Increase In Tropospheric Ozone Research Articles

Sulphate particles from subsonic aviation: impact on upper tropospheric and lower stratospheric ozone

An assessment study is presented here on the impact of sulphate particles emitted by subsonic aircraft. The strategy has been to design a tracer fuel experiment to deduce a first approximation of the surface area density change of sulphate aerosols produced by subsonic aviation. Five three-dimensional chemical-transport models (CTM) have run this tracer experiment. Effects on upper tropospheric - lower stratospheric (UTLS) chemistry have then been studied with appropriate full chemistry runs performed by three CTMs. The participating models are from: University of L'Aquila, University of Utrecht, University of Cambridge, University of Oslo, Deutsches Zentrum für Luft- und Raumfahrt, Meteo - France. The calculated average aerosol perturbation has a maximum of approx. 0.8 × 10−8 cm−1 at an altitude of 11 km and poleward of 50°N and shows some upward transport into the lowermost stratosphere from the upper troposphere: physical and possible numerical reasons for this behaviour are discussed in the paper. This effect has implications on the chemical budget of the UTLS region, making heterogeneous chemical reactions faster. The magnitude of ozone changes is found to be comparable but smaller (up to 60% globally) than that produced by NOx emissions from subsonic aircraft, and of the opposite sign. Sulphate particle formation from subsonic aircraft would then have the overall effect of lowering the upper tropospheric ozone increase produced by the additional NOx from the aircraft.

Ozone/biotic disease interactions: molecular biomarkers as a new experimental tool

Current climate change scenarios predict a further increase of tropospheric ozone which is well known to inhibit plant photosynthesis and growth processes. Ozone can also predispose plants to enhanced biotic attack, as proposed in particular for necrotrophic fungi, root-rot fungi and bark beetles. However, at present it does not seem possible to predict whether increased ambient ozone will lead to a higher or lower disease likelihood in particular plant–pathogen systems. It has been stated repeatedly in the literature that periods of high ambient ozone are essentially non-coincident with infection periods of most fungal pathogens. This implies minimal interactive risks. However, it now appears that the various ozone-induced metabolic changes can persist in plants over days or months. Visible ozone symptoms also may be greatly delayed. Certain stress transcripts, proteins and metabolites have been developed as ozone biomarkers in controlled exposure experiments, but these biomarkers remain to be examined on field sites. A simple epidemiological scenario based on ‘memory’ time spans of ozone effects is proposed as a tool to make ozone–plant disease interactions more predictable.

Recent changes observed in column ozone concentrations over India

Ozone observations taken during the past 23-39 years by Dobson Spectrophotometers at Delhi, Varanasi, Pune and Kodikanal have been analysed to examine its long-term trend over Indian stations. An increasing trend of this species over the years has been noticed at all the places, except at Varanasi, where a decreasing trend has been found. The cause of these trends could be attributed, partly, to the trends of ozone in the troposphere. The results also indicate that there are certain recent changes in ozone levels at the Indian stations. These changes are less apparent in long-term trend analysis of total ozone data as the increase in tropospheric ozone has a compensating effect to the decrease in ozone at stratospheric levels.

Total ozone increase associated with forest fires over the Indonesian region and its relation to the El Niño-Southern oscillation

Significant increases of total ozone were observed both by the total ozone mapping spectrometer (TOMS) and by the Brewer spectrophotometer in Indonesia in September and October of 1994 and 1997, during the El Niño periods, when extensive forest fires were reported in Sumatra Island, Kalimantan (the southern part of Borneo Island) and south New Guinea. The two observations were consistent with each other, and the total ozone increases were attributed to the tropospheric ozone increases because their amplitudes agreed with those of integrated tropospheric ozone increases derived from ozonesonde observations. The TOMS data indicated that the horizontal distributions and temporal variations of the ozone increases were similar in both years; the ozone increases were found mainly over Sumatra Island and the Malay Peninsula in September, and spread out from Kalimantan to the central Indian Ocean in October. This ozone distribution was partly different from the reported fire areas. This difference suggested the importance of the horizontal advection due to the easterly wind in the lower troposphere and of the vertical transport due to the upward wind at the west of Sumatra Island, in the ozone maximum area. Distinctive total ozone increases similar to those in 1994 and 1997 repeatedly appeared over the Indonesian region in the TOMS data between 1979 and 1998. The average ozone increase in this region was estimated by subtracting the background structure of total ozone in the tropics, and this analysis showed that large ozone increases mostly occurred in the dry season during the El Niño periods when the precipitation decreased significantly and extensive forest fires occurred frequently in Indonesia.

