1. State of the Models

To update information and provide a basis for further discussion, an inventory of current models and their capabilities was constructed. The resulting table is shown below.  Further information on GChM-O, MOZART, IMAGES, and an additional chemical-transport model is given in the breakout session report titled "Progress in Global and Hemispheric Modeling of Oxidants and Aerosols."
 

Model	Scale Resolved	Thrust	Aerosols
STEM	1o by 1o	Oxidant cycle	Size distribution
GChM-O (global)	1o by 1o	Sulfur cycle	Size distribution
GChM (regional)	5 km by 5 km	Oxidants	Size distribution
BOX	NA	Oxidants	No
IMPACT	1o by 1o	Oxidants	Not yet
GRANTOUR	300 km by 300 km	VOCs	Not yet; separate
MOZART	2.5o by 2.5o	Oxidants	Not yet
IMAGES	5o by 5o	Oxidants	-

 
Aside from the regional version of GChM, which is in early stages of construction, the ACP chemical models noted above address scales appropriate for global studies. The global scales are considerably larger than the regional scales addressed in most of the field experiments in which ACP participates. As a result, present ACP modeling activities do not interface well with the needs of ACP field observational efforts. Many of the above models could in principle be adapted to address the scales of about 5 km by 5 km that would be necessary for regional-scale studies. A simple scale shrinkage is usually inappropriate or difficult, however, because new processes that are important only on a local scale would have to be introduced. Moreover, the present models resulted from many years of research, are used in global studies for programs other than ACP, and strongly represent the current interests of the principal investigators. Hence, very few of these global models are likely to be adapted for regional-scale studies in the near future.

2. Scientific issues that ACP might address

Several overarching scientific questions are addressed in this report:

What is the best method to evaluate models?
To what extent can urban plumes be used to test understanding?
How important is it to represent heterogeneous chemistry?
How important are the scales of interaction? What is the importance of nonlinearity of chemical processes in going from plume to urban, regional, and larger scales?

Several strategies and activities were mentioned:

a. Measurements upwind and downwind of an area being modeled should be attempted.
b. Ratios of species concentrations estimated in the models should be compared to measurements.
c. Models should be used to design the best location for measurements and to derive diagnostic indicators that may transcend scale. This latter issue was emphasized because many of the models in the ACP program are not well suited to being tested through comparison with data from the field programs.
d. The models should be used to calculate the concentration of intermediate species so that researchers making measurements will know how well they need to be measured.

a. Additional, more fundamental information might be needed, such as VOC oxidation leading to aerosol formation.
b. An intercomparison of models should be carried out to evaluate their treatment of VOCs.
c. The optimal vertical resolution in models needs to be investigated, which is particularly important for studies of nighttime chemistry.
d. The importance of aerosol-oxidant interactions needs to be investigated. It is not known whether there are testable consequences that result from this interaction.

2.2 Issues Involved in Understanding the Chemistry of Urban Plumes

Several questions were put forward that might be the focus of future field campaigns:

a. To what extent do oxidants increase nucleation and secondary aerosol formation?
b. Does the presence of aerosols exert a significant effect on the chemistry? How would it do so? For example, does an optically dense boundary layer change the actinic flux and therefore the photochemistry?
c. Do the models correctly predict the growth of pre-existing particles and nucleation of new particles? It was suggested that knowledge is particularly primitive in this area.
d. What is the effect of industrial chlorine emissions and/or the presence of sea salt?

Scientific knowledge is still lacking on several fundamental processes, making it difficult to incorporate the processes into models:

a. What are the rates of reaction of NO₂ with aerosol particles such as dust and what are the products?
b. Is there special chemistry taking place at the liquid interface on aerosol surfaces? Models could be used to address how important this might be.
c. Is the production of HONO on aerosol surfaces important?
d. A need exists for data on the properties of aerosol surfaces and the rates of chemical transformation in these surfaces. The surface composition properties of ambient aerosols is not known.
e. Although the gaseous phase processes that produce the precursors of sulfate and nitrate aerosols seem to be well understood, the level of knowledge for corresponding processing involving organics is much lower.
f. What are the organics in aerosols? How are these particulate organics created? What is the role of organics in the formation of new particles?

Field measurements typically require models with grid scales of less than or equal to 5 km but very few of the models in the program typically are run at this scale. Two approaches were discussed under this topic:

a. One suggestion is that ACP devote resources to develop an all-purpose or "community" model that could be used by ACP researchers. A "plug-and-play" model composed of individual modules that could be modified by the users could be developed. Experience by the U.S. Environmental Protection Agency (EPA) indicates that the level of resources required might be quite large and that the amount of time needed to achieve a sufficiently user-friendly model could be up to 10 years. An EPA effort has been devoted to producing a plug-and-play model that can operate on various scales, even within a single run, be able to utilize various types of emission inventory datasets, meteorological datasets, etc. This model, which is due to be released in late 1998, can be made available to ACP researchers.

b. Another suggestion is that ACP enlist the large-scale modelers to apply their models in a "box model" approach to try to simulate a particular data set. This approach would allow both a model-to-model intercomparison and a models-measurements comparison. Careful thought would be needed initially on identifying a set of measurements that would help constrain the models.