The modeling and theoretical group held a series of breakout sessions during the 1996 annual ACP meeting. We held sessions as a separate focus group, as well as joint sessions with the other ACP groups. A copy of the agenda for our activities is attached to this report.
The participants in the Modeling and Theoretical sessions included: Cynthia Atherton (Lawrence Livermore National Laboratory), Guy Brasseur (NCAR), Greg Carmichael (University of Iowa), Richard Easter (Battelle Pacific Northwest Laboratory), Weigang Gao (Argonne National Laboratory), Allen Grossman (Lawrence Livermore National Laboratory), Jake Hales (Envair), Lawrence Kleinman (Brookhaven national Laboratory), H. Lee (Environmental Measurements Laboratory), Mike McElroy (Harvard), Huiting Mao (State University of New York-Albany), William Marley (Texas A & M), Joyce Penner (University of Michigan), Stephen Schery (New Mexico Institute of Mining and Technology), H. Schneider (Harvard), Jeffrey Shorter (Mission Research Corporation), Gregory Smith (SRI International), XueXi Tie (NCAR), Wei-Chyung Wang (State University of New York-Albany), Marvin Wesely (Argonne National Laboratory), and Yang Zhang (Battelle Pacific Northwest Laboratory).
Strengths and Successes
One of our first activities was to identify and articulate the present strengths of the modeling and theoretical activities within the ACP program and to summarize major accomplishments. One distinguishing strength of the program is that activities cover the spectrum from point analysis to regional to global scales, span areas from highly polluted environments, to global tropospheric and stratospheric chemistry issues, includes chemistry-climate interactions, and covers fundamental processes such as dry deposition, boundary layer dynamics and molecular interactions with surfaces. A unique component of the program is the state-of-the-art sensitivity and uncertainty analysis activities which are being applied to important atmospheric chemistry problems.
These modeling activities are an essential element of ACP activities and provide a framework for interpretation of laboratory and field studies, and a means to extend, extrapolate and evaluate the impacts of energy usage on local, regional and global environments. Further details are presented below in the form of brief summaries.
Point (Observations-Based) Modeling Activities
Kleinman (BNL) is actively developing and applying new observation-based modeling techniques. These concepts combined with the use of indicator species and ratios provide a powerful tool for atmospheric chemistry studies. Through the use of these ideas one can better identify the chemical regime (e.g., NMHC or NOx controlled chemistry). This analysis, by its nature, works closely with the field measurements, and also provides valuable guidance for future field studies.
Global Chemistry-Climate Studies
Activities in this general category span the spectrum from climate-chemistry interactions, to atmospheric chemistry issues focused in the stratosphere and in the troposphere.
Climate-chemistry. Wang (SUNY) is examining the climatic effects of ozone changes through the use of climate and chemical transport models (CMT). Two GCM studies have been conducted using 1980s and 1990s observed ozone trend and changes in surface NOx emissions in selected regions. Development of coupled climate-chemistry GCM is continuing with a focus on the consistency between physical and chemical processes.
Grossman and Long (LLNL) are developing a fast running solar radiative transfer model for sulfate and biomass aerosol radiative forcing calculations. This includes a diagnostic tool to minimize the number of UV-visible bands needed for appropriate ozone/aerosol overlapping across the chapuis band spectral region. They are also involved in the development of models for analysis of direct and indirect radiative forcing effects of methane and methyl bromide.
Stratospheric ozone. McElroy, Schneider and Jones (Harvard) have analyzed transport between the tropics and mid latitudes of the stratosphere, using an interactive 2-D model with a minimum amount of parameters. The model has been validated, using measurements (SPADE, ASHOE/MAESA) of the propagation of the annual/seasonal signal of CO2 into the stratosphere. The characteristics of the CO2 signal are reproduced by the model without new assumptions. They have also investigated the variability of the transport induced by the QBO and have reproduced a QBO in our 2-D model using a parameterization for the Kelvin and Rossby-gravity wave momentum in the tropical stratosphere. The modeled QBO has a reasonable signal in ozone, with a subtropical signal that is comparable to observations in magnitude. They have found that the subtropical ozone anomaly is produced by the meridional advection of ozone out of the tropics by the QBO induced meridional circulation. This circulation favors the winter hemisphere due to the asymmetry in the forcing across the equator, synchronizing the ozone anomaly with the seasonal cycle (as observed). When easterly winds are a maximum in the lower stratosphere there is enhanced poleward transport in the lower stratosphere. In the middle stratosphere there is reduced poleward transport. When the westerly winds are a maximum in the lower stratosphere, the direction of transport reverses.
