E X P O S U R E A N A L Y S I S M O D E L I N G S Y S T E M User's Guide for EXAMS II Version 2.94 by Lawrence A. Burns, Ph.D. Research Ecologist Environmental Research Laboratory U.S. Environmental Protection Agency Athens, Georgia 30613-7799 ENVIRONMENTAL RESEARCH LABORATORY OFFICE OF RESEARCH AND DEVELOPMENT U.S. ENVIRONMENTAL PROTECTION AGENCY ATHENS, GEORGIA 30613-7799 E X P O S U R E A N A L Y S I S M O D E L I N G S Y S T E M User's Guide for EXAMS II Version 2.94 I N T R O D U C T I O N Overview EXAMS is an interactive computer-based system for specifying and storing the properties of chemicals and ecosystems, modifying them using simple commands, and conducting rapid evaluations and error analyses of the probable aquatic fate of synthetic organic chemicals. EXAMS constructs simulation models by combining the loadings, transport, and transformations of a chemical into a set of differential equations, using the law of conservation of mass as an accounting principle. This is accomplished by computing the total mass of chemical entering and leaving each section of a body of water as the algebraic sum of external loadings, transport processes that distribute chemicals through the system and export them across its external boundaries, and transformation processes that convert chemicals to daughter products. The differential equations are then assembled and solved to give a picture of the behavior of chemicals in an aquatic ecosystem. The program produces output tables and simple graphics describing chemical o exposure: the expected environmental concentrations (EECs) resulting from a particular pattern of chemical loadings, o fate: the distribution of the chemical in the system and the fraction of the loadings consumed by each transport and transformation process, and o persistence: the time required for purification of the system (via export/transformation processes) should the chemical loadings cease. EXAMS includes separate mathematical models of the kinetics of the physical, chemical, and biological processes governing transport and transformations of chemicals. This set of unit process equations is the central core of EXAMS. EXAMS' "second-order" or "system-independent" approach makes it possible to study the fundamental chemistry of materials in the laboratory and then, based on independent studies of the levels of driving forces in aquatic systems, evaluate the potential fate of materials in systems that have not yet been exposed (Baughman and Burns 1980). EXAMS treats ionization, and partitioning of the compound with sediments and biota, as thermodynamic properties or purely local equilibria peculiar to each segment of the environmental model--as opposed to a treatment as system-wide "global" equilibria. In this way, EXAMS allows for the impact of spatial variation in sediment properties, pH, etc. on chemical reactivity. EXAMS computes the behavior of trivalent organic acids, bases, and ampholytes; each ionic species can have its own distinctive pattern of sorption and complexation with naturally occurring particulates and dissolved organic matter. Reaction pathways can be entered for the production of transformation products, whose further transport and transformations are then also simulated by EXAMS. EXAMS computes the kinetics of transformations by direct photolysis, hydrolysis, biolysis, and oxidation reactions. The input chemical data for hydrolytic, biolytic, and oxidative reactions can be entered either as single valued, second-order rate constants, or as pairs of values defining the rate constant as an Arrhenius function of the temperature in each segment of the water body. EXAMS has been designed to accept standard water-quality parameters and system characteristics that are commonly measured by limnologists throughout the world, and chemical datasets conventionally measured or required by United States Environmental Protection Agency regulatory procedures. Functional Capabilities EXAMS is a computer-based system for building models of aquatic ecosystems and running simulation studies on the behavior of chemical contaminants. EXAMS' environmental models are maintained in a file composed of concise ("canonical") descriptions of aquatic systems, in which a body of water is described as a set of N segments or distinct zones in the system. By applying the principle of conservation of mass to the transport and transformation process equations, EXAMS compiles a differential equation for the net rate of change of chemical concentration in each segment. The resulting system of N differential equations describes the mass balance for the entire system. EXAMS includes a descriptor language that simplifies the specification of system geometry and connectedness. The code is written in a general (N-segment) form. The software is available in 32-segment (MS-DOS) and 