Carl Berkowitz, Jerome Fast
Department of Atmospheric Sciences
Pacific Northwest National Laboratory
P.O Box 999, MSIN K9-30
Richland, WA 99352
PNNL has developed a modeling system to address issues relevant to DOE’s Atmospheric Chemistry Program. This system is based on two components: a mesoscale meteorological model and a chemical transport model (CTM).
The mesoscale model is the Regional Atmospheric Modeling System (RAMS) developed at Colorado State University. RAMS is a primitive, nonhydrostatic model based on a terrain following coordinate system. Subgrid-scale turbulent diffusion is parameterized using a 2.5-level turbulence closure with prognostic turbulent kinetic energy equation. Shortwave and longwave parameterizations are used to determine the heating or cooling due to radiative fluxes. Prognostic soil-vegetation relationships are used to calculate the diurnal variation of temperature and moisture at the ground-atmosphere interface. Turbulence sensible heat, latent heat, and momentum fluxes in the surface layer are based on similarity equations. RAMS also contains cumulus and explicit microphysics parameterizations. The model can be configured to cover an area as large as a hemisphere to simulate synoptic-scale atmospheric systems. Two-way interactive grid nesting allows fine mesh grids to resolve mesoscale atmospheric systems using a grid spacing as small as 1 km, while simultaneously simulating the large-scale environment on a coarser grid. A nudging four-dimensional data assimilation technique has been incorporated into the model so that mesoscale analyses, which combine predicted and observed variables, can be produced if needed.
The meteorological quantities produced by the mesoscale model are used as input for the CTM to simulate the chemical production and destruction, transport, and deposition of various trace gas species. The CTM coordinate system is the same as the RAMS model so that no interpolation of the meteorological quantities is necessary. The model presently employs the Lurmann, Lloyd, and Atkinson chemical mechanism, with several modifications, updated kinetic expressions and reaction rate coefficients, and the addition of isoprene chemistry. The mechanism includes 126 gas phase reactions and 65 separate chemical species. The model simulates gas phase ozone, various peroxides, NOx and NOy compounds, hydroxyl radicals, and hydrocarbons, and is designed for regional-scale analyses. Clear air photolysis rate constants are modified in the presence of clouds. The emissions data depend upon the specific application. The CTM has recently used emissions based on the U.S. Environmental Protection Agency’s 1990 Interim Emissions Inventory, the 1985 Global Atmospheric Inventory Activity (GEIA), and smaller-scale databases developed for specific field campaigns (i.e., the Southern Oxidants Study). Spatially and temporally varying lateral boundary conditions for the trace gas species can be specified. The model can be run in a one-way nested configuration, similar to RAMS. Individual terms of the governing equations can be saved to determine the budget for each chemical specie. The code is begin used on both serial and parallel machines.
More details on the modeling system, related publications, and recent applications can be found at http://www.pnl.gov/atmos_sciences/as_acp.html.
Atmospheric Chemistry Box Model
Department of Applied Science/Environmental Chemistry Division
Brookhaven National Laboratory
75 Rutherford Drive
Upton, NY 11973-5000
Our calculations are done with a zero dimension box model. The chemical mechanism is based on RADM2 with the addition of a reaction scheme from Paulson and Seinfeld for isoprene oxidation. Most of our calculations are done in the constrained steady state mode. In this case we constrain the concentrations of stable species (i.e., O3, CO, VOCs, NO, HCHO, peroxides) to the values that are observed in our field experiments. A model run then consists of integrating the chemical equations to steady state yielding the concentrations of free radicals that are in equilibrium with the observed compounds. This process also gives us rate information, such as P(O3) and the sensitivity of P(O3) to changes in NOx and VOCs. Model calculations are also done in the time dependent mode where observation are used for initialization. The focus of this work has been on developing observation based relations that provide an easily visualized translation between atmospheric concentrations (which can be observed) and rates and sensitivities (which cannot be directly observable).