Catalyst Enhanced Oxidation of VOCs and Methane in Cold-Plasma Reactors

Volatile organic compounds, VOCs, are used in a number of industrial processes in a variety of roles, for instance as solvents, cleaners and liquid fuels. During these processes, some of the VOCs, which are known to be air toxins, inevitably escape into the surrounding air. It is the release of this VOC-laden air into the atmosphere that is partly responsible for stratospheric ozone depletion, tropospheric ozone increase, global warming (the Greenhouse Effect) and some health problems. Here, we briefly review some applications of platinum metals catalysts in the abatement of VOCs and in the conversion of methane using cold-plasma techniques. Some information on the abatement of acid gases, the generation and application of non-thermal plasmas is also presented.

Airborne Particulate Matter

Philip H. Abelson's editorial “Airborne particulate matter” (11 Sept., p. [1609][1]) raises timely questions about the Environmental Protection Agency's (EPA's) proposed standards regarding airborne particulate matter (PM). The EPA is preparing an expensive program to sample and weigh particulates having a size of less than 2.5 micrometers (PM 2.5) at 1500 locations ([1][2]). Chemical analysis of particulates will be performed at only 300 sites. As observed by Abelson, the weight of sampled particulates reveals nothing about their composition and origin. Thus, considerable inequities will arise when the EPA begins to enforce its proposed PM regulations. For example, almost daily aerosol optical thickness measurements from South Texas since 1989 reveal various particulate events of distant origin superimposed on the seasonal cycle. These events, which are confirmed by satellite images, include haze from the south central United States, volcanic eruptions, significant smoke from Mexico and Central America, and dust events from Africa, China, and Mexico. During May 1998, smoke from major forest fires in Mexico caused severe air pollution over much of Texas and other states. The smoke, which at my site caused significant optical depth (4.2 at 540 nanometers on 14 May) and reduced visibility (less than 1.5 kilometers on 14 May), was associated with increased tropospheric ozone, a phenomenon that caused violations of EPA air-quality standards in San Antonio and elsewhere. Although tropospheric ozone increases have been observed during burning seasons in Africa ([2][3]) and Brazil ([3][4]), officials in Texas have thus far failed to persuade the EPA to discount the ozone violations over which Texas cities had no control. How will a single national PM 2.5 standard affect communities that have no control over the particulates that arrive from distant sources? Will Florida and Texas be penalized by the EPA when dust arrives each spring from the Sahara? Will Hawaii and the western states be penalized when dust arrives from the Gobi desert? Will Texans be penalized when smoke arrives from Central America? In short, will citizens downwind of forest fires, volcanic eruptions, dust storms, and coal-fired power plants be penalized for PM 2.5 violations beyond their control? When the lives and commerce of citizens are regulated by scientific measurements, the citizens and businesses who are taxed to pay for such observations have every right to expect that the science behind the measurements will be appropriate. As Abelson observes, the EPA's plan to measure PM 2.5 particulates is inadequate. It is time that Congress follows Abelson's suggestion that the National Academy of Sciences address the matter. 1. [↵][5]Overview of National PM 2.5 Network, 1998 (Washington, DC, 1998) (Environmental Protection Agency, [www.epa.gov/ttnamti1/supsites.html][6]). 2. [↵][7]1. A. M. Thompson 2. et al. , J. Geophys. Res 101, 793 (1996) 23,. [OpenUrl][8] 3. [↵][9]1. J. Fishman, 2. V. G. Brackett, 3. E. V. Brownell, 4. W. B. Grant , ibid p. 24,069. [1]: /lookup/doi/10.1126/science.281.5383.1609 [2]: #ref-1 [3]: #ref-2 [4]: #ref-3 [5]: #xref-ref-1-1 View reference 1 in text [6]: http://www.epa.gov/ttnamti1/supsites.html [7]: #xref-ref-2-1 View reference 2 in text [8]: {openurl}?query=rft.jtitle%253DJ.%2BGeophys.%2BRes%26rft.volume%253D101%26rft.spage%253D793%26rft.atitle%253DJ%2BGEOPHYS%2BRES%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [9]: #xref-ref-3-1 View reference 3 in text

