Plume Dispersion Research Articles

How does resolution of sedimentary architecture data affect plume dispersion in multiscale and hierarchical systems?

The potential impacts of the spatial resolution of sedimentary structure data on solute dispersion in heterogeneous porous formations are investigated with Lagrangian-based transport models. The models rely on a covariance function that represent a hierarchical organization of sedimentary facies types by using facies physical properties, such as volume proportions and mean length as well as auto- and cross-transition probabilities and their log-permeability covariance. A detailed sedimentary architecture data (i.e. indicator data) provides better representation of the spatial correlation structures of the global covariances through capturing its underlying structure defined by transition probabilities more accurately. However, the extent to which such data affect time-dependent transport parameters (i.e. dispersivity) is unclear. In this study, we parameterize transport models using detailed collocated sedimentary architecture and permeability data from an outcrop in Española Basin, NM. In addition, we perform global sensitivity analysis based on Polynomials Chaos Expansion to understand the significance of parameters in the transport models. The results show that dispersivity and particle displacement variance are under-estimated if less resolved facies (i.e. indicator) data are used even if the global covariance structure is well captured. Dispersivity is sensitive to the correlation scale that is directly calculated from sedimentary architecture data, and the mean log-permeability. However, anisotropy ratio and mean log-permeability are the most sensitive parameters for the transverse dispersivity.

Report of Particulate Matter Emissions During the 2015 Fire at Fuel Tanks in Santos, Brazil

We report a study of the particulate matter emissions related to the 2015 fire outbreak at a fuel tank storage facility located at Santos, a coastal city in Brazil. The facility, managed by the company Ultracargo, had oil tanks (filled with gasoline and ethanol) destroyed by fire that lasted more than a week, between 2nd and 9th April. In this article, we present the atmospheric concentration analysis of particulate matter (PM10) measured over the entire month of April 2015 by the 2 closest stations that integrate the air quality monitoring system of the São Paulo State Environmental Company (CETESB). The results were compared with similar data from the same period of the yesteryear (April 2014). The results were also complemented by the air masses trajectories over the region (obtained with HYSPLIT/NOAA software). Our results do indicate a subtle increase in the particulate concentration during the days of the fire, followed by a fast dissipation over the subsequent weeks. The observed plume dispersion discussion is made considering the meteorological patterns of the region and other environmental and health reported impacts related to the accident.

Qualitative assessment on premature human mortality due to the emission of fine particulate matter from the Matarbari coal power plant

AbstractThis study reports the probability of increased mortality of people within the political border of Bangladesh due to the emission of fine particulate matter with diameters of 2.5 microns or less (PM2.5) from the Matarbari coal power plant (MCPP). A Gaussian plume dispersion model has been used for this estimation. The PM2.5 emission rate data are unavailable as the construction of MCPP is still in its initial stage; therefore, the anticipated PM2.5 emission rate has been estimated based on data from a number of coal‐fired power plants in India and China. To make this study more meaningful, two different emission rates have been considered representing the best‐case and worst‐case scenarios. In both cases, the intake fraction has been found to be 0.12×10−2, and the value of relative risk varies between 1.134 and 1.374, respectively. Finally, it is estimated that approximately 11.5 million people inside Bangladesh will be exposed to the PM2.5 emission from MCPP, and between 7,667 and 17,675 people will experience premature death every year.

Atmospheric modeling and source reconstruction of radioactive ruthenium from an undeclared major release in 2017

In October 2017 unusual 106Ru detections across most of Europe prompted the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) to analyze the event in order to locate the origin and identify the magnitude of the release. This paper presents the inverse modeling techniques used during the event to achieve this goal. The method is based on a variational approach and consists of using air concentration measurements with the ldX long-range dispersion model included in the IRSN's C3X operational platform. The method made it possible to quickly identify the southern Urals as the most likely geographical origin of the release. Despite uncertainties regarding the starting date of the release, calculations show that it potentially began on 23 September, while most of the release was emitted on 26 September. Among the nuclear plants identified in the southern Urals, the Mayak complex is that from which the dispersion of the 106Ru plume is most consistent with observations. The reconstructed 106Ru source term from Mayak is ∼250 TBq. In total, it was found that for 72% of the measurements simulated and observed air concentration agreed within a factor of 5. In addition, the simulated deposition of 106Ru agrees with the observed deposition. Outside the southern Urals, the simulations indicate that areas with highest deposition values are located in southern Scandinavia and southeastern Bulgaria and are explained by rainfall events occurring while the plume was passing over.