Sulfur Transport Eulerian Model (STEM)
Center for Global & Regional Environmental Research 202 IATL
University of Iowa
Iowa City, IA 52240
The STEM comprehensive model has been developed to provide a theoretical basis to investigate the relationships between the emissions, atmospheric transport, chemical transformation, removal processes, and the resultant distribution of air pollutants and deposition patterns on meso and regional scales. The STEM model has then been used to address a wide series of policy issues in U.S., Asia and Europe, related to acidification, cloud chemistry, tropospheric ozone, and aerosols formation. Starting from emissions (area and point sources), meteorology (wind, temperature, humidity, precipitation, etc.) and a set of chemical initial and boundary conditions it simulates the pollutants behavior in the selected domain. The results (concentrations fields and deposition fluxes, pollutant balances) can be used to analyze in detail pollutant formation and exchange mechanisms, to detect concentration levels and trends and to study effects of alternative emissions scenarios.
Several pre- and post-processing tools have been developed for preparation, analysis, and visualization of i/o data, as well to interface the code with meteorological and emissions models. Beside operational applications, model development is continuously going on.
Complete details are found at www.cgrer.uiowa.edu/people/carmichael/stem2_desc.html.
Dry Deposition and Natural Emissions Module
Yiwen Xu and Marvin L. Wesely
Environmental Research Division
Argonne National Laboratory
9700 South Cass Avenue
Argonne, IL 60439
Argonne National Laboratory’s dry deposition model extends the previous version of the dry deposition model by incorporating remote sensing data from satellites to infer surface properties such as leaf area index and absorbed photosynthetically active radiation in vegetative canopies. In addition, biogenic emissions from vegetation and soils are estimated with BEIS 2.2 coupled with the satellite data. In its current version, the model uses Pathfinder estimates of the normalized vegetative difference index (NDVI) with a spatial resolution of 8 km by 8 km. Land use data are also necessary and are obtained from a standard data product derived in part from advanced very high resolution radiometer data. Inputs to the model consist of near-surface quantities such as friction velocity, atmospheric stability, temperature, and humidity, which can be supplied by a mesoscale meteorological model. Outputs include deposition velocities for ozone and several other substances and biogenic emission rates of isoprene, monoterpene, and nitric oxide. At the present time, methods of coupling the dry deposition and biogenic emissions model with a mesoscale meteorological model is being tested with MM5.
IMPACT, A Global Three-Dimensional Atmospheric Model
Atmospheric Science Division
Lawrence Livermore National Laboratory
Livermore, CA 94550
LLNL has developed a global, three-dimensional model, IMPACT, that contains both a chemically prognostic troposphere and stratosphere. IMPACT is driven by meteorology supplied by either a general circulation model or actual assimilated meteorological fields. Using assimilated meteorology allows us to directly compare model results with field observations. IMPACT contains the tropospheric photochemical reactions necessary to predict the concentration of a full suite of species, including O3, OH, PAN, NO, NO2, CO, CH4, HNO3, isoprene, ethane, propane, higher alkanes, ethene, propene, higher alkenes, and others. IMPACT also contains the chemistry of stratospheric species such as Ox, NOy, ClOy, HOy, BrOy, and CH4, and their oxidation products. The chemical concentrations are obtained using the SMVGEARII solution technique of Jacobson.
Physical processes included in IMPACT are advection, vertical diffusion, dry deposition, and wet deposition. The model resolution is determined by the meteorological fields used. Using the currently available NASA DAO GEOS-STRAT assimilated meteorology allows us a horizontal resolution of 2 degrees by 2.5 degrees, with 46 vertical levels extending to 0.1 mbar. We have recently compared our model with observations from NASA SONEX mission (fall 1997) to understand the role of tropospheric in-situ production and stratospheric transport on the production of ozone and other tropospheric species over the North Atlantic Ocean. We will compare our model results with other field measurements (including ACP) to assess our model’s capabilities and quantify the larger synoptic scale conditions under which the measurements were made. We also plan to compare our model predictions with those obtained by ACP models of different scales. In particular, we will interact closely with regional and urban scale modeling groups in the ACP (by providing inflow boundary conditions) to assess the role of long range tracer transport in observed quantities.
Global Chemistry Model Driven by Observation-Derived Meteorological Data (GChM-O)
Carmen Benkovitz, Bob McGraw, Stephen Schwartz
Brookhaven National Laboratory
Department of Applied Science/Environmental Chemistry Division
Brookhaven National Laboratory
75 Rutherford Drive
Upton, NY 11973-5000
The Brookhaven GChM-O is a hemispheric scale chemical transport and transformation model that has been previously described [Benkovitz and Schwartz, 1994; 1997]. This summary describes the latest version (Version 3) of GChM-O, which includes refined representations of the sulfur chemistry and representation of aerosol microphysical processes using the quadrature method of moments (QMOM) [McGraw, 1997].
