Measurements of trace gases in the tropical tropopause layer

A unique dataset of airborne in situ observations of HCl, O₃, HNO₃, H₂O, CO, CO₂ and CH₃Cl has been made in and near the tropical tropopause layer (TTL). A total of 16 profiles across the tropopause were obtained at latitudes between 10°N and 3°S from the NASA WB-57F high-altitude aircraft flying from Costa Rica. Few in situ measurements of these gases, particularly HCl and HNO₃, have been reported for the TTL. The general features of the trace gas vertical profiles are consistent with the concept of the TTL as distinct from the lower troposphere and lower stratosphere. A combination of the tracer profiles and correlations with O₃ is used to show that a measurable amount of stratospheric air is mixed into this region. The HCl measurements offer an important constraint on stratospheric mixing into the TTL because once the contribution from halocarbon decomposition is quantified, the remaining HCl (>60% in this study) must have a stratospheric source. Stratospheric HCl in the TTL brings with it a proportional amount of stratospheric O₃. Quantifying the sources of O₃ in the TTL is important because O₃ is particularly effective as a greenhouse gas in the tropopause region.     

Beryllium-7 Measurements in the Houston and Phoenix Urban Areas: An Estimation of Upper Atmospheric Ozone Contributions

Abstract

Natural radionuclides have been proposed as a means of assessing the transport of ozone (O₃) and aerosols in the troposphere. Beryllium-7 (⁷Be) is produced in the upper troposphere and lower stratosphere by the interaction of cosmogenic particles with atmospheric nitrogen and oxygen. ⁷Be has a 53.29-day half-life (478 keV γ) and is known to attach to fine particles in the atmosphere once it is formed. It has been suggested that O₃ from aloft can be transported into rural and urban regions during stratospheric–tropospheric folding events leading to increased background levels of O₃ at the surface. ⁷Be can be used as a tracer of upper atmospheric air parcels and the O₃ associated with them. Aerosol samples with a 2.5-µm cutoff were collected during 12-hr cycles (day/night) for a 30-day period at Deer Park, TX, near Houston, in August– September of 2000, and at Waddell, AZ, near Phoenix, in June–July of 2001. A comparison of ⁷Be levels with 12-hr O₃ averages and maxima shows little correlation. Comparison of nighttime and daytime O₃ levels indicate that during the day, when mixing is anticipated to be higher, the correlation of ⁷Be with O₃ in Houston is approximately twice that observed at night. This is consistent with mixing and with the anticipated loss of O₃ by reaction with nitric oxide (NO) and dry deposition. At best, 30% of the O₃ variance can be explained by the correlation with ⁷Be for Houston, less than that for Phoenix where no significant correlation was seen. This result is consistent with the intercept values obtained for ⁷Be correlations with either O₃ 24-hr averages or O₃ 12-hr maxima and is also in the range of the low O₃ levels (25 ppb) observed at Deer Park during a tropical storm event where the O₃ is attributable primarily to background air masses. That is, maximum background O₃ level contributions from stratospheric sources aloft are estimated to be in the range of 15–30 ppb in the Houston, TX, and Phoenix, AZ, area, and levels above these are because of local tropospheric photochemical production.     

The Impact of Stratospheric Ozone on Tropospheric Air Quality

A background of ozone (O₃), principally of stratospheric origin, is present in the lower free troposphere. Typical mean O₃ levels of 50 ppb, 40 ppb, and 30 ppb are encountered here in spring, summer, and fall, respectively. Maximum hourly O₃ concentrations which are twice these mean values can be expected. Ozone from the free troposphere is routinely brought down to ground level under turbulent atmospheric conditions. Deep and rapid Intrusions of stratospheric air into the lower troposphere are associated with low-pressure troughs and occur regularly. In the mid troposphere, O₃ levels as high as 300 ppb are found within these intrusions. Observational data showing these intrusions, containing high O₃ concentrations, to directly reach ground level are currently lacking. Over the United States, an intrusion was present aloft on 8 9% of the days in 1978. The frequency, however, is somewhat reduced in summer and a northward movement is evident. During 1978, no intrusion occurred south of 30°N between June and August and none south of 40 °N in August.

