Good Absolute Agreement Research Articles

Free-breathing myocardial T2* mapping using GRE-EPI and automatic non-rigid motion correction.

BackgroundMeasurement of myocardial T2* is becoming widely used in the assessment of patients at risk for cardiac iron overload. The conventional breath-hold, ECG-triggered, segmented, multi-echo gradient echo (MGRE) sequence used for myocardial T2* quantification is very sensitive to respiratory motion and may not be feasible in patients who are unable to breath-hold. We propose a free-breathing myocardial T2* mapping approach that combines a single-shot gradient-echo echo-planar imaging (GRE-EPI) sequence for T2*-weighted image acquisition with automatic non-rigid motion correction (MOCO) of respiratory motion between single-shot images.MethodsECG-triggered T2*-weighted images at different echo times were acquired by a black-blood, single-shot GRE-EPI sequence during free-breathing. A single image at a single TE is acquired in each heartbeat. Automatic non-rigid MOCO was applied to correct for in-plane respiratory motion before pixel-wise T2* mapping. In a total of 117 patients referred for clinical cardiac magnetic resonance exams, the free-breathing MOCO GRE-EPI sequence was compared to the breath-hold segmented MGRE approach. Image quality was scored independently by 2 experienced observers blinded to the particular image acquisition strategy. T2* measurements in the interventricular septum and in the liver were compared for the two methods in all cases with adequate image quality.ResultsT2* maps were acquired in all 117 patients using the breath-hold MGRE and the free-breathing MOCO GRE-EPI approaches, including 8 patients with myocardial iron overload and 25 patients with hepatic iron overload. The mean image quality of the free-breathing MOCO GRE-EPI images was scored significantly higher than that of the breath-hold MGRE images by both reviewers. Out of the 117 studies, 21 breath-hold MGRE studies (17.9 % of all the patients) were scored to be less than adequate or very poor by both reviewers, while only 2 free-breathing MOCO GRE-EPI studies were scored to be less than adequate image quality. In a comparative evaluation of the images with at least adequate quality, the intra-class correlation coefficients for myocardial and liver T2* were 0.868 and 0.986 respectively (p < 0.001), indicating that the T2* measured by breath-hold MGRE and free-breathing MOCO GRE-EPI were in close agreement. The coefficient of variation between the breath-hold and free-breathing approaches for myocardial and liver T2* were 9.88 % and 9.38 % respectively. Bland-Altman plots demonstrated good absolute agreement of T2* in the interventricular septum and the liver from the free-breathing and breath-hold approaches (mean differences -0.03 and 0.16 ms, respectively).ConclusionThe free-breathing approach described for T2* mapping using MOCO GRE-EPI enables accurate myocardial and liver T2* measurements and is insensitive to respiratory motion.

Validation of the automated method VIENA: An accurate, precise, and robust measure of ventricular enlargement

In many retrospective studies and large clinical trials, high-resolution, good-contrast 3DT1 images are unavailable, hampering detailed analysis of brain atrophy. Ventricular enlargement then provides a sensitive indirect measure of ongoing central brain atrophy. Validated automated methods are required that can reliably measure ventricular enlargement and are robust across magnetic resonance (MR) image types. To validate the automated method VIENA for measuring the percentage ventricular volume change (PVVC) between two scans. Accuracy was assessed using four image types, acquired in 15 elderly patients (five with Alzheimer's disease, five with mild cognitive impairment, and five cognitively normal elderly) and 58 patients with multiple sclerosis (MS), by comparing PVVC values from VIENA to manual outlining. Precision was assessed from data with three imaging time points per MS patient, by measuring the difference between the direct (one-step) and indirect (two-step) measurement of ventricular volume change between the first and last time points. The stringent concordance correlation coefficient (CCC) was used to quantify absolute agreement. CCC of VIENA with manual measurement was 0.84, indicating good absolute agreement. The median absolute difference between two-step and one-step measurement with VIENA was 1.01%, while CCC was 0.98. Neither initial ventricular volume nor ventricular volume change affected performance of the method. VIENA has good accuracy and good precision across four image types. VIENA therefore provides a useful fully automated method for measuring ventricular volume change in large datasets. VIENA is a robust, accurate, and precise method for measuring ventricular volume change.

Long-term validation of tropospheric column-averaged CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; mole fractions obtained by mid-infrared ground-based FTIR spectrometry

Abstract. At the Izaña Atmospheric Research Center, high-resolution mid-infrared solar absorption spectra have been recorded for more than 12 yr using Fourier Transform InfraRed (FTIR) spectrometers. We use the spectral fitting algorithm PROFFIT to retrieve long-term time series of methane (CH4) from the measured spectra. We investigate the total column-averaged dry air mole fractions of methane (totXCH4) obtained from a profile scaling and a profile retrieval, and apply two approaches for deriving the tropospheric column-averaged dry air mole fractions: firstly, we use the FTIR hydrogen fluoride (HF) total column amounts as an estimator for the stratospheric CH4 contribution and a posteriori correct the totXCH4 data of a profile scaling retrieval accordingly (troXCH4post); secondly, we directly determine the tropospheric column-averaged dry air mole fractions of methane (troXCH4retr) from retrieved CH4 profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH4 amounts that are subject to a large smoothing error, which can be widely avoided by applying a profile retrieval (for the latter we estimate an overall precision of 0.41%). We compare the different FTIR CH4 data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH4 data (CH4GAW), which in the case of the Izaña Atmospheric Research Center high mountain observatory are very representative for the free tropospheric CH4 amounts. Concerning totXCH4, the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH4 is not a valid free tropospheric CH4 proxy, as it is significantly affected by the varying stratospheric CH4 contribution and it rather follows the variation in the tropopause altitude. The a posteriori correction method as applied here only removes a part of this stratospheric CH4 contribution. In contrast the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH4 amounts. Results of the profile retrieval analysis correlate well with the CH4GAW data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97%), and both data sets show very similar annual cycles and trend behaviour for the 2001–2010 time period. Furthermore, we find a very good absolute agreement between the troXCH4retr and CH4GAW (mid-infrared FTIR/GAW scaling factor of 0.9987) suggesting that mid-infrared FTIR data can be well combined with the surface in-situ GAW data. Our study strongly supports the value of mid-infrared ground-based FTIR CH4 profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH4 data of high quality.

Ultra-high extinction ratio micropolarizers using plasmonic lenses

The design of a new type of plasmonic ultra-high extinction ratio micropolarizing transmission filter is presented along with an experimental demonstration. A pair of dielectric coated metal gratings couple incident TM polarized light into surface plasmons, which are fed into a central metal-insulator-metal (MIM) waveguide, followed by transmission through a sub-wavelength aperture. Extinction ratios exceeding 10¹¹ are predicted by finite element simulation. Good absolute agreement for both the spectral and polarization response is obtained between measurement and simulations using measured geometric parameters. The filters can be easily fabricated and sized to match the pixel pitch of current focal plane arrays.

Measuring physical activity during pregnancy

BackgroundCurrently, little is known about physical activity patterns in pregnancy with prior estimates predominantly based on subjective assessment measures that are prone to error. Given the increasing obesity rates and the importance of physical activity in pregnancy, we evaluated the relationship and agreement between subjective and objective physical activity assessment tools to inform researchers and clinicians on optimal assessment of physical activity in pregnancy.Methods48 pregnant women between 26-28 weeks gestation were recruited. The Yamax pedometer and Actigraph accelerometer were worn for 5-7 days under free living conditions and thereafter the International Physical Activity Questionnaire (IPAQ) was completed. IPAQ and pedometer estimates of activity were compared to the more robust and accurate accelerometer data.ResultsOf 48 women recruited, 30 women completed the study (mean age: 33.6 ± 4.7 years; mean BMI: 31.2 ± 5.1 kg/m2) and 18 were excluded (failure to wear [n = 8] and incomplete data [n = 10]). The accelerometer and pedometer correlated significantly on estimation of daily steps (ρ = 0.69, p < 0.01) and had good absolute agreement with low systematic error (mean difference: 505 ± 1498 steps/day). Accelerometer and IPAQ estimates of total, light and moderate Metabolic Equivalent minutes/day (MET min-1 day-1) were not significantly correlated and there was poor absolute agreement. Relative to the accelerometer, the IPAQ under predicted daily total METs (105.76 ± 259.13 min-1 day-1) and light METs (255.55 ± 128.41 min-1 day-1) and over predicted moderate METs (-112.25 ± 166.41 min-1 day-1).ConclusionCompared with the accelerometer, the pedometer appears to provide a reliable estimate of physical activity in pregnancy, whereas the subjective IPAQ measure performed less accurately in this setting. Future research measuring activity in pregnancy should optimally encompass objective measures of physical activity.Trial RegistrationAustralian New Zealand Clinical Trial Registry Number: ACTRN12608000233325. Registered 7/5/2008.