50-segment (VAX) versions. The second-order process models used to compute the kinetics of chemicals are the central core of EXAMS. Each includes a direct statement of the interactions between the chemistry of a compound and the environmental forces that shape its behavior in aquatic systems. Most of the process equations are based on standard theoretical constructs or accepted empirical relationships. For example, the light intensity in the water column of the system is computed using the Beer-Lambert law, and temperature corrections for rate constants are computed using Arrhenius functions. Ionization of organic acids and bases, complexing with dissolved organic carbon (DOC), and sorption of the compound with sediments and biota, are treated as thermodynamic properties or (local) equilibria that modify the speed of the kinetic processes. For example, an organic base in the water column may occur in a number of molecular species (as dissolved ions, sorbed with sediments, etc.), but only the uncharged, dissolved species can be volatilized across the air-water interface. EXAMS allows for the simultaneous treatment of up to 28 molecular species of a chemical--the parent uncharged molecule, and singly, doubly, or triply charged cations and anions, each of which can occur in a dissolved, DOC-complexed, sediment-sorbed, or biosorbed form. The program computes the fraction of the total concentration of a compound that is present as each of the 28 molecular structures ("distribution coefficients," ALPHA). These values enter the kinetic equations as multipliers on the rate constants. The program thus completely accounts for differences in reactivity that depend on the molecular form of the chemical, as a function of the spatial distribution of environmental parameters controlling molecular speciation. EXAMS makes no internal assumptions about the relative transformation reactivities of the 28 molecular species. These assumptions are controlled through entry of species-specific rate constants in the chemical input data. EXAMS includes two algorithms for computing the rate of photolytic transformation of a synthetic organic chemical. These algorithms accommodate the two more common kinds of laboratory data and chemical parameters used to describe photolysis reactions. The simpler algorithm requires only an average pseudo-first-order rate constant (KDP) applicable to near-surface waters under cloudless conditions at a specified reference latitude (RFLAT). To control reactivity assumptions, KDP is coupled to nominal (normally unit-valued) reaction quantum yields (QUANT) for each molecular species of the compound. This approach makes possible a first approximation of photochemical reactivity, but neglects the very important effects of changes in the spectral quality of sunlight with increasing depth in a body of water. The more complex photochemical algorithm computes photolysis rates directly from the absorption spectra (molar extinction coefficients) of the compound and its ions, measured values of the reaction quantum yields, and the environmental concentrations of competing light absorbers (chlorophylls, suspended sediments, DOC, and water itself). When using a KDP, please be aware that data from laboratory photoreactors usually are obtained at intensities as much as one thousand times larger than that of normal sunlight. The total rate of hydrolytic transformation of a chemical is computed by EXAMS as the sum of three contributing processes. Each of these processes can be entered via simple rate constants, or as Arrhenius functions of temperature. The rate of specific-acid-catalyzed reactions is computed from the pH of each sector of the ecosystem, and specific-base catalysis is computed from the environmental pOH data. The rate data for neutral hydrolysis of the compound is entered as a set of pseudo-first-order rate coefficients (or Arrhenius functions) for reaction of the 28 (potential) molecular species with the water molecule. EXAMS computes biotransformation of the chemical in the water column, and in the bottom sediments, of the system as entirely separate functions. Both functions are second-order equations that relate the rate of biotransformation to the size of the bacterial population actively degrading the compound. The second-order rate constants (KBACW for the water column, KBACS for benthic sediments) can be entered either as single-valued constants or as functions of temperature. When a non-zero value is entered for the Q10 of a biotransformation (parameters QTBAW and QTBAS, respectively), KBAC is interpreted as the rate constant at 20 degrees Celsius, and the biolysis rate in each sector of the ecosystem is adjusted for the local temperature (TCEL). Oxidation reactions are computed from the chemical input data and the total environmental concentrations of