The hypothesis that low levels of stratospheric O₃ produce disproportionately large amounts of O₃ in the polluted atmosphere cannot be supported from currently known chemistry but should be studied further. The experimental technique involving a ⁷Be/O₃ ratio to estimate the daily stratospheric component of ground level O₃ is unverified and considered to be inadequate for air quality applications. Estimates resulting from such a technique are considered uncertain by a factor of more than three. Specially designed aircraft studies provide the best means to determine quantitatively the impact of stratospheric O₃ on ground level air quality.     

What is the lesson from the unprecedented event over antarctica in 2002

Varotsos (2002a,b), suggested that both the smaller-sized ozone hole over Antarctica and its splitting in two holes in September 2002 occurred due to an unprecedented major sudden stratospheric warming caused by very strong planetary waves propagated in the southern hemisphere. Subsequently, a NASA press release of December 6, 2002, also reported the prevalence of very strong planetary waves in Antarctica. The aim of this Letter is to further discuss the morphology of the Antarctic ozone hole, to detect the causes that allowed the Antarctic stratosphere to exhibit this exceptional warming and to examine what it denotes about its mechanisms. Concerning the morphology, among the principal findings is that the ozone hole split occurred not only in the stratosphere but extended in the lower altitudes (upper troposphere). As to the causes of the major sudden stratospheric warming of 2002, a comparison with the previous warmings in Antarctica since 1964 is made. The smaller-sized Antarctic ozone hole of 2002 is approximately equal to that of 1988 when a strong sudden stratospheric warming occurred. If only the destruction of ozone by chlorofluorocarbons resulted in the delayed sudden stratospheric warmings in Antarctica, then the early sudden stratospheric warmings of 1988 and 2002 would not have occurred, since chlorofluorocarbon loading of the stratosphere has remained relatively stable in recent years. Furthermore, it appears that the El Nino characteristics in 1988 and 2002 are not similar.     

On the forest fires and the analysis of air quality data and total atmospheric ozone

In Canada about 1.3 million hectares (M ha) of forests are destroyed by wildfires each year, and about 63 % of all these fires are man-caused. During the 1980 and 1981 fire seasons, however, about 10 M ha were damaged; estimated annual emissions from forest fires were ~ 224 million tonnes (M t) of CO₂; and over 22 M t of CO, total suspended particulates (TSP), hydrocarbons (HC), nitrogen oxides (NO_x), etc.One of the major problems resulting from these forest fires was the severe reduction of visibility over large areas. Daily values of TSP recorded at Fort McMurray, Alberta were in the range of 163–257 μg m⁻³, while TSP observed at Edmonton, about 850km downstream from large fires, were in the range of 134–220 μg m⁻³. Nevertheless, surface ozone (O₃) and total O₃ in vertical air columns had evidently decreased in the area affected by smoke plumes. It is plausible that the O₃ depletion might have occurred in the lower troposphere from the overwhelming existence of forest fire smoke in the region.     

The radiative effect of atmospheric pollution: Latitudinally dependent radiation calculations including cloud contribution

In this paper, we have developed a radiation scheme based on the discrete-ordinate method into which a comparatively thick cloud layer can be incorporated for inhomogeneous aerosol atmospheres. Using the above radiation scheme, we performed calculations of the effect of clouds upon the inhomogeneous aerosol atmospheres, including the effects of optical depth, vertical distribution and extent of cloud layer. Calculations were also carried out for the local albedo, total absorption and diffuse transmission at each latitude belt, considering a land-sea distribution and a latitudinal variation of cloudiness, for six realistic model aerosol atmospheres. A 0.50 μm flux of the solar radiation is used for this study. The main conclusions of the radiative calculations may be summarized as follows.

