Typical Measurement Uncertainties Research Articles

Thermal-Conductivity Measurement of Thermoelectric Materials Using $$3{{\upomega }}$$ 3 ω Method

In this work, a measurement system for high-temperature thermal-conductivity measurements has been designed, constructed, and characterized. The system is based on the $$3{\upomega }$$ method which is an ac technique suitable for both bulk and thin-film samples. The thermal-conductivity measurements were performed in a horizontal three-zone tube furnace whose sample space can be evacuated to vacuum or alternatively a protective argon gas environment can be applied to prevent undesired oxidation and contamination of the sample material. The system was tested with several dielectric, semiconductor, and metal bulk samples from room temperature up to 725 K. The test materials were chosen so that the thermal-conductivity values covered a wide range from $$0.37\,\hbox {W}\!\cdot \! \hbox {m}^{-1}\!\cdot \! \hbox {K}^{-1}$$ to $$150\,\hbox {W}\!\cdot \! \hbox {m}^{-1}\!\cdot \!\hbox {K}^{-1}$$ . An uncertainty analysis for the thermal-conductivity measurements was carried out. The measurement accuracy is mainly limited by the determination of the third harmonic of the voltage over the resistive metal heater strip that is used for heating the sample. A typical relative measurement uncertainty in the thermal-conductivity measurements was between 5 % and 8 % ( $$k=2$$ ). An extension of the $$3{\upomega }$$ method was also implemented in which the metal heater strip is first deposited on a transferable Kapton foil. Utilizing such a prefabricated sensor allows for faster measurements of the samples as there is no need to deposit a heater strip on each new sample.

Investigating temporal field sampling strategies for site-specific calibration of three soil moisture–neutron intensity parameterisation methods

Abstract. The Cosmic-Ray Neutron Sensor (CRNS) can provide soil moisture information at scales relevant to hydrometeorological modelling applications. Site-specific calibration is needed to translate CRNS neutron intensities into sensor footprint average soil moisture contents. We investigated temporal sampling strategies for calibration of three CRNS parameterisations (modified N0, HMF, and COSMIC) by assessing the effects of the number of sampling days and soil wetness conditions on the performance of the calibration results while investigating actual neutron intensity measurements, for three sites with distinct climate and land use: a semi-arid site, a temperate grassland, and a temperate forest. When calibrated with 1 year of data, both COSMIC and the modified N0 method performed better than HMF. The performance of COSMIC was remarkably good at the semi-arid site in the USA, while the N0mod performed best at the two temperate sites in Germany. The successful performance of COSMIC at all three sites can be attributed to the benefits of explicitly resolving individual soil layers (which is not accounted for in the other two parameterisations). To better calibrate these parameterisations, we recommend in situ soil sampled to be collected on more than a single day. However, little improvement is observed for sampling on more than 6 days. At the semi-arid site, the N0mod method was calibrated better under site-specific average wetness conditions, whereas HMF and COSMIC were calibrated better under drier conditions. Average soil wetness condition gave better calibration results at the two humid sites. The calibration results for the HMF method were better when calibrated with combinations of days with similar soil wetness conditions, opposed to N0mod and COSMIC, which profited from using days with distinct wetness conditions. Errors in actual neutron intensities were translated to average errors specifically to each site. At the semi-arid site, these errors were below the typical measurement uncertainties from in situ point-scale sensors and satellite remote sensing products. Nevertheless, at the two humid sites, reduction in uncertainty with increasing sampling days only reached typical errors associated with satellite remote sensing products. The outcomes of this study can be used by researchers as a CRNS calibration strategy guideline.

Methane standards made in whole and synthetic air compared by cavity ring down spectroscopy and gas chromatography with flame ionization detection for atmospheric monitoring applications.

There is evidence that the use of whole air versus synthetic air can bias measurement results when analyzing atmospheric samples for methane (CH4) and carbon dioxide (CO2). Gas chromatography with flame ionization detection (GC-FID) and wavelength scanned-cavity ring down spectroscopy (WS-CRDS) were used to compare CH4 standards produced with whole air or synthetic air as the matrix over the mole fraction range of 1600-2100 nmol mol(-1). GC-FID measurements were performed by including ratios to a stable control cylinder, obtaining a typical relative standard measurement uncertainty of 0.025%. CRDS measurements were performed using the same protocol and also with no interruption for a limited time period without use of a control cylinder, obtaining relative standard uncertainties of 0.031% and 0.015%, respectively. This measurement procedure was subsequently used for an international comparison, in which three pairs of whole air standards were compared with five pairs of synthetic air standards (two each from eight different laboratories). The variation from the reference value for the whole air standards was determined to be 2.07 nmol mol(-1) (average standard deviation) and that of synthetic air standards was 1.37 nmol mol(-1) (average standard deviation). All but one standard agreed with the reference value within the stated uncertainty. No significant difference in performance was observed between standards made from synthetic air or whole air, and the accuracy of both types of standards was limited only by the ability to measure trace CH4 levels in the matrix gases used to produce the standards.

On the Axisymmetric Counterflow Flame Simulations: Is There an Optimal Nozzle Diameter and Separation Distance to Apply Quasi One-Dimensional Theory?

Two-dimensional axisymmetric numerical analysis of counterflow flames was employed to better understand the applicability of the quasi one-dimensional theory of Seshadri and Williams to flames produced by small diameter convergent nozzles. The computational domain considered included the convergent sections of two opposed nozzles as well as the surrounding inert annular co-flows. For computational efficiency, the fuel-oxidizer system of diluted hydrogen versus air in non-premixed flame mode, with a detailed chemical kinetic model and mixture-averaged transport property description, was considered. With an increase of nozzle diameter from 6.5 mm to larger values (with plug flow velocity profiles at the nozzle exit), the influence of the radial terms on eigenvalue and scalar variables has been compared. Error metric on nozzle diameter effects is presented with comparison to typical experimental measurement uncertainties. The analysis also showed that, for nozzle separation distances below the free-floating limit, the self-similar function in quasi one-dimensional formulation can be preserved by specifying radial velocity boundary conditions, as long as the radial gradients are negligible.

Role of data uncertainties in identifying the logarithmic region of turbulent boundary layers

Composite expansions based on the log-law and the power-law were used to generate synthetic velocity profiles of zero pressure gradient turbulent boundary layers (TBLs) in the range of Reynolds number \(800 \le Re_{\theta } \le 860{,}000,\) based on displacement thickness and freestream velocity. Several artificial errors were added to the velocity profiles to simulate typical measurement uncertainties. The effects of the simulated errors were studied by extracting log-law and power-law parameters from all these pseudo-experimental profiles. Various techniques were used to establish a measure of the deviations in the overlap region. When parameters extracted for the log-law and the power-law are associated with similar levels of deviations with respect to their expected values, we consider that the profile leads to ambiguous conclusions. This ambiguity was observed up to \(Re_{\theta }=16{,}000\) for a 4 % dispersion in the velocity measurements, up to \(Re_{\theta }=8.6 \times 10^{5}\) for a 400 \(\upmu\)m uncertainty in probe position (in air flow at atmospheric pressure), and up to \(Re_{\theta }=32{,}000\) for 3 % uncertainty in the determination of \(u_{\tau }.\) In addition, a new method for the determination of the log-law limits is proposed. The results clearly serve as a further note for caution when identifying either a log or a power-law in TBLs. Together with a number of available studies in the literature, the present results can be seen as a additional reconfirmation of the log-law.

Effects of Environmental and Aircraft Parameters on Wake Vortex Behavior

This study investigates the impact of meteorological parameters (crosswind, headwind, wind shear, thermal stratification, turbulent kinetic energy, dissipation rate, and density) and aircraft parameters (position, heading, speed, weight, span, and spanwise load factor) on wake-vortex behavior. Typical measurement uncertainties of these parameters, on one hand, and the range in which they typically reside, on the other hand, are mapped on variations of lateral and vertical position and lifetime of the wake vortices. For this mapping process, the deterministic two-phase wake-vortex model is employed, complemented by some simple considerations, dimensional analysis, and sources from literature. Three scenarios comprising cruise conditions, flight within the atmospheric boundary layer, and ground proximity are considered. From these investigations, the following ranking of the impact parameters has been deduced: wind, thermal stratification, turbulence, position, mass, and spanwise load factor. The relevance of the remaining parameters appears small.

Strong shift from HCO3 (-) to CO 2 uptake in Emiliania huxleyi with acidification: new approach unravels acclimation versus short-term pH effects.

Effects of ocean acidification on Emiliania huxleyi strain RCC 1216 (calcifying, diploid life-cycle stage) and RCC 1217 (non-calcifying, haploid life-cycle stage) were investigated by measuring growth, elemental composition, and production rates under different pCO2 levels (380 and 950 μatm). In these differently acclimated cells, the photosynthetic carbon source was assessed by a 14C disequilibrium assay, conducted over a range of ecologically relevant pH values (7.9–8.7). In agreement with previous studies, we observed decreased calcification and stimulated biomass production in diploid cells under high pCO2, but no CO2-dependent changes in biomass production for haploid cells. In both life-cycle stages, the relative contributions of CO2 and HCO3 − uptake depended strongly on the assay pH. At pH values ≤ 8.1, cells preferentially used CO2 (≥ 90 % CO2), whereas at pH values ≥ 8.3, cells progressively increased the fraction of HCO3 − uptake (~45 % CO2 at pH 8.7 in diploid cells; ~55 % CO2 at pH 8.5 in haploid cells). In contrast to the short-term effect of the assay pH, the pCO2 acclimation history had no significant effect on the carbon uptake behavior. A numerical sensitivity study confirmed that the pH-modification in the 14C disequilibrium method yields reliable results, provided that model parameters (e.g., pH, temperature) are kept within typical measurement uncertainties. Our results demonstrate a high plasticity of E. huxleyi to rapidly adjust carbon acquisition to the external carbon supply and/or pH, and provide an explanation for the paradoxical observation of high CO2 sensitivity despite the apparently high HCO3 − usage seen in previous studies.Electronic supplementary materialThe online version of this article (doi:10.1007/s11120-014-9984-9) contains supplementary material, which is available to authorized users.

THE INTRINSIC SCATTER ALONG THE MAIN SEQUENCE OF STAR-FORMING GALAXIES ATz∼ 0.7

A sample of 12614 star-forming galaxies (SFGs) with stellar mass >10^9.5 M_sun between 0.6<z<0.8 from COSMOS is selected to study the intrinsic scatter of the correlation between star formation rate (SFR) and stellar mass. We derive SFR from ultraviolet (UV) and infrared (IR) luminosities. A stacking technique is adopted to measure IR emission for galaxies undetected at 24 micron. We confirm that the slope of the mass-SFR relation is close to unity. We examine the distributions of specific SFRs (SSFRs) in four equally spaced mass bins from 10^9.5 M_sun to 10^11.5 M_sun. Different models are used to constrain the scatter of SSFR for lower mass galaxies that are mostly undetected at 24 micron. The SFR scatter is dominated by the scatter of UV luminosity and gradually that of IR luminosity at increasing stellar mass. We derive SSFR dispersions of 0.18, 0.21, 0.26 and 0.31 dex with a typical measurement uncertainty of <~ 0.01 dex for the four mass bins. Interestingly, the scatter of the mass-SFR relation seems not constant in the sense that the scatter in SSFR is smaller for SFGs of stellar mass <10^10.5 M_sun. If confirmed, this suggests that the physical processes governing star formation become systematically less violent for less massive galaxies. The SSFR distribution for SFGs with intermediate mass 10^10-10^10.5 M_sun is characterized by a prominent excess of intense starbursts in comparison with other mass bins. We argue that this feature reflects that both violent (e.g., major/minor mergers) and quiescent processes are important in regulating star formation in this intermediate mass regime.

WE‐C‐108‐12: Accounting for Intrinsic Energy Dependence of TLDs When Establishing Dose Rate Constants for Brachytherapy Seeds

Purpose: To retrospectively account for the intrinsic energy dependence of TLDs when used to measure dose rate constants (DRC) for I–125 and Pd‐103 brachytherapy seeds, to establish new measured values for all seeds and compare the new values with Monte Carlo (MC) values and consensus values published by TG‐43U1. Methods: To calculate the relative absorbed‐dose energy dependence, f^rel, the ratio D(TLD)/D(w) at brachytherapy energies relative to Co‐60 is calculated using the EGSnrc BrachyDose code for TLD sized voxels at 1 cm from the seed centered in a water phantom. A similar geometry is used at high energy with the detector at 5 cm depth and DOSXYZnrc code. The original energy dependence applied to DRC measurements is replaced by the calculated f^rel values. By comparing the revised measured DRCs to the MC values, one can determine an appropriate relative intrinsic energy dependence, k^rel. The new relative absorbed‐dose sensitivity, S^rel, calculated as the product of f^rel and k^rel, is to give the measured DRC for comparison with Monte Carlo calculated values and TG‐43U1 consensus values. Results: The calculated f^rel values are 1.389 and 1.354 and the deduced k^rel values are 1.067 and 1.066 for I‐125 and Pd‐103 seed models, respectively. Using these new values with the literature DRC measurements, differences between calculated and revised measured DRCs are within 5% (typical measurement uncertainty) and the average discrepancy is 0.5%. The average of both DRC sets are on average 3.4% lower than the the consensus values reported in TG‐43U1. Conclusion: This work demonstrates that the energy response dependence, measured 25 years ago, applied to recent DRC measurements of I–125 and Pd‐103 brachytherapy seeds needs to be updated, thereby leading to a revision of the DRC consensus values reported by TG‐43U1.

Spatially Resolved Thermal Conductivity Measurements Using a Thermoreflectance Microprobe

A transient thermoreflectance technique capable of creating maps of the out-of-plane thermal conductivity of materials with spatial resolution of 5 μm is presented. The applied noncontact optical microprobe uses a thin reflective metal film as temperature transducer and heat reservoir. The parameters of the experimental setup guarantee that the heat flow is one dimensional. Thus, relatively simple mathematical relations can be used to extract the thermal conductivity from the recorded signal shape, which is important for thermal conductivity maps with high pixel count. For reference materials (soda-lime glass and PbTe), the determined thermal conductivities were in good agreement with bulk values, indicating a typical measurement uncertainty of 10%. For low-conductivity substrates, the measuring scheme is relatively insensitive to thermal interface resistance. Thermal conductivity maps of lead-antimony-silver-tellurium surfaces showed distinct features which could be correlated with optical surface micrographs. The local conductivity fluctuations can possibly be attributed to local changes in chemical composition and secondary phase formation.

Unfolding linac photon spectra and incident electron energies from experimental transmission data, with direct independent validation

In a recent computational study, an improved physics-based approach was proposed for unfolding linac photon spectra and incident electron energies from transmission data. In this approach, energy differentiation is improved by simultaneously using transmission data for multiple attenuators and detectors, and the unfolding robustness is improved by using a four-parameter functional form to describe the photon spectrum. The purpose of the current study is to validate this approach experimentally, and to demonstrate its application on a typical clinical linac. The validation makes use of the recent transmission measurements performed on the Vickers research linac of National Research Council Canada. For this linac, the photon spectra were previously measured using a NaI detector, and the incident electron parameters are independently known. The transmission data are for eight beams in the range 10-30 MV using thick Be, Al and Pb bremsstrahlung targets. To demonstrate the approach on a typical clinical linac, new measurements are performed on an Elekta Precise linac for 6, 10 and 25 MV beams. The different experimental setups are modeled using EGSnrc, with the newly added photonuclear attenuation included. For the validation on the research linac, the 95% confidence bounds of the unfolded spectra fall within the noise of the NaI data. The unfolded spectra agree with the EGSnrc spectra (calculated using independently known electron parameters) with RMS energy fluence deviations of 4.5%. The accuracy of unfolding the incident electron energy is shown to be ∼3%. A transmission cutoff of only 10% is suitable for accurate unfolding, provided that the other components of the proposed approach are implemented. For the demonstration on a clinical linac, the unfolded incident electron energies and their 68% confidence bounds for the 6, 10 and 25 MV beams are 6.1 ± 0.1, 9.3 ± 0.1, and 19.3 ± 0.2 MeV, respectively. The unfolded spectra for the clinical linac agree with the EGSnrc spectra (calculated using the unfolded electron energies) with RMS energy fluence deviations of 3.7%. The corresponding measured and EGSnrc-calculated transmission data agree within 1.5%, where the typical transmission measurement uncertainty on the clinical linac is 0.4% (not including the uncertainties on the incident electron parameters). The approach proposed in an earlier study for unfolding photon spectra and incident electron energies from transmission data is accurate and practical for clinical use.

A simplified approach to characterizing a kilovoltage source spectrum for accurate dose computation

To investigate and validate the clinical feasibility of using half-value layer (HVL) and peak tube potential (kVp) for characterizing a kilovoltage (kV) source spectrum for the purpose of computing kV x-ray dose accrued from imaging procedures. To use this approach to characterize a Varian® On-Board Imager® (OBI) source and perform experimental validation of a novel in-house hybrid dose computation algorithm for kV x-rays. We characterized the spectrum of an imaging kV x-ray source using the HVL and the kVp as the sole beam quality identifiers using third-party freeware Spektr to generate the spectra. We studied the sensitivity of our dose computation algorithm to uncertainties in the beam's HVL and kVp by systematically varying these spectral parameters. To validate our approach experimentally, we characterized the spectrum of a Varian® OBI system by measuring the HVL using a Farmer-type Capintec ion chamber (0.06 cc) in air and compared dose calculations using our computationally validated in-house kV dose calculation code to measured percent depth-dose and transverse dose profiles for 80, 100, and 125 kVp open beams in a homogeneous phantom and a heterogeneous phantom comprising tissue, lung, and bone equivalent materials. The sensitivity analysis of the beam quality parameters (i.e., HVL, kVp, and field size) on dose computation accuracy shows that typical measurement uncertainties in the HVL and kVp (±0.2 mm Al and ±2 kVp, respectively) source characterization parameters lead to dose computation errors of less than 2%. Furthermore, for an open beam with no added filtration, HVL variations affect dose computation accuracy by less than 1% for a 125 kVp beam when field size is varied from 5 × 5 cm(2) to 40 × 40 cm(2). The central axis depth dose calculations and experimental measurements for the 80, 100, and 125 kVp energies agreed within 2% for the homogeneous and heterogeneous block phantoms, and agreement for the transverse dose profiles was within 6%. The HVL and kVp are sufficient for characterizing a kV x-ray source spectrum for accurate dose computation. As these parameters can be easily and accurately measured, they provide for a clinically feasible approach to characterizing a kV energy spectrum to be used for patient specific x-ray dose computations. Furthermore, these results provide experimental validation of our novel hybrid dose computation algorithm.

Understanding and reducing statistical uncertainties in nebular abundance determinations

Whenever observations are compared to theories, an estimate of the uncertainties associated with the observations is vital if the comparison is to be meaningful. However, many or even most determinations of temperatures, densities and abundances in photoionized nebulae do not quote the associated uncertainty. Those that do typically propagate the uncertainties using analytical techniques which rely on assumptions that generally do not hold. Motivated by this issue, we have developed Nebular Empirical Analysis Tool (neat), a new code for calculating chemical abundances in photoionized nebulae. The code carries out a standard analysis of lists of emission lines using long-established techniques to estimate the amount of interstellar extinction, calculate representative temperatures and densities, compute ionic abundances from both collisionally excited lines and recombination lines, and finally to estimate total elemental abundances using an ionization correction scheme. neatuses a Monte Carlo technique to robustly propagate uncertainties from line flux measurements through to the derived abundances. We show that, for typical observational data, this approach is superior to analytic estimates of uncertainties. neat also accounts for the effect of upward biasing on measurements of lines with low signal-to-noise ratio, allowing us to accurately quantify the effect of this bias on abundance determinations. We find not only that the effect can result in significant overestimates of heavy element abundances derived from weak lines, but also that taking it into account reduces the uncertainty of these abundance determinations. Finally, we investigate the effect of possible uncertainties in R, the ratio of selective-to-total extinction, on abundance determinations. We find that the uncertainty due to this parameter is negligible compared to the statistical uncertainties due to typical line flux measurement uncertainties.

SPECKLE OBSERVATIONS OF BINARY STARS WITH THE WIYN TELESCOPE. VII. MEASURES DURING 2008-2009

Five hundred thirty-one speckle measures of binary stars are reported. These data were taken mainly during the period 2008 June through 2009 October at the WIYN 3.5 m Telescope at Kitt Peak and represent the last data set of single-filter speckle observations taken in the WIYN speckle program prior to the use of the current two-channel speckle camera. The astrometric and photometric precision of these observations is consistent with previous papers in this series: we obtain a typical linear measurement uncertainty of approximately 2.5 mas, and the magnitude differences reported have typical uncertainties in the range of 0.1-0.14 mag. In combination with measures already in the literature, the data presented here permit the revision of the orbit of A 1634AB (= HIP 76041) and the first determination of visual orbital elements for HDS 1895 (= HIP 65982).

Fixed-Point Comparison Uncertainties for Two Cell Geometries

To realize the ITS-90 according to its definition, among others, the melting and freezing temperatures of ideally pure metals are needed. Therefore, many national metrology institutes (NMIs) utilize a group of cells instead of one single cell as the national reference for each temperature. With direct fixed-point cell comparisons on a regular basis, it is feasible to account for the small differences between the individual fixed-point temperatures and to detect possible temperature drifts of the cells. At PTB (the German NMI), in recent years, these groups of national standard cells and the so-called transfer cells for calibrations have been complemented by newly developed slim fixed points. These cells typically contain 75% to 80% less fixed-point material compared with standard cells. Slim cells are used for homogeneity investigations of large batches of fixed-point material, for doping experiments to determine the influence of very small amounts of impurities on the fixed-point temperature with very small uncertainties, and for the investigation of contamination or purification effects after the manufacture of a fixed-point cell. These investigations have shown that the main limitation of slim cells is the quality of the phase boundary. The small dimensions of the cell do not allow the formation of a closed phase boundary (or even two of them). However, this can be compensated using a quasi-adiabatic realization procedure, and in this way, uncertainties comparable to those of standard fixed-point cells can be achieved. In this article, the design of the cells as well as typical measurement results and uncertainties for the direct comparison of fixed-point cells of both types, the standard size and slim design, are presented.

Pressure sensing in vacuum hermetic micropackaging for MOEMS-MEMS

Packaging constitutes one of the most costly steps of MEMS/MOEMS manufacturing. Uncooled IR bolometers require a vacuum atmosphere of <10 mTorr to operate at their highest sensitivity. The bolometer response is also dependent on the package temperature. In order to minimize cost, real estate, and power consumption, temperature stabilization is typically not provided to the package. Hence, long-term high-sensitivity operation of IR bolometric radiometers requires a calibration as function of in-package pressure and temperature. A low-cost and accurate means of measuring the pressure in the package without being affected by the operating temperature is therefore needed. The Institut National d'Optique (INO) has developed a low-cost, low-temperature hybrid vacuum micropackaging technology. An equivalent flow rate of 4 × 10−14 Torr L /s for storage at 80°C has been obtained without getter. Even with such low flow, the long-term stabilization of residual pressure variations affects the sensitivity and calibration of the IR bolometers. INO has developed microelectromechanical systems pressure sensors that allow for real-time measurement of package pressure of >1 mTorr and can be integrated with the IR bolometers in a die-level packaging process or microfabricated simultaneously on the same die. We present the typical performance and measurement uncertainty of these pressure sensors along with a reading method that provides a pressure measurement with a dependence on the package temperature as low as 0.7%/°C. A complex reading circuit or temperature control of the packages are not required, making the pressure sensor well adapted for low-cost high-volume production and integration with IR bolometer arrays.

Determining the eccentricity of the Moon’s orbit without a telescope

Prior to the invention of the telescope many astronomers worked out models of the motion of the Moon to predict the position of the Moon in the sky. These geometrical models implied a certain range of distances of the Moon from Earth. Ptolemy’s most quoted model predicted that the Moon was nearly twice as far away at apogee than at perigee. Measurements of the angular size of the Moon were within the capabilities of pretelescopic astronomers. Such measurements could have helped refine the models of the motion of the Moon, but hardly anyone seems to have made any measurements that have come down to us. We use a piece of cardboard with a small hole in it which slides up and down a yardstick to show that it is possible to determine the eccentricity ϵ≈0.039±0.006 of the Moon’s orbit. A typical measurement uncertainty of the Moon’s angular size is ±0.8 arc min. Because the Moon’s angular size ranges from 29.4 to 33.5 arc min, carefully taken naked eye data are accurate enough to demonstrate periodic variations of the Moon’s angular size.

Performance of the ground‐based total ozone network assessed using satellite data

Dobson and Brewer spectrophotometer and filter ozonometer data available from the World Ozone and Ultraviolet Data Centre (WOUDC) were compared with satellite total ozone measurements from TOMS (onboard Nimbus 7, Meteor 3, and Earth Probe satellites), OMI (AURA satellite) and GOME (ERS‐2 satellite) instruments. Five characteristics of the difference with satellite data were calculated for each site and instrument type: the mean difference, the standard deviation of daily differences, the standard deviation of monthly differences, the amplitude of the seasonal component of the difference, and the range of annual values. All these characteristics were calculated for five 5‐year‐long bins and for each site separately for direct sun (DS) and zenith sky (ZS) ozone measurements. The main percentiles were estimated for the five characteristics of the difference and then used to establish criteria for “suspect” or “outlier” sites for each characteristic. About 61% of Dobson, 46% of Brewer, and 28% of filter stations located between 60°S and 60°N have no “suspect” or “outlier” characteristics. In nearly 90% of all cases, Dobson and Brewer sites demonstrated 5‐year mean differences with satellites to be within ±3% (for DS observations). The seasonal median difference between all Brewer DS measurements at 25°–60°N and GOME and OMI overpasses remained within ±0.5% over a period of more than 10 years. The satellite instrument performance was also analyzed to determine typical measurement uncertainties. It is demonstrated that systematic differences between the analyzed satellite instruments are typically within ±2% and very rarely are they outside the ±3% envelope. As the satellite instrument measurements appear to be better than ±3%, ground‐based instruments with precision values worse than ±3% are not particularly useful for the analyses of long‐term changes and comparison with numerical simulations.

Direct aerosol forcing: Calculation from observables and sensitivities to inputs

Understanding sources of uncertainty in aerosol direct radiative forcing (DRF), the difference in a given radiative flux component with and without aerosol, is essential to quantifying changes in Earth's radiation budget. We examine the uncertainty in DRF owing to measurement uncertainty in the quantities on which it depends: aerosol optical depth, single scattering albedo, asymmetry parameter, solar geometry, and surface albedo. Direct radiative forcing at the top of the atmosphere and at the surface is calculated at three locations representing distinct aerosol types and radiative environments. Sensitivities, the changes in DRF in response to unit changes in individual aerosol or surface properties, are also calculated for these conditions. The uncertainty in DRF associated with a given property is computed as the product of the sensitivity and typical measurement uncertainty in the respective property. Sensitivity and uncertainty values permit estimation of total uncertainty in calculated DRF and identification of properties that most limit accuracy in estimating forcing. Absolute total uncertainties in modeled local diurnally averaged forcing range from 0.2 to 3.1 W m−2 for the ranges of properties examined here. Relative total uncertainties range from ∼20 to 80% with larger values at higher latitudes, where fluxes are low. The largest contributor to total uncertainty in DRF is single scattering albedo; however, decreasing measurement uncertainties for any property would increase accuracy in DRF. Comparison of two radiative transfer models suggests the contribution of modeling error is small compared to the total uncertainty although comparable to uncertainty arising from some individual properties.

Monte Carlo study of neutron dose equivalent during passive scattering proton therapy

Stray radiation exposures are of concern for patients receiving proton radiotherapy and vary strongly with several treatment factors. The purposes of this study were to conservatively estimate neutron exposures for a contemporary passive scattering proton therapy system and to understand how they vary with treatment factors. We studied the neutron dose equivalent per therapeutic absorbed dose (H/D) as a function of treatment factors including proton energy, location in the treatment room, treatment field size, spread-out Bragg peak (SOBP) width and snout position using both Monte Carlo simulations and analytical modeling. The H/D value at the isocenter for a 250 MeV medium field size option was estimated to be 20 mSv Gy−1. H/D values generally increased with the energy or penetration range, fell off sharply with distance from the treatment unit, decreased modestly with the aperture size, increased with the SOBP width and decreased with the snout distance from the isocenter. The H/D values from Monte Carlo simulations agreed well with experimental results from the literature. The analytical model predicted H/D values within 28% of those obtained in simulations; this value is within typical neutron measurement uncertainties.