CLIMATE (CHEMICAL PROCESSES)

Solar radiation delivers not only the energy to drive the global circulation of the oceans and the atmosphere, but it is also responsible for maintaining the temperature of the earth. The average temperature is determined by the amount of radiation which is absorbed, and this in turn is a function of the chemical composition of the atmosphere. The average temperature is 33 oC (OR 60 oF) greater than it would be because of the presence of the so-called greenhouse gases (see figure). These include water vapor, carbon dioxide, methane, ozone, and the halocarbons. Of these, water vapor has the largest influence, followed by carbon dioxide.

Carbon dioxide levels in the atmosphere, as measured from gas bubbles trapped in ice cores, has increased since the start of the industrial revolution in the 19th century from about 280 to about 360 parts per million (by volume). Direct measurements over the last 30 years show an average increase of 1.5 ppm/year. This increase, which contributes to global warming, has been ascribed to burning of fossil fuels and to a lesser extent to deforestation. However, incomplete understanding of the global carbon cycle limits the capability to evaluate the sources and sinks of carbon dioxide. We know that carbon is exchanged on decadal to centennial time scales between the terrestrial biosphere, the atmosphere, and the oceans. The exchange is much slower between the oceans and its sediments and sedimentary rocks. Fossil fuel burning, cement manufacture, and changing land-use are activities that release carbon (as carbon dioxide) to the atmosphere. The flux of carbon dioxide generated by these activities is small compared with mean natural gross fluxes, but it is sufficient to rapidly increase atmospheric levels by 29 % over the past 200 years. Because the net uptake of carbon dioxide occurs slowly (time-scale of centuries for the deep ocean), any additions will have a long-lasting effect on concentration in the atmosphere. For example, if carbon dioxide emissions were held constant at present day levels, atmospheric concentrations are estimated to continue to rise for at least two centuries.

Key processes in the carbon cycle include:

The oceans of the world are believed to be absorbing about 40% of the annual release from fossil fuel burning. This number is derived from atmospheric measurements and fossil fuel combustion statistics, and not from direct carbon measurements in the oceans. On decadal scales, we presently do not know the accuracy of this estimate nor do we know the relative roles of the terrestrial biosphere and oceanic sinks. Without this knowledge, we are unlikely to ever produce accurate forecasts of the effects of carbon dioxide.

For the last several years, NOAA has pursued a program that seeks to quantify the uptake and partitioning of carbon in the three main reservoirs, the atmosphere, the ocean, and the terrestrial biosphere . This program, called the Ocean-Atmosphere Carbon Exchange Study (OACES), was established by the NOAA Office of Global Programs. OACES is a collaborative study between AOML, the NOAA Climate Monitoring and Diagnostics Laboratory, the NOAA Pacific Marine Environmental Laboratory, and university-based investigators. the oceanic component evaluates the role of the oceans relative to the exchange and eventual penetration of carbon dioxide across the air-sea interface and into the depths. The field component of OACES has been underway for seven years and includes: extending the data base for partial pressure of carbon dioxide in the surface ocean and overlying atmosphere; providing global coverage of total dissolved inorganic carbon dioxide measurements in the surface and deep ocean water masses in order to quantify the total amount of carbon in the ocean and its anthropogenic inventory ; providing a better understanding of physical and biological processes that affect seasonal variations in the distribution of carbon species in the oceans. Sequestration of carbon dioxide in the ocean is a complicated process and the direction of exchange between the atmosphere and the ocean varies both seasonally and geographically.

AOML has assumed the lead role for observations taken on cruises in the Atlantic and Indian Oceans and has also participated in cruises in the Pacific. These observations are taken along pre-arranged routes that span the ocean basins. As such, they supply a base line of total carbon inventory in the ocean against which future increases can be measured. Several interesting results have recently emerged from studies of these base line data. The specific inventory and depth of penetration of anthropogenic carbon has unequivocally been determined in the Atlantic and Indian oceans using two independent techniques. Semi-annual studies of carbon dioxide in surface waters of the equatorial Pacific have shown that this region is the largest and most variable oceanic source of CO2 to the atmosphere. The release from the ocean near the equator was strongly suppressed during the 1992 El Nio and the same effect is observed during the current 1997 El Nio. Over 70 % of the interannual variability in oceanic uptake is attributed to the el Nio effect in the equatorial Pacific, and subsequently returned to larger values.

Other gases than carbon dioxide, such as methane and ozone, have a role to play in the greenhouse effect. Unlike carbon dioxide, the concentration and distribution of these gases are highly dependent on physical and chemical reactions in the atmosphere. In the lower part of the atmosphere, the concentration of methane depends on the rate of its destruction by the hydroxyl radical. The concentration of ozone depends on various short lived precursor gases (carbon monoxide, oxides of nitrogen, and non-methane hydrocarbons) and transport from the stratosphere.

The handful of observations of ozone concentration over the oceans, especially in the southern hemisphere, make it difficult to construct an accurate global mass balance for fluxes, sources, and sinks of this important chemical species. In response to this deficiency and with support from the NOAA Radiatively Important Trace Species program, AOML initiated a program to document the distribution of ozone in the marine atmosphere.