• 1.(1) For the same optical thickness, a densely stratified cloud layer within the lower aerosol troposphere is apt to reflect the solar radiation much less effectively than other stratifications of cloud. If cloud layers are present at higher levels, absorption of solar radiation within the atmosphere would decrease considerably.
• 2.(2) Cloudiness and/or cloud thickness play a very important role upon the global heat balance problems and should never be ignored in studying the effects of increased aerosols upon climate, because contrary to the cloudless condition, heating of the earth-atmosphere system would tend to be induced by an increase of aerosols in the atmosphere where clouds are present. Because of the high surface albedo, an increase of aerosols reduces the reflectivity at the snow-covered high latitude belts, regardless of the effects of cloud.
• 3.(3) By the perturbation of adding aerosols into the troposphere or the stratosphere the diffuse transmission increases at the ground level. However, this effect is offset by the direct exponential attenuation of solar flux, and as a result, the total solar radiation reaching the ground is somewhat reduced.
• 4.(4) The absorption of solar radiation within the atmospheres due to aerosols would reach near 10% of incident solar flux for injection of a fairly massive amount of aerosols into the troposphere or the stratosphere due to great volcanic eruptions or man's impact.

     

Estimation of the Natural Background of Ozone Present at Surface Rural Locations

The natural background in the ozone concentration at rural locations in the United States and western Europe has been estimated by use of several approaches. The approaches utilized include the following: (1) historical trends in ozone concentration measurements, (2) recent ozone measurements at remote sites, (3) use of tracers of air originating in the stratosphere or upper troposphere and (4) results from applications of tropospheric photochemical models. While each of these approaches has its own limitations it appears that the natural background of ozone during the warmer months of the year is in the range of 10 to 20 ppb. Most of the ozone originating in the lower stratosphere or upper troposphere is lost by chemical or physical removal processes as well as undergoing dilution by air in the lower troposphere before reaching ground level rural locations. Lower tropospheric photochemical processes, those below 5 km, are likely to account for most of the ozone measured at rural locations during the warmer months of the year.

A key aspect to improved quantitation of the contributions from lower tropospheric photochemical processes to ozone concentrations continues to be more extensive atmospheric measurements of the distribution of reactive species of nitrogen. The emission densities of anthropogenic sources of NO_x are known to be highly variable over populated areas of continents as well as between continental areas and the oceans. The emission densities of biogenic sources of NO_x are small, likely to be highly variable, but poorly quantitated. These wide variations indicate the need for use of three dimensional tropospheric photochemical models over large continental regions.

Available results do indicate higher efficiencies for ozone formation at lower NO_x concentrations, especially below 1 ppb.     

Intrusions of stratospheric air into Alaska's troposphere,March 1983

During the Arctic Gas and Aerosol Sampling Program (AGASP) in March 1983, two distinctly different mechanisms for transporting stratospheric air into the Arctic troposphere were documented. A tropopause folding event, associated with an Arctic front, injected ‘perturbed’ polar stratospheric air into the troposphere. This perturbed polar stratospheric air was characterized by enhanced condensation nuclei concentrations (up to 1800 cm⁻³), enhanced aerosol light scattering (up to 90 × 10⁻⁶m⁻¹), and crustal aerosol particles of probable volcanic origin.The second mechanism, large-scale anticyclonic subsidence, transported relatively ‘clean’ stratospheric air into the Arctic troposphere. This clean stratospheric air was characterized by relatively low condensation nuclei concentrations (maximum of 300 cm⁻³), low aerosol light scattering ([5–7] × 10⁻⁶ m⁻¹), and the absence of detectable crustal particles.     