Monitoring and analysis of millisecond laser drilling process and performance with and without longitudinal magnetic assistance and/or assist gas

A systematic investigation was reported for observing and analyzing the process and performance of millisecond laser drilling with and without magnetic assistance and/or assist gas using a self-developed real-time monitoring system based on high-speed photography. The systematic and corresponding comparisons between the effects of longitudinal magnetic assistance and co-axial assist gas for millisecond laser drilling of both blind holes and through holes using high-speed photography were first reported. Using a direct comparable visual method based on comparable observations using high-speed camera, the effects of the co-axial assist gas and/or the longitudinal magnetic assistance on millisecond laser drilling process, drilling efficiency and drilling quality were systematically analyzed by correspondingly comparing and correlating the real-time observations with the measurements for laser drilled specimens, including the characterizations and analyses for material removal, plume expansion and dispersion, specimen spatters, hole geometry/morphology, hole sidewall recast layer and micro hardness performance. It was shown that the metal vapor and plasma induced by a millisecond laser mixed with each other as a hybrid plume. The co-axial assist gas flow was observed to be much more effective than the longitudinal magnetic field for promoting plume dispersion and enhancing drilling efficiency during the processes of blind-hole drilling and through-hole drilling. When using the assistance of a co-axial argon flow together with a longitudinal magnetic field, the blowing and cooling effects of assist gas were dominant during the laser drilling process, and the longitudinal magnetic assistance further vertically squeezed and horizontally stretched the plasma, greatly reducing the shielding effect of laser-induced plume for effectively increasing laser drilling efficiency and quality.

Effect of oil plumes on upper-ocean radiative transfer — A numerical study

Oil plumes released from subsea blowouts can induce strong optical extinction effect and significantly affect radiative transfer in the ocean euphotic zone where photosynthesis occurs. In this paper, the effect of oil plumes on the ocean radiative transfer is studied numerically by modeling the dispersion of oil plume in the ocean mixed layer and the radiative transfer through the oil-contaminated seawater. Due to the interactions with flows in the ocean mixed layer, oil plumes of different droplet diameters exhibit considerable differences in plume size, shape and oil concentration. With the same volumetric release rate, plumes of large oil droplets exhibit highly intermittent local concentration in near-surface regions and cause strong but intermittent extinction effect over relatively small horizontal area; plumes of small oil droplets diffuse more smoothly over large horizontal and vertical extensions, resulting in significant overall extinction effect to the radiative transfer in the ocean euphotic zone.

A large contribution of anthropogenic organo-nitrates to secondary organic aerosol in the Alberta oil sands

Abstract. The oil sands industry in Alberta, Canada, represents a large anthropogenic source of secondary organic aerosol (SOA). Atmospheric emissions from oil sands operations are a complex mixture of gaseous and particulate pollutants. Their interaction can affect the formation and characteristics of SOA during plume dispersion, but their chemical evolution remains poorly understood. Oxidative processing of organic vapours in the presence of NOx can lead to particulate organo-nitrate (pON) formation, with important impacts on the SOA budgets, the nitrogen cycle and human health. We provide the first direct field evidence, from ground- and aircraft-based real-time aerosol mass spectrometry, that anthropogenic pON contributed up to half of SOA mass that was freshly produced within the emission plumes of oil sands facilities. Using a top-down emission-rate retrieval algorithm constrained by aircraft measurements, we estimate the production rate of pON in the oil sands region to be ∼15.5 t d−1. We demonstrate that pON formation occurs via photo-oxidation of intermediate-volatility organic compounds (IVOCs) in high-NOx environments, providing observational constraints to improve current SOA modelling frameworks. Our ambient observations are supported by laboratory photo-oxidation experiments of IVOCs from bitumen vapours under high-NOx conditions, which demonstrate that pON can account for 30 %–55 % of the observed SOA mass depending on the degree of photochemical ageing. The large contribution of pON to freshly formed anthropogenic SOA illustrates the central role of pON in SOA production from the oil and gas industry, with relevance for other urban and industrial regions with significant anthropogenic IVOC and NOx emissions.