reactive oxidizing species (alkylperoxy and alkoxyl radicals, etc.), corrected for ultra-violet light extinction in the water column. The chemical data can again be entered either as simple second-order rate constants or as Arrhenius functions. Oxidations due to singlet oxygen are computed from chemical reactivity data and singlet oxygen concentrations; singlet oxygen is estimated as a function of the concentration of DOC, oxygen tension, and light intensity. Reduction is included in the program as a simple second-order reaction process driven by the user entries for concentrations of reductants in the system. Internal transport and export of a chemical occur in EXAMS via advective and dispersive movement of dissolved, complexed, sediment-sorbed, and biosorbed materials, and by volatilization losses at the air-water interface. EXAMS provides a simple set of vectors (JFRAD, etc.) for specifying the location and strength of both advective and dispersive transport pathways. Advection of water through the system then is computed from the water balance, using hydrologic data (rainfall, evaporation rates, stream-flows, groundwater seepages, etc.) supplied as part of the definition of each environment. Dispersive interchanges within the system, and across system boundaries, are computed from a conventional geochemical specification of the characteristic length (CHARL), cross-sectional area (XSTUR), and dispersion coefficient (DSP) for each active exchange pathway. EXAMS can compute transport of a chemical via whole-sediment bedloads, suspended sediment wash-loads, ground-water infiltration, transport through the thermocline of a lake, losses in effluent streams, etc. Volatilization losses are computed using a two-resistance model. This computation treats the total resistance to transport across the air-water interface as the sum of resistances in the liquid and vapor phases immediately adjacent to the interface. EXAMS allows for entry of external loadings of chemicals via point sources, non-point sources, dry fallout or aerial drift, atmospheric wash-out, and ground-water seepage entering the system. Any type of chemical load can be entered for any system segment, but the program will not implement a loading that is inconsistent with the system definition. For example, the program will automatically cancel a rainfall load entered for the hypolimnion or benthic sediments of a lake ecosystem. When this type of corrective action is executed, the change is reported to the user via an information message. EXAMS provides three operating "modes" of increasing complexity. In the simplest case (mode 1), EXAMS executes a direct steady-state solution of the dynamic system equations, thus generating a long-term analysis using a single set of environmental conditions (e.g., annual average driving forces). In mode 2, EXAMS makes available initial-value approaches that can be used to set initial conditions and introduce immediate "pulse" chemical loadings. To the extent that changes in hydrographic volumes (e.g., during spates) can be neglected, this mode can be used to evaluate shorter-term transport and transformation events by segmenting the input datasets and simulation intervals according to time-slices under full user control. In mode 3, EXAMS uses a set of 12 monthly values of all environmental parameters, with input loads that can change monthly and can also include pulse events on individual dates, to compute the dynamics of chemical contamination over the course of one or more years' time. The outputs produced by the system are analogous for all modes of operation, although they differ in detail. For example, in mode 1, a summary table and sensitivity analyses of system fluxes are reported for steady-state conditions; in mode two the reports are generated for conditions at the close of each time slice, and in mode 3, the program reports annual (or interannual) average values and the size and location of exposure extrema. Basic Assumptions EXAMS has been designed to evaluate the consequences of longer-term, primarily time-averaged chemical loadings that ultimately result in trace-level contamination of aquatic systems. EXAMS generates a steady-state, average flow field (long-term or monthly) for the ecosystem. The program thus cannot fully evaluate the transient, concentrated EECs that arise, for example, from chemical spills. This limitation derives from two factors. First, a steady flow field is not always appropriate for evaluating the spread and decay of a major pulse (spill) input. Second, an assumption of trace-level EECs, which can be violated by spills, has been used to design the process equations used in EXAMS. The following assumptions were used to build the program. o A useful evaluation can be executed independently of the chemical's actual effects on the system. In