The fluorocarbon-ozone theory—IV. Fluorocarbon mixing and photolysis. The effects of eddy diffusion and tropospheric lifetime on stratospheric odd chlorine mixing ratios

Odd chlorine concentrations in the stratosphere arising from CCl₂F₂ and CCl₃F are calculated using a one-dimensional model of diffusion and photolysis together with accurate production and release data. Odd chlorine mixing ratios are mapped as a function of eddy diffusion profile and tropospheric lifetime. The results allow quick conversion of ozone depletion estimates, obtained from complete one-dimensional modelling, from a given assumed tropospheric lifetime and diffusion profile to other assumed lifetimes.In addition to contributions to CIX from CCl₂F₂ and CCl₃F, contributions from other chlorocarbons which have been detected in the troposphere, are assesssed. Our calculated estimate for the currently expected (January, 1977) total stratospheric CIX level is 1.5 ppb. Assuming there are no major calibration problems with current stratospheric CIX measurements, the measured stratospheric C1X levels are higher than calculated suggesting the possibility of other unidentified chlorine sources.     

Natural and anthropogenic sources and fate of atmospheric ethylene

This study attempted to estimate the amount of ethylene emitted into the atmosphere from natural and anthropogenic sources and to determine the fate of atmospheric ethylene. The total emission from the global surface was estimated to be 18–45 × 10⁶ t y⁻¹, of which 74% was released from natural sources and 26% from the anthropogenic sources. Releases from the terrestial and aquatic ecosystems comprised 89 and 11% of the natural emissions, respectively. Biomass burning in terrestial ecosystems to clear land for agriculture was the largest anthropogenic source (77%); the combustion of various fossil fuels amounts to only a small fraction (21%) of anthropogenic emissions. The relative amounts of ethylene destroyed by reactions with OH radical and O₃ in the troposphere were estimated to be 89 and 8%, respectively. The remaining 3% of the atmospheric ethylene was transported into the stratosphere. The atmospheric lifetime of ethylene was estimated to be between 2 and 4 days. We discuss the possibility that degradation by bacteria in the soil is a sink of atmospheric ethylene. The physiological effects of elevated ethylene concentrations on plants because of large-scale destruction of the terrestial ecosystem by forest fires is also discussed.     

Effects of climate change on surface-water photochemistry: a review

Information concerning the link between surface-water photochemistry and climate is presently very scarce as only a few studies have been dedicated to the subject. On the basis of the limited knowledge that is currently available, the present inferences can be made as follows: (1) Warming can cause enhanced leaching of ionic solutes from the catchments to surface waters, including cations and more biologically labile anions such as sulphate. Preferential sulphate biodegradation followed by removal as organic sulphides in sediment could increase alkalinity, favouring the generation of the carbonate radical, CO₃ ^·?. However, this phenomenon would be easily offset by fluctuations of the dissolved organic carbon (DOC), which is strongly anticorrelated with CO₃ ^·?. Therefore, obtaining insight into DOC evolution is a key issue in understanding the link between photochemistry and climate. (2) Climate change could exacerbate water scarcity in the dry season in some regions. Fluctuations in the water column could deeply alter photochemistry that is usually favoured in shallower waters. However, the way water is lost would strongly affect the prevailing photoinduced processes. Water outflow without important changes in solute concentration would mostly favour reactions induced by the hydroxyl and carbonate radicals (·OH and CO₃ ^·?). In contrast, evaporative concentration would enhance reactions mediated by singlet oxygen (¹O₂) and by the triplet states of chromophoric dissolved organic matter (³CDOM*). (3) In a warmer climate, the summer stratification period of lakes would last longer, thereby enhancing photochemical reactions in the epilimnion but at the same time keeping the hypolimnion water in the dark for longer periods.     