Enhancements to Version 1 are as follows:
SUNYA-CCM3 and SUNYA-ReCM
Atmospheric Sciences Research Center
State University of New York-Albany
251 Fuller Road
Albany, NY 12203
Two types of atmospheric models, SUNYA-CCM3 and SUNYA-ReCM, are used in the ACP program.
The SUNYA-CCM3 is NCAR-CCM3 version with the incorporation of SUNYA 3-D ozone climatology [Wang et al.,1995]. AMIP-II experiments are being conducted with the objective of examining the effect of interannual ozone variation on the interannual climate variability. We are also in the process of incorporating atmospheric chemistry to study a variety issues related to climate-chemistry interaction.
The SUNYA-ReCM is based on MM5 with improved cloud-radiation treatment, and land surface processes. Extensive numerical experiments have been conducted to study the capability of simulating the meteorological fields [Wang et al.,1999; Gong and Wang, 1999]. The ReCM is currently being used to conduct offline study of the effect of changes in ozone and water vapor due to aircraft emissions. Work is also planned to incorporate atmospheric chemistry into the regional model.
Wang W.-C., X. Z. Liang, M. P. Dudek, D. Pollard, and S. L. Thompson, Atmospheric ozone as a climate gas, Atmos. Res., 37, 247-256, 1995.
Wang, W.-C., W. Gong, and H. Wei, A regional model simulation of the 1991 sever precipitation event over the Yangtze-Huai river valley. Part I: Precipitation and circulation statistics, J. Climate, 1999, in press.
Gong, W., and W.-C. Wang, A regional model simulation of the 1991 sever precipitation event over the Yangtze-Huai river valley. Part II: Model bias, J. Climate, 1999, in press.
Model for Ozone and Related Chemical Tracers and Particles (MOZART-PT)
Department of Atmospheric Chemistry
National Center for Atmospheric Research
P.O. Box 3000
Boulder, CO 80307-3000
The integrated model is called MOZART-PT (Model for OZone And Related chemical Tracers and ParTicles). The model is fully interactive between tropospheric chemical species and formation of aerosols, which provides a useful tool to study the effect of aerosols on tropospheric chemistry, including the heterogeneous reactions occurring on the surface of aerosols. The model is global model and extends from the surface to the approximate altitude of 35 km. The spatial resolution is 2.8 degrees in latitude and 2.8 degrees in longitude, with 25 levels in the vertical The timestep used to solve the continuity equations associated with each chemical compound is 20 minutes.
The transport of tracers is performed by using the three-dimensional semi-Lagrangian formulation with shape-preserving interpolation of Rasch and Williamson . The dynamical fields used in the model can be either from GCM calculated results or from assimilated results (NCEP). At the present, the meteorological fields used to drive the model are taken from a simulation using the NCAR CCM3. The model also includes a representation of boundary layer exchanges [Holstag and Boville, 1993] and of convective transport [Hack, 1994], based on the dynamical output of CCM3.
The model has a detailed representation of tropospheric chemistry, including 56 chemical species (CO, N2O, nitrogen, HOx, Ox, hydrocarbons, etc.) with more than 130 photochemical reactions. The details of the chemistry scheme of the model are described by Brasseur et al. . The distribution of water vapor in the troposphere is taken from the CCM3 simulation.
The chemical transport equations are solved by a splitting operator technique; at each time step, the mixing ratios of the chemical species are successively updated in the following order: (1) advection by the semi-Lagrangian technique, (2) chemical transformations, (3) mixing accounting for vertical subgrid transport, and (4) redistribution by convection.
The mass density of sulfate, black carbon, and ammonium nitrate particles are calculated in the model. The processes of the formation of sulfate aerosol represented in the model include sulfuric chemical reactions (gas and aqueous phase), surface emissions, wet and dry depositions, and transport. The heterogeneous reactions occurring on sulfate particles (such as N2O5 + (sul), HO2 + (sul), and NH3 + (sul)) are taking into account in the model. The processes of the formation of black carbon aerosol represented in the model include transformation between hydrophobic into hydrophilic black carbon aerosols, surface emissions, wet and dry depositions, and transport. The heterogeneous reactions occurring on black carbon particles (such as HNO3 + (BCl), and O3 + (BC)) are taking into account in the model. Finally, the ammonium nitrate aerosol concentrations are calculated according to the method suggested by Seinfeld . The formation of ammonium nitrate has an important effect on HNO3 concentrations.