Changes in ultraviolet radiation due to stratospheric and tropospheric ozone changes since preindustrial times

This paper estimates global changes in ultraviolet radiation since preindustrial times, resulting from anthropogenic stratospheric and tropospheric ozone changes. A sophisticated radiative transfer scheme is employed to calculate changes in the diurnally integrated UVB (defined here as 280–320 nm) and erythemally weighted (EER) irradiance reaching the Earth's surface. Stratospheric ozone changes are obtained from combining total ozone mapping spectrometer satellite data with Dobson ground‐based instrument data. Tropospheric ozone changes are estimated from the University of Oslo three‐dimensional chemical transport model. These ozone trend data are combined in three ways to calculate the associated ultraviolet trend and estimate its uncertainty. We show that in low latitudes a significant (up to 5% for UVB and 9% for EER) decrease in surface UV may have occurred, due to tropospheric ozone increases. Large UV increases are found at high latitudes, with up to 60% increases in October and 20% increases in April. The possible effects of soot and sulphate aerosol changes on the UV are also briefly examined; increased aerosol amounts may have decreased surface UVB and EER by up to 7% locally and about 2% on a global average.

Impact of aircraft NO x emissions on tropospheric and stratospheric ozone. part II : 3-D model results

The global three-dimensional dynamic-chemical model ECHAM3/CHEM is employed to estimate the impact of present and future sub- and supersonic aircraft NO x emissions on ozone. Multinual simulations are performed to investigate changes of large-scale climatological features. An increase of tropospheric ozone of 3–4% is found in the Northern Hemisphere for a present day scenario, independently from season. No indications for significant ozone changes are indicated in the lower stratosphere. NO x emissions expected for the year 2015 by sub- and supersonic (500 civil aircraft, Mach 2.4, EI NO x =15) air traffic yield much larger ozone changes in the model. An ozone increase of approximately 15% is predicted near the cruise altitude of future subsonic air traffic. This is accompanied by a clear ozone reduction of 3–4% in the lower stratosphere caused by supersonic aircraft. A stronger decrease of ozone is detected in polar winter due to the impact of heterogeneous reactions on polar stratospheric clouds.

Impact of aircraft emissions on tropospheric and stratospheric ozone. Part I: chemistry and 2-D model results

The impact of air-traffic-induced NO x and water vapor emissions on the chemical composition of the global troposphere and stratosphere is investigated for current conditions (1991) and a future scenario (2015). The NO x dependence of ozone chemistry is studied using photochemical steady-state calculations for a typical upper tropospheric chemical composition. These calculations demonstrate that above a critical NO x mixing ratio of about 0.3 ppbv, additional NO x emitted by aircraft can actually decrease the net ozone production, whereas below this value there is the commonly accepted increase in ozone production. Subsequently, we assess the impact of aircraft emissions on photochemical ozone production using the Mainz two-dimensional photochemical model including effects of heterogeneous chemistry in the lower stratosphere. Based on not well-represented convection, 2-D models generally underestimate background values of NO x in the free troposphere, hence overestimate the ozone increase caused by subsonic aircraft. In particular, convection might shift the NO x mixing ratio above the critical 0.3 ppbv level. To correctly reproduce the impact of this non-linear relation on ozone, a 3-D model calculation is essential, especially for mid-latitude summer, where significant convection take place. For northern winters, where due to weak convection the 2-D calculations are most appropriate, current aircraft emissions are calculated to yield a tropospheric ozone increase of about 3% and little effect on stratospheric ozone. For the case of installation of 500 commercial supersonic transport in the year 2015 (flight altitude 18–21 km, cruise speed Mach 2.4, emission index 15 g NO 2 kg -1 fuel), ozone decreases of 3% in the lower polar stratosphere are predicted leading to decreases in ozone columns by up to 1.5%.

