Tropospheric Ozone

Ozone in the stratosphere (~15 to 55 km altitude) protects Earth from the Sun's harmful ultraviolet radiation, but in the troposphere (0 to ~15 km altitude), ozone can have detrimental effects on both animals and plants. For this reason, stratospheric ozone has been called the "good ozone", while the same ozone in the troposphere has been called "bad ozone."

Tropospheric ozone plays a pivotal role in the chemistry of the lower atmosphere. Even in small quantities (<20 parts per billion [ppb]), tropospheric ozone accelerates the oxidation of hydrocarbons and other pollutants. However, as concentrations increase, the effect of ozone becomes markedly more detrimental. Tropospheric ozone is harmful because it is a strong oxidant and because it is a greenhouse gas. In fact, its ability to oxidize has served as the grounds for its historical usage as a disinfectant. Unfortunately, along with literally oxidizing bacteria and other pathogens to death, it will also oxidize animal or plant tissue with which it comes into contact. These effects are most noticeable in the human respiratory system and the interiors of plant cells.

Chemistry of Tropospheric Ozone

Tropospheric ozone, whether from natural or anthropogenic sources, is formed via the same chemical reactions. Burning fossil fuels or biomass releases CO, hydrocarbons, and NO_x (NO and NO₂) into the atmosphere. The hydrocarbons can then be photooxidized in the presence of NOx to form ozone (O₃). The series of reactions for the oxidation of carbon monoxide (CO) to carbon dioxide (CO₂) is shown at right.

Ozone formation as the result of carbon monoxide (CO) oxidation in the presence of nitrogen oxides (NO and NO₂)

This reaction series produces one O₃ molecule, as well as oxidizing one CO molecule to CO₂. The OH and NO are critical to these reactions, but there is no net production or destruction of either species in this sequence.

At night, NO can react with ozone to produce NO₂ and O₂, decreasing the concentration of ozone.

An important aspect of these reactions is that OH and NO are both produced and consumed, resulting in no net loss of either molecule. This cycling allows the reactions to continue until one of these two molecules is lost in a termination step (such as OH+NO₂ 6 HNO₃). Hydrocarbons other than CO can be oxidized by similar chain reaction mechanisms (click here to see the oxidation of methane).

Transport of ozone from the stratosphere is another important source of tropospheric ozone. Its relative importance is the subject of much current research (e.g., Oltmans et al., 1996).

Surface-level ozone in areas with sufficient NOx typically varies on a diurnal cycle with maximum levels in the late afternoon and minima in the early morning. The high levels in the late afternoon are the result of the solar-driven reactions discussed above. The low levels in the morning are caused by a combination of three factors. The first is deposition to land and marine surfaces. The second is the reaction between NO and ozone to produce NO₂ and O₂. Finally, tropospheric mixing becomes slower at night. This prevents ozone in the free troposphere from replenishing whatever ozone has been deposited or chemically consumed in the lower troposphere.

In the remote marine environment (with low NO_x), ozone is photochemically destroyed during the day, leading to a diurnal cycle with a daytime minimum. Here, ozone destruction is balanced by transport from the free troposphere (e.g., Ayers et al., 1992).

The Current Situation

Average tropospheric ozone levels are thought to be increasing over much of the world. In 1998, 51 million people lived in areas of the United States with O₃ levels exceeding the EPA standard of 120 ppb (1-hour average), and 130 million people lived in areas where O₃ concentrations exceeded the newly proposed standard of 80 ppb over an 8-hour average (Lin et al., 1999). Regulation of tropospheric ozone levels is necessary to protect public health and the environment, but it also comes at an economic cost to industry, the transportation sector, and consumers.

It is believed that significant amounts of ozone and ozone precursors may be transported from Asia to North America and from North America to Europe. For instance, in mid-spring approximately 14% of the ozone off the western coast of the United States is thought to have been carried across the Pacific Ocean from East Asia (Berntsen et al., 1999). Europe may be similarly affected as a result of emissions in the eastern United States. This transport makes it difficult to link tropospheric ozone levels to their sources, which may be several thousand miles away. However, it is crucial to quantify intercontinental pollution transport, because of its potential effects on continental ozone levels.

How Buoys Will Help

Although surface ozone monitoring stations are numerous on land, they come to an abrupt end at all coastlines, leaving most of Earth's surface without ozone measurements. The goal of our research is to implement buoy-mounted instruments capable of running autonomously at sea for several months to measure ozone and other chemicals.

By measuring tropospheric ozone in the marine boundary layer (MBL), we will gain insight into daily and seasonal ozone patterns and cycles. This will help us to understand anthropogenic and natural factors affecting tropospheric ozone concentrations. Improvements in our knowledge of intercontinental ozone and ozone precursor transport will allow distinctions to be more easily made between locally-produced and imported ozone. Buoy-mounted detectors will also allow us to study natural factors affecting ozone cycles and provide baseline ozone data for long-term studies of global change.