Capturing Plume Rise and Dispersion with a Coupled Large-Eddy Simulation: Case Study of a Prescribed Burn

Current understanding of the buoyant rise and subsequent dispersion of smoke due to wildfires has been limited by the complexity of interactions between fire behavior and atmospheric conditions, as well as the uncertainty in model evaluation data. To assess the feasibility of using numerical models to address this knowledge gap, we designed a large-eddy simulation of a real-life prescribed burn using a coupled semi-emperical fire–atmosphere model. We used observational data to evaluate the simulated smoke plume, as well as to identify sources of model biases. The results suggest that the rise and dispersion of fire emissions are reasonably captured by the model, subject to accurate surface thermal forcing and relatively steady atmospheric conditions. Overall, encouraging model performance and the high level of detail offered by simulated data may help inform future smoke plume modeling work, plume-rise parameterizations and field experiment designs.

Numerical investigation of plume dispersion characteristics for various ambient flow velocities

Floating LNG bunkering offshore terminal (FLBT) has been adopted as a floating unit by the marine industry equipped with the natural gas liquefaction plant on the ship’s deck with the practical interest in controlling the plume emission discharged from the ship. The downwash of emitted plumes has adverse consequences on the ship’s engine intake and ventilation system followed by the interference of the smoke with helicopter operations. Owing to this fact, understanding of the plume behavior is considered to be so significant in the aspect of ship design and thereby, flow visualization techniques assist to study the plume path ensuring the safety of the personnel and functioning system. The unignited flare gas emitted from the tower invites heat transfer due to the temperature difference between the atmospheric wind and the ship’s exhaust, which follows the examination of mixing enhancement of fluid mediums as well as the mitigation of plume concentration depending upon wind speeds. The parametric study is intended to investigate the plume dispersion pattern around a ship based on the flare motion and bending angle for light and strong wind speeds. It is observed that the plume gas rises higher and disperses over the larger area with wider streamline separation due to the effect of buoyancy for light wind speeds. On the other hand, the motion of flare gas is found to be narrow for strong winds restricting vertical movements due to dominant inertial forces than gravity pull. Thus, the current numerical investigation facilitates in understanding the configuration and plume distribution by the variation in streamline behavior with contours plots. In this work, the calculated results are analyzed systematically in pretty realistic conditions and simple measures are obtained, which will be applied to the preliminary design of plume stack depending on the ship’s deck arrangement.

Simulating Forest Fire Plume Dispersion, Chemistry, and Aerosol Formation Using SAM-ASP version 1.0

Simulating Forest Fire Plume Dispersion, Chemistry, and Aerosol Formation Using SAM-ASP version 1.0

Environmental and economic assessment of traffic-related air pollution using aggregate spatial information: A case study of Balneário Camboriú, Brazil

IntroductionTransportation is one of the main determinants of atmospheric pollutant emissions in urban areas. This externality has direct environmental, economic and public health consequences. This paper aims at investigating the space-time patterns of traffic air pollution in Balneário Camboriú (Brazil) over projected temporal scenarios and at estimating the damage costs of traffic air pollution to support transport policy-making. MethodsTo estimate the emission rates of pollutants, emission factors and traffic data were jointly used, whereas the pollutant concentrations were estimated using the Gaussian plume dispersion model. To identify the affected areas as well as possible spatial heterogeneity patterns of air pollution within clustered areas, an exploratory spatial analysis was also conducted. To assess the economic impact of air pollution, damage costs were calculated for various pollutants. ResultsThe modeling results show that oxides of nitrogen (NO2) and oxides of sulphur (SO2) pollutants exceed the limits of air quality legislation, especially at a distance up to 10 m from the roads, while 60% and 71% of the intersections are found to yield pollutant concentrations above the thresholds, primarily during peak hours. The analysis also confirmed that homogeneous traffic zones with similar emission rates are spatially clustered exhibiting positive autocorrelation patterns. The results of the economic appraisal showed that the estimated costs of traffic-related emissions were $0.89, $1.38 and $1.43 million/year, respectively, for the current, short-term and long-term scenarios. ConclusionsThis study serves as the first comprehensive analysis of traffic air pollution for the specific study region, providing implications and modeling tools that can be leveraged in public policies focusing on the elimination of the transportation-generated health burden. The developed analysis framework can also serve as a supporting tool for Public Agencies focusing on the high-level evaluation of traffic-related air pollution using limited and aggregate spatial and traffic data.