other words, the chemical is assumed not to itself radically change the environmental variables that drive its transformations. Thus, for example, an organic acid or base is assumed not to change the pH of the system; the compound is assumed not to itself absorb a significant fraction of the light entering the system; bacterial populations do not significantly increase (or decline) in response to the presence of the chemical. o EXAMS uses linear sorption isotherms, and second-order (rather than Michaelis-Menten-Monod) expressions for biotransformation kinetics. This approach is known to be valid for low concentrations of pollutants; its validity at high concentrations is less certain. EXAMS controls its computational range to ensure that the assumption of trace-level concentrations is not grossly violated. This control is keyed to aqueous-phase (dissolved) residual concentrations of the compound: EXAMS aborts any analysis generating EECs that exceed (the lesser of) 50% of the compound's aqueous solubility or 10 micromolar (10-5M) concentrations of a dissolved unionized molecular species. This restraint incidentally allows the program to ignore precipitation of the compound from solution and precludes inputs of solid particles of the chemical. o Sorption is treated as a thermodynamic or constitutive property of each segment of the system, that is, sorption/desorption kinetics are assumed to be rapid compared to other processes. The adequacy of this assumption is partially controlled by properties of the chemical and system being evaluated. Extensively sorbed chemicals tend to be sorbed and desorbed more slowly than weakly sorbed compounds; desorption half-lives may approach 40 days for the most extensively bound compounds. Experience with the program has indicated, however, that strongly sorbed chemicals tend to be captured by benthic sediments, where their release to the water column is controlled by their availability to benthic exchange processes. This phenomenon overwhelms any accentuation of the speed of processes in the water column that may be caused by the assumption of local equilibrium. Input and Output Input parameters include: 1) A set of chemical loadings on each sector of the ecosystem. 2) Molecular weight, solubility, and ionization constants of the compound. 3) Sediment-sorption and biosorption parameters: Kp, Koc or Kow, biomasses, benthic water contents and bulk densities, suspended sediment concentrations, sediment organic carbon, and ion exchange capacities. 4) Volatilization parameters: Henry's Law constant or vapor pressure data, wind speeds, and reaeration rates. 5) Photolysis parameters: reaction quantum yields, absorption spectra, stratospheric ozone, cloudiness, relative humidity, atmospheric dust content and air-mass type, scattering parameters, suspended sediments, chlorophyll, and dissolved organic carbon. 6) Hydrolysis: second-order rate constants or Arrhenius functions for the relevant molecular species, pH, pOH, and temperatures. 7) Oxidation: rate constants, temperatures, surface oxidant concentrations, dissolved organic carbon, and oxygen tension. 8) Biotransformation: rate constants, temperatures, bacterial population densities. 9) Parameters defining strength and direction of advective and dispersive transport pathways. 10) System geometry and hydrology: volumes, areas, depths, rainfall, evaporation rates, entering stream and non-point-source flows and sediment loads, and ground-water flows. Although EXAMS allows for the entry of extensive environmental data, the program can be run with a much reduced data set when the chemistry of a compound of interest precludes some of the transformation processes. For example, pH and pOH data can be omitted in the case of neutral organics that are not subject to acid or alkaline hydrolysis. EXAMS produces 20 output tables; these include an echo of the input data, and integrated analyses of the exposure, fate, and persistence of the chemical or chemicals under study. The program prints a summary report of the results obtained. Printer-plots of longitudinal and vertical concentration profiles, as well as time-based graphics, can be invoked by the user. System Resource Requirements EXAMS has been implemented in FORTRAN 77 and can be run on computers with Fortran compilers that adhere to the full standard. The MS-DOS version of EXAMS was compiled under Ryan-McFarland v. 2.43, and linked with PLINK86plus version 2.24. The program requires available DOS memory of 435 Kbytes to load plus 32Kb for program operations; its size thus precludes co-residence with many of the popular PC "TSR" (terminate and stay resident) programs. EXAMS is overlaid to run in the DOS environment but, after reserving its 32 Kb for program operations, will establish memory caches in available extended memory space to minimize disk