Brassier and Tie (NCAR) are developing and applying a variety of three-dimensional CTM for use in stratospheric and tropospheric chemistry studies. They have developed a global 3-D chemical/transport model of the stratosphere called STARS and have conducted a detailed modeling study of the role of heterogeneous reactions affecting bromine compounds in the lower stratosphere. They have also modeled the impacts in the Antarctic stratosphere on mid-latitude stratospheric and tropospheric ozone, and conducted a study of the cross-tropopause transport of ozone. Furthermore they have developed a global 3-D chemical/transport model of the troposphere called MOZART and are using this model to look at a variety of tropopsheric chemistry issues related to ozone and the photochemical oxidant cycle.
Tropospheric chemistry. Penner (UM) and Atherton (LLNL) are developing and applying global tropopsheric chemistry models to studies related to impact of energy use on the global environment. They are involved in the development of the first global scale model that includes detailed chemical represented for NMHCs, they are investigating the response of the global tropospheric ozone distribution to a 50% reduction in fossil NOx emissions, and are involved in the evaluation/development of dynamic tropopause for use in ozone budget studies (i.e., trans-tropopause contribution relative to photochemical production in the troposphere). They are also coupling their CTM with a GCM that includes prognostic water for future development of aqueous chemistry studies.
Regional Scale Modeling Efforts
Berkowitz and Fast (PNNL) are using a mesoscale meteorological model with FDDA to study transport during the NARE93 experiment. Particle tracer simulations successfully reproduced arrival of elevated pollutant levels aloft and downwind of the NE US. In addition a regional-scale version of GChM with photochemistry was coupled with RAMS and successfully simulated the observed ozone plumes aloft. This work is helping to characterize how the processes involved in continental outflow of trace species.
Further activities in this category are discussed in the following section.
Sensitivity and Uncertainty Analysis
A variety of new sensitivity analysis techniques are being used in ACP atmospheric chemistry studies. Smith (SRI) has applied O-D sensitivity analysis to a 2-D ozone model and coupled this analysis with an evaluation of rate uncertainties. In this way they were able to map out uncertainties in ozone predictions to determine kinetic uncertainty in HSCT O3 perturbation assessment calculations. These techniques are also being used to examine sensitivity of other observations (and model failures) in stratosphere, such as OH, ClO, etc., and to explore damping of the O-D sensitivity values in the lower stratosphere 2-D model and diurnal averaging effects.
Shorter (MRC) and Rabitz (Princeton) have developed and applied a new technique to evaluate multi-dimensional model sensitivities to all parameters in the model. This method has been used to calculate spatially dependent ozone sensitivities to chemical reactions in a stratospheric ozone model. They are also extending these techniques for use in evaluating emission reduction scenarios in regional air quality modeling applications.
Carmichael and Potra (UI) are studying the interactions between ozone, aerosols, and ultraviolet radiation using a detailed radiation model combined with the STEM-II, regional-scale three-dimensional atmospheric chemistry model. These models are joined with Automatic-Differentiation software to enable desired sensitivities to be calculated on-line with the radiation/chemistry computations. The combined methodology is used to investigate: UV-B radiation at the Earth's surface as a function of changes in ozone and aerosols in the stratosphere and troposphere; and the effects of changes in solar actinic flux on the photochemical oxidant cycle of the troposphere. They are also developing new more efficient chemical integrators for use in atmospheric chemistry studies.
Aerosol Modeling Activities
Many of the studies discussed above also have aerosol components as an integral part of there study. In addition there are additional activities whose primary focus is aerosols.
