolving power near 500,000 at 300 nm to provide high-resolution spectra from 290 to 675 nm for the quantification of column abundances of trace atmospheric species. The measurement technique used is spectral analysis of molecular absorptions of solar radiation. The instrument, accompanying systems designs, and results of the atmospheric hydroxyl column observations are described.

Water vapor density effect on measurements of trace gas mixing ratio and flux with a massflow controller

One configuration in measurements of trace species in the atmosphere uses a massflow controller to regulate airflow through a device that traps the species of interest. Because the flow controller is calibrated with dry air, correction should be made to account for the density effect of water vapor in the ambient air. A common type of massflow controller regulates the flow by measuring temperature difference along a heated tube. It is shown that for these units the true mixing ratio Sc (ratio of mass of trace species c to mass of dry air) is related to the apparent mixing ratio Sc (measurement before correction), as Sc = (1+1.85r)Sc, where r is water vapor mixing ratio of the ambient air, typically varying in the range 0–0.04. Correction should also be made to the surface‐air flux measured with such an apparatus in conjunction with micrometeorological flux gradient or relaxed eddy accumulation method and flux chambers; this involves a correction term proportional to surface evaporation rate and for some measurement configurations the same scale factor (1+1.85r) as for Sc. Examples are given for gaseous mercury to illustrate the magnitude of these corrections. No correction is needed if moisture is removed from the airstream before it enters the massflow controller.