During the 1970s and into the 1980s, observations of ozone concentrations in mid-latitudes suggested that its rate of increase was about 1% per year, with the increase presumed to be caused by anthropogenic activity. However, a few aircraft and shipboard field programs documented the existence of extremely low ozone concentrations (defined as less than or equal to 10 parts per billion by volume) within the boundary layer of the equatorial Pacific Ocean. Detection of these low ozone concentrations helped investigators to hypothesize that the boundary layer over the equatorial Pacific ocean was a sink for ozone. A possible mechanism for this sink involves the photochemical destruction of volatile species containing halogens that are produced by biological processes in the ocean and are then released to the atmosphere. The species (iodine in the marine boundary layer) resulting from this destruction could then consume ozone by a mechanism similiar to that involved in the destruction of stratospheric ozone by chlorine species. Free tropospheric ozone might also be destroyed by bromine.

Beginning in 1984 and continuing to the present, AOML conducted a set of shipborne and aircraft experiments that covered oceans in the tropics, the southern hemisphere and the North Atlantic basin. These experiments measured the photochemical and dynamic processes that determine the distribution of ozone in the marine boundary layer and in the free troposphere. The major results of these investigations include the following:

AOML is working to describe the chemistry of oxides of nitrogen in the near-surface marine atmosphere. The single most important factor controlling ozone production is the concentration of nitrogen oxides--low concentrations lead to ozone destruction while high concentrations lead to ozone production. In general, nitrogen oxides in high concentrations are found in industrial and heavily populated regions; low concentrations are found in rural and oceanic regions. To understand the impact of anthropogenic inputs of nitrogen oxides to the atmosphere, it is vital to know the concentrations of these gases in the vast expanse of the marine atmosphere and to describe their role in ozone generation and destruction. Measuring low concentrations of nitrogen oxides on land is a very delicate task; shipboard measurements are even more difficult but AOML has constructed an instrument that has proved successful on several cruises over the last few years. During the summer of 1992, the photochemical environment over the North Atlantic was found to result in the destruction of ozone while encountering plumes of polluted air from the European and American continents, as well as during clean air episodes. NO levels were much higher than those observed in the remote Pacific troposphere, but similar to that found previously in the North Atlantic.

Greenhouse gases tend to absorb radiation while clouds tend to scatter or reflect radiation. Consequently, clouds play a very important role in the radiation balance for the earth. A recent hypothesis suggested that the formation of clouds could depend substantially on aerosol particles that originated from the oxidation of sulfur dioxide. Sulfur dioxide is produced not only from the combustion of fuels containing sulfur, but also from the oxidation of biogenically-produced sulfur compounds. One of these, dimethyl sulfide, is produced by numerous species of plankton throughout the oceans. This offers the possibility of a large flux of dimethyl sulfide from ocean to atmosphere where a considerable portion can be oxidized to produce sulfur dioxide. Sulfur dioxide is further oxidized, but this oxidation is a complicated process which occurs along several pathways. Only one of these leads to formation of the aerosol particles required for cloud formation.

Studies of this process suggest that ammonia plays a large part in the formation of aerosol particles from oxidation of sulfur dioxide. Measuring gaseous ammonia in the marine atmosphere is difficult; therefore, atmospheric ammonia concentrations are not well characterized. AOML is currently field testing a system that will be used to determine ambient levels of gaseous ammonia in the marine atmosphere. With this information, better estimates of the amount of aerosol particles produced as a result of oxidation of sulfur dioxide can be made. This information will enhance the accuracy of coupled ocean-atmosphere climate models.

The effects of halocarbons on the earth's radiation balance are two-fold. First, the chlorofluorocarbons (CFCs) and their hydrochlorfluorocarbon (HCFC) replacements are strong infrared (IR) absorbers. This, as with any other IR absorber, would tend to lead toward global warming. Second, these trace gases destroy stratospheric ozone, and that in itself can cause a significant radiative forcing in the opposite direction (i.e. Cooling). The reduction in ozone may also have another effect in that tropospheric hydroxyl radical (OH) concentrations should increase in the presence of enhanced ultraviolet (UV) radiation, normally absorbed by stratospheric ozone. An increase in tropospheric OH also has a net cooling effect.

The ability of a trace gas to cause global warming is described by its Global Warming Potential (GWP), and the ability of a gas to destroy ozone is determined by its ozone depletion potential (ODP). Both the GWP and the ODP for a trace gas are functions of the atmospheric lifetime of that gas as well as their ability to absorb IR radiation (in the case of GWP) and reactivity in the stratosphere (in the case of ODP). The lifetime of a trace gas is a function of the sink strengths for the gas. For those gases that are solely anthropogenic, man-made, and react only in the stratosphere, the calculation of lifetime is simple. However, there are a number of reactive halocarbons that have natural (terrestrial and oceanic) sources and sinks in addition to their anthropogenic sources (e.g. methyl bromide). The complicated nature of these source/sink distributions makes it difficult to calculate the atmospheric lifetimes for these gases.

Scientists at AOML, in collaboration with scientists at the NOAA Climate Monitoring and Diagnostics Laboratory, are now able to model the oceanic uptake of some of these reactive halocarbons. These scientists, working with collaborators at universities are conducting field experiments that look at the balance between oceanic uptake and emission of many naturally occurring halocarbons. During an upcoming cruise (GasEx98) in the North Atlantic, these scientists will be making direct measurements of the flux of these trace gases across the air-sea interface. Collaborating scientists will be measuring the in situ degradation and production of one of these gases, methyl bromide (CH3Br). The information collected during this and subsequent cruises will be incorporated into a global model being developed at AOML to study the impact that the distribution of sources and sinks for these gases has on their atmospheric distribution both horizontally and vertically. Understanding how regional sources and sinks can effect global atmospheric distributions of these gases will help policymakers better determine how to reduce the impact that anthropogenic emissions may have on the natural system.

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