Scale Dependence and Atmospheric Chemistry

Donald J. Wuebbles

University of Illinois

Urbana, Illinois

A number of issues in atmospheric chemistry relate to scale. In global change research, there is concern about changing atmospheric composition due to emissions of carbon dioxide, nitrogen oxides, chlorofluorocarbons and their replacements, methane, and other gases potentially important to climate. The relationship between the biogeochemical cycles at the land-biosphere-ocean interface with the atmosphere are of critical interest with regard to atmospheric composition. Local emissions, particularly from large urban areas, can lead to regional changes, such as acid rain, as well as to global changes. There is still much that is not understood about this interface between the local level and the regional and global levels.

There is a need to downscale and see how changes occurring globally will lead all the way down to changes in the biosphere.

A hierarchy of methods and models are used in biogeochemical cycle research, and there is a range of transitions in going from one type of model to another. Table 1.39 summarizes the scales of different types of models. There is a need to downscale and see how changes occurring globally will lead all the way down to changes in the biosphere. An example of this kind of effect is that the period from 1991 to 1993 saw sharp reductions in the rates of increase of carbon dioxide, carbon monoxide, nitrous oxide, and methane, probably as a result of the 1991 Mt. Pinatubo eruption, the aerosols from which led to a cooling effect and a decrease in the amount of solar radiation reaching Earth's surface, and to feedbacks in the biosphere.

Spatial and Temporal Scales in Atmospheric Chemistry

Atmospheric chemists study processes affecting atmospheric chemistry at spatial scales ranging from urban (~10 km) to global (~106 km) and time scales extending from hours to years. The spatial and temporal scale of interest is set primarily by the lifetime of the trace gas affecting the problem and the relevant transport scales. Among the important transport timescales: north-south mixing in the troposphere is of the order of one year; mixing from the boundary layer to the upper troposphere (the so-called turn over time for the troposphere) is of the order of one month; and the time scale for exchange between the stratosphere and troposphere is estimated to be around three years.

These transport timescales are then related to the atmospheric lifetimes of gases. For example, many halocarbons have lifetimes extending into tens of years, so the only relevant transport scale of interest would be the stratosphere-troposphere exchange. For trace gases with lifetimes less than five years, the removal process in the troposphere by oxidation and physical mechanisms becomes relevant. For trace gases with lifetimes close to a year (the interhemispheric exchange time) the location of the sources is important as there is not enough time to mix the trace gases uniformly in the troposphere (e. g., emissions from the northern hemisphere that do not reach the southern hemisphere). With trace gases that have lifetimes of months, the vertical mixing gains importance as the lifetimes approach the scales of the vertical mixing. Thus, the problem steadily cascades down to smaller and smaller scales as the lifetime of the trace gas under consideration decreases.

Wuebbles discussed the emissions of interest with regard to global change (summarized in Table 1.40). Gases with long atmospheric lifetimes and important effects on climate are of particular concern, such as the fluorocarbons which are being proposed as substitutes for CFCs but have lifetimes of thousands of years and are strong greenhouse gases (GHGs).

The hydroxyl radical (OH) is the primary oxidizing species in the atmosphere, making it a very prominent gas in terms of atmospheric control of many gases of interest. Carbon monoxide (CO), for example, is largely controlled by OH. Methane and CO have both natural and anthropogenic sources. OH concentration is affected by ozone, water vapor and nitrogen oxides. OH, CO and CH₄ form a triangle of interactions that control their decay rates. In addition, the lifetimes of HCFCs and HFCs are sensitive enough to the abundance of OH that if human activity were to dramatically decreases the OH level in the atmosphere, it could allow HCFCs and HFCs to reach the stratosphere where they could destroy ozone (sufficient OH would otherwise destroy these in the troposphere). With a lifetime of seconds (determined locally), and many important interactions, including potential climatic feedbacks, OH has significant effects globally, but we do not have a thorough understanding of it yet.

CO₂ carbon dioxide

OH hydroxyl

N₂O nitrous oxide

NO nitric oxide

NO₂ nitrogen dioxide

NO_X nitrogen oxides (NO + NO₂)

CH₄ methane

CO carbon monoxide

O₃ ozone

SO₂ sulfur dioxide

NMHC non-methane hydrocarbons

CFC chlorofluorocarbon

HCFC hydrochlorofluorocarbon

HFC hydrofluorocarbon

PFC perfluorocarbon

PAN peroxyacetyl nitrate

GHG greenhouse gas

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Scale Dependence of Ozone Production in the Troposphere

One way that scale dependence in the troposphere occurs is due to nonlinear production location efficiency of ozone (O₃) from NO and NO₂ (NO_X). NO_X destroys O₃ in the stratosphere but produces it in the troposphere due to the different type and amount of solar radiation present. Ozone production efficiency depends on the concentration of NO_X and non-methane hydrocarbons (NMHC) and the ratio of NMHC to NO_X. When NO_X concentrations are greater than 1 part per billion (ppb), there is less efficient production of O₃ per molecule of NO_X . When NO_X concentrations are less than 1 ppb there is more efficient production of O₃ per molecule of NO_X. Therefore, diluting NO_X emissions over a large domain in a model results in excess production of O₃. For this reason, current atmospheric chemistry models tend to produce too much ozone in the troposphere.

