Hand-considered hypotheses, 2501–2600

Misattribution
H2501. The loss of nitric oxide in a long sampling line is usually charged to reaction with ambient ozone, but in lines that have carried high concentrations before, much of it is uptake by deposits already on the wall, whose capacity depends on what the line last sampled. A conditioned wall can take up NO for hours before it saturates, while the ozone reaction depends only on concentration and residence time. Two identical lines, one flushed with zero air and one with a high-NO mixture the day before, sampled side by side at the same ozone level, separate the two.

Hidden term
H2502. Reported inlet transmission carries a thermophoretic term whenever the inlet is warmer or colder than the air it samples, and that term is missing from the laboratory calibration because laboratory air and inlet are at one temperature. Particles migrate down the gradient toward the cold wall, so a chilled inlet loses fine particles that a neutral one passes. Reversing the temperature difference at fixed flow and aerosol should change transmission by a similar amount in the opposite direction.

Bound
H2503. At a given flow, an inlet's transmission of ultrafine particles cannot exceed a ceiling set by diffusion to the walls, and lengthening the inlet buys nothing once the diffusion length exceeds the tube radius. Deposition velocity rises as the tube narrows, so shrinking the diameter to shorten residence time does not preserve transmission. Two inlets matched in residence time but differing in diameter span the bound with no change in chemistry.

Reversal
H2504. Heating an inlet to stop water condensing on its walls usually raises particle transmission, but a few kelvin above ambient it begins to lower the transmission of ammonium nitrate and semi-volatile organics by evaporating them off the particles. The same heat that keeps the line dry drives the size distribution toward the gas phase. Sampling the same air through heated and unheated parallel inlets and comparing downstream filter mass puts the crossing temperature in view.

Scale argument
H2505. Residence time and surface-to-volume ratio are separate variables in an inlet, and the chemical loss of a reactive gas depends on the second more than the first, so a short narrow line and a long wide one with the same residence time are not interchangeable. Loss scales with wall area times exposure per unit volume. Building two lines to equal residence time while changing only their bore, and feeding both one mixture, should produce a loss difference the rate constant predicts.

Inversion of standard practice
H2506. Downward-facing inlet heads keep rain out, yet for particles larger than a few micrometres the downward-facing head is itself the dominant loss, because the bend throws coarse particles onto the wall by inertia. An upward-facing head with a rain cap transmits more coarse mass at the cost of a different wetting problem. Running both heads on one stack for a week and comparing size-resolved mass identifies which penalty is larger.

Count
H2507. Line loss in a sampling manifold is often budgeted from total length, but the number of bends matters more, because each bend imposes a nearly fixed fractional loss on particles above a size set by the bend radius. Eight gentle bends can cost more than ten metres of straight tube, since straight length loses mainly by diffusion and settling. Manifolds of equal total length and unequal bend count, tested with a polydisperse aerosol, order themselves by bend count if this is right.

Identity
H2508. Inlet efficiency and line loss are the same deposition measurement made at two positions, so a study that measures both and adds them into a transmission budget charges the wall twice. The wall does not know where the inlet ends. A single deposition tracer injected at the inlet mouth and counted at the analyser, compared with the sum of the two reported terms, exposes the double count.

Threshold
H2509. Two regimes of inlet transmission meet at a flow at which settling and inertial impaction trade places: below it, coarse particles settle along the horizontal run; above it, they impact at the bends. The sign of the size dependence changes across the crossover, so a flow chosen to spare one size class may penalize another. Sweeping flow through one geometry with a size-resolved inlet and outlet comparison places the crossover at the flow where the size dependence changes sign.

Selection effect
H2510. Efficiency data come from monodisperse test aerosols chosen for their ability to survive generation and transport, so the sizes tested are the ones that already pass and the ambient range that matters is underweighted. A test aerosol that reaches the inlet has usually been dried and neutralized, which is not the state of ambient particles. Comparing the laboratory efficiency curve with a field curve from a natural aerosol of known distribution, at the same inlet, measures the gap.

Artefact mimicking effect
H2511. A midday dip in measured nitrogen oxides at a fixed site can look like photochemistry or a traffic lull, but a mass-flow-controlled inlet holds mass flow while volumetric flow rises with temperature, so residence time shortens and wall loss changes across the day. The dip then follows the inlet's temperature rather than the atmosphere. Holding the inlet at a fixed temperature, or plotting the residual against inlet temperature after removing a traffic proxy, separates them.