Evidence for cloud venting of mixed layer ozone and aerosols

Observations are presented which substantiate the hypothesis that significant vertical exchange of ozone (O₃) and aerosol pollutants occurs between the mixed layer and the free troposphere during cumulus cloud convective activity. Flight experiments conducted in July 1981 utilized the airborne UV-DIAL (Ultra-Violet Differential Absorption Lidar) system developed by NASA. This system provides simultaneous range resolved O₃ concentration and aerosol backscatter profiles with high spatial resolution. Data were obtained during the afternoon along east-west and south-north intersecting transects over North Carolina in the presence of active, non-precipitating cumulus clouds. Evening transects were obtained in the area indicated by trajectory calculations to be the current position of the air mass sampled earlier in the day. Space-height cross-section analyses for the evening flight show the cloud ‘debris’ as patterns of aerosol and O₃ in excess of the ambient free tropospheric background. The O₃ excess was approximately the value of the concentration difference between the afternoon mixed layer and free troposphere measured in the afternoon from independent in-situ vertical soundings made by another aircraft.     

The effects of increasing atmospheric ozone on biogenic monoterpene profiles and the formation of secondary aerosols

Monoterpenes are biogenic volatile organic compounds (BVOCs) which play an important role in plant adaptation to stresses, atmospheric chemistry, plant–plant and plant–insect interactions. In this study, we determined whether ozonolysis can influence the monoterpenes in the headspace of cabbage. The monoterpenes were mixed with an air-flow enriched with 100, 200 or 400 ppbv of ozone (O₃) in a Teflon chamber. The changes in the monoterpene and O₃ concentrations, and the formation of secondary organic aerosols (SOA) were determined during ozonolysis. Furthermore, the monoterpene reactions with O₃ and OH were modelled using reaction kinetics equations. The results showed that all of the monoterpenes were unequally affected: α-thujene, sabinene and d-limonene were affected to the greatest extend, whereas the 1,8-cineole concentration did not change. In addition, plant monoterpene emissions reduced the O₃ concentration by 12–24%. The SOA formation was dependent on O₃ concentration. At 100 ppbv of O₃, virtually no new particles were formed but clear SOA formation was observed at the higher ozone concentrations. The modelled results showed rather good agreements for α-pinene and 1,8-cineole, whereas the measured concentrations were clearly lower compared to modelled values for sabinene and limonene. In summary, O₃-quenching by monoterpenes occurs beyond the boundary layer of leaves and results in a decreased O₃ concentration, altered monoterpene profiles and SOA formation.     

Ozone destruction by catalysis: Credibility of the threat

In an effort to assess the credibility of predictions concerning the O₃ layer and to pinpoint avenues for improving current stratospheric models, a review was undertaken of observational data believed to have a bearing on the validity of destruction of stratospheric O₃ by catalysis. Aside from short period responses of O₃ above 20 km to PCAs and changes in solar flux, no data were found to confirm catalytic destruction both qualitatively and quantitatively. Predicted changes following such perturbations as nuclear tests, solar cycle modulation of cosmic rays, PCA's and volcanic eruptions cannot be clearly identified in the total O₃ observations. Observed variations in total O₃ appear to have been controlled by processes not included in current stratospheric models. As a minimum, it is concluded that predictions regarding stratospheric O₃ do not, at present, warrant the confidence implied by CIAP.     

Sensitivity of tropospheric oxidants to global chemical and climate change

A photochemical model has been used to quantify the sensitivity of the tropospheric oxidants O₃ and OH to changes in CH₄, CO and NO emissions and to perturbations in climate and stratospheric chemistry. Coefficients of the form ∂1n[O₃]/∂1n[X] and ∂1n[OH]/∂1n[X], where [X] = flux of CH₄, CO, NO; stratospheric O₃ and H₂O have been calculated for a number of “chemically coherent” regions (e.g. nonpolluted continental, nonpolluted marine, urban) at low and middle latitudes. Sensitivities in O₃ and OH vary with regional emissions patterns and are nonlinear within a given region as [X] changes. In most cases increasing CH₄ and CO emissions will suppress OH (negative coefficients) and increase O₃ (positive coefficients) except in areas where NO and O₃ influenced by pollution are sufficient to increase OH. Stratospheric O₃ depletion will tend to decrease O₃ (except in high NO_x areas) and increase OH through enhanced u.v. photolysis. Increased levels of water vapor (one possible outcome of a global warming) will also decrease O₃ and increase OH. We conclude that in most regions, NO, CO and CH₄ emission increases will suppress OH and increase O₃, but these trends may be opposed by stratospheric O₃ depletion and climate change. A regional survey of OH and O₃ levels suggests that the tropics have a pivotal role in determining the earth's future oxidizing capacity.     