Gases with long atmospheric lifetimes and important effects on climate are of particular concern, such as the fluorocarbons.

Regarding scale and dimensionality, Wuebbles says that two-dimensional atmospheric chemistry models are adequate for understanding many processes in the stratosphere but that three-dimensional models are needed for most tropospheric research. A recent study by Crutzen, et al. comparing 2-D and 3-D models shows that 2-D models don't represent tropospheric processes very well. The ratio of results from the 2-D to the 3-D model reveals that one can get a 100 percent difference in OH concentration near the equator in the troposphere. The 2-D model has too much mixing and shows different responses for a variety of species than the 3-D; the largest differences are in lower troposphere though significant differences are found in the upper troposphere as well. However, this conclusion may be affected by the particular models used and may not be a general rule.

Effects of Small Scale Processes on Global Chemistry

Small scale processes effect global chemistry through the transport of reservoir species from source regions to remote regions and the subsequent effect on chemistry. For example, peroxyacetyl nitrate (PAN), a complex hydrocarbon with a lifetime of the order of weeks to months, is transported from the polluted regions in which it is formed to relatively cleaner regions of the troposphere. This leads to a redistribution of ozone precursors from small scale polluted urban areas to the remote troposphere.

In addition, rapid transport of ozone precursors from the boundary layer to the upper troposphere occurs through deep cumulus convection, thereby affecting global scale chemistry. Deep convection occurs on small spatial and time scales and is essentially a sub-grid scale process in terms of global scale models. NO_X has a lifetime of a few hours in the boundary layer and a lifetime of over a week in the upper troposphere. Thus, the small scale convection process extends the lifetime of this ozone precursor and can lead to a spatially extended impact on background atmospheric ozone levels due to anthropogenic emissions.

How can such processes be treated in a model when such a small fraction of the grid space is involved? There is ongoing work to improve model parameterizations of such sub-grid scale processes so that they more closely match the data (which are of increasing quality). A recent observational study shows that cumulus convection can actually bring stratospheric ozone down into the troposphere; this could have negative effects on agricultural production, for instance.

Solving Scaling Problems

Wuebbles discussed a number of methods of solving scaling problems in atmospheric chemistry. One option is to reduce the model grid size or use special grids. Another is the use of nested grids or telescoping grids. This method, which involves increasing the resolution of a limited subdomain while leaving the larger domain at a coarser resolution is illustrated in Figure 1.41. A third is the coupling of local/regional /urban models with global scale models. Such coupling is a challenge because the global models can not include as detailed chemistry as the smaller scale models.

Two-dimensional atmospheric chemistry models are adequate for understanding many processes in the stratosphere but three-dimensional models are needed for most tropospheric research.

As an example, Wuebbles showed results from the Air Quality Model (AQM) of Julius Chang and others developed at State University of New York at Albany. The modeled area is central California, including the San Francisco area, at a 12 x 12 km grid resolution. This resolution could not capture the detail necessary, so, within this, three regions of particular interest (Bakersfield, Fresno and the Bay area) were treated with 4 x 4 km two-way nested grids in which special calculations were applied. This captures the needed detail in most important areas without the tremendous computational expense (9 times greater) of going to a 12 x 12 km grid for the entire domain. The differences between the 12 x 12 results and the results using the nested 4 x 4 grids become greater the longer the model is run; by the fifth day the differences are large.

In another example, adding a Surface Layer Submodel (sls) to the AQM model added important information to its results. Urban/regional models generally have a 50-100 meter surface layer (nearest the ground). However, in many regions, especially at night, the bottom boundary layer is actually very close to the ground - just tens of meters. By dividing the surface layer into four sub-layers with the bottom zone being just 10 meters, a greater simulation of reality is achieved. In the less-resolved model, large amounts of NO_X appear all the way to the ground. But when the sls is added, the NO_X appears in the second layer from the ground, which represents with more accuracy what actually happens at night in urban areas. During the day, there is not much difference between the two models, but at night, it is quite significant. With the sls included, the NO_X available for producing ground level ozone compares well with observations. It is important to understand the mechanisms of the chemistry involved and attempt to reproduce them in the models.

With regard to global change issues, scaling up from the local to the global scale involves atmospheric chemistry with significant effects on the troposphere (far less in stratosphere). Scaling down from the global to the local and urban scales involves many important climate-related changes including changes in water vapor, local temperature , clouds, etc., and these can then cause feedbacks up to the global scale.

A recent observational study shows that cumulus convection can actually bring stratospheric ozone down into the troposphere; this could have negative effects on agricultural production.

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