Estimates of radiative forcing due to modeled increases in tropospheric ozone

The GFDL R30 general circulation model (GCM) and a fixed dynamical heating model (FDHM) are used to assess the instantaneous and adjusted radiative forcing due to changes in tropospheric ozone caused by anthropogenic activity. Ozone perturbations from the GFDL global chemical transport model are applied to the GCM, and the instantaneous solar and terrestrial radiative forcings are calculated excluding and including clouds. The FDHM is used to calculate the adjusted radiative forcing at the tropopause. The net global annual mean adjusted radiative forcing, including clouds, ranges from +0.29 to +0.35 W m−2 with ∼80% of this forcing being in the terrestrial spectrum. If stratospheric adjustment is ignored, the forcing increases by ∼10%, and if clouds are ignored, the radiative forcing increases by a further 20–30%. These results are in reasonable agreement with earlier studies and suggest that changes in tropospheric ozone due to anthropogenic emissions exert a global mean radiative forcing that is of similar magnitude but of opposite sign to the direct forcing of sulfate aerosols.

Stratosphere‐troposphere ozone exchange observed with the Indian MST radar and a simultaneous balloon‐borne ozonesonde

The stratosphere‐troposphere (ST) mode operation of the Indian mesosphere‐stratosphere‐troposphere (MST) radar provided evidence of multiple stable layer structures near the tropopause and also of its “weakening” during late night hours on several occasions. Mass exchange can take place between the troposphere and the stratosphere during periods of such weakening. To examine whether there is any transport of ozone from the stratosphere to the troposphere at the time of tropopause weakening at tropical latitudes, simultaneous observations were carried out using the Indian MST radar located at Gadanki (13.5°N, 79.2°E), now fully operational in MST mode, and ozonesonde flights from Trivandrum (8.9°N, 76.6°E). Four campaigns of simultaneous observations were conducted during January 3–8, November 9–16, and December 5–10, 1994, and June 7–24, 1995. The results show occurrence of the tropopause weakening several times during January 3–8, November 9–16, and December 5–10, 1994. Simultaneous observations of ozone profiles, particularly on January 7–8, November 10 and 16, and December 7–8, 1994, the days of tropopause weakening, show evidence of a decrease in stratospheric ozone and a corresponding increase in tropospheric ozone after this event, the total ozone remaining essentially constant on these days with respect to other days. Tropopause weakening on January 4–5, 1994, however, does not show any considerable change in the ozone profiles. The results are presented here relating the degree of tropopause weakening to the extent of vertical mass exchange between the troposphere and the stratosphere. An effort has been made to examine the role of horizontal advection of ozone in increasing tropospheric ozone at the region of interest; the geographical distribution of tropospheric ozone residual (TOR) has been analyzed by the two data sets obtained independently from Meteor 3 total ozone mapping spectrometers (Meteor 3 TOMS) and the Upper Air Research Satellite (UARS). Simultaneously, the possibility of horizontal transport of ozone from the areas surrounding Tirupati and Trivandrum in increasing tropospheric ozone due to synoptic scale circulation has also been verified by close inspection of synoptic weather charts at 1000, 700, 500, and 300 hPa obtained from the National Center for Medium‐Range Weather Forecasting general circulation model at 0000 and 1200 UTC on January 7 and 0000 and 1200 UTC on January 8, 1994.