Comparison of body composition with bioelectric impedance (BIA) and dual energy X-ray absorptiometry (DEXA) among Singapore Chinese

The purpose of the study was to determine the agreement of bioelectric impedance (BIA) with dual energy X-ray absorptiometry (DEXA) among Singapore Chinese adults. Hundred subjects [63 healthy males (age: 25.2 ± 0.5 yrs; weight: 69.7 ± 1.9 kg; BMI: 22.8 ± 0.4 kg m −2) and 37 females (age: 22.9 ± 0.3 yrs; weight: 51.8 ± 0.8 kg; BMI: 20.2 ± 0.4 kg m −2)] were selected and body fat percentage (%BF) was measured by DEXA and BIA. Paired t test and coefficient of correlation statistics were utilized to compare the relationship between %BF BIA and %BF DEXA. Bland and Altman plot was employed to investigate the agreement of %BF BIA with %BF DEXA. The limits of agreement between different methods were defined as mean ( M; bias) ± 1.96SD of the difference between the methods (95% confidence interval; CI). %BF BIA, when compared to %BF DEXA, revealed non-significant underestimation of %BF in females (24.1% < 24.4%, p > 0.05) and significant overestimation in males (15.5% < 17.7%, p < 0.01). There was good absolute agreement between %BF BIA and %BF DEXA among the whole cohort (1.3 ± 6.9%) as well as among both genders (male: 2.2 ± 6.7%, female: −0.3 ± 6.1%) due to small mean differences between both methods. However, wider limits of agreement were revealed for %BF BIA among whole cohort and as well as on gender basis. The results indicate a good agreement between BIA and DEXA in measuring %BF among Singapore Chinese adults, but may not be a suitable method of measuring %BF for clinical purposes among this population due to wider limits of agreement.

Comparison of body composition methods in overweight and obese Brazilian women

The purpose of this study was to compare skinfold thickness (SKF) and bioelectrical impedance analysis (BIA) of body composition using three different equations against dual-energy X-ray absorptiometry (DXA) in overweight and obese Brazilian women. Thirty-four women (age 43.8 +/- 10.9 years; body mass index [BMI] 32.1 +/- 4.3 kg/m(2)) had percentage body fat (BF%), fat mass (FM) and fat-free mass (FFM) estimated by DXA, SKF and BIA (BIA-man: manufacturer's equation; and predictive obesity-specific equations of Segal and of Gray). Regression analysis, Bland-Altman plot analysis and intra-class correlation coefficient (ICC) were used to compare methods. Absolute agreement between DXA and BIA-man was poor for all measures of body composition (BF% -6.8% +/- 3.7%, FM -3.1 +/- 3.6 kg, FFM 5.7 +/- 2.8 kg). BIA-Segal equation showed good absolute agreement with DXA for BF% (1.5% +/- 1.5%), FM (1.0 +/- 3.2 kg) and FFM (1.5 +/- 2.6 kg), albeit the limits of agreement were wide. BIA-Gray equation showed good absolute agreement with DXA for FM (2.3 +/- 4.1 kg), and smaller biases for BF% (0.05% +/- 4.4%) and FFM (0.2 +/- 2.9 kg), although wide limits of agreement. BIA-Gray and DXA showed the highest ICC among the pairs of methods. A good absolute agreement was observed between DXA and SKF for BF% (-2.3% +/- 5.8%), FM (0.09 +/- 4.7 kg), and FFM (2.4 +/- 4.4 kg), although limits of agreement were wider and ICC between DXA and SKF for BF% indicated poor degree of reproducibility. These findings show that both BIA-Segal and BIA-Gray equations are suitable for BF%, FM and FFM estimations in overweight and obese women.

