1a. What are four basic processes that form the components of a photochemical model?

1b. In one sentence, briefly explain each of the processes above.


2a. Describe the photochemical box model.

2b. How is this model limited?

2c. How is this model beneficial?


3a. Describe the trajectory model.

3b. How is this model limited?

3c. How is this model beneficial?


4a. Describe the zonal mean model (2-D model).

4b. How is this model limited?

4c. How is this model beneficial?

4d. Why do the Residual models work better than the Eulerian?


5a. What is the significance of the Montreal Protocol?

5b. What is the difference between the 1987 Montreal Protocol, the 1990 London Agreement, and the Copenhagen Amendment of 1992?

5c. Why was international policy on CFC usage changed from 1987 to 1992?


6a. How can scientists validate a model to see if it's correct?

6b. According to all models, what will the difference of total ozone amounts be between 1979-2050?



1a. the four basic processes that form the components of the photochemical model are (1) radiation, (2) gas phase chemistry, (3) heterogeneous chemistry, and (4) dynamics.

1b. The radiation of a model refers to the way the model represents visible and ultraviolet solar energy entering the atmosphere and how that energy is absorbed and reemitted as thermal radiation by the surface and various trace gas species, including ozone.

The gas phase chemistry of a model refers to the way the model represents the chemical species and their reactions in the atmosphere.

The heterogeneous chemistry of a model refers to the way the model represents with reactions that occur on surfaces, either liquid or solid, such as are provided by aerosols in the atmosphere.

The dynamics of a model refer to the way the model represents the real-world behavior of air circulation and mixing.

2a. The photochemical box model is a moedl that calculates what happens to temperature, chemical concentrations, and reaction rates in an isolated parcel or "box" of air. The only external aspect to the model is solar shortwave radiation or "radiative flux" that passes through the model and drives the photochemical reactions. Equilibrium is achieved within a box model when all the constituents reach a constant value with time.

2b. The box model is limited in that "dynamics" or transport processes in the real atmosphere are not considered. For chemical reactions that proceed quickly, it is okay to disregard these atmospheric motions and think of an air parcel as being effectively isolated. However, for long-lived trace species, we do not want to completely discount the fact that transport processes are advecting (moving) such species into and out of the box. That is, the "integrity" of the air parcel is not maintained over long periods of time.

2c. A box model can be run through a daily or diurnal cycle of time-varying solar radiation input in order to make predictions about how concentrations of different species will change throughout the course of a day. The box model can thus aid in the interpretation of observations of a wide variety of species. The results can be compared to measurements made by actual missions (such as NASA's ER-2) to see if the photochemistry of a region of atmosphere isindeed well understood.

3a. The trajectory model takes information on the location of air parcels and wind field analyses as its input and calculates both whence the air parcels originated and where they are going.

3b. The trajectory model assumes that the integrity of the air parcel is maintained over the entire course of its projected path, forward and backward. Mixing processes that occur in the real atmosphere are not considered. Consider the example of puff of smoke leaving the tailpipe of a car. Mixing processes quickly cause the smoke to lose its integrity so that it isn't visible after a short time. A trajectory model would calculate the forward path of the puff of smoke far longer than it would actually be visible to our eyes.

To be useful, the spatial scales over which trace gas species (such as ozone) varies must be sufficiently long and the dynamics of motion must not cause too much mixing. Both criteria are met for stratospheric ozone, and so the trajectory model is useful for calculating ozone trajectories over the course of days or weeks in the stratosphere.

3c. The trajectory model is useful for (1) determining the possible history of a given air parcel using back trajectories, (2) studying mixing and transport processes in the atmosphere, (3) combining observations made over a wide variety of places and times into a consistent map representing the state of the atmosphere at a single time (i.e., a synoptic map), and (4) providing a dynamical component to a photochemical box model so that the chemical evolution of an air parcel can be reliably studied.

4a. A zonal mean or 2-D model neglects variability in the zonal component of some variable and calculates changes of the variable only as a function of latitude and altitude. Since many trace gases in the stratosphere have zonal (i.e., east-west) variations in concentration that are much smaller than their meridional (i.e., north-south) and vertical variations, we can neglect zonal variations and instead take a zonal mean value. This is the key feature of a zonal mean or 2-D model. The model only calculates the change in time of two dimensions -- meridional concentration and vertical concentration.

4b. Early 2-D models were limited in that they could not successfully represent eddy advective transport processes in the atmosphere, instead using the much slower eddy diffusive transport representation. Diffusion is a molecular level process in which individual molecules pass along heat and momentum. Eddies, on the other hand, are large-scale deviations in the zonal wind field from the zonal mean value. In the real atmosphere, trace gas constituents are transported both by the zonal wind and by eddy advection. Indeed, eddy driven advective transport of trace gas constituents is significant and cannot be neglected. 2-D transport models must include accurate eddy advective transport terms.

Another limitation of 2-D models occurred in the "ClO overprediction problem." By not including particular reactions in the model chemistry, the amount of ClO predicted was too high. Overpredicting ClO has implications for the amount of ozone predicted. This particular problem was solved, but the larger, conceptual problem remains: any lack of knowledge about reactions in the atmosphere can limit the predictive usefulness of the 2-D model.

4c. A main advantage of the 2-D model is its computational economy compared to trajectory models and more complicated 3-D global models (in which zonal variability is considered). 2-D models are inexpensive to run and they can be run multiple times or for many year predictions. This means that 2-D models can be used to study the sensitivity of the model to changes in assumptions about the atmosphere. The 2-D model allows us also to make assessment studies about the impact of natural and manmade forced changes (perturbations) to the stratosphere (such as volcanic eruptions or adding CFCs). Finally, the 2-D model lets us make long-term trend predictions about important trace gas species, such as ozone.

4d. Residual 2-D models take into account the advective nature of eddy transport processes.

5a. The 1987 Montreal Protocol was the first time the international community took action to reduce the amount of manmade CFCs in the atmosphere and did so partly on the basis of model predictions of future ozone concentrations. The models permitted different predictions of total ozone to be made on the basis of different assumptions about CFC usage (e.g., continued increase in CFCs versus reducing CFC usage). Ozone predictions were made 50 years into the future and, as a result, politicians were able to see the likely effects of different courses of action. The result of this was a first-ever major change in policy on CFC useage as was agreed to by the 27 nations that signed the Montreal Protocol.

5b. The 1987 Montreal Protocol called for the signing nations to agree to a 50% reduction in CFC production by 1999. The 1990 London Agreement called for a complete elimination of CFC production by 2000 among the 80 signing nations. The Copenhagen Amendment of 1992 called for a complete phaseout of all CFCs by 1996 and an eventual phaseout of HCFCs by 2030.

5c. The policy was changed based on additional scientific evidence that showed the severity of the ozone loss problem caused by CFCs. Model predictions using the new data again enabled policymakers to decide the best course of action for protecting stratospheric ozone.

6a. Scientists can validate a model by "predicting the past" or "hindcasting." If a model can accurately simulate some known variable from the past -- such as climate or atmospheric trace constituent concentrations -- then we can be better assured that the simulations of this variable in the future are reasonable.

6b. Although the models show a variety of predictions for ozone in 2050 relative to 1979, they all agree that the difference will still be negative: ozone will not recover globally to the 1979 levels until after 2050.