The model is under development in several aspects: (1) Other type of aerosols, s uch as mineral dust and seas-salt, will be calculated and included in the model. (2) The heterogeneous reactions on dust and seas-salt aerosol particles will be studied, (3) The observation-derived meteorological data (NCEP) will be used to drive the model (MOZART). (4) The horizontal resolution will be enhanced to 1.4 degree x 1.4 degree (T128). With the improvements, we intend to use the model t o participate in field campaign programs, such as NARSTO etc.
Brasseur, G. P., D. A. Hauglustaine, S. Walters, P. Rasch, J. Muller, C. Granier, and X. Tie, MOZART, a global chemical transport model for ozone and related chemical tracers: 1. Model description, J. Geophys. Res., 103, 28265-28289, 1998.
Hack, J. J., B. A. Boville, B. P. Briegleb, J. T. Kiehl, P. J. Rasch, and D. L. Williamson, Description of the NCAR community climate model (CCM2), Tech Note, NCAR/TN- 382+STR, 108 pp, Natl. Cent. Atmos. Res., Boulder, CO, 1993.
Holtslag, A. A. M., and B. A. Boville, Local versus nonlocal boundary-layer diffusion in a global climate model, J. Clim., 6, 1825-1842, 1993.
Rasch, P.J., and D.L. Williamson, Sensitivity of a general circulation model climate to the moisture transport formulation, J. Geophys. Res., 96, 13123-13137, 1991.
Seinfeld, J. H., Atmospheric Chemistry and Physics of Air Pollution, N.Y., John Wiley & Sons, Inc., 1986.
Joyce E. Penner
Dept. of Atmospheric Oceanic and Space Sciences
The University of Michigan
2455 Hayward Street
Ann Arbor, MI 48109-2143
GRANTOUR is a global three-dimensional chemistry and transport Lagrangian parcel model in which the atmosphere is treated as a set of constant mass air parcels advected by the winds on a fixed Eulerian grid generated either by a climate model or a data assimilation model. The model uses a split-operator technique to compute the effects of sources, chemistry, interparcel mixing, large-scale diffusion, vertical convective mixing, precipitation scavenging, dry deposition, and advection upon parcel mixing ratios for each prognostic species. Specific descriptions of the representation of these processes in GRANTOUR are given in Walton et al.  and Penner et al. [1991a]. A large number of studies have used the Livermore/NCAR CCM1 general circulation model to provide global flow fields and meteorological data. For example, we have simulated the global transport and deposition of 222Rn and 210Pb [Jacob et al., 1997], the complex photochemistry of O3 and OH [Atherton et al., 1996; Penner et al., 1994], soot aerosols from biomass burning [Penner et al., 1991b], black and organic carbon aerosols [Penner et al., 1993; Liousse et al., 1996], the global nitrogen budget [Penner et al., 1991a] and the global sulfur cycle [Erickson et al., 1991; Penner et al., 1994], the effect of anthropogenic sulfate aerosols on climate [Taylor and Penner, 1994], and climate forcing by carbonaceous and sulfate aerosols [Penner et al., 1998].
The GRANTOUR/CCM1 version of the model has been simplified in order to allow us to study the large scale distribution of OH and to better quantify the interactions of OH, CO, CH4 and volatile organic compounds. Uncertainties in the source distributions of these compounds continue to hamper efforts to predict future concentrations of CH4 as well as O3, both of which act on as greenhouse gases on a global scale. To develop our simplified method for CO and CH4, ambient distributions of 17 species were calculated for a prescribed methane and CO distribution using the 3-D chemistry-transport model GRANTOUR [Penner et al., 1994]. The chemistry represented in Penner et al.  includes 47 reactions representing the chemistry of O3, OH, HO2, NO, NO2, HNO3, CO, CH4, as well as other minor species in the O3-CH4-CO-HOx-NOx. This simulation was used to diagnose reaction rate coefficients as a function of latitude, longitude, height and time, for the following simplified system:
d[CH4]/dt = SCH4 - k1 [OH] [CH4]
d[CO]/dt = SCO + k1 [OH] [CH4] - k2 [OH] [CO]
d[OH]/dt = SOH - k1 [OH] [CH4] - k2 [OH] [CO] - k3 [OH] [X],
where k3[X] accounts for the sum of
all the reactions for OH sinks that are independent of the CH4-CO
system, SOx accounts for all reactions which are sources for
OH, and SCH4 and SCO
are the source terms for CH4 and CO, respectively.
This 3-species model was solved using GRANTOUR with the CO source terms
described in Table 1. In addition, CH4
sources from Fung et al.  were prescribed.
Table 1. Prescribed sources of CO used in the base model (Tg
|Source type||NH||SH||Global Average|
We are now in the process of updating this model to include reactions of volatile organic species and their impact on OH as well as feedbacks from changes in HOx to the NOx system. The product will be a fast, efficient model for studying the long term consequences of changes in emissions of CO, CH4 and other volatile organic compounds.
We have also adapted the model to use the ECHAM model [Deutsches
Klimarechenzentrum, 1993] as the meteorological driver for GRANTOUR.
This change has allowed us to improve the representation of several of
the processes in the model (paper in preparation). ECHAM includes three-dimensional
vertical mass fluxes, cloud water mixing ratios, and precipitation production
and evaporation rates for convective clouds; for large-scale clouds it
provides three-dimensional cloud fraction, prognostic cloud water mixing
ratio, and precipitation rate. The specific representation of liquid water
mixing ratio facilitates an improved representation of aqueous phase gas-to-particle
conversion of sulfur dioxide to sulfate. The large-scale cloud fractions
and precipitation rates permit an improved parameterization of precipitation
scavenging by stratiform clouds, and the convective mass fluxes and precipitation
rates allow for an improved representation of mixing and scavenging by
Table 2. ECHAM Variables Passed to GRANTOUR
|U||Zonal Velocity||km hr-1|
|V||Meridional Velocity||km hr-1|
|qv||Water Vapor Mixing Ratio|
|ql||Liquid Water Mixing Ratio Large- Scale Cloud|
|PLS||Large-Scale Precipitation Rate||cm hr-1|
|PCV||Convective Precipitation Produc tion Rate||cm hr-1|
|Mu||Convective Mass Flux Up||kg m-2 s-1|
|Md||Convective Mass Flux Down||kg m-2 s-1|
|qlc||Liquid Water Mixing Ratio Con vective Cloud|
|Fc||Large-Scale Cloud Fraction|
|Kz||Vertical Diffusion Coefficient||m2 s-1|
We currently use a data set of one year of meteorological data from ECHAM at T21 resolution (5.625 deg in latitude and longitude). Four hour averages of the 13 variables in Table 2 constitute the meteorological data set; all, except the surface albedo which was saved for use in climate forcing calculations, are three-dimensional variables defined on the ECHAM grid. The present version treats the following as prognostic species: H2O2, H2SO4, SO2, DMS, dust, sea salt, and organic and black carbon aerosols.
Atherton, C. A., S. Grotch, D. D. Parrish, J. E. Penner, and J. J. Walton, The role of anthropogenic emissions of NOx on tropospheric ozone over the North Atlantic Ocean: A three dimensional, global model study, Atmos. Environ., 30, 1739-1749, 1996.
Deutsches Klimarechenzentrum, The ECHAM 3 Atmospheric General Circulation Model, Report No. 6, Deutsches Klimarechenzentrum, Hamburg, 189 pp, 1993.
Erickson III, D. J., J. J. Walton, S. J. Ghan, and J. E. Penner, Three-dimensional modeling of the global atmospheric sulfur cycle: A first step, Atmos. Environ., 25A, 2513—2520, 1991.
Fung, I., J. John, J. Lerner, E. Matthews, and others, Three-dimensional model synthesis of the global methane cycle, J. Geophys. Res., 96, 13,033-13,065, 1991.