Analytical Study of Fuel Switching from Heavy Fuel Oil to Natural Gas in clay brick factories at Arab Abu Saed, Greater Cairo

Arab Abu Saed area in Giza governorate, south to Cairo contains more than 228 clay brick kilns represent the largest cluster of brickworks in Egypt. Burning of Heavy Fuel Oil (HFO) in such kilns is the main source of air pollution in the surrounding locations. In this study, investigation of switching the fuel used in brick kilns from (HFO) to Natural Gas (NG) is carried out and the pollution loads are assessed in both cases. In addition, two Gaussian dispersion plume models are employed to estimate the concentration of primary pollutants; PM10, SO2, and NO2 at seven locations in the vicinity of Arab Abu Saed to determine the most adversely affected locations. Statistical analysis is applied to evaluate the correlation and conformity of the results of both models. Results show that using of NG leads to a significant reduction of pollution loads of PM10, SO2 and NO2 reaches 96%, 72%, and 24% respectively. In addition, the reduction of naturally occurring radionuclides in air is analyzed. Activity concentrations of Ra-226, Th-232 and K-40 in Bq/l for HFO were measured using HPGe detector for six HFO samples. Exposure due to air submersion of naturally occurring radionuclides in the study area leads to annual equivalent dose ranged between 2.16 mSv/y (received by Uterus) and 14 mSv/y (received by skin), and average effective dose 2.65 mSv/y which represent valuable exposure.

Anisotropic dispersion with a consistent smoothed particle hydrodynamics scheme

A consistent smoothed particle hydrodynamics (SPH) approach is used to simulate the anisotropic dispersion of a solute in porous media. Consistency demands using large numbers of neighbors with increasing resolution. The method is tested against the anisotropic dispersion of a Gaussian contaminant plume. With irregularly distributed particles, the solution for isotropic dispersion converges to second-order accuracy when at sufficiently high resolution a large number of neighbors is used within the kernel support. For low to moderate anisotropy, the convergence rates are close to second-order, while for large anisotropic dispersion the solutions converge to better than first-order. For randomly distributed particles, the solutions are also better than first order independently of the degree of anisotropy. When negative concentrations arise, they are several orders of magnitude smaller than those encountered with standard SPH and comparable to those obtained with the MWSPH scheme of Avesani et al. The method is also insensitive to particle disorder and achieves an overall accuracy comparable to the MWSPH method using a much simpler approach.

Transport mechanism of urban plume dispersion

Winds (advection) and turbulence (diffusion) are the driving forces for urban plume dispersion whose partitioning is complicated by urban morphology and the conditions of atmospheric surface layer (ASL). Wind tunnel parametric tests are conducted to examine the plume behavior in the turbulent boundary layers (TBLs) over hypothetical buildings (roughness elements of ribs and bricks) with drag coefficient Cd (= 2uτ2/U∞2; where uτ and U∞ are, respectively, the friction velocity and the freestream velocity) varying from 4.7 × 10−3 to 10 × 10−3. Vehicular emission is modeled by atomizing water vapor via a ground-level line source in crossflows. The laboratory measurements show that the mean tracer concentrations ψ‾ exhibit the theoretical Gaussian form, supporting the classic plume dispersion framework. The horizontal advection tracer flux u‾ψ‾ dominates the transport processes compared with the vertical turbulent tracer flux w''ψ''‾. Increasing Cd leads to a decrease in u‾ψ‾ and an increase in w''ψ''‾ simultaneously, and vice versa. The changes in partitioning of transport processes in response to urban morphology imply that the conventional parameterization of dispersion coefficient σz (determined by atmospheric stability only) should be applied cautiously for urban setting. A new simple scheme of σz parameterization, which specifically accounts for the influence from urban roughness on the plume dispersion, is proposed.