I/O overhead. Applications EXAMS can be used to assess the fate, exposure, and persistence of synthetic organic chemicals in aquatic ecosystems in which the chemical loadings can be time-averaged or event loaded, and chemical residuals are at trace levels. The program has been used, for example, by EPA to evaluate the behavior of relatively field-persistent herbicides and to evaluate dioxin contamination downstream from paper mills. EXAMS has been successfully used to model volatilization of organics in specific field situations and for a general assessment of the behavior of phthalate esters in aquatic systems. EXAMS has been implemented by a number of manufacturing firms for environmental evaluations of newly synthesized materials and has been used in an academic setting for both teaching and research. The Bibliography section of this document lists application and validation studies that can be consulted for additional detail. Technical questions: contact the author, Lawrence A. Burns, Ph.D. Research Ecologist US EPA/Athens-ERL College Station Road Athens, Georgia 30613-7799 USA Telephone: (404) 546-3511, (FTS) 250-3511 Documentation and Software Availability The computer program for version 2.94 of the Exposure Analysis Modeling System II (EXAMS-II, v. 2.94) is available gratis from the U.S. Environmental Protection Agency. A user manual is provided with the program; technical documentation for the program is available from the National Technical Information Service (NTIS) in the publication "Exposure Analysis Modeling System (EXAMS): User Manual and System Documentation" (EPA-600/3-82-023, NTIS PB 82 258096 (US $34.00)) The given price is for purchasers on the North American continent, who can obtain the document from the U.S. Department of Commerce National Technical Information Service Springfield, Virginia 22161 USA NTIS also maintains overseas depositories for the convenience of non-USA organizations wishing to acquire their publications. The EXAMS computer program can be obtained from the author at the address given above. The program is supplied on microcomputer diskette containing an MS-DOS executable image for use on an IBM PC or "compatible." To use the PC/MS-DOS run-time version, you will need a microcomputer (IBM-PC/AT or "Compatible") with at least 512 kilobytes of RAM (Random Access Memory), a 1.2 megabyte or 360 kilobyte diskette drive, a mass-storage device (5+ megabyte hard disk). Although not required, a math co-processor (80x87) is strongly recommended. The EXAMS executable image runs under MS-DOS 2.12+ on the Intel 8086 chip family; note that you do NOT need a Fortran compiler. Along with a request letter, please send one high-density (2S/HD, 5.25 inch) or two 360 K (2S/DD, 5.25 inch) diskettes. In addition, the software is available through the Center for Exposure Assessment Modeling (CEAM) bulletin board system (BBS). The CEAM BBS can be accessed at no charge by calling (404)-546-3402 (8N1). Bibliography Baughman, G.L., and L.A. Burns. 1980. Transport and transformation of chemicals: a perspective. pp. 1-17 In: O. Hutzinger (Ed.). The Handbook of Environmental Chemistry, vol.2, part A. Springer-Verlag, Berlin, Federal Republic of Germany. Burns, L.A. 1989. Method 209--Exposure Analysis Modeling System (EXAMS-- Version 2.92). pp. 108-115 In: OECD Environment Monographs No. 27: Compendium of Environmental Exposure Assessment Methods for Chemicals. Environment Directorate, Organisation for Economic Co-Operation and Development, Paris, France. Burns, L.A. 1986. Validation methods for chemical exposure and hazard assessment models. pp. 148-172 In: Gesellschaft fuer Strahlen- und Umwelt forschung mbH Muenchen, Projektgruppe "Umwelt gefahrdungspotentiale von Chemikalien" (Eds.) Environmental Modelling for Priority Setting among Existing Chemicals. Ecomed, Muenchen-Landsberg/Lech, Federal Republic of Germany. Burns, L.A. 1985. Models for predicting the fate of synthetic chemicals in aquatic systems. pp. 176-190 In: T.P. Boyle (Ed.) Validation and Predictability of Laboratory Methods for Assessing the Fate and Effects of Contaminants in Aquatic Ecosystems. ASTM STP 865, American Society for Testing and Materials, Philadelphia, Pennsylvania. Burns, L.A. 1983a. Fate of chemicals in aquatic systems: process models and computer codes. pp. 25-40 In: R.L. Swann and A. Eschenroeder (Eds.) Fate of Chemicals in the Environment: Compartmental and Multimedia Models for Predictions. Symposium Series 225, American Chemical Society, Washington, D.C. Burns, L.A. 1983b. Validation of exposure models: the role of conceptual verification, sensitivity analysis, and alternative hypotheses. pp. 255-281 In: W.E. Bishop, R.D. Cardwell, and B.B. Heidolph (Eds.) Aquatic Toxicology and Hazard Assessment. ASTM STP 802, American Society for Testing and Materials, Philadelphia,, Pennsylvania. Burns, L.A. 1982. Identification and evaluation of fundamental transport and transformation process models. pp. 101-126 In: K.L. Dickson, A.W. Maki, and J. Cairns, Jr. (Eds.). Modeling the Fate of Chemicals in the Aquatic Environment. Ann Arbor Science Publ., Ann Arbor, Michigan. Burns, L.A., and G.L. Baughman. 