Tropospheric Ozone: The Missing GHG in the UNFCCC Equation

Pak Sum Low, BE (Chem), MEnvSt, Ph.D,
Senior Programme Officer
Climate Change/Ozone Depletion
GEF Coordination Office, UNEP, Nairobi, Kenya

Ozone (O₃) can be both good and bad.

In the stratosphere (10-50 km above the surface of the Earth) where 90% of ozone is located, ozone is produced by the photolysis of oxygen molecules, as follows:

hu

O₂ ------------------> O + O (1)

l < 242 nm

O + O₂ ----> O₃ (2)

Ozone can be destroyed through photolysis:

hu

O₃ -------------------> O₂ + O(¹D) (3)

l < 315 nm

In this sense, the stratospheric ozone (or ozone layer commonly referred to) "absorbs" or "filters" or eliminates the most damaging UV-C (< 290 nm) and greatly reduces the amount of harmful UV-B (280-315 nm) reaching the Earth's surface, and hence protecting the terrestrial ecosystems. Thus, stratospheric ozone is good ozone.

As shown in (1) to (3) above, stratospheric ozone is constantly being created and destroyed by natural photochemical processes that are in steady-state (i.e., input and output fluxes are equal). However, the introduction of anthropogenic chemicals such as chlorofluorocarbons (CFCs) are disrupting this steady-state.

The excited oxygen atom produced from (3) above will then react with water vapour to form the hydroxyl, OH, radicals, the short-lived but the most powerful oxidizing and cleansing agent in the atmosphere:

O(¹D) + H₂O ------> OH + OH (4)

Because of the maximum abundance of UV-B radiation and water vapour, the concentration of OH radicals (globally averaging about 4 x 10¹⁴ by volume) is largest in the tropics and subtropics (Ref. 1).

However, in the troposphere (0-10 km above the surface of the Earth), ozone is produced photochemically via a different mechanism: the photolysis of nitrogen dioxide, as follows:

hu

NO₂ ------------------> NO + O (5)

l < 400 nm

O + O₂ + M ------> O₃ + M, where M = N₂, O₂ (6)

The ozone formed in this way can be removed by nitric oxide, NO:

NO + O₃ = NO₂ + O₂ (7)

Thus a photochemical equilibrium exists and the ozone equilibrium concentration can be calculated by a certain chemical equation. This equilibrium is disturbed by the oxidation of NO to NO₂ by peroxy radicals (HO₂) formed in the course of oxidation of carbon monoxide (CO) and hydrocarbons by OH radicals:

For CO:

CO + OH -----> CO₂ + H

H + O₂ ------> HO₂

HO₂ + NO ------> OH + NO₂

For hydrocarbons (e.g., CH₄):

CH₄ + OH ------> CH₃ + H₂O

CH₃ + O₂ ------> CH₃O₂

CH₃O₂ + NO -------> CH₃O + NO₂

CH₃O + O₂ -------> CH₂O + HO₂

HO₂ + NO ------> OH + NO₂

The net result of this chemistry is to produce NO₂ from NO by other means than by reaction (7), thus leading to enhanced ozone concentrations following reactions (5) and (6). Tropospheric ozone accounts for about 10% of the total ozone.

Ozone affects human health (exposure to ozone can cause mucosal irritation, headache, reduced physical performance, reduction in resistance to infections and respiratory diseases particularly if the concentration is higher than 100 ppb). It is phytotoxic and causes damage to materials (e.g., it accelerates rubber cracking, fading of textile dye, loss of tensile strength in fabrics and the deterioration of works of art). In addition, tropospheric (including surface) ozone is a main oxidant in photochemical smog, a good indicator for air pollution. Thus, with all these effects, tropospheric ozone is bad.

Tropospheric ozone is also a greenhouse gas (GHG) as it absorbs upward directed terrestrial radiation. It plays a key role in atmospheric chemistry. As a strong oxidant, it affects the lifetimes and hence the concentrations of most atmospheric trace gases, including CH₄ and the replacements of the CFCs such as hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs), which, in turn, are greenhouse gases that have implications for global climate.

The relatively short lifetime of ozone has made it both a regional climate forcing mechanism and a relatively highly variable greenhouse gas, hence the need for frequent regional measurements of vertical profiles.