Diversity of soluble salt concentrations on volcanic ash aggregates from a variety of eruption types and deposits

Ash aggregation is a common phenomenon in particle-laden environments of volcanic eruption plumes and pyroclastic density currents. Many of these initially fragile aggregates gain sufficient mechanical strength to remain intact after atmospheric transport and deposition. Several processes contribute to ash aggregate stability, including electrostatic and hydrostatic bonding, ice formation, and cementation by salt precipitates. Here, we compare leachate chemistry from aggregates from a variety of eruption and sedimentation conditions, ranging from dry magmatic eruptions with immediate deposition, to eruptions through seawater. The leachate data shows that the broad window of opportunity for aggregation and aggregate break-up may be used to qualitatively constrain suspended ash concentration and its temporal evolution. We show that aggregation rate and aggregate stability largely depend on the availability of external water and salt source. In particular, high humidity and extensive salt precipitation in seawater environments, such as during the Surtseyan eruptions, promote high aggregation rates and aggregate stability, with accordingly accentuated proximal deposition and aggregate concentration in the deposits. On the other hand, low humidity and salt concentrations during dry magmatic eruptions promote less aggregation and more efficient aggregate break-up, explaining the rarity of aggregates in the deposits. These results have strong implications for the ash budget in volcanic plumes and associated models of plume dispersal and related hazards.

Numerical study of plume emission from an elevated line source in a wall-bounded flow

Purpose This paper aims to study turbulent dispersion of a passive plume emitting from a single elevated line source of different elevations in a plane channel flow by using direct numerical simulation (DNS). Design/methodology/approach The investigation was conducted in both physical and spectral spaces, which includes an analysis of statistical moments and pre-multiplied spectra of the velocity and concentration fields. The pre-multiplied power spectra of the velocity and concentration fields are compared to identify the transition of the plume development from the turbulent convective stage to the turbulent diffusive stage. Findings It is observed that due to the presence of wall shear, the mean plume drifts toward the wall for the near-wall source release case. It is also observed that streamwise development of the plume is sensitive to both the source elevation and the downstream distance from the source. For the line source placed near the center of the channel, the plume development is dominated by the bulk meandering effects. However, for the plume emitting from the near-wall line source, it hits the ground soon after its release and becomes dominated by the wall shear. As the downstream distance from the line source increases, the streamwise development of the plume released from the near-wall line source transitions from a turbulent convective stage to a turbulent diffusive stage. Originality/value This paper represents an original DNS study of turbulent mixing and dispersion of a passive plume emitting from a line source of different elevations in a wall-bounded flow. This paper proposes a practical method to identify the transition of the plume development from the turbulent convective to the turbulent diffusive stages.

Numerical Simulation of the Water Temperature in the Al-Zour Area of Kuwait

The Al-Zour coastal area, located in southern Kuwait, is a region of concentrated industrial water use, seawater intake, and the outfall of existing power plants. The Al-Zour LNG import facility project is ongoing and there are two issues regarding the seawater temperature in this area that must be considered: variations in water temperature under local meteorology and an increase in water temperature due to the expansion of the thermal discharge of expanded power plant. MIKE 3 model was applied to simulate the water temperature from June to July, based on re-analysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the thermal discharge input from adjacent power plants. The annual water temperatures of two candidate locations of the seawater intake for the Al-Zour LNG re-gasification facility were measured in 2017 and compared to the numerical results. It was determined that the daily seawater temperature is mainly affected by thermal plume dispersion oscillating with the phase of the tidal currents. The regional meteorological conditions such as air temperature and tidal currents, also contributed a great deal to the prediction of seawater temperature.

DNS study of passive plume interference emitting from two parallel line sources in a turbulent channel flow

Turbulent mixing of dual plumes emitting simultaneously from line sources in a turbulent channel flow has been studied using direct numerical simulation (DNS). Three test cases have been compared to investigate the effects of the source separation on turbulent mixing of the two instantaneous plumes. The dispersion and interference of dual plumes are investigated in both physical and spectral spaces, which include an analysis of statistical moments of the concentration field, cross-correlation between the two instantaneous plumes, pre-multiplied spectra of the velocity and concentration fields, and co-spectrum and coherency spectrum of the dual plumes. As the downstream distance from the line source increases, the plume development associated with a single source emission transitions from a turbulent convective stage to a turbulent diffusive stage. It is observed that a plume released from a ground-level source reaches the turbulent diffusive stage faster than that released from an elevated source. It is also observed that a smaller separation between the two line sources tends to facilitate a more rapid growth in the cross-correlation coefficient of two instantaneous plumes. In the near-source region, the maximum coherency spectrum is produced at lower frequencies indicating that dual-plume mixing is dominated by the external flapping effects of large-scale eddy motions. However, in the far downstream region of the sources, the coherency spectrum in the higher frequency range increases significantly, indicating that the spread of the total plume is larger than all scales of turbulent eddies, such that they all contribute to the in-plume mixing of the dual plumes.