No-go claim
H2512. No passive inlet can simultaneously preserve coarse particles and semi-volatile vapours, because the measures that keep particles airborne — short residence time, warm walls, no impaction — are the ones that let vapours pass or evaporate from the deposit. The two requirements pull the same variables in opposite directions. A single inlet tested for both coarse mass and semi-volatile recovery across temperatures and flows should trace a frontier rather than show an optimum.

Hidden term
H2513. Filter blanks are not constants of the filter type: they fall as the filter accumulates material, because deposited particles occupy the sites that would otherwise adsorb vapours during and after sampling. A blank measured on a clean filter therefore over-corrects a loaded one, and the error grows with loading. Weighing filters before and after an inert preload, then measuring the apparent blank, maps the dependence with no ambient sampling.

Misattribution
H2514. Negative organic artefact on quartz filters is usually attributed to adsorption of gas-phase organics, but part of it is mass shed from the fibre mat itself, loosened by the handling and heating the method requires. Shedding is a loss and adsorption is a gain, and the method reports one number. Pre-weighing pairs handled identically, only one of which is heated, separates the two without a denuder.

Bound
H2515. A denuder's capacity is bounded by its coated area times the monolayer coverage of the reagent, and a coating thicker than a few tens of micrometres adds capacity but also adds a particle loss that rises with thickness. The optimum maximizes capacity minus loss, not capacity. Denuders coated at several thicknesses, tested for both breakthrough and fine-particle transmission, locate it.

Reversal
H2516. Thickening a denuder coating is supposed to improve capture, but for the most volatile analytes the coating becomes a reversible sink: it re-emits what it captured when the incoming concentration falls, producing an outlet tail that a thin coating would not. Capacity and retention are different properties and only one improves with thickness. Alternating high and low spikes through denuders of two thicknesses and watching the low phase exposes the re-emission.

Identity
H2517. Breakthrough volume and retention volume of a sorbent tube are the same quantity approached from opposite ends, so a method that measures breakthrough directly and quotes a safe sampling volume at half of it is using one number, not two. The half is a safety factor, not a second measurement. Running a tube in reverse after a frontal run should release the analyte over the same volume at which it broke through.

Threshold
H2518. Two analytes on one sorbent can reverse their elution order as the tube warms, because the more volatile compound's retention falls faster with temperature than the less volatile one's. Below the crossing temperature one is held longer; above it the other is. Frontal runs at three temperatures with a mixed standard show the swap, and the swap temperature belongs to the pair, not to the tube.

Artefact mimicking effect
H2519. Back-diffusion from the pump line can make a sorbent tube's blank rise over a sampling period, which looks like ambient contamination but is a transport artefact with the shape of a source. It is independent of the tube's inlet concentration and grows with the time the pump runs after the sample. A tube fitted with an outlet plug, or a blank tube pulled at the same flow, separates the two.

Count
H2520. How many sorbent beds a method needs is set not by the number of analytes but by the number of distinct retention volumes among them, because analytes whose retention volumes differ by less than the frontal width travel as one band and can share a bed. Extra beds add dead volume and loss without adding resolution. A mixed standard run on one bed and on two, compared band by band, shows whether the second bed did any work.

Unit trap
H2521. Sorbent capacity is quoted both per gram and per tube, and a safe sampling volume derived from one is not convertible to the other without the bed mass, which differs between tube sizes. A volume quoted in litres at one flow is a mole quantity in disguise, so the same tube at half the flow does not have twice the safe time. Running one analyte at two flows to breakthrough and comparing with the quoted per-gram capacity exposes the mismatch.

Scale argument
H2522. Whether a filter's collected semi-volatile mass is limited by evaporation or by reaction depends on face velocity: at low velocity the deposit equilibrates with the gas phase and loses mass, while at high velocity the deposit's residence time is short and reaction with co-collected oxidant dominates. The two regimes have opposite temperature dependences, so an Arrhenius fit to the wrong one yields a negative apparent activation energy. Sampling at several face velocities at one temperature and looking at the sign of the temperature coefficient separates them.

Selection effect
H2523. Denuder and filter methods are validated with gases and aerosols that are easy to generate cleanly, and those are the species with high vapour pressure and simple chemistry, while the semi-volatile and reactive species that dominate field artefact are exactly the ones missing from the validation set. A method can pass every test and still fail on ambient air. Running the validated method beside a direct spectroscopic measurement on the same ambient manifold, rather than on a generated standard, prices the gap.

Hysteresis claim
H2524. The mass a loaded filter registers depends on whether the weighing room warmed or cooled before the weighing, because the deposit and the fibre mat relax toward equilibrium from different directions. Two filters of equal true loading can differ by more than the repeatability the method quotes. Weighing one batch on an ascending and then a descending temperature ramp, without re-equilibrating, shows the loop.

Misattribution
H2525. Oxygen loss in a stored sample is normally charged to biological consumption, but part of it can be the bottle's headspace exchanging with the water each time the bottle is opened for the measurement itself. Every opening replaces a little water with air and changes the partial pressure the sensor sees. Filling to zero headspace and reading with an in-bottle optode, against a bottle opened at each reading, separates the two.

Hidden term
H2526. Carry-over in a grab sample comes from the sampler's own tubing, which retains analyte from the previous sample and releases it into the next, so the reported value depends on the sampling sequence rather than on the water. The term is largest when a high sample precedes a low one and smallest after a blank. Sampling a fixed low standard immediately after a high one, at several rinse volumes, gives the washout curve directly.

Bound
H2527. A peristaltic sampler's contamination floor is bounded by the tubing's leaching rate per unit wetted area, not by the analytical method's sensitivity, and the bound scales with the tube's residence time. A longer tube at the same flow leaches more. Pumping ultrapure water through tubes of two lengths and two materials, and measuring the plateau, yields the bound without a field sample.

Reversal
H2528. Low-flow purging is meant to reduce turbidity and preserve in-situ redox, but below a certain rate the pump's suction lowers pressure enough to degas carbon dioxide, raising pH and shifting the redox state the method was trying to preserve. The effect grows as flow falls, opposite to the assumption that slower is gentler. Sampling at several pumping rates while measuring pH and dissolved oxygen in a flow-through cell, before any air contact, shows the reversal.

Scale argument
H2529. Purge volume is usually expressed in well volumes, which hides that the volume to be removed scales with the square of the well radius while the time to remove it scales inversely with pumping rate. A wide well and a narrow one therefore need the same number of well volumes and different times. Purging two wells of different diameter at equal flow, with a tracer at the screen, tests which quantity tracks the arrival of formation water.

Inversion of standard practice
H2530. Field filtration is standard for dissolved metals, but for redox-sensitive species the filtration itself can be the contaminant: the filter's trace metal content and the air drawn through it during filtration can dominate at low concentrations. Acidifying in the field and filtering later in a glove box is slower but can be cleaner. Splitting one sample into field-filtered and later-filtered aliquots, with filter blanks for both, decides the case rather than the rule.

Count
H2531. Purging to a fixed count of well volumes assumes the count stands for a residence time, and in a low-yield well the proxy fails because the water reaching the pump has had time to equilibrate with the casing and the atmosphere. Time, not volume, is the variable that matters there. Purging one low-yield well to a fixed volume and to a fixed time, and measuring a conservative parameter in both, shows which reaches formation water first.

Identity
H2532. Suspended-sediment mass and turbidity are different measurements that coincide only over a narrow range of particle size and refractive index, so a sampler triggered on turbidity selects a different population of events than one triggered on mass. The trigger defines the sample, not the river. Deploying both triggers at one station and comparing the size distributions of the samples they collect, rather than their turbidity values, shows the selection.

Threshold
H2533. Above a suspended load set by the intake's screen area and the sampler's lift, the intake clogs and what gets through is finer than the stream, because the coarse fraction is held at the screen. The composition bias therefore grows with load instead of staying proportional, so a rating curve fitted across the threshold bends. Sampling a range of loads with two screen areas and sizing what arrives separates clogging from settling.

Artefact mimicking effect
H2534. An apparent diurnal pH cycle in a stream, with afternoon maxima, can be the sensor's own membrane equilibrating with the biofilm that grows on it in daylight rather than a photosynthetic signal in the water. The biofilm shifts the junction potential on a schedule set by light, not by carbonate chemistry. Two pH sensors at one depth, one shielded from light and one cleaned daily, separate the three explanations.

Selection effect
H2535. Samples that reach the laboratory in good condition are the ones whose bottles were full, cold and dark, which are also the samples with less biological activity, so the preserved subset is a biased subset of the campaign. Analyses that use only the valid samples condition on the variable being studied. Comparing the analyte distribution of all samples with that of samples that passed holding-time criteria, for one site, prices the selection.

Conservation argument
H2536. Alkalinity measured on a stored sample closes only if the bottle's headspace carbon dioxide is accounted for, because the headspace is a reservoir that exchanges with the water and shifts the titration's endpoint. A bottle with a large headspace can lose or gain measurable alkalinity with no biological activity at all. Titrating samples bottled at two headspace fractions, with total inorganic carbon measured on the same bottles, tests the closure.

Misattribution
H2537. Zero offsets that appear after a site visit are often charged to sensor aging, but a cylinder near exhaustion fractionates: the more volatile minor component leaves first, so the zero gas can carry a small and growing concentration of the analyte. The offset then tracks the cylinder's remaining pressure, not the sensor's history. Checking one sensor against a fresh cylinder and against a scrubbed ambient stream at several cylinder pressures tells the two apart.

Hidden term
H2538. Every span check contains the regulator's contribution: elastomer seats and diaphragms permeate a small mole fraction of ambient air or of the cylinder's contents into the delivery line, and at ppb levels that term is not negligible. A two-stage regulator adds two such surfaces. Spanning through a regulator purged for different times, or through a heated one, sizes the term.

Bound
H2539. Permeation tubes are quoted as constant sources, but the rate is constant only above a flow at which the boundary layer over the tube is thin enough that emission is set by diffusion through the wall rather than by removal from the surface. Below that flow the rate falls with flow, and the constant is a plateau rather than a property of the tube. Sweeping dilution flow while weighing the tube over days locates the plateau's lower edge.

Reversal
H2540. Diluting a span gas with mass-flow controllers is most accurate at high dilution ratios because the span flow is small, but the relative uncertainty of the ratio is then dominated by that small flow's calibration, so accuracy reverses and the best ratio is mid-range. The two errors cross. Spanning an analyser at a series of ratios made with two controller pairs, compared against a direct standard at each point, locates the crossing.

Scale argument
H2541. Gas in a cylinder stratifies by molecular weight, and the relevant scale is the cylinder's height, so a tall cylinder's first draws differ from a short one's at the same nominal mixture. Stratification relaxes by diffusion, slowly compared with the interval between draws. Drawing from the top and the bottom of a tall cylinder, or inverting it for a day, tests homogeneity on the height scale.

Inversion of standard practice
H2542. Calibration is normally done at the sample flow, on the reasoning that the sensor should be characterized as it is used, but a diffusion-limited sensor's response is set by the boundary layer at its own face, and a calibration at the sample flow can be less representative than one at a flow where that layer is stable. The rule optimizes the plumbing, not the sensor. Calibrating one sensor at three flows and then running it at the field flow shows which calibration predicts the field value.

Count
H2543. A calibration point is worth taking only when it separates mechanisms, so the count needed is set by the number of independent response mechanisms rather than by the order of polynomial that will fit the data. A sensor with one mechanism is a line and an offset, and extra points sharpen two numbers. Adding points one at a time to one dataset and watching the fitted parameters stop moving identifies the count.

Identity
H2544. Zero and baseline are treated as two adjustments on many analysers but are one parameter observed under two conditions, so adjusting both creates a gain error that neither adjustment reveals. The second absorbs whatever the first left, and the pair stops being identifiable. Adjusting zero twice and then span twice, in both orders, on one instrument and comparing the resulting slopes shows whether the knobs are independent.

Threshold
H2545. Below a concentration set by the intercept's uncertainty divided by the slope's, a two-point calibration is worse than a one-point calibration, because the fitted intercept moves the low-end prediction more than the span does. Two-point methods are not uniformly better. Calibrating one sensor by both methods at several span levels and comparing prediction error at low concentration locates the threshold.

Artefact mimicking effect
H2546. Span loss over a season can be the regulator's seat taking a permanent set, which changes the delivered concentration at a fixed setting, rather than the sensor losing sensitivity. The regulator's change is invisible at the cylinder and visible only downstream. Swapping regulators between two analysers mid-season and seeing whether the loss follows the regulator or the analyser settles it.

Selection effect
H2547. Calibration certificates describe units that passed the manufacturer's screen, so the certificate file understates the spread of span among units actually shipped, because the rejects are missing from it. A campaign that treats certificate values as the population is using a truncated distribution. Buying a lot and measuring every unit, including returns, gives the untruncated spread for comparison.

Identity
H2548. For a membrane-covered sensor, sensitivity and response time both come from the membrane's diffusion resistance: sensitivity falls as thickness rises and the time constant rises as its square, so the square of the sensitivity times the time constant should be the same for membranes of one material. A membrane that breaks the product is leaking or has a different permeability. Measuring both quantities on one sensor with three membranes of one material and different thicknesses tests it.

Misattribution
H2549. Electrochemical carbon monoxide sensors respond to hydrogen as well as to CO, so an apparent response to traffic can be largely a hydrogen response that rises with vehicle exhaust. The two gases differ in diffusion coefficient, so the ratio of their responses changes with the membrane. Co-locating the sensor with a hydrogen scrubber, or with a CO-only reference, at a kerbside site separates the contributions.

Hidden term
H2550. Ozone sensors carry a term from the counter electrode's own chemistry, which sets a floor independent of the sensing electrode's potential and therefore invisible to a zero-air check. The counter electrode runs at whatever potential the cell needs to balance its current. Running the cell at two bias voltages, or with a guard electrode, shows how much signal survives when the sensing reaction is suppressed.

Identity
H2551. Cross-sensitivity coefficients and the response matrix of a sensor array are the same object written two ways, so a sensor that is selective for one gas over another has a fixed ratio no firmware can retune, and a two-gas correction is a matrix inversion that amplifies noise by the matrix's condition number. Claiming better selectivity from software is claiming a different matrix. Inverting calibration matrices of different condition number for two co-located arrays and comparing output noise shows the amplification.

Bound
H2552. NDIR stability is bounded by the detector's thermal time constant and the source's flicker, not by the electronics' sampling rate, so averaging beyond the slower of the two adds no information and can add bias if the source changes during the average. The bound is a time, not a number of samples. Recording the raw detector at high rate and computing the deviation of increasing averages from their neighbours shows where the curve turns up.

Reversal
H2553. Raising sample flow through a photoionization detector is supposed to speed the response by shortening residence time, but above a flow the dilution inside the lamp housing lowers the concentration at the electrode, so the steady amplitude falls while the time constant has already stopped improving. The two effects have different flow exponents. Stepping concentration at several flows and measuring both the time constant and the steady amplitude puts the reversal in view.

Threshold
H2554. Molybdenum converters are self-poisoning: efficiency for nitrogen dioxide depends on the surface's oxidation state, and the surface is oxidized cumulatively by the NO2 it converts, after which the analyser under-reports. The threshold is cumulative rather than instantaneous, so a pass at the start of a run says little about the end. Challenging the converter with a known NO2 standard at intervals through a long run, and integrating exposure, plots efficiency against cumulative moles.

Artefact mimicking effect
H2555. Nighttime maxima in converted nitrogen oxides can be the converter's own residence time lengthening as the sample cools, since a mass-flow controller holds mass while volumetric flow falls and more of the semi-volatile nitrate and peroxyacetyl nitrate in the sample is converted. The instrument's conversion efficiency then follows its temperature rather than the atmosphere's chemistry. Holding the converter at fixed temperature while the inlet cools tests it.

Count
H2556. Manual interference lists overstate the correction burden, because the species that matter at a site are the ones that co-vary with the target there, and that is usually two: one tracking the target's source and one tracking its sink. Listings enumerate everything the sensor responds to, not everything that matters. Regressing the sensor against a suite of measured co-pollutants at one site and counting coefficients that survive out-of-sample prediction gives the site's number.

Scale argument
H2557. Diffusion path length sets a sensor's speed and its sensitivity together: response time falls as the square of the path while the signal rises as its inverse, so shortening helps twice, until the path is short enough that the electrode's own time constant dominates and further shortening buys no speed while the signal keeps rising. The crossover is a length, not a flow. Measuring response time against path length for three membranes of one material shows where the points leave the square law.

Unit trap
H2558. Converting between ppb and micrograms per cubic metre uses a molar volume evaluated at a temperature and pressure that are usually assumed rather than measured, so a network converting at a fixed molar volume writes a temperature-correlated bias into every reported mass concentration. At a mountain site the bias is larger than at sea level and changes sign between summer and winter. Recomputing one station's series with local pressure and temperature and differencing it against the original shows how much of the seasonal cycle was the conversion.

Selection effect
H2559. Field validation studies report the sensors that survived the deployment, and survival correlates with the stability being measured, so published stability statistics are conditional on the outcome and the failures, which are the informative cases, are absent from the figure. Following an entire purchased batch through a deployment, including units removed, gives the unconditional distribution.

Null claim
H2560. For a diffusion-limited electrochemical cell the temperature coefficient of sensitivity should be small, because the diffusion coefficient rises with temperature while solubility falls and the two partly cancel; a large measured coefficient therefore indicates a blocked diffusion path or a dry electrolyte, not a temperature effect. The null is a diagnostic. Measuring the coefficient before and after deliberately partial blockage of the membrane shows the sign and size of what a blocked path does.

Misattribution
H2561. Cooking aerosol is larger and of lower refractive index than the size and refractive index a low-cost optical sensor assumes, so a peak during cooking is a size-classification error read as a mass increase. The sensor infers mass from scattering and never measures it. Co-locating the sensor with filter mass and a size spectrometer through one cooking event separates the classification error from the mass.

Hidden term
H2562. Coincidence loss in an optical particle counter has a term from the detector amplifier's dead time in addition to the optical sampling volume, and at high count rates the usual correction, which assumes only the volume, underestimates the loss by the ratio of the two dead times. The amplifier term is invisible at low rates. Counting a monodisperse aerosol at rising concentration and fitting the departure from linearity with one and then two dead-time parameters shows which is present.

Bound
H2563. The smallest particle an optical counter can size at a given wavelength is bounded by stray light in the collecting optics rather than by diffraction, so quoting the diffraction limit overstates the instrument's reach. Stray light sets a background that a small particle's scattering cannot exceed. Measuring the pulse-height distribution for a filtered airstream and for a monodisperse aerosol at the quoted smallest size shows where the background sits relative to the signal.

Reversal
H2564. For a fixed mass, a nephelometer's scattering rises as particles shrink toward the wavelength, because the scattering cross-section per unit mass peaks there, so a size shift caused by evaporation can look like a mass increase and the instrument reports the wrong direction. Measuring scattering and mass concurrently while the distribution shifts, for instance by gentle heating, shows the two moving apart.

Scale argument
H2565. Personal monitors sample still indoor air and windy outdoor air through the same inlet, and aspiration efficiency depends on the ratio of wind speed to inlet velocity, so an indoor-outdoor comparison with one monitor compares two sampling regimes as well as two atmospheres. The ratio, not the inlet, is the variable. Running the monitor indoors and outdoors while raising the indoor air speed to the outdoor value, at one aerosol, separates the regimes.

Count
H2566. Size bins earn their keep only when they separate modes: subdividing one mode adds no independent information and multiplies counting noise per bin, so more bins with fewer counts can make a unimodal distribution look multimodal. Rebinning one dataset at half the resolution and testing whether the modes survive shows which features were real.

Identity
H2567. PM2.5 from an optical sensor and PM2.5 from a filter are not the same quantity: one is a calibrated proxy for mass derived from scattering, the other is a mass, and their agreement at one site is a property of the calibration and the aerosol there. Applying both at a second site with a different size distribution and refractive index, without refitting, tests whether the agreement travels.

Threshold
H2568. An optical counter has a middle band of count rates that is trustworthy: below it the flow's pulsation and the sheath's instability dominate, above it coincidence loss dominates, and the band can be narrow. Operating in either tail gives a number that looks stable but is set by the instrument. Running a monodisperse aerosol across a hundredfold range of count rate and plotting each rate against a reference draws the band.

Artefact mimicking effect
H2569. Fan wear, not darkness, can explain a nighttime rise in a low-cost particle sensor's output: a slowing fan lengthens residence time in the optical chamber and lets the same air be counted more than once, and the effect grows with runtime rather than with the hour. Logging the fan tachometer, or swapping the fan mid-deployment, shows whether the rise follows the fan or the night.

Selection effect
H2570. Sensor evaluation campaigns tend to deploy where power and access are easy, which is often where the air is cleaner, and the slope fitted there is then applied at polluted sites where the aerosol is larger and more absorbing. Nothing in the instrument records that dependence. Refitting one sensor type at a polluted site against a co-located reference, and comparing slopes, shows how far the calibration travels.

No-go claim
H2571. A single-wavelength scattering sensor cannot separate black carbon from sulfate at the same mass, because the two differ mainly in absorption and a scattering measurement is nearly blind to absorption, so the ratio is not identifiable from one channel. Any separation claimed from one wavelength is a prior, not a measurement. Adding a second wavelength or a filter-based absorption measurement and testing whether the separation survives is the only check available.

Misattribution
H2572. pH electrodes at remote sites acquire a slow bias that is usually charged to the glass bulb, but the reference junction's chloride supply depletes and its potential shifts as junction resistance rises, while the bulb is unchanged. The bias then follows time in service rather than calibration history. Replacing only the reference electrolyte and comparing the potential before and after separates the junction from the bulb.

Hidden term
H2573. Scratches and aged coatings in a turbidity sensor's optical path add a term to every reading, and the term changes when the window is cleaned or the shroud's black coating dulls, so a sensor can report rising turbidity while the water is unchanged. The term belongs to the optical path, not the sample. Measuring the same water with two windows of different scratch density, or with the shroud removed, exposes the term.

Bound
H2574. Low-ionic-strength accuracy of a conductivity cell is bounded by the cell's own dissolution and by the water's uptake of carbon dioxide, both of which add ions the cell counts. The blank is a concentration, not a voltage, so no electronics improvement moves it. Recirculating ultrapure water through the cell in a closed, CO2-free loop and watching the reading climb gives the bound directly.

Reversal
H2575. Copper anti-fouling guards keep biofilm off optical windows, but the copper they release is what a copper ion-selective electrode at the same station is trying to measure, so the guard protects one sensor by contaminating another. The two instruments cannot share a housing without a partition. Measuring copper at two distances from a guard, and with the guard removed, maps the contamination.

Scale argument
H2576. Chlorophyll fluorometers divide their signal by a sampled volume set by the excitation light's penetration depth, which shrinks as turbidity rises, so the reported concentration rests on a volume calibration that depends on the water's clarity. The effect is largest where blooms and sediment coincide. Comparing a fluorometer with extracted chlorophyll across a turbidity range, without recalibration, shows the volume effect.

Inversion of standard practice
H2577. Formazin is the turbidity standard, but a site's particles differ in size and refractive index from formazin, so the standard is a poor surrogate there and a calibration made by diluting the site's own settled sediment would be more representative. The standard exists for comparability, not for accuracy at a site. Running both calibrations on one instrument and comparing each against suspended-sediment mass shows which predicts better.

Count
H2578. Maintenance intervals are set by the number of cleaning cycles a station tolerates before its error exceeds tolerance, not by the number of fouling species, because a species count says nothing about growth rate or the sensor's sensitivity to coverage. Recording the error after each cleaning across a season and counting cycles to failure gives the operational number.

Threshold
H2579. An ammonium ion-selective electrode responds Nernstian down to a concentration set by the reference electrode's ammonium leakage and the membrane's own dissolution, below which the slope flattens and the electrode reports a false floor. The floor is a property of the cell, not of the water. Measuring a dilution series made with ammonium-free water, then repeating with a double-junction reference, locates the floor and its source.

Artefact mimicking effect
H2580. Diel nitrate cycles at a river station can be temperature compensation applied to a membrane whose selectivity biofilm has changed, so the compensation corrects the wrong thing on a daily schedule and the cycle follows water temperature rather than the nitrogen cycle. Comparing a cleaned sensor with a fouled one at the same temperature, over one day, separates the two cycles.

Identity
H2581. ORP and dissolved oxygen are two views of the same redox system only where the water is poised by one couple; where they disagree the site has two couples or a mixed potential, not a faulty probe. Reading the disagreement as instrument error discards the chemistry. Measuring both through a redoxcline, alongside a platinum electrode in a poised standard, shows whether the disagreement is chemical or instrumental.

Unit trap
H2582. NTU, FNU and FTU coincide numerically only for formazin in a specified geometry, and field instruments with different source angles and path lengths produce different numbers for the same water, so mixing units across a network creates apparent spatial structure. The units are not interchangeable. Sending one sample to three instruments of different geometry and comparing the ratios to their formazin calibrations exposes the conversion.

Null claim
H2583. At a site with steady flow and chemistry, moving an in-situ sensor a few centimetres within the water column should not change its reading, and any change is a siting artefact that a swap of two identical sensors resolves. If the reading follows the position rather than the sensor, the column is not well mixed at that scale. The test costs one dive.

Misattribution
H2584. Site-to-site differences in a network are often read as differences in emissions, but part of the difference is inlet height: a sensor at three metres and one at ten sample different footprints, and the difference between them is a vertical gradient rather than a source. The gradient is largest at night when the boundary layer is shallow. Co-locating two identical sensors at two heights for a week measures it, and the network differences can then be compared against it.

Hidden term
H2585. Solar and mains power give a low-cost node different heater and fan duty cycles, and the sensor's response depends on its chamber temperature, so a network's spatial average carries a term from each site's power supply that is correlated with its land use. Running one sensor type on both power arrangements at one site, and comparing, isolates the term.

Rank argument
H2586. Independent information from a network of N sensors is bounded by the number of distinct air masses it samples during the period, not by N, so a dense cluster inside one air mass adds precision but not spatial information. The bound is set by wind and network extent. Computing the covariance matrix of the network's residuals and counting eigenvalues above the noise gives the effective number, which can be far below N.

Reversal
H2587. Densifying a network improves the estimate of the spatial mean but can degrade the estimate of spatial variance, because nearby sensors share calibration error that does not average out and added sensors are correlated with existing ones. More instruments can mean more confidence in the mean and less in the spread. Estimating variance from progressively denser subsets of one deployment, with calibration error held fixed, measures the degradation.

Scale argument
H2588. Footprints scale with wind speed times averaging time, so a one-minute average and a one-hour average at one site describe different areas, and comparing either with a model grid of fixed size compares two different things. The mismatch is largest on windy days. Computing the footprint for both averaging times from the site's wind record and testing which predicts a co-located reference sizes the mismatch.

Inversion of standard practice
H2589. Averaging to fixed clock hours is standard, but when a sensor's response time is comparable to the interval, the reported hourly value is a convolution of the true signal with the sensor's response, which shifts peaks later and lowers them, so the average is not the hour's mean. The distortion belongs to the instrument. Deconvolving one record with the measured response function, or comparing sensors of two response times at one site, shows the shift.

Count
H2590. A count of hours above a level depends on the sampling interval, because a five-minute sampler can miss a two-minute plume that a one-minute sampler catches, so the count is partly an instrument's property. The missing hours are not random; they are the shortest events. Running two samplers at one site at different intervals through a season and comparing counts, and the durations each missed, quantifies the dependence.

Artefact mimicking effect
H2591. Weekday-weekend differences in a network's record can be its maintenance schedule, because sensors are serviced and zeroed on weekdays and a service changes the reading for a day or two afterwards, so the pattern follows the calendar rather than the traffic. Comparing sites with different service days, or shifting one site's service day mid-year, shows whether the weekly pattern moves with it.

Selection effect
H2592. Sites that survive in a multi-year network are those with reliable power, access and a landlord who stays, and those conditions correlate with land use, so the surviving network is not a random sample of the city and trends estimated from it inherit the selection. Comparing the land-use mix of surviving sites with that of sites planned or dropped prices the bias.

Threshold
H2593. Beyond a decorrelation distance set by boundary-layer depth and wind, the correlation between two sites falls faster than the mean difference between them grows, so a spacing exists beyond which adding sites adds difference without shared signal. That distance, not an administrative boundary, sets useful spacing. Computing correlation and mean absolute difference against separation for one network locates the crossing.

Null claim
H2594. Two identical co-located sensors should differ only by their calibrations; if the difference is larger and structured, the cause is their positions within the shelter, and swapping the positions should reverse its sign. The null is a siting test that costs one swap. Applying it to a pair before trusting either as a reference is the cheapest check in a network.

Unit trap
H2595. Converting ppb to micrograms per cubic metre at a fixed molar volume mis-states concentrations by up to about a tenth across a seasonal temperature range (estimate), and the error is systematic, so it appears as a seasonal cycle no atmospheric process produced. The conversion is a hidden multiplicative term with a calendar. Recomputing one site's series with hourly pressure and temperature, and differencing the two series, shows the artificial cycle's amplitude and phase.

Quantization
H2596. Eight-bit storage imposes a step that a histogram reveals as evenly spaced modes, and a real atmosphere rarely produces evenly spaced modes, so apparent multimodality can be the recorder. The step belongs to the storage, not the air. Reading the raw register values and plotting their differences, or re-recording one channel at higher resolution, reveals the comb.

Hidden term
H2597. A blank subtracted as a constant concentration actually scales with sample volume, so a sampler run twice as long collects twice the blank mass while the same concentration is subtracted, and the corrected value depends on run length. The correction is only constant if the blank is carried as a mass. Running blanks at two sample volumes from one filter lot shows whether the subtracted quantity should be a mass or a concentration.

Covariance argument
H2598. Flow-weighted composites and arithmetic means of grabs differ by the covariance of concentration and flow, so at a site where high flows carry high concentrations the two means answer different questions and neither is wrong; reporting one as the other is a category error. Computing the covariance from paired grab and flow measurements at one site, and predicting the composite from it, tests the arithmetic.

Reversal
H2599. Longer averaging reduces random noise, but for a sensor with a nonlinear calibration, averaging the raw signal and then inverting the calibration biases the mean upward by the curvature, while averaging after inversion does not, and the two orders converge only as variance goes to zero. The bias grows with signal variance, so it is worst in patchy conditions. Averaging one high-rate record both ways and comparing both against a reference shows the size of the bias.

No-go claim
H2600. No single co-location calibration transfers across seasons for a sensor whose response depends on the analyte's speciation, because the slope is a property of the mixture as well as the instrument and the mixture changes with season. A calibration is a function of the air it was fitted in. Refitting one sensor against a reference in two seasons and applying each season's slope to the other season's data shows the transfer failure directly.