The UN Framework Convention on Climate Change (UNFCCC) calls for the stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. However, the major greenhouse gases focused in the UNFCCC and its Kyoto Protocol are carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF₆). Tropospheric ozone is the missing GHG in the UNFCCC equation. Perhaps it is understandable, as unlike other greenhouse gases, tropospheric ozone is unique in the sense that it is not emitted directly to the atmosphere, and hence its control is not very straight forward. Rather, as shown above, it is produced photochemically in the troposphere via its precursors, notably nitrogen oxides (NO_x), carbon monoxide (CO), CH₄ and non-methane hydrocarbons, all of which have large anthropogenic sources (e.g., fuel combustion; large scale of biomass burning, including forest and grassland fires in dry seasons, especially in the tropics and subtropics, etc.). Thus, any control measure regarding the reduction of tropospheric ozone must imply strong controls on the emissions of these precursors.

Long range transport of the above-mentioned precursors can cause large scale production of ozone far from the source regions, while the ozone produced can, in turn, through long range transport, influence the ozone concentration in large regions of the troposphere in both hemispheres. Since the atmospheric lifetime of NO_x increases strongly with altitude, hence the ozone production efficiency, this implies a possibility for a substantial increase in upper tropospheric ozone concentrations, and it is also in these altitude regions that ozone is most effective as a GHG, especially in the tropics (Ref. 2).

However, significant uncertainties remain in the budget of tropospheric ozone, its precursors, and the chemical and physical processes involved. Large spatial and temporal variability is observed in tropospheric ozone, resulting from important regional differences in the factors controlling its concentration. For example, recent assessment reveals that trends in tropospheric ozone since 1970 in the Northern Hemisphere show large regional differences, with increases in Europe and Japan, decreases in Canada, and only small changes in the U.S. (Ref. 3).

Earlier IPCC (Intergovernmental Panel on Climate Change) Assessment estimated that the radiative forcing of tropospheric ozone were of order a few tenths of W m^-2 (tentatively put at 0.2 to 0.6 W m^-2, which translates to much stronger forcing at the regional scale), making it of comparable importance to methane, which has an estimated radiative forcing of 0.5 W m^-2 (Ref. 4) However, the most recent scientific assessment has indicated that the global average radiative forcing due to increases in tropospheric ozone since pre-industrial times is 0.35± 0.15 W m^-2, which is about 10% to 20% of the forcing due to long-lived greenhouse gases over the same period (Ref. 3).

Article 4.1 (g) of the UNFCCC urges the Parties to "Promote and cooperate in scientific, technological, technical, socio-economic and other research, systematic observation and development of data archives related to the climate system and intended to further the understanding and to reduce or eliminate the remaining uncertainties regarding the causes, effects, magnitude and timing of climate change and the economic and social consequences of various response strategies;"

One of the major "remaining uncertainties regarding the causes, effects, magnitude and timing of climate change" is the contribution of tropospheric ozone as a greenhouse gas, as the global distribution of tropospheric ozone is still poorly known, especially in the tropics and subtropics where data are sparse. Thus, any research effort to reduce this major scientific uncertainty would be a useful contribution to our understanding of our climate system.

Recognizing the importance of tropospheric ozone in atmospheric chemistry and in climate change and the lack of capacity in developing countries in undertaking ozone research, Professor Paul Crutzen, a Nobel Laureate for Chemistry in 1995 for his pioneering work on the chemistry of ozone formation and decomposition, and Professor Henning Rodhe of Stockholm University, first proposed a global research programme known as International Tropospheric Ozone Years (ITOY) at the First Scientific Conference of IGAC (International Global Atmospheric Chemistry) of IGBP (International Geosphere Biosphere Programme) in April 1993. Since then, the ITOY concept and proposed activities have been further developed and widely supported by the international atmospheric scientific community.

The aim of the ITOY is to assess the global distribution of tropospheric ozone, its present and future, direct and indirect importance as a greenhouse gas, especially in the tropics and subtropics, where there has been a growing intensity of agricultural and industrial activities, and the emissions of tropospheric ozone precursors resulted from these activities could lead to the high potential of the photochemical production of tropospheric ozone. At these latitudes, enhanced photochemistry, strong convective processes, and absorption of radiation in the upper troposphere are major contributors that affect the global self-cleansing capability of the atmosphere and the Earth's climate. Yet in these regions, very little ozone measurement capability currently exists.

Thus, the ITOY programme will consist of two phases. Phase I aims to improve the tropospheric ozone observational basis of developing countries and build-up their capacity so that they are in a position to fully participate in the Phase II of the ITOY programme, which will include global measurements (mainly balloon-borne ozonesondes augmented by lidars where available and aircraft samplings, etc), supplemented by special measurement campaigns which would determine, among others, the atmospheric concentrations of compounds that are most critical for constraining the ozone budget, the rate of fast vertical transport, the tropospheric-stratospheric exchange of ozone, the quantities of NO_x that are produced by lightning, and the contribution of large scale biomass burning to the formation of tropospheric ozone (Refs 2, 5). The results of the global measurement programme will be used as inputs to validate and improve models, so that the actual role of tropospheric ozone as a greenhouse gas can be assessed and predicted (Refs 2, 5). The findings of this research could have significant implications for policy decisions especially in the implementation of the UNFCCC.