Volcanic ash generation: Effects of componentry, particle size and conduit geometry on size-reduction processes

Grain size distributions (GSD) of pyroclastic materials are the product of processes ranging from primary fragmentation efficiency to tephra transport. As such, a detailed description of their physical and chemical state can provide pivotal information regarding such processes. By constraining the GSD of volcanic deposits one can thereby deliver powerful constraints on the energetics and dynamics of eruptions. In order to do so we must distinguish between two primary controls: 1) fragmentation of magma to tephra, and 2) secondary transport-related processes (such as abrasion and comminution), within the conduit and until deposition of the particles. Variations in particle interactions in the conduit together with conduit geometry, may be major factors in modifying the GSD. As in all physicochemical processes, for eruptive dynamics an experimental basis is an essential element of calibration and quantification. Here, we have conducted the first experimental investigation linking fragmentation and ash production via the influence of 1) particle-componentry and particle-size, and 2) the conduit geometry (constricted versus unconstricted) on the GSD. Rapid decompression experiments with loose tephra material from the fall deposit of Pomici Principali eruption (10.3 ka, Campi Flegrei) have been conducted in an optically transparent setup that enables the optical monitoring of particle dynamics with a high-speed camera. The samples employed can be classified into two main groups; 1) pumices, and 2) dense clasts (including crystals, lava clasts and wall rock fragments). Our results indicate that 1) a conduit diameter constriction (simulating obstacles in the conduit walls) is likely to reduce the average diameter of individual clasts and increase the generation of ash (<63 μm, fine ash); 2) the presence of larger pumice particles in the starting sample results in more efficient ash production; 3) the presence of dense particles is linked to a higher efficiency of size-reduction processes (whereby the presence of crystal-rich pumice appears to have a counterproductive effect). These findings correlate well with existing studies, enabling a general assessment of possible secondary size-reduction processes controlling the GSD of a volcanic deposit. Further, our results also suggest that secondary-generated ash should be considered for hazard assessment and modeling, since increased amounts of ash may affect, e.g., the plume dispersion dynamics, and in turn the area affected by ash deposition.

Hydraulic conductivity and porosity heterogeneity controls on environmental performance metrics: Implications in probabilistic risk analysis

Heterogeneities in natural porous formations, mainly manifested through the hydraulic conductivity (K) and, to a lesser degree, the porosity (ϕ), largely control subsurface flow and solute transport. The influence of the heterogeneous structure of K on transport processes has been widely studied, whereas less attention is dedicated to the joint heterogeneity of conductivity and porosity fields. Our study employs Monte Carlo simulations to investigate the coupled effect of K−ϕ spatial variability on the transport behavior (and uncertainty) of conservative and reactive plumes within a 3D aquifer domain. We explore multiple scenarios, characterized by different levels of heterogeneity of the geological properties, and compare the computational results from the joint K−ϕ heterogeneous system with the results originating from generally adopted constant ϕ conditions. In our study, the spatially variable K−ϕ fields are positively correlated. We statistically analyze key Environmental Performance Metrics: first arrival times and peak mass fluxes for non-reactive species and increased lifetime cancer risk for reactive chlorinated solvents. The conservative transport simulations show that considering coupled K−ϕ fields decreases the plume dispersion, increases both the first arrival times of solutes and the peak mass fluxes at the observation planes. A positive correlation between aquifer connectivity and peak mass fluxes is identified for both homogeneous and heterogeneous ϕ. Our conservative transport results indicate that the relevance of ϕ variability can depend on the metric of interest, the control plane-source distance as well as the level of heterogeneity of the conductivity field. The analysis on reactive transport shows that ϕ variability only slightly affects the mean increased lifetime cancer risk at the control planes but leads to a considerable reduction of the cancer risk uncertainty. We also see that the sensitivity of cancer risk towards ϕ heterogeneity can be influenced by the level of variability of the conductivity field, the source-to-control plane distance, but is not affected by the manner in which the contaminant concentration is computed.