Lower tropospheric ozone trend observed in 1989–1997 at Okinawa, Japan

In order to elucidate recent tropospheric ozone trends in Northeast Asia, 8 year‐long ozone sounding data obtained between 1989 and 1997 at Naha (Okinawa Island), Japan, were analyzed incorporating with backward trajectory categorization. Focusing on the regionally polluted continental outflow, only data associated with air masses that reached Naha from northern, northwestern, and western directions during autumn/winter/early spring seasons (from October to March) were selected for analysis. The concentration of ozone shows an increase of about 2.5±0.6% (one standard deviation) per year for a 0‐2 km layer from the ground representing the planetary boundary. The surface ozone concentrations obtained at Cape Hedo (northern tip of Okinawa Island) at the same timing as the selected ozone sounding also showed an increasing trend of 2.6% per year but with a large statistical uncertainty of ±2.0%. This result is in accordance with the 22‐year period (1969–1990) of tropospheric ozone trends, 1.5–2.5% per year, obtained previously at three other Japanese ozone sounding stations, Kagoshima, Tsukuba, and Sapporo [Akimoto et al. 1994], demonstrating the continuous increase of tropospheric ozone in Northeast Asia in 1990s. It is suggested that this tropospheric ozone trend would be related to the increasing emission of NOx from Northeast Asian region (China, Japan, South Korea, Taiwan) during this period with a rate of 3.9% per year.

Trend analysis of the homogenized total ozone series of Arosa (Switzerland), 1926–1996

Total ozone measurements have been made at Arosa, Switzerland (47°N), from 1926 through the present day, forming the longest total ozone series in the world. The record has been recently homogenized. Ozone trends are calculated to be −(2.3±0.6)% per decade for annual means (larger losses are found in winter and spring, approximately −4% per decade for trends in January, February, and March) when a simple linear change from 1970 to 1996 is assumed. In addition, total ozone trends are calculated using multiple regression models involving combinations of explanatory variables for the 11‐year solar cycle, local meteorological conditions (the Mount Säntis high‐altitude temperature record), stratospheric aerosol loading from volcanoes, and stratospheric chlorine loading. When the terms for the solar cycle, the stratospheric aerosol loading and the high mountain temperature record were included, the annually averaged ozone trends were found to be −(1.9±0.6)% per decade. While a statistically significant relation is found between total ozone and indices of aerosol loadings of the stratosphere, the recent decrease in total ozone cannot be accounted for by the higher average aerosol content in the second half of the century. Finally, the decrease in ozone in the stratosphere is estimated to be approximately 30% larger than that found for total ozone, when a crude estimate of the increase in tropospheric ozone is included. The acceleration observed in total ozone trends between the 1970s and the 1980s over northern midlatitudes [e.g., Harris et al., 1997] might be partially attributed to the larger increase in tropospheric ozone in the 1970s.

Effects of anthropogenic emissions on tropospheric ozone and its radiative forcing

Tropospheric ozone changes since preindustrial times due to changes in emissions have been calculated by the University of Oslo global three‐dimensional photochemical model. The radiative forcing caused by the increase in ozone has been calculated by two independent radiative transfer models; the University of Reading model (Reading), and the University of Oslo/Norwegian Institute for Air Research model (OsloRad). Significant increases in upper tropospheric ozone concentrations are found at northern midlatitudes (15–40 ppbv, depending on season) at about 10 km altitude. In the tropical regions the largest increase (about 20 ppbv for all seasons) is found at about 15 km altitude. The increase is found to be caused mainly by enhanced in situ production due to transport of precursors from the boundary layer, with a smaller contribution from increased transport of ozone produced in the boundary layer. The lifetime of ozone in the troposphere decreased by about 35% as a result of enhanced concentrations of HO2. The calculated increase in surface ozone in Europe is found to be in good agreement with observations. The calculations of radiative forcing include the effect of clouds and allow for thermal adjustment in the stratosphere. The global and annual averaged radiative forcing at the tropopause from both models (0.28 W m−2 and 0.31 W m−2, for the Reading and OsloRad models, respectively) are in the lower part of the Intergovernmental Panel on Climate Change [1995] estimated range. The calculated radiative forcing is similar in magnitude to the negative radiative forcing by sulfate aerosols, but displaced southward in source regions at northern midlatitudes. The increase in tropospheric ozone is calculated to have cooled the lower stratosphere by up to 0.9 K, with possibly half of this cooling occurring in the past 2 to 3 decades.