1985. Fate modeling. pp. 558-584 In: G.M. Rand and S.R. Petrocelli (Eds.) Fundamentals of Aquatic Toxicology: Methods and Applications. Hemisphere Publ. Co., New York, New York. Burns, L.A., and D.M. Cline. 1985. Exposure Analysis Modeling System: Reference Manual for EXAMS II. EPA/600/3-85/038, U.S. Environmental Protection Agency, Athens, Georgia. 83 pp. Burns, L.A., D.M. Cline, and R.R. Lassiter. 1982. Exposure Analysis Modeling System (EXAMS): User Manual and System Documentation. EPA-600/3-82-023, U.S. Environmental Protection Agency, Athens, Georgia. 443 pp. Games, L.M. 1982. Field validation of Exposure Analysis Modeling System (EXAMS) in a flowing stream. pp. 325-346 In: K.L. Dickson, A.W. Maki, and J. Cairns, Jr. (Eds.) Modeling the Fate of Chemicals in the Aquatic Environment. Ann Arbor Science Publ., Ann Arbor, Michigan. Games, L.M. 1983. Practical applications and comparisons of environmental exposure assessment models. pp. 282-299 In: W.E. Bishop, R.D. Cardwell, and B.B. Heidolph (Eds.) Aquatic Toxicology and Hazard Assessment, ASTM STP 802. American Society for Testing and Materials, Philadelphia, Pennsylvania. Lassiter, R.R., R.S. Parrish, and L.A. Burns. 1986. Decomposition by planktonic and attached microorganisms improves chemical fate models. Environmental Toxicology and Chemistry 5:29-39. Paris, D.F., W.C. Steen, and L.A. Burns. 1982. Microbial transformation kinetics of organic compounds. pp. 73-81 In: O. Hutzinger (Ed.). The Handbook of Environmental Chemistry, v.2, pt.B. Springer-Verlag, Berlin, Federal Republic of Germany. Plane, J.M.C., R.G. Zika, R.G. Zepp, and L.A. Burns. 1987. Photochemical modeling applied to natural waters. pp. 250-267 In: R.G. Zika and W.J. Cooper (Eds.) Photochemistry of Environmental Aquatic Systems. ACS Symposium Series 327, American Chemical Society, Washington, D.C. Pollard, J.E., and S.C. Hern. 1985. A field test of the EXAMS model in the Monongahela River. Environmental Toxicology and Chemistry 4:362-369. Sanders, P.F., and J.N. Seiber. 1984. Organophosphorus pesticide volatilization: Model soil pits and evaporation ponds. pp. 279-295 In: R.F. Kreuger and J.N. Seiber (Eds.) Treatment and Disposal of Pesticide Wastes. ACS Symposium Series 259, American Chemical Society, Washington, D.C. Schramm, K.-W., M. Hirsch, R. Twele, and O. Hutzinger. 1988. Measured and modeled fate of Disperse Yellow 42 in an outdoor pond. Chemosphere 17:587- 595. Slimak, M.W., and C. Delos. 1982. Predictive fate models: their role in the U.S. Environmental Protection Agency's water program. pp. 59-71 In: K.L. Dickson, A.W. Maki, and J. Cairns, Jr. (Eds.) Modeling the Fate of Chemicals in the Aquatic Environment. Ann Arbor Science Publ., Ann Arbor, Michigan. Staples, C.A., K.L. Dickson, F.Y. Saleh, and J.H. Rodgers, Jr. 1983. A microcosm study of Lindane and Naphthalene for model validation. pp. 26-41 In: W.E. Bishop, R.D. Cardwell, and B.B. Heidolph (Eds.) Aquatic Toxicology and Hazard Assessment: sixth Symposium, ASTM STP 802, American Society for Testing and Materials, Philadelphia, Pennsylvania. Wolfe, N.L., L.A. Burns, and W.C. Steen. 1980. Use of linear free energy relationships and an evaluative model to assess the fate and transport of phthalate esters in the aquatic environment. Chemosphere 9:393-402. Wolfe, N.L., R.G. Zepp, P. Schlotzhauer, and M. Sink. 1982. Transformation pathways of hexachlorocylcopentadiene in the aquatic environment. Chemosphere 11:91-101. EXAMS COMMAND LANGUAGE USER'S GUIDE This section describes the EXAMS command language, including usage and reference information. The first part provides an overview of the command language and its grammar. The second part contains detailed descriptions of each command. The commands are listed in alphabetical order. Conventions used in this Section: Convention Meaning CTRL/x The phrase CTRL/x indicates that you must press the key labeled CTRL while simultaneously pressing another key, for example, CTRL/Q. EXAMS-> LIST 7 Vertical series of periods, or ellipsis, mean that . not all the data EXAMS would display in response to . the particular command is shown, or that not all . the data a user would enter is shown. keyword,... Horizontal ellipsis indicates that additional key- words, command parameters, or data can be entered in a command sequence, or that EXAMS displays additional data as part of the sample output line. [keyword] Square brackets indicate that the item enclosed is optional, that is, the entity can be omitted from the command line altogether.