Modelling NOx from lightning and its impact on global chemical fields

We have used a three-dimensional off-line chemical transport model (CTM) to assess the impact of lightning emissions in the free troposphere both on NO_x itself and on other chemical species such as O₃ and OH. We have investigated these effects using two lightning emission scenarios. In the first, lightning emissions are coupled in space and time to the convective cloud top height calculated every 6 h by the CTM's moist convection scheme. In the second, lightning emissions are calculated as a constant, monthly mean field. The model's performance against observed profiles of NO_x and O₃ in the Atlantic and Pacific ocean improves significantly when lightning emissions are included. With the inclusion of these emissions, the CTM produces a significant increase in the NO_x concentrations in the upper troposphere, where the NO_x lifetime is long, and a smaller increase in the lower free troposphere, where the surface NO_x sources dominate. These changes cause a significant increase in the O₃ production in the upper troposphere and hence higher calculated O₃ there. The model indicates that lightning emissions cause local increases of over 50 parts per 10¹² by volume (pptv) in NO_x, 200 pptv in HNO₃ and 20 parts per 10⁹ by volume (ppbv) (>40%) in O₃. In addition, a smaller increase of O₃ in the lower troposphere occurs due to an increase in the downward transport of O₃. The O₃ change is accompanied by an increase in OH which is more pronounced in the upper troposphere with a corresponding reduction in CO. The method of emission employed in the model does not appear to have a significant effect globally. In the upper troposphere (above about 300 hPa) NO_x concentrations are generally lower with monthly mean emissions, because of the de-coupling of emissions from the model's convection scheme, which vents NO_x aloft more efficiently in the coupled scheme. Below the local convective outflow altitude, NO_x concentrations are larger when using the monthly mean emissions than when coupled to the convection scheme, because the more dilute emissions, and nighttime emissions, lead to a slower NO_x destruction rate. Only minor changes are predicted in the monthly average fields of O₃ if we emit lightning as a monthly constant field. However, the method of emission becomes important when we make a direct comparison of model results with time varying data. These differences should be taken into account when a direct comparison of O₃ with measurements collected at particular times and locations is attempted.     

Modeling of ozone reactions on aircraft-related soot in the upper troposphere and lower stratosphere

Several studies in modeling atmospheric processes have suggested that heterogeneous chemistry on soot emitted from high altitude aircraft could affect stratospheric ozone depletion. However, these modeling studies were limited because they did not adequately consider the decrease in reaction probability with time as the surface of the soot becomes “poisoned” by its interactions with various gases. Here we extend UIUC's two-dimensional chemical-transport model to investigate possible effects of heterogeneous reactions of ozone on aircraft-generated carbon particles, including a treatment of soot poisoning in the model. We generally follow literature recommendations for ozone uptake probabilities and determine the available active sites on soot given partial pressures of the reactants, temperature, and time since soot emission in order to investigate ozone decrease. The regeneration of soot active sites is also taken into account in this study. We find that, even if active sites on soot surfaces are regenerated, upper troposphere and lower stratosphere ozone losses on aircraft emitted soot occurring through heterogeneous reactions are insignificant once poisoning effects are considered.     