Online gait event detection using a large force platform embedded in a treadmill

Gait research and clinical gait training may benefit from movement-dependent event control, that is, technical applications in which events such as obstacle appearance or visual/acoustic cueing are (co)determined online on the basis of current gait properties. A prerequisite for successful gait-dependent event control is accurate online detection of gait events such as foot contact (FC) and foot off (FO). The objective of the present study was to assess the feasibility of online FC and FO detection using a single large force platform embedded in a treadmill. Center-of-pressure, total force output and kinematic data were recorded simultaneously in 12 healthy participants. Online FC and FO estimates and spatial and temporal gait parameters estimated from the force platform data—i.e., center-of-pressure profiles—were compared to offline kinematic counterparts, which served as the gold standard. Good correspondence was achieved between online FC detections using center-of-pressure profiles and those derived offline from kinematic data, whereas FO was detected 31 ms too late. A good relative and absolute agreement was achieved for both spatial and temporal gait parameters, which was improved further by applying more fine-grained FO estimation procedures using characteristic local minima in the total force output time series. These positive results suggest that the proposed system for gait-dependent event control may be successfully implemented in gait research as well as gait interventions in clinical practice.

Accuracy of Bioelectrical Impedance Consumer Devices for Measurement of Body Composition in Comparison to Whole Body Magnetic Resonance Imaging and Dual X-Ray Absorptiometry

Objective: To compare body composition determined by bioelectrical impedance (BIA) consumer devices against criterion estimates determined by whole body magnetic resonance imaging (MRI) and dual energy X-ray absorptiometry (DXA) in healthy normal weight, overweight and obese adults. Methods: In 106 adults (54 females, 52 males, age 54.2 ± 16.1 years, BMI 25.8 ± 4.4 kg/m²) fat mass (FM), skeletal muscle mass (SM), total body bone-free lean mass (TBBLM), and level of visceral fat mass (VF) were estimated by 3 single-frequency bipedal (foot-to-foot) and one tretrapolar BIA device, and compared to body composition measured by MRI and DXA. Bland-Altman and simple linear regression analyses were used to determine agreement between methods. Results: %FMDXA, SMMRI or TBBLMDXA showed good relative and absolute agreement with two bipolar and one tetrapolar instrument (r² = 0.92–0.96; all p < 0.001; mean bias <1.5 %FM and <1 kg SM or TBBLM) and less relative and absolute agreement for another bipolar device (r² = 0.82 and 0.84, mean bias ∼3 %FM and ∼3 kg SM). The 95% limits of agreement (bias ± 2 SD) were narrowest for the tetrapolar device (–6.59 to 4.61 %FM and –4.62 to 4.74 kg SM) and widest for bipolar instruments (up to –14.54 to 8.58 %FM and –9.52 to 3.92 kg SM). Systematic biases for %FM were found for all bipedal devices, but not for the tetrapolar instrument. Conclusion: Because of the lower agreement between foot-to-foot BIA and DXA or MRI for the assessment of body composition in individuals, tetrapolar electrode arrangement should be preferred for individual or public use. Bipolar devices provide accurate results for field studies with group estimation.

Differential cross sections for positron-xenon elastic scattering

Absolute measurements of differential cross sections for the elastic scattering of positrons from xenon are made at 2, 5 and $8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ using a trap-based beam and the technique of measuring scattering cross sections in a strong magnetic field. Calculations are carried out using the relativistic Dirac equations with a static plus polarization potential. Generally good absolute agreement is found between experiment and theory.

Climacteric symptoms and quality of life: validity of women's health questionnaire

To evaluate the reliability and validity of the Portuguese version of the Women's Health Questionnaire. In order to evaluate the Women's Health Questionnaire (WHQ), an analytical cross-sectional study was carried out at the women's menopause outpatient clinic of a university hospital in São Paulo, Brazil. There were studied 87 women in perimenopause or menopause, defined as experiencing at least one year's absence of menstrual flow. The following variables were collected: demographic data, clinical variables (Kupperman index and correlate numeric scale) and quality of life indexes (SF-36 and utility). The WHQ proved to be a questionnaire easily translated into Portuguese and well-adjusted to Brazilian women. The internal consistency of the overall WHQ was excellent (Cronbach alpha =0.83; 95% CI: 0.71-0.91). Test-retest reliability was also excellent (intraclass correlation coefficient [ICC]=0.92; 95% IC: 0.86-0.96) and had good absolute agreement (0.84; 95% CI: 0.71-0.92). A satisfactory clinical validity was observed. The construct validity was corroborated by clear associations with others scales. A good index of responsiveness after the intervention was reached. The Portuguese version of the WHQ is of easy and fast administration and understanding. Its measuring properties were related, allowing its use in the evaluation of Brazilian climacteric women's quality of life for various purposes.