A new approach to the characterization of long-term changes in total atmospheric ozone: Determination and application of frequency distributions: Discussion

The extreme value analysis in the subject paper may be summarized as follows: More low values are observed during one 6 yr period than another, so there has been a change in the frequency of low values. The changes are not significant. The situation for distributional analysis is more complex. Had the distributional analysis simply provided information on the changes in the means and standard deviations of the total ozone values and presented the changes in the frequency of various ranges of these values from year to year, it would have been an interesting data article. Instead the authors attempted to relate their results to tropospheric ozone increases and stratospheric decreases. The posited relations are either trivial or not supported by their discussions. The methods have been misapplied and the results of their treatment of the data have been misinterpreted.

A 3D chemistry transport model study of changes in atmospheric ozone due to aircraft NO x emissions

The effect of present day aircraft emissions of nitrogen oxides (NO x = NO + NO 2) on atmospheric NO x and ozone concentrations is investigated with the global three-dimensional chemistry transport model CTMK. This model uses 12-hourly meteorological data from the ECMWF analysis and includes parameterizations for subgrid scale processes such as convection. CTMK includes an ozone chemistry module containing the methane and carbon monoxide oxidation chain for the troposphere and lower stratosphere. It is found by using the ANCAT aircraft NO x emission invertory that aviation contributes to 20–100 pptv of the NO x at cruise altitudes in northern mid-latitudes, which corresponds to 30–80 and 20–50% of the background mixing ratios for January and July, respectively. This perturbation in NO x occurs mainly in the North Atlantic Flight Corridor and is transported eastwards. The resulting increase in upper tropospheric ozone is 2–3 ppbv (2%) in January and 5–10 ppbv (3%) in July. The ozone perturbation is almost zonally symmetric and attains maximum values at northern mid-latitudes in January and in the polar region in July. The calculated effect of aircraft emissions is found to be small (i.e. less than 5 pptv for NO x and less than 1 ppbv for 0 3) in the southern hemisphere. The perturbation of NO x by the aircraft emissions at cruise altitudes in northern mid-latitudes is large compared to the standard deviation. Therefore, it is expected that the effect of aviation on NO x is distinguishable from the contribution from other NO x sources. As the modelled natural variability of ozone is already about 30%, it will not be easy to detect the ozone perturbation due to aircraft NO x emissions.

Radiative forcing and climate response

We examine the sensitivity of a climate model to a wide range of radiative forcings, including changes of solar irradiance, atmospheric CO2, O3, CFCs, clouds, aerosols, surface albedo, and a “ghost” forcing introduced at arbitrary heights, latitudes, longitudes, seasons, and times of day. We show that, in general, the climate response, specifically the global mean temperature change, is sensitive to the altitude, latitude, and nature of the forcing; that is, the response to a given forcing can vary by 50% or more depending upon characteristics of the forcing other than its magnitude measured in watts per square meter. The consistency of the response among different forcings is higher, within 20% or better, for most of the globally distributed forcings suspected of influencing global mean temperature in the past century, but exceptions occur for certain changes of ozone or absorbing aerosols, for which the climate response is less well behaved. In all cases the physical basis for the variations of the response can be understood. The principal mechanisms involve alterations of lapse rate and decrease (increase) of large‐scale cloud cover in layers that are preferentially heated (cooled). Although the magnitude of these effects must be model‐dependent, the existence and sense of the mechanisms appear to be reasonable. Overall, we reaffirm the value of the radiative forcing concept for predicting climate response and for comparative studies of different forcings; indeed, the present results can help improve the accuracy of such analyses and define error estimates. Our results also emphasize the need for measurements having the specificity and precision needed to define poorly known forcings such as absorbing aerosols and ozone change. Available data on aerosol single scatter albedo imply that anthropogenic aerosols cause less cooling than has commonly been assumed. However, negative forcing due to the net ozone change since 1979 appears to have counterbalanced 30–50% of the positive forcing due to the increase of well‐mixed greenhouse gases in the same period. As the net ozone change includes halogen‐driven ozone depletion with negative radiative forcing and a tropospheric ozone increase with positive radiative forcing, it is possible that the halogen‐driven ozone depletion has counterbalanced more than half of the radiative forcing due to well‐mixed greenhouse gases since 1979.