Jacob, D. J., M. J. Prather, P. J. Rasch, R.-L. Shia, Y. J. Balkanski, S. R. Beagley, D. J. Bergmann, W. T. Blackshear, M. Brown, M. Chiba, M.P. Chipperfield, J. de Grandpré, J. E. Dignon, J. Feichter, C. Genthon, W. L. Grose, P. S. Kasibhatla, I. Köhler, M. A. Kritz, K. Law, J. E. Penner, M. Ramonet, C. E. Reeves, D. A. Rotman, D. Z. Stockwell, P. F. J. Van Velthoven, G. Verver, O. Wild, H. Yang, and P. Zimmermann, Evaluation and intercomparison of global atmospheric transport models using 222Rn and other short-lived tracers, J. Geophys. Res., 102, 5953-5970, 1997.
Liousse, C., J. E. Penner, C. Chuang, J. J. Walton, H. Eddleman, and H. Cachier, A Three-dimensional model study of carbonaceous aerosols, J. Geophys. Res., 101, 19,411-19,432, 1996.
Penner, J. E., C. S. Atherton, J. Dignon, S. J. Ghan, J. J. Walton, and S. Hameed, Tropospheric nitrogen: A three-dimensional study of sources, distribution, and deposition, J. of Geophys. Res., 96, 959—990, 1991a.
Penner, J. E., S. J. Ghan, and J. J. Walton, The role of biomass burning in the budget and cycle of carbonaceous soot aerosols and their climate impact, in Global Biomass Burning, edited by J. Levine, MIT press, Cambridge, MA, 387—393, 1991b.
Penner, J. E., H. Eddleman and T. Novakov, Towards the development of a global inventory of black carbon emissions, Atmos. Environ., 27A, 1277—1295, 1993.
Penner, J. E., C. A. Atherton, and T. E. Graedel, Global emissions and models of photochemically active compounds, in Global Atmospheric-Biospheric Chemistry, edited by R. Prinn, Plenum Publishing, NY, 223-248, 1994.
Penner, J. E., C.Chuang, and K. Grant, Climate forcing by carbonaceous and sulfate aerosols, Climate Dynamics, 14, 839-551, 1998.
Taylor, K. E. and J. E. Penner, Response of the climate system to atmospheric aerosols and greenhouse gases, Nature, 369, 734-737, 1994.
Walton, J. J., M. C. MacCracken, and S. J. Ghan, A global-scale Lagrangian trace species model of transport, transformation, and removal processes, J. Geophys. Res., 93, 8339-8354, 1998.
Tropospheric Ultraviolet Visible (TUV) Model
Atmospheric Chemistry Division
National Center for Atmospheric Research
P. O. Box 3000
Boulder, Colorado 80303
Solar ultraviolet (UV) radiation is the driving force for all tropospheric photochemical processes. Photons at UV wavelengths have the potential to break usually fairly stable molecules into very reactive fragments (photolysis) and thus initiate reaction chains otherwise unlikely or even impossible. UV radiation is also harmful to living organisms and detrimental to human health. High doses of UV radiation are considered the major contributing factor for the development of skin cancer or cataracts. UV radiation can weaken the human immune system and can affect crop yields and phytoplankton activity, among other effects.
The Tropospheric Ultraviolet Visible (TUV) model computes UV and visible radiation levels, and related quantities, for the troposphere and now also in the stratosphere. Specifically, the model calculates the spectral irradiance and actinic flux at any altitude. Diffuse and direct components are available separately. The model also computes wavelength-integrated quantities such as photolysis rate coefficients (J-values) for a number of molecules of interest to atmospheric chemistry, and biologically weighted radiation (e.g., erythemal exposure, DNA damage, UV-Index). The model atmosphere allows for absorption by gases (O2, O3, NO2, and SO2), scattering by air molecules (Rayleigh), and absorption and scattering by cloud and aerosol particles (Mie scattering). Wavelength and altitude grids may be defined by the user. Extensive spectral data bases are included, including several options for extraterrestrial spectral irradiances, molecular cross sections and quantum yields, and biological sensitivity functions. Several different radiation schemes may be used interchangeably, and currently include a generalized two-stream method, and a multi-stream discrete ordinates method, both with fast pseudo-spherical correction for atmospheric curvature.
The code (tar file) is available at http://acd.ucar.edu/models/open/tuv/tuv.html It is written in Fortran-77 and normally operates on a UNIX platform, but users have reported that conversion to other operating systems is straightforward (Cray, PC, Mac). Extensive online documentation is available. A global UV climatology calculated with the TUV model is available at http://acd.ucar.edu/UV. Questions should be e-mailed to email@example.com.