SU-FF-T-76: Implementation and Initial Testing of a Monte Carlo Based Algorithm for IMRT Inverse Treatment Planning

Purpose: To report on the implementation of a Monte Carlo (MC) based algorithm and to compare this system with a convolution/superposition-based algorithm (CS) for IMRT inverse planning. Method and Materials: The DPM MC code was modified using a fluence matrix approach to perform beamlet calculations for IMRT planning. The code was integrated within our in-house inverse treatment planning system and compared with the TPS (CS) algorithm. Initial testing involved the computation of 6 MV beamlet depth doses for 1×1, 2×2 and 10×10 (100, 1 cm beamlets) in a water phantom. MC and CS calculations were then performed for an example lung treatment plan to examine dosimetric differences between these algorithms. MC statistical uncertainties were on average less than 2% (in the depth doses) for all beamlet calculations. Optimization of beamlet doses is carried out using simulated annealing with quadratic cost functions derived from our clinical protocols. Results: Beamlet depth doses calculated with MC and CS are in good absolute agreement for field sizes larger than 2×2 cm2. Significant differences exist for 1×1 beamlets because CS is unable to accurately model lateral electron transport. For the example lung plan, much smaller differences were found. This is likely due to the fact that with larger field sizes (∼10×10 cm in the example), effects of lateral electron scattering are much less pronounced. Conclusion: We have implemented a fluence matrix method to perform MC-based beamlet calculations for IMRT planning. Initial testing for an example lung plan and field sizes larger than 2×2, revealed good agreement between MC and CS. However, larger differences were found for 1×1 beamlets due to lateral electron transport issues. Testing is currently being performed for a variety of treatment plans, spanning a range of field sizes to thoroughly investigate dosimetric differences between MC and CS in IMRT planning. Supported by NIH P01-CA59827

Rotational energy transfer in collisions of CH A2Δ, v = 0 with Ar, N2and CO2

Rotational energy transfer (RET) in collisions of CH A2Δ, v=0 with Ar, N2 and CO2 has been investigated experimentally. Laser excitation was used to prepare selected initial levels N=2,3,5 (all colliders), 8 (Ar and CO2) and 14 (Ar only). Additional fine-structure state selection was achieved for the lower levels, and Λ-doublet selection for the higher levels. Total and state-to-state RET rate constants were extracted from time-resolved dispersed fluorescence spectra. Partial fine-structure-state resolution was possible for N′⩽4, and Λ-doublet resolution for the highest levels. For Ar, good absolute agreement was found with previous state-selective experimental work and ab initio theoretical predictions. RET is substantially more efficient for N2 and CO2, for which the first reliable absolute rate constants are provided. A change of fine-structure state with ΔN=0 is the most probable single state-to-state channel at low N for all three partners. Partial conservation of the fine-structure label during ΔN≠0 collisions, previously observed for Ar, was found to extend to the other partners. The relative ΔN propensities are similar for all three partners. This is consistent with a simple impulsive model of the collision dynamics, in which energy constraints dominate because of the large rotational energy spacing characteristic of hydrides. This model does not, however, provide a straightforward explanation for the substantial differences in RET efficiency for Ar, N2 and CO2.

Intercomparison of NO2 photolysis frequency measurements by actinic flux spectroradiometry and chemical actinometry during JCOM97

A first intercomparison of JNO2 measurements by actinic flux spectroradiometry (SR) and chemical actinometry (CA) was performed in summer 1997 in Jülich, Germany. During this experiment (JCOM97) a large set of data was collected with a time resolution of ∼ 1 min, showing a high linear correlation (R = 0.998) in JNO2. Linear regression indicates a good absolute agreement with the CA being 6% higher than SR if the quantum yield recommended by NASA/JPL (1997) and recently published data of the NO2 absorption spectrum (σNO2) is used for the evaluation of the SR‐data. The influence of using previously published σNO2 data is discussed. Overall the comparison suggests that JNO2 can be measured by either CA or SR with an accuracy of better than 10%.

Study of Quantum Dynamics in the Transition from Classical Stability to Chaos.

We report an experimental and theoretical study of momentum transfer from a modulated standing wave of light to a sample of ultracold atoms. This system is a quantum realization of the periodically driven rotor where the underlying classical phase space goes from stable to chaotic as a control parameter is varied. Our experimental results are in good absolute agreement with a quantum Floquet analysis and with a quantum simulation. We relate the quantum evolution to the underlying classical dynamics in this mixed phase space regime.