Easter, Ghan, and Laulainen (PNNL) are involved in the development of a global aerosol model for use in climate-chemistry studies. Their global aerosol model has been extended to include dust, sea salt, and carbonaceous aerosol species. Satellite and surface based observations of aerosol optical depth and surface concentration and deposition data are used to evaluate the model. Aerosol loadings are predicted fairly well but water uptake and optical depths are overestimated.
Marlow (Texas A&M) has performed a direct simulation of aerosol aggregation including compositional and temperature dependencies of "shape" for aggregates of 3nm and 30 nm spheres. He has also studied the interaction energy of water on 1nm, 1.5nm, and 2nm spheres (Monte Carlo calculations) on substrate spheres of tetradezane, and aerosol condensation processes as a function of size and compositional polydispersity. This work provides fundamental insights into aerosol condensation processes.
Air-Surface Exchange Processes
The modeling activities within ACP also include studies focused on processes, such as air-surface exchange, which are integral components of atmospheric chemistry studies.
Lee (EML) is developing a new boundary layer model for impriving the boundarylayer prediction in the GChM (Global Chemistry Model), which has been used in this ACP scientific community. Simple and efficient formulae for calculating surface fluxes are formulated for the nondimensional profile funcitonal forms of wind and temperature in teh atmospheric surface layer. The stability parameter z/L where L is the Monin-0bukhov length scale, can be expressed in terms of a bulk Richardson nuimber which can be computed from the model variables of wind and surface temperature difference. The surface fluxes are then calculated explicitly from the formulations developed. The boundary layer model predicts growth of the boundary layer and diurnal changes of vertical eddy diffusivity.
Wesely & Gao (ANL) are improving parameterization of spatial and temporal changes in dry deposition by using satellite observations and associate inversion algorithms. They have also conducted field measurements of eddy correlation fluxes of NOx and O3 under influence of fast chemical reactions associated with NO emission from soil. This information is being used to develop parameterization of net fluxes of NO, NO2, and O3. They are also developing an improved air-surface exchange model which will be made available to the modeling community.
Future Plans
Much of our time was spent discussing basic research questions and future focus areas. The discussions tended to focus of ozone, aerosols, and ways to further foster interactions among the various modeling activities, and between the field, laboratory and UV-B components of ACP.
Ozone
Much of the discussion revolved around ozone. Ozone is a key atmospheric species. Not only does it play a prominent role in the photochemical oxidant cycle, but it also is an important greenhouse gas, and in the stratosphere shields the surface of the Earth from harmful UV-B radiation.
Specifically we posed the question: What are the outstanding questions in ozone & which ones are best addressed in ACP? The general opinion was that understanding ozone change and its relation to anthropogenic energy usage remains a central and priority research focus. Many key questions remain including: what is the role and importance of stratospheric-tropospheric exchange in the tropospheric ozone budget?; how well do we understand the chemical processes which control ozone in urban, rural, and remote locations, and under daytime and night time conditions?; and how do urban, regional and global processes influence one another?
Answers to these questions involve processes which occur at a variety of spatial and temporal time scales. We discussed the idea of looking at these issues with an integrated view which combines and utilizes the breadth of modeling expertise within ACP. Ultimately this could include nesting and coupling of climate and chemical models at regional and global scales. However in the near term this could take the form of a coordinated study where a set of specific questions and conditions would be addressed using present models. The models would be used in an off-line mode with results shared between models (e.g., global results are used as boundary conditions for a regional analysis, etc.). Such an activity would include sensitivity analysis as an integral component. We feel that an activity which studied regional-global scale interactions would be an appropriate focus for such an integrated analysis, and one which can be uniquely addressed by the ACP program.
Along these lines we discussed at some length the utility of indicator species in ozone studies. Indicator species can provide important insights into model performance and into the processes which are controlling atmospheric chemistry. For example identifying whether chemical production in and around major source regions is limited by NOx or NMHC is central to the development of meaningful emissions control strategies. While indicator species/ratios are proving to be valuable in interpretation of field studies, there remain import questions as to how robust such ratios are in space and time, how they are impacted by various processes such as deposition and cloud processes, etc. One intriguing possible focus for the integrated study discussed above would be to use the ACP assets to examine such issues, and to identify additional indicators. This is an area where modeling and sensitivity analysis could be fully utilized, and which could be nicely combined with field studies.
