William Chameides

Georgia Institute of Technology, School of Earth & Atmospheric Sciences

Global change is an observational fact, not a theoretical possibility. Humankind is a major factor in this change. Examples of human impacts include the increase in atmospheric carbon dioxide, stratospheric ozone depletion, and increases in tropospheric ozone.

The 1990s have already witnessed a significant global change surprise - the unexpected slowing in the upward trend in some greenhouse gases (GHGs), namely carbon dioxide, methane and nitrous oxide. Is this just a blip in the long-term upward trend or a new regime? What is the cause of this surprise? Mt. Pinatubo effects? Other ideas? Chameides points to a remote possibility (perhaps a 1 to 5% probability) that by some global process, the Earth system is accommodating anthropogenic emissions.

On the subject of atmospheric chemistry and global change, the public's concerns are centered on 1) personal health, 2) human welfare, and 3) ecological impacts. The subject of tropospheric ozone touches all of these concerns. There are different regimes of tropospheric ozone. Ozone in the upper free troposphere (some of which comes from the stratosphere) is an effective GHG and measures about 40-80 parts per billion by volume (ppbv). In the boundary layer, it occurs at high levels (80-200 ppbv) and is mainly a result of urban smog. In rural areas, the levels are relatively low and the concerns are related to production of food and human health. In remote areas, the concerns are mainly about ecosystem impacts. There are currently significant effects of ozone pollution.

In the area of human health:

• 98 U.S. cities are out of compliance with EPA limits on ozone, and
• 70 million Americans are exposed to unhealthy concentrations of ozone. Agricultural impacts include:
• $3 to 5 billion lost annually in crop production, and
• significant losses in forest products.

A clear upward trend in tropospheric ozone concentrations in the Northern Hemisphere is probably due to anthropogenic activities. A doubling or tripling of background levels of tropospheric ozone may have caused an increase in radiative forcing of some 0.2 to 0.5 watts per square meter (W/m²) and could also be causing a decrease in net production in forest ecosystems of 1 to 3% per year. This represents a significant increase in radiative forcing and a significant decrease in forest production. What is causing the ozone increase and what are its implications?

VOC+ NO_x + Sunlight = ozone (O₃₎ Nitrogen oxides (NO_x) and volatile organic compounds (VOCs), the vast majority of which come from anthropogenic sources, are the precursors of tropospheric ozone. The amount of NOx most closely controls the amount of ozone. Lower VOC levels cause the system to react more slowly by lowering reactivity but will result in the same concentration in the end. Conversely, lowering NO_x levels causes the reaction to build up more quickly but results in less ozone in the end. Therefore, Chameides believes that what we really must do is lower NOx emissions to control ozone pollution. Current policy, however, is mostly concerned with lowering VOC levels, which merely exports the ozone somewhere else and raises background levels of atmospheric ozone. Will urban ozone-reducing policies actually increase global levels of tropospheric ozone? We need to make connections between spatial scales and be concerned about regional and global impacts as well as local ones.

Empirical evidence indicates that ozone level is a function of NO_x emitted. This relationship exists even in remote areas and the increase in ozone globally is a result of NO_x emissions. Humans are changing the chemistry of the atmosphere; 75% of NO_x comes from fossil fuel burning, biomass burning, fertilizer-induced soil emissions, and aircraft emissions. The observed increase in ozone levels highlights a series of specific concerns and suggests some potential surprises. In the area of climate, upper tropospheric ozone is an effective greenhouse gas, and aircraft NO_x emissions could be enough to perturb upper tropospheric ozone. (Particularly in winter months, there is a significant contribution by aircraft to NOx in the upper troposphere.) New aircraft may be more fuel-efficient, but emit more NO_x. There is also evidence that ozone changes may be affecting hydroxyl (OH) levels and thus, the oxidative capacity of the troposphere.

Increasing Tropospheric Ozone: Specific Concerns Climate

• Upper tropospheric ozone (O₃) is an effective greenhouse gas.
• Are aircraft NOx emissions perturbing upper tropospheric O₃?
• Some greenhouse gases are affected by the oxidative capacity of the troposphere.
• Are O₃ changes affecting the oxidative capacity? How?

• Increasing O₃ on regional and hemispheric scales may be suppressing net productivity and growth in ecosystems.
• Is there a significant perturbation of biospheric trace gas emissions?
• Is there a significant impact on carbon storage and ultimately on atmospheric CO₂?
• Is O₃ pollution affecting agriculture and world food production?

• Will urban ozone pollution mitigation policies that emphasize VOC reductions rather than NO_x reductions implemented in the US and elsewhere accelerate the build-up of tropospheric ozone?

Generally, we burn fossil fuels in the same places we produce food and use fertilizer. On 23% of the world's land mass we burn 74% of the fossil fuels and grow 62% of the food (using 77% of the nitrogenous fertilizer). These regions are the economic drivers of the globe. Globally, 10 to 30% of all cereals are produced in regions subject to ozone impacts. These ozone impacts could triple in the next 25 years. A back-of-the-envelope calculation supports a prediction that 3 to 5% of the world's food production could thus be eliminated through loss of plant productivity.

Tropospheric ozone increases could result in total food crop losses similar in magnitude to estimates of those due to global warming. If a net global crop loss of 1 to 5% resulted from each of these factors, there could be more than a doubling effect since they may occur in opposite regions from each other.