Impact of vertical transport processes on the tropospheric ozone layering above Europe.: Part II: Climatological analysis of the past 30 years

Using the set of multivariate criteria described in a companion paper, ozone-rich layers detected in tropospheric soundings are clustered according to their stratospheric or boundary layer origin. An additional class for aged tropospheric air masses is also considered. This analysis is exclusively based on the measured physical properties of the layers. The database includes 27,000 ozone profiles collected above 11 European stations—two of which provide measurements since 1970. The seasonal cycle of the tropospheric ozone stratification exhibits a clear summer maximum. This increase is due to aged tropospheric air masses that are more frequently detected, suggesting an enhanced lifetime of layers in summer. In terms of ozone content, the relative impact of stratospheric ozone compared to the other sources is highest in winter while export from the boundary layer presents a uniform seasonal cycle. Altitude and thickness distributions of the layers are consistent with the dynamical processes involved in the layering. Northernmost and southernmost stations are more exposed to stratospheric air intrusions into the free troposphere. Long-term trends show that transport from the tropopause region has increased since the mid 1980s. This trend being concomitant with lower ozone content of such layers, a moderate trend of the transport efficiency from the stratosphere on total tropospheric ozone is observed. The increase of ozone detected in tropospheric layers since the mid 1980s cannot be attributed to any recent export process from either the stratosphere or the boundary layer but rather to enhanced photochemical production in aged air masses or to an increase in the lifetime of the layers.     

Comparison of measured and modeled surface ozone concentrations at two different sites in Europe during the solar eclipse on August 11, 1999

The effects of the solar eclipse on 11 August 1999 on surface ozone at two sites, Thessaloniki, Greece (urban site) and Hohenpeissenberg, Germany (elevated rural site) are investigated in this study and compared with model results. The eclipse offered a unique opportunity to test our understanding of tropospheric ozone chemistry and to investigate with a simple photochemical box model the response of surface ozone to changes of solar radiation during a photolytical perturbation such as the solar eclipse. The surface ozone measurements following the eclipse display a decrease of around 10–15 ppbv at the urban station of Eptapyrgio at Thessaloniki while at Hohenpeissenberg, the actual ozone data do not show any clear effect of eclipse on surface ozone. For Thessaloniki, the model results suggest that solely photochemistry can account for a significant amount of the observed surface ozone decrease during the eclipse but transport effects mask part of the photochemical effect of eclipse on surface ozone. For Hohenpeissenberg, the box model predicted an ozone decrease, due to the eclipse, of about 2 ppbv in relative agreement with the magnitude of the observed ozone decrease from the 2 h moving average while at the same time it inhibits the foreseen diurnal ozone increase. However, this modeled ozone decrease during the eclipse is small compared to the diurnal ozone variability due to transport effects, and hence, transport really masks such relative small changes. The different magnitude of the surface ozone decrease between the two sites indicates mainly the role of the NO_x levels. Measured and modeled NO and NO₂ concentrations at Hohenpeissenberg during the eclipse are also compared and indicate that the partitioning of NO and NO₂ in NO_x is influenced clearly from the eclipse. This is not observed at Thessaloniki due to local NO_x sources.     

Greenhouse effect of NOX

Through various processes the nitrogen oxides (NO_X) interact with trace gases in the troposphere and stratosphere which do absorb in the spectral range relevant to the greenhouse effect (infrared wavelengths). The net effect is an enhancement of the greenhouse effect. The catalytic role of NO_X in the production of tropospheric ozone provides the most prominent contribution. The global waming potential is estimated as GWP (NO_X = 30 – 33 and 7 – 10 for the respective time horizons of 20 and 100 years, and is thereby comparable to that of methane. NO_X emissions in rural areas of anthropogenically influenced regions, or those in the vicinity of the txopopause caused by air traffic, cause the greenhouse effectivity to be substantially more intense. We estimate an additional 5–23 % for Germany’s contribution to the anthropogenic greenhouse effect as a result of the indirect greenhouse effects stemming from NO_X. Furthermore, a small and still inaccurately defined amount of the deposited NO_X which has primarily been converted into nitrates is again released from the soil into the atmosphere in the form of the long-lived greenhouse gas nitrous oxide (N₂O). Thus, anthropogenically induced NO_X emissions contribute to enhanced greenhouse effect and to stratospheric ozone depletion in the time scale of more than a century.