Vertical ozone distribution over southern Africa and adjacent oceans during SAFARI‐92

A set of four ozonesonde stations located at Ascension Island, Brazzaville, Okaukuejo, and Irene, operational during the TRACE A and SAFARI‐92 experiments has provided an opportunity to investigate the vertical distribution of ozone over southern Africa and adjacent oceans. All stations display a springtime maximum in tropospheric ozone. Enhanced tropospheric ozone, which occurs between June and September at Brazzaville and between July and October at Ascension Island, is linked to dry season biomass burning. The influence of tropical biomass burning is delayed until September at Okaukuejo when a sharp increase in tropospheric ozone is experienced. The biomass burning influence at Irene is less because of its more southerly location. A general tropospheric enhancement is observed at all stations. It is manifest as an enriched layer in the upper troposphere at Okaukuejo (9–12 km) and Brazzaville (11–14 km) and in the lower troposphere (2–8 km) at Ascension Island. At Ascension Island lower tropospheric ozone values are about 20 parts per billion by volume greater than elsewhere and the tropospheric component here accounts for about 18% of the total column ozone. A series of tethersonde soundings conducted at hourly intervals at Okaukuejo revealed ozone to be well mixed in the lower boundary layer during the day, but to display marked vertical stratification at night.

Radiative forcing due to increased tropospheric ozone concentrations

Increasing tropospheric ozone concentrations have been observed in the past decades in industrialized and remote areas of the Northern Hemisphere. Since ozone absorbs both solar and infrared radiation, several studies concerning the tropospheric ozone-climate problem have been recently conducted mainly with one- and two-dimensional models. In this study, pre-industrial and present-day tropospheric ozone concentrations simulated by a three-dimensional Chemistry Transport Model (3-D CTM) IMAGES (Intermediate Model for the Annual and Global Evolution of Species) are used in conjunction with the Laboratoire de Météorologie Dynamique General Circulation Model (LMD GCM) to determine the ozone radiative forcing since the pre-industrial era. We find that the ozone forcing is regionally heterogeneous with a marked interhemispheric difference and a strong seasonal variation, peaking over the Northern Hemisphere continents during summer and reaching locally more than 1 W m −2. Sensitivity simulations confirm that the major contributions to the tropopause forcing arise from ozone changes occurring in the high troposphere. Changes of ozone concentration in the planetary boundary layer are about 10 times less efficient than in the high troposphere in terms of radiative perturbation. These 3D results also confirm the quasi-linear relationship between the radiative forcing and the tropospheric ozone increase for both hemispheres. Some previsions of future forcing change considering a critical constant global rate of ozone increase equal to 10% per decade and the IS92a IPCC emission scenario are realized for the next century.

CLIMATE IMPLICATIONS OF THE OBSERVED CHANGES IN OZONE VERTICAL DISTRIBUTION

Satellite and ozone-sonde observations indicated a decreasing trend in the ozone concentration in the lower stratosphere and an increasing trend in the troposphere, especially the upper troposphere. We have used a one-dimensional radiative – convective model (RCM) to examine the climate implications of these observed changes in vertical distribution of ozone. Instantaneous radiative forcing calculated by the one-dimensional radiative transfer model indicated a net warming of the surface–troposphere system due to stratospheric ozone losses. The increase in tropospheric ozone causes an additional positive forcing. However, the radiative forcing due to increases in trace gases during the same period is larger than the forcing due to ozone changes. There is marked decrease of the instantaneous solar and longwave heating rates in the lower stratosphere due to stratospheric ozone losses. Increases in tropospheric ozone causes additional decrease in longwave heating rate, in the lower stratosphere. Consistent with the changes in the heating rates, the equilibrium temperature profile computed by the radiative–convective model predicted a cooling in the stratosphere of the order of 0ċ6°C and a very slight warming in the troposphere. The increase of tropospheric ozone causes an additional cooling in the stratosphere. The temperature decrease caused by ozone decrease in the lower stratosphere is larger than that caused by increase of trace gases concentration during the same period. The stratospheric cooling effect due to trace gases is, however, better presented near 30 km altitude. Note: Erratum to this article printed: does change abstract Volume 16 Issue 8 pp 959