Aerosols
Another topic discussed in some detail was aerosols and their role in atmospheric chemistry. We discussed this topic both in our modeling sessions and in a joint session with the laboratory group. Several aerosol questions were put on the table including: what are the roles/importance of heterogeneous processes in atmospheric chemistry?; how well can we answer this question at present?; do we have sufficient models, fundamental data, field observations, understanding, etc., to evaluate their role?; where is the state of the science in modeling aerosols for use in climate studies? for atmospheric chemistry studies? for health related assessment studies?; and what priority should be assigned to these issues within ACP.
The modeling and theoretical group generally viewed this subject as very important, and one in which there is both substantial interest as well as significant capability within ACP. Like ozone, aerosols play a critical role in climate, can be produced by gas phase chemical processes involving energy-related precursors, and have adverse environmental effects on urban and regional scales. Thus studies involving aerosols can (and should) be conducted in conjunction with ozone since their cycles are coupled and similar modeling tools are needed. It was also recognized that aerosols is a topic for which there are rather broad strengths within ACP, an area which could help to distinguish ACP activities, and one which could interface with other important DOE projects such as ARM. The idea of fostering further collaborations with ARM was discussed. This appears to be a promising area to pursue. The modeling group would support such activities, especially those which would lead to chemical measurements at the ARM sites.
Much of the present interest in aerosols has focused on their radiative properties. Even less is known regarding their role in atmospheric chemistry. However there are growing indications that reactions on aerosol surfaces may play an important role in tropospheric chemistry, including ozone chemistry. This area provides an ideal interface for fostering interactions between the modeling and laboratory groups. Models provide a means of evaluating the importance of various reaction pathways, and through sensitivity analysis, can help guide laboratory studies to those areas (parameters, processes, etc.) which contribute the largest uncertainty to model behavior. Conversely, very little is known at present regarding what chemistry occurs on these surfaces and at what rates. Results from laboratory studies could be rapidly assimulated into exisiting models. This is also an area where theoretical/fundamental studies are needed. The idea of a focused workshop on this topic involving the laboratory and modeling groups was discussed. This idea will be flushed out over the next few months by email, with the possibility of a workshop conducted sometime before the next ACP meeting.
Field Studies
The desire for the modeling and field programs to work more closely together was an active topic of discussion throughout the meeting. The modeling group certainly endorses this concept. Clearly the way to maximize such interaction is to have the modeling component firmly embedded in the planning process. The meteorological modeling component of ACP (which hasn't been discussed very much) is one area which is of great value in interpretation and planning of the field experiments. We discussed various generic experiments. These included: experiments to evaluate atmospheric chemistry under various conditions including night time and seasonal differences, and different geographical locations; experiments to characterize the interrelations between regional and global scale chemistry; and to evaluate/test indicator species and the processes which control their utility. We also discussed the idea of a megacities experiment, perhaps focused in China, where a variety of the energy-chemistry relationships could be explored.
UVB & Ozone
We also held a joint session with the UVB-Ozone group. Many common interests between the modeling and UV-B groups were identified. As discussed previously ozone is a primary focus of the ACP modeling activities, with a common theme of understanding changes in atmospheric ozone. The ACP UVB-Ozone work regarding quantification of ozone trends and the calculation of radiative transfer, including estimates of photolysis rates, are of particular interest. Global and regional models use information on ozone and its trends both in initializing the models (say for photolysis calculations) and in the evaluation of model performance. Conversely, models are needed to understand the underlying causes of the ozone trends. Photolysis rates are needed by all chemistry models and the new techniques being developed within ACP can be used to improve the estimation of photoylsis reaction rates used within the various atmospheric chemistry models. We feel that there is great potential for further collaboration between these activities and recommend that more time be spent together at the next ACP meeting.
Endnotes
We also discussed ways to further foster interactions within the modeling group. We plan to develop a ACP modeling homepage upon which we can describe our modeling activities and successes. This will also serve as a means to share models, output, etc., and to keep informed of the various activities. We also strongly encourage ACP to develop a data archive of ACP data (which could include modeling products). Other programs are doing this and it is felt to be an effective way to get ACP products out to the community.