Photochemical Events Research Articles

Photomodulation of liquid crystal orientations for photonic applications

Two types of photomodulation of orientations of liquid crystals (LCs) are reviewed: 1) order–disorder phase transitions of LCs induced by photochemical reactions of photochromic molecules and 2) order–order alignment change of LCs (change in LC directors) induced by photochemical reactions or without photochemical events. Both processes produce a large refractive-index modulation, which forms the basis of a range of photonic applications. Various modes of photomodulation of orientations of LCs with plausible mechanisms and their possible applications in photonics are described.

Quality assessment of urban trees: A comparative study of physiological characterisation, airborne imaging and on site fluorescence monitoring by the OJIP-test

The purpose of this paper is to demonstrate the complementary utility of chlorophyll fast fluorescence OJIP transient (from 50 microns to 1s) measurements in the aerial study of rows of trees. We identify limitations in photochemical events induced by urban injuries on Platanus acerfolia L. using the JIP-test procedure. The Performance Index (PIABS) showed the largest dynamic range to characterise the vitality of trees. Individual trees were graded into three quality groups based on the individual PIABS value compared to the overall average for trees in the alley. These groups are: high performers, with PIABS 50% higher than the alley average; normal trees, with a deviation from the alley average between -50% and %; and poor performers, whose deviation from the alley average was -50% or less. Trees also were grouped into five vitality categories on the basis of a stereoscopic and morphologic observation of the symmetry of tree crowns, percentage of defoliation and reflectance property in the visible and infra-red range. Here, we report a remarkable correlation between the airborne remote sensing data and the on-site fluorescence measurements.

THE INFLUENCE OF POWDERY MILDEW INFECTION ON PHOTOSYNTHESIS, CHLOROPHYLL FLUORESCENCE, LEAF CHLOROPHYLL AND CAROTENOID CONTENT OF THREE WOODY PLANT SPECIES

Summary The effