Current Measurement Uncertainties Research Articles

Sensitivity of global direct aerosol shortwave radiative forcing to uncertainties in aerosol optical properties

Abstract. New satellite missions promise global reductions in the uncertainties in aerosol optical properties, but it is unclear how those reductions will propagate to uncertainties in the shortwave (SW) direct aerosol radiative effect (DARE) and direct aerosol radiative forcing (DARF), which are currently large, on the order of at least 20 %. In this work, we build a Monte Carlo framework to calculate the impact of uncertainties in aerosol optical depth (AOD), single scattering albedo (SSA), and the asymmetry parameter on the uncertainty in shortwave DARE and DARF. This framework uses the results of over 2.3 million radiative transfer simulations to calculate global clear-sky DARE and DARF based on a range of uncertainties in present-day and pre-industrial aerosol optical properties, representative of existing and future global observing systems. We find the 1σ uncertainty varies between ±0.23 and ±1.91 W m−2 (5 % and 42 %) for the top-of-atmosphere (TOA) clear-sky DARE and between ±0.08 and ±0.47 W m−2 (9 % and 52 %) for the TOA DARF. At the TOA, AOD uncertainty is the main contributor to overall uncertainty except over bright surfaces where SSA uncertainty contributes most. We apply regionally varying uncertainties to represent current measurement uncertainties and find that aerosol optical property uncertainties represent 24 % of TOA DARE and DARF. Reducing regionally varying optical property uncertainties by a factor of 2 would reduce their contributions to TOA DARE and DARF uncertainty proportionally. Applying a simple scaling to all-sky conditions, aerosol optical property uncertainty contributes to about 25 % total uncertainty in TOA, all-sky SW DARE, and DARF. Compared to previous studies which considered uncertainties in non-aerosol variables, our results suggest that the aerosol optical property uncertainty accounts for one-third to half of the total direct SW uncertainty. Recent and future progress in constraining aerosol optical properties using ground-based or satellite retrievals could be translated into DARE and DARF uncertainty using our freely available framework.

Upgrade of the medium energy dump geometry for the SPIRAL2 single bunch selector

The medium energy beam transport of the SPIRAL2 superconducting linac contains a single bunch selection system equipped with a 7.5 kW beam dump. This device, originally designed with a long flat slope to decrease the power density so that the maximum operating temperature was 170 °C, was impacted by Coulomb scattering generating two side effects: heating of the downstream beam transport components and degrading of the beam current measurement uncertainty. The paper relates the way these two problems were solved.

Sphygmomanometer Dynamic Pressure Measurement Using a Condenser Microphone.

There is a worldwide need to improve blood pressure (BP) measurement error in order to correctly diagnose hypertension. Cardiovascular diseases cause 17.9 million deaths annually and are a substantial monetary strain on healthcare. The current measurement uncertainty of 3 mmHg should be improved upon. Dynamic pressure measurement standards are lacking or non-existing. In this study we propose a novel method of measuring air pressure inside the sphygmomanometer tubing during BP measurement using a condenser microphone. We designed, built, and tested a system that uses a radiofrequency (RF) modulation method to convert changes in capacitance of a condenser microphone into pressure signals. We tested the RF microphone with a low-frequency (LF) sound source, BP simulator and using a piezoresistive pressure sensor as a reference. Necessary tests were conducted to assess the uncertainty budget of the system. The RF microphone prototype has a working frequency range from 0.5 Hz to 280 Hz in the pressure range from 0 to 300 mmHg. The total expanded uncertainty (k = 2, p = 95.5%) of the RF microphone was 4.32 mmHg. The proposed method could establish traceability of BP measuring devices to acoustic standards described in IEC 61094-2 and could also be used in forming dynamic BP standards.

How to assess the primordial origin of single gravitational-wave events with mass, spin, eccentricity, and deformability measurements

A population of primordial black holes formed in the early Universe could contribute to at least a fraction of the black-hole merger events detectable by current and future gravitational-wave interferometers. With the ever-increasing number of detections, an important open problem is how to discriminate whether a given event is of primordial or astrophysical origin. We systematically present a comprehensive and interconnected list of discriminators that would allow us to rule out, or potentially claim, the primordial origin of a binary by measuring different parameters, including redshift, masses, spins, eccentricity, and tidal deformability. We estimate how accurately future detectors (such as the Einstein Telescope and LISA) could measure these quantities, and we quantify the constraining power of each discriminator for current interferometers. We apply this strategy to the GWTC-3 catalog of compact binary mergers. We show that current measurement uncertainties do not allow us to draw solid conclusions on the primordial origin of individual events, but this may become possible with next-generation ground-based detectors.

On the Co-orbitation of Satellite Galaxies along the Great Plane of Andromeda: NGC 147, NGC 185, and Expectations from Cosmological Simulations

Abstract Half of the satellite galaxies of Andromeda form a narrow plane termed the Great Plane of Andromeda (GPoA), and their line-of-sight velocities display a correlation reminiscent of a rotating structure. Recently reported first proper-motion measurements for the on-plane satellites NGC 147 and NGC 185 indicate that they indeed co-orbit along the GPoA. This provides a novel opportunity to compare the M31 satellite system to ΛCDM expectations. We perform the first detailed comparison of the orbital alignment of two satellite galaxies beyond the Milky Way with several hydrodynamical and dark-matter-only cosmological simulations (Illustris TNG50, TNG100, ELVIS, and PhatELVIS) in the context of the Planes of Satellite Galaxies Problem. In line with previous works, we find that the spatial flattening and line-of-sight velocity correlation are already in substantial tension with ΛCDM, with none of the simulated analogs simultaneously reproducing both parameters. Almost none (3%–4%) of the simulated systems contain two satellites with orbital poles as well aligned with their satellite plane as indicated by the most likely proper motions of NGC 147 and NGC 185. However, within current measurement uncertainties, it is common (≈70%) that the two best-aligned satellites of simulated systems are consistent with the orbital alignment. Yet, the chance that any two simulated on-plane satellites have as well-aligned orbital poles as observed is low (≈4%). We conclude that confirmation of the tight orbital alignment for these two objects via improved measurements, or the discovery of similar alignments for additional GPoA members, holds the potential to further raise the tension with ΛCDM expectations.

The effect of inhomogeneous reionization on the Lyman α forest power spectrum at redshift z > 4: implications for thermal parameter recovery

ABSTRACT We use the Sherwood–Relics suite of hybrid hydrodynamical and radiative transfer simulations to model the effect of inhomogeneous reionization on the 1D power spectrum of the Lyman α (Lyα) forest transmitted flux at redshifts 4.2 ≤ z ≤ 5. Relative to models that assume a homogeneous ultraviolet background, reionization suppresses the power spectrum at small scales, k ∼ 0.1 km−1 s, by ∼10 per cent because of spatial variations in the thermal broadening kernel and the divergent peculiar velocity field associated with overpressurized intergalactic gas. On larger scales, $k\lt 0.03\rm \, km^{-1}\, s$, the power spectrum is instead enhanced by 10–50 per cent by large-scale spatial variations in the neutral hydrogen fraction. The effect of inhomogeneous reionization must therefore be accounted for in analyses of forthcoming high precision measurements. We provide a correction for the Lyα forest power spectrum at 4.1 ≤ z ≤ 5.4 that can be easily applied within other parameter inference frameworks using similar reionization models. We perform a Bayesian analysis of mock data to assess the extent of systematic biases that may arise in measurements of the intergalactic medium if ignoring this correction. At the scales probed by current high-resolution Lyα forest data at z > 4, $0.006 \le k \le 0.2 \rm \, km^{-1}\, s$, we find inhomogeneous reionization does not introduce any significant bias in thermal parameter recovery for the current measurement uncertainties of ∼10 per cent. However, for 5 per cent uncertainties, ∼1σ shifts between the estimated and true parameters occur.

Zero‐Sequence current measurement uncertainty in three‐phase power systems during normal operation

Zero-sequence currents in high-voltage power systems during normal operation can have a significant influence on nearby infrastructure. It is therefore necessary to gain insight into typical levels of zero-sequence currents in a variety of operational situations. Measurements can provide this insight. When performing measurements, one needs to know the uncertainty associated with the measurement to draw accurate conclusions. The measurement uncertainty associated with three different practical strategies to measure zero-sequence currents during normal operation are studied: a direct and indirect measurement strategy using current clamps and oscilloscopes and a measurement strategy using phasor measurement units (PMU). Measurement uncertainty is studied using two methods: by analytical expressions and by Monte Carlo simulations. Both methods give consistent outcomes for the measurement uncertainty and show the same tendencies of the zero-sequence current measurement uncertainty as a function of the positive-sequence current. This paper studies measurement strategies with error sources based on realistic measurement devices that could be used in practice. The following outcomes were found: in the direct strategy, the primary current transformers are the largest uncertainty sources; in the indirect strategy, the current clamps are the largest uncertainty sources; in the PMU strategy, the PMU total vector error is the largest uncertainty source.

Potential of Clumped Isotopes in Constraining the Global Atmospheric Methane Budget

AbstractAtmospheric methane (CH4) and its isotopic composition trends over the last decades are explained by various flux scenarios, from tropical wetland emission increases through to reductions in global hydroxyl (OH). In this study, we develop a modeling framework to assess the potential usefulness of clumped isotope measurements to distinguish between the main drivers of change in the CH4 burden. We model interhemispheric differences of 0.12‰ and 0.38‰ and seasonal cycles of 0.02‰–0.04‰ and 0.21‰–0.32‰ for Δ13CH3D and Δ12CH2D2, respectively, which is insignificant relative to the uncertainty of measurements that could eventually be made. We show, however, that measurements of Δ12CH2D2 specifically could provide constraints for understanding trends in the global total source and sink magnitudes, which has not been possible with the current sets of observables. Changes in OH concentration of 10% developed across three decades result in a difference of up to 2‰ in Δ12CH2D2, which would be observable given current measurement uncertainty limits. For this type of global‐scale analysis, we show that measurements of Δ13CH3D would be unlikely to provide additional useful information. We suggest an emphasis should now be on developing the methods to make measurements from ambient air samples, followed by measurements of Δ13CH3D and Δ12CH2D2 from sampling at clean Northern and Southern Hemisphere sites, combined with more accurate and precise laboratory measurements of the clumped kinetic isotope effects relevant for the atmospheric sinks.

SDSS-IV MaNGA: Uncovering the Angular Momentum Content of Central and Satellite Early-type Galaxies

Abstract We study 379 central and 159 satellite early-type galaxies with two-dimensional kinematics from the integral-field survey Mapping Nearby Galaxies at APO (MaNGA) to determine how their angular momentum content depends on stellar and halo mass. Using the Yang et al. group catalog, we identify central and satellite galaxies in groups with halo masses in the range . As in previous work, we see a sharp dependence on stellar mass, in the sense that ∼70% of galaxies with stellar mass tend to have very little rotation, while nearly all galaxies at lower mass show some net rotation. The ∼30% of high-mass galaxies that have significant rotation do not stand out in other galaxy properties, except for a higher incidence of ionized gas emission. Our data are consistent with recent simulation results suggesting that major merging and gas accretion have more impact on the rotational support of lower-mass galaxies. When carefully matching the stellar mass distributions, we find no residual differences in angular momentum content between satellite and central galaxies at the 20% level. Similarly, at fixed mass, galaxies have consistent rotation properties across a wide range of halo mass. However, we find that errors in classification of central and satellite galaxies with group finders systematically lower differences between satellite and central galaxies at a level that is comparable to current measurement uncertainties. To improve constraints, the impact of group-finding methods will have to be forward-modeled via mock catalogs.

Uncertainties in Current Measurements in the Northern North Sea

AbstractA met–ocean measurement program of waves and current profiles at five locations in the northern North Sea was performed over a period of approximately 5 years. Despite quality control, the measured current speed data contained more noise than expected and large discrepancies were observed between overlapping current speed data measured by different current profilers at the same locations and water depths. Some of the noise and discrepancies can be explained by the influence from surface waves. The current measurements from instruments attached to a surface buoy indicated that these suffered from the influence of surface waves. Further investigations of the uncertainties in current speed data were carried out through three phases of a current verification study, where both additional current measurements and data analysis were done. Comparisons of overlapping measured current speed showed large deviations, suggesting that the accuracy of current measurements is not as good as the user expects. These presented results are in contrast to previous studies of overlapping current measurements.

On the relative contributions of bias and noise to lithium-ion battery state of charge estimation errors

This article examines the problem of quantifying lithium-ion battery state of charge (SOC) estimation errors. The article is motivated by the need for accurate SOC estimation in battery management and diagnostics applications. Given a statistical ensemble of SOC estimation experiments, one can classify these experiments’ estimation errors into an average bias plus estimation noise. Previous theoretical work by the authors quantifies both of these error types, and suggests that in the presence of reasonable voltage and current measurement uncertainties, bias effects are likely to dominate compared to noise. The main goal of this article is to extend this theoretical finding to a higher-order battery model, and furthermore validate it experimentally. In particular, the article shows experimentally that the SOC estimation errors for a LiFePO4 battery are much closer in magnitude to the SOC estimation bias predicted by our theoretical analyses. This is important not only because it furnishes an experimentally-supported analytic characterization of lithium-ion battery SOC estimation accuracy, but also because it contrasts to previous studies in the literature where SOC estimation accuracy is evaluated using the Cramér–Rao theorem, which assumes unbiased estimation.

Collaborative guarded-hot-plate tests between the Laboratoire national de métrologie et d’essais and the National Institute of Standards and Technology

A collaborative study to compare the long-term measurement performance between guarded-hot-plate facilities at the Laboratoire national de métrologie et d’essais (LNE) in France and the National Institute of Standards and Technology (NIST) in the United States is presented. Thermal conductivity data were compiled from three international comparisons organized from 1997 to 2014. Measurements were conducted in accordance with standardized test methods (ISO 8302 or ASTM C 177) over a temperature range from 280 K to 320 K. Nine thermal insulating materials (either mineral fiber or expanded polystyrene) were examined covering broad ranges of bulk densities (13 kg · m−3–200 kg · m−3) and thicknesses (13 mm–70 mm). A different set of specimens was utilized for each comparison. Results of this study indicate that, over a 17 year interval, the majority of test data from LNE and NIST agreed to within ±1.0%, or less, for mineral fiber materials and to within ±0.5%, or less, for expanded polystyrene. The long-term variability limit of 1% between the two laboratories is in good agreement with their current measurement uncertainties. Regression coefficients and their standard uncertainties for a straight-line model relating thermal conductivity to temperature from 280 K to 320 K were computed by material and laboratory. Graphical analysis of the data and corresponding fits exhibit consistent behavior by material type between the two laboratories. Sources of measurement variability are addressed.

DISCOVERY OF LOW-METALLICITY STARS IN THE CENTRAL PARSEC OF THE MILKY WAY

We present a metallicity analysis of 83 late-type giants within the central 1 pc of the Milky Way. K-band spectroscopy of these stars were obtained with the medium-spectral resolution integral-field spectrograph NIFS on Gemini North using laser-guide star adaptive optics. Using spectral template fitting with the MARCS synthetic spectral grid, we find that there is large variation in metallicity, with stars ranging from [M/H] $<$ -1.0 to above solar metallicity. About 6\% of the stars have [M/H] $<$ -0.5. This result is in contrast to previous observations, with smaller samples, that show stars at the Galactic center have approximately solar metallicity with only small variations. Our current measurement uncertainties are dominated by systematics in the model, especially at [M/H] $>$ 0, where there are stellar lines not represented in the model. However, the conclusion that there are low metallicity stars, as well as large variations in metallicity is robust. The metallicity may be an indicator of the origin of these stars. The low-metallicity population is consistent with that of globular clusters in the Milky Way, but their small fraction likely means that globular cluster infall is not the dominant mechanism for forming the Milky Way nuclear star cluster. The majority of stars are at or above solar metallicity, which suggests they were formed closer to the Galactic center or from the disk. In addition, our results indicate that it will be important for star formation history analyses using red giants at the Galactic center to consider the effect of varying metallicity.

Effect of core–mantle and tidal torques on Mercury’s spin axis orientation

The rotational evolution of Mercury’s mantle plus crust and its core under conservative and dissipative torques is important for understanding the planet’s spin state. Dissipation results from tidal torques and viscous, magnetic, and topographic torques contributed by interactions between the liquid core and solid mantle. For a spherically symmetric core–mantle boundary (CMB), the system goes to an equilibrium state wherein the spin axes of the mantle and core are fixed in the frame precessing with the orbit, and in which the mantle and core are differentially rotating. This equilibrium exhibits a mantle spin axis that is offset from the Cassini state by larger amounts for weaker core–mantle coupling for all three dissipative core–mantle coupling mechanisms, and the spin axis of the core is separated farther from that of the mantle, leading to larger differential rotation. Relatively strong core–mantle coupling is necessary to bring the mantle spin axis to a position within the uncertainty in its observed position, which is close to the Cassini state defined for a completely solid Mercury. Strong core–mantle coupling means that Mercury’s response is closer to that of a solid planet. Measured or inferred values of parameters in all three core–mantle coupling mechanisms for a spherically symmetric CMB appear not to accomplish this requirement. For a hydrostatic ellipsoidal CMB, pressure coupling dominates the dissipative effects on the mantle and core positions, and dissipation with pressure coupling brings the mantle spin solidly to the Cassini state. The core spin goes to a position displaced from that of the mantle by about 3.55arcmin nearly in the plane containing the Cassini state. The core spin lags the precessing plane containing the Cassini state by an increasing angle as the core viscosity is increased. With the maximum viscosity considered of ν∼15.0cm2/s if the coupling is by the circulation through an Ekman boundary layer or ν∼8.75×105cm2/s for purely viscous coupling, the core spin lags the precessing Cassini plane by 23arcsec, whereas the mantle spin lags by only 0.055arcsec. Larger, non-hydrostatic values of the CMB ellipticity also result in the mantle spin at the Cassini state, but the core spin is moved closer to the mantle spin. Current measurement uncertainties preclude using the mantle offset to constrain the internal core viscosity.

A Current Consumption Measurement Approach for FPGA-Based Embedded Systems

In this paper, an approach for the current-consumption measurement of a field-programmable gate array (FPGA)-based embedded system is presented. This approach is based on the current conversion to pulsewidth using the external-to-FPGA capacitor charging circuit and comparator. The pulsewidth is then measured using the timer synthesized inside the FPGA. Measurement uncertainty budget analysis is performed. It reveals the parameters mostly affecting steady-state current measurement uncertainty. Sources contributing to the budget of current measurement uncertainty include directly measured pulsewidth, manufacturing scattering of measurement setup component parameters, and their fluctuations in response to the measured current value. A calibration procedure enabling to reduce the influence of charging circuit component manufacturing tolerances on measurement uncertainty is suggested. The current measurement prototype is developed and tested. Measured pulsewidth jitter is estimated experimentally using the prototype. The jitter influence on measurement uncertainty is modeled by including the corresponding quantity in the measurement model. The influence of the temperature on measurement uncertainty is estimated using Monte Carlo simulation.

Neutron-induced235U fission spectrum measurements using liquid organic scintillation detectors

Measurements have been performed at Los Alamos Neutron Science Center to acquire neutron fission spectra of ${}^{235}$U in the energy range between 0.5 and 10 MeV. These new data complement the currently available experimental data, which are not well known in the energy range below 1 and above 5 MeV. Organic liquid scintillation detectors (EJ-309s) were used together with a digital data-acquisition system. The EJ-309 detectors show excellent pulse shape discrimination capabilities and this is vital for identifying only neutron pulses and rejecting $\ensuremath{\gamma}$-ray pulses. The measurement data show the dependence of average fission neutron energy as a function of the inducing neutron energy. The data agree well with previously published measurements and Watt spectra fits for energies up to tens of MeV. At high fission-inducing neutron energies the spectrum deviates from Watt-spectra fits. The increased energy deposited in higher energy neutron-induced fissions does not significantly increase emitted fission neutron energies for up to 10 MeV. A significant decrease in neutron energy around second-chance fission cannot be observed beyond the current measurement uncertainties.

Direct Measurement of AC Current by Measuring the Voltage Drop on the Coaxial Current Shunt

An accurate direct measurement of ac current that avoids complex and time-consuming ac-dc current transfer measurements by directly measuring the ac voltage drop of coaxial current shunts is described. Evaluation of custom-developed coaxial current shunts for long-term drift, self-heating effects, and loading using a commercial ac voltmeter is presented. By correcting only the shunts' long-term drift, direct ac current measurement uncertainties below 55 muA/A are attained in the frequency range of 20 Hz-30 kHz and for currents ranging from 100 mA up to 1.0 A.

Prospects for constraining cosmology with the extragalactic cosmic microwave background temperature

Observers have demonstrated that it is now feasible to measure the cosmic microwave background (CMB) temperature at high redshifts. We explore the possible constraints on cosmology which might ultimately be derived from such measurements. In addition to providing a consistency check on standard and alternative cosmologies, possibilities include constraints on the inhomogeneity and anisotropy of the universe at intermediate redshift $z\ensuremath{\lesssim}10,$ an independent probe of peculiar motions with respect to the Hubble flow, constraining the epoch of reionization, and the time variability of the fine structure constant. We argue that the best possibility is as a probe of peculiar motions. We show, however, that the current measurement uncertainty $(\ensuremath{\Delta}T=\ifmmode\pm\else\textpm\fi{}0.002\mathrm{K})$ in the local present absolute CMB temperature imposes intrinsic limits on the use of such CMB temperature measurements as a cosmological probe. At best, anisotropies at intermediate redshift could only be constrained at a level of $\ensuremath{\gtrsim}0.1%$ and peculiar motions could only be determined to an uncertainty of $\ensuremath{\gtrsim}311{\mathrm{km}\mathrm{}\mathrm{}\mathrm{s}}^{\ensuremath{-}1}.$ If the high z CMB temperature can only be measured with a precision comparable to the uncertainty of the local interstellar CMB temperature, then peculiar motions could be determined to an uncertainty of ${1101(1+z)}^{\ensuremath{-}1}[\ensuremath{\Delta}{T}_{\mathrm{CMB}}(z)/0.01\mathrm{K}]{\mathrm{km}\mathrm{}\mathrm{s}}^{\ensuremath{-}1}.$

Constraints on Warm Dark Matter from Cosmological Reionization

We study the constraints that high-redshift structure formation in the universe places on warm dark matter (WDM) dominated cosmological models. We modify the extended Press-Schechter formalism to derive the halo mass function in WDM models. We show that our predictions agree with recent numerical simulations at low redshift over the halo masses of interest. Applying our model to galaxy formation at high redshift, we find that the loss of power on small scales, together with the delayed collapse of low-mass objects, results in strong limits on the root-mean-square velocity dispersion vrms,0 of the WDM particles at redshift zero. For fermions decoupling while relativistic, these limits are equivalent to constraints on the mass mX of the particles. The presence of a ≈4 × 109 M ☉ supermassive black hole at redshift 5.8, believed to power the quasar SDSS 1044-1215, implies mX 0.5 keV (or vrms,0 0.10 km s-1), assuming that the quasar is unlensed and radiating at or below the Eddington limit. Reionization by redshift 5.8 also implies a limit on mX. If high-redshift galaxies produce ionizing photons with an efficiency similar to their redshift-three counterparts, we find mX 1.2 keV (or vrms,0 0.03 s-1). However, given the uncertainties in current measurements from the proximity effect of the ionizing background at redshift three, values of mX as low as 0.75 keV (or vrms,0 = 0.06 s-1) are not ruled out. The limit weakens further to mX 0.4 keV (or vrms,0 0.14 s-1), if, instead, the ionizing-photon production efficiency is 10 times greater at high redshift, but this limit will tighten considerably if reionization is shown in the future to have occurred at higher redshifts. WDM models with mX 1 keV (or vrms,0 0.04 s-1) produce a low-luminosity cutoff in the high-redshift galaxy luminosity function that is directly detectable with the Next Generation Space Telescope, and which serves as a direct constraint on mX.

On the use of HF as a reference for the comparison of stratospheric observations and models

Hydrogen fluoride (HF) is often used as a simple reference for other column observations of chemically active stratospheric species. However, seasonal and shorter timescale variations in column HF make its use as a reference more complicated. In this paper we characterize the expected magnitude of these variations in HF, and variations of ratio quantities involving HF, using a two‐dimensional (2‐D) photochemical model and two versions of a three‐dimensional (3‐D) transport model. The 2‐D model predicts that the column ratios HNO3/HF and HCl/HF increase from midlatitudes to the tropics, although this is very sensitive to HCl and HNO3 abundances in the tropical upper troposphere. Seasonal variations in vertical motion modifys the predicted ratios; for example, wintertime descent at high latitudes decreases HCl/HF. The ratio HNO3/HF at high latitudes is strongly modified by seasonal variations in the chemical partitioning of the odd nitrogen (NOy) species. We compare these model predictions with ground‐based Fourier transform infrared spectroscopy (FTIR) observations of HF along with HCl, ClONO2 and HNO3 obtained at eight northern hemisphere sites between October 1994 and July 1995. We investigate quantitatively how HF can be used as a tracer to follow the evolution of observations at a single station and to intercompare results from different stations or with photochemical models. The magnitude of the 3‐D model HF column agrees well with the observations, except on some occasions at high latitudes, giving indirect support for the important role of COF2 in the stratospheric inorganic fluorine budget. The observed day‐to‐day variability in the column ratios HCl/HF and HNO3/HF is much larger at high latitudes. This variability is reproduced in the 3‐D models and is due to horizontal motion. Short timescale vertical displacement of the species profiles is estimated to have a small effect on the column ratios. In particular, we analyze the usefulness of the observed column ratio (ClONO2 + HCl)/HF as an indicator for chlorine activation. Current measurement uncertainties limit the degree of activation which can be unambiguously detected using this observed quantity, but we can determine that chlorine‐activated air was observed above Aberdeen (58°N) on 6 days in late January 1995.