The ITOY programme will complement the efforts taking place within the WMO/Global Atmosphere Watch (WMO/GAW) which are aimed at the reliable detection of long-term trends in the global ozone distribution. Indeed, the Rio Conference in 1992 had called specifically for improvement for the WMO ozone system in the tropics.

The ITOY programme will also complement and contribute to the Global Tropospheric Ozone Project (GTOP) recently initiated by IGAC, chaired by Professor Guy Brasseur of NCAR (National Centre for Atmospheric Research). A comprehensive science plan is being developed by GTOP, which is led by Dr. Jack Fishman of NASA (National Aeronautics and Space Administration), and this plan will form the scientific basis for the ITOY programme.

In short, ITOY will serve as the observational centerpiece for GTOP, the purpose of which is to establish a truly global climatology of tropospheric ozone and to quantify the components of its global budget. The knowledge and insight obtained from ITOY for GTOP should provide the basis for a more reliable understanding of the factors that have led to changes in its abundance and distribution, as well as providing the observational foundation for future assessments and predictions.

Atmospheric scientists from both developed and developing countries will actively participate in the ITOY programme and GTOP, which will provide a framework for international coordination in tropospheric ozone research. In addition to the enhancement of observational capabilities as a means of gaining a more comprehensive understanding of the science, ITOY and GTOP will advocate for an awareness of the tropospheric ozone issue, and encourage education and capacity building for the next generation of scientists.

Currently, a project proposal entitled "Improve the observational basis for studies of the impact of tropospheric ozone on climate in developing countries and build-up of capacity as part of the global assessment of tropospheric ozone as a greenhouse gas" is being developed by UNEP, IGAC and WMO (World Meteorological Organization) and to be submitted to the Global Environment Facility (GEF) for funding. The major objective of the proposal is to establish 24 ozonesonde stations in 21 developing countries in the tropics and subtropics so that they are in a position to participate in the Phase II activities of the ITOY programme to be undertaken during the designated International Tropospheric Ozone Years (to be decided). The 24 stations have been identified and selected based on the scientific rationale and criteria discussed during the earlier ITOY planning meetings. The selected countries are located in three regions: Africa (Algeria, Botswana, Cape Verde, Côte d'Ivoire, Kenya, Sudan), Asia and the Pacific (Bangladesh, China, India, Malaysia, Papua New Guinea, Philippines, Solomon Islands, Vanuatu), and Latin America and the Caribbean (Argentina, Brazil, Chile, Costa Rica, Ecuador, Mexico, Venezuela). This list is still tentative and it will be finalized later by the Scientific Committee of ITOY with representatives from both developed and developing countries.

In order to complete the proper design and formulation of the GEF project proposal, a GEF PDF/A (Project Preparation and Development Facility/Block A) grant of US$25,000 is being applied. The GEF Research Committee, which was chaired by Professor Pier Vellinga, the former Chair of the Scientific and Technical Advisory Panel (STAP) of the GEF and participated by the representatives of the GEF Secretariat and the Implementing Agencies, as well as the current Chair of SBSTA (Subsidiary Body for Scientific and Technological Advice), Dr. Chow Kok Kee, endorsed the project concept in April 1998.

It is hoped that under the GEF targeted research window, seed money can be solicited from the GEF to fund the selected developing countries and build-up their scientific and technical capacity, so that their scientists can fully participate in this global tropospheric assessment project of great importance and significance.

Acknowledgement: This article has been kindly reviewed by Dr. Jack Fishman of NASA Langley Research Centre, Hampton, Virginia, USA, Professor Peter Brimblecombe, School of Environmental Sciences, University of East Anglia, Norwich, U.K. and Professor Paul Crutzen of Max Planck Institute for Chemistry, Atmospheric Chemistry Division, Mainz, Germany, who have kindly provided many useful comments. Dr. Fishman has also kindly provided additional information on GTOP.

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2. International Tropospheric Ozone Years, Global Change Newsletter No. 19, September 1994, pp. i-iv (pull-out section), IGBP, Stochholm.
3. Scientific Assessment of Ozone Depletion: 1998: Executive Summary (Full Report to be published by WMO).
4. IPCC (1994) Radiative Forcing of Climate Change: The 1994 Report of the Scientific Assessment Working Group of IPCC: Summary for Policymakers; UNEP and WMO.
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