Mass resolution of recoil fragment detector telescopes for 0.05–0.5 A MeV heavy recoiling fragments

The factors governing the mass resolution for 0.05–0.5 A MeV recoil nuclei have been investigated for detector telescopes in which carbon-foil time zero detectors and ion-implanted silicon detectors are used to determine the time of flight and energy respectively. Experimentally determined second moments of the mass distribution have been compared with theoretical estimates based on literature data. The experimental mass resolution is in reasonably good absolute agreement with theoretical estimates. For low energy (< 0.3 A MeV) particles the mass resolution is dominated by the contribution from the silicon detector and thus largely independent of timed flight length. In fact for detection of very low energy (0.1 A MeV) recoil nuclei timed flight lengths of less than 0.22 m are sufficient.

Oxygen aging of SRC-III heavy distillate

The aging of SRC-II heavy distillae has been studied under oxygen or nitrogen atmospheres at a temperature of 60‡C for one month. 1R spectra, viscosities, calorific values, and elemental analyses have been determined to characterize the aging behavior of samples. The oxygen aged samples exhibit a new 1R band at 1700-1cm which is not shown in the nitrogen aged samples. The viscosity increases by 325% in oxygen, but only 18% in nitrogen, after one month of aging at 60‡C. The sample aged in oxygen for one month shows a significant decrease in the heat of combustion, 106 cal/g. On the other hand, the decrease for the nitrogen aged sample is only 13 cal/g. Elemental analyses show that the content of carbon decreased by 1.03 wt% and that of hydrogen by 0.2 wt% in the oxygen aged sample compared with the unaged sample. No significant change occurs in the nitrogen aged sample. An examination has been made of the extent to which the experimental values of combustion heat can be fitted by various literature equations relating heats of combustion of coals to the elemental analyses. It is found that the equation of Mott and Spooner gives good absolute agreement (within 0.47% err) with our experimental values.

Characteristics of electron-beam-excited Xe*2 at low pressures as a vacuum ultraviolet source

The performance of Xe*2 as a 172-nm fluorescence or laser source when pumped by a low-current, long-pulse electron beam was determined. The fluorescence efficiency of Xe*2 is near the theoretical limit of ∼50% at modest pressures over a range of pump rates up to 106 W/cm. The laser efficiency is limited to values &amp;lt;1% by a very strong medium absorption that is probably due to Xe*2 photoionization. Laser performance is further degraded by early pulse termination that appears related to mirror degradation. An improved kinetics and extraction code was developed to model the performance of the Xe*2 system. A key component of the model is a more detailed treatment of the interactions between secondary electrons and excited atomic and molecular xenon states. Rates for these processes were derived as described herein. With this model, good absolute agreement was obtained between experiments and calculated parameters at pressures as low as 0.5 atm.

Characteristics of electron-beam-excited Kr*2 at low pressures as a vacuum ultraviolet source

The performance of Kr*2 as a 146 nm fluorescence or laser source when pumped by a low-current, long-pulse electron beam has been determined. The fluorescence efficiency of Kr*2 is near the theoretical limit of ∼46% at modest pressures over a range of pump rates up to 3×105 W/cm3. Lasing could not be achieved under the conditions studied here. An improved kinetics and extraction code has been developed to model the performance of the Kr*2 system. A key component of the model is a more detailed treatment of the interactions between secondary electrons and excited atomic and molecular xenon states. Estimated rates for these processes are presented. With this model, good absolute agreement of model predictions with experimental results has been obtained at pressures as low as 0.5 atm.

Atmospheric CO2 variations at Barrow, Alaska, 1973?1982

The first 10 years (1973–1982) of atmospheric CO2 measurements at Barrow, Alaska, by the NOAA/GMCC program are described. The paper updates and extends the Barrow CO2 record presented in Tellus (1982). The data are given in final form, based on recent calibrations of the Scripps Institution of Oceanography, with selected values identified as representative of large, spacescale conditions. Analyses of the data show: (1) a long-term CO2 average increase of 1.3 ppm per year, but with large year-to-year variations in that growth rate; (2) a suggestion, not statistically significant, of a secular increase in the amplitude of the annual cycle, presumably a reflection of global-scale biospheric variability; and (3) good absolute agreement between the Barrow results and those from four neighboring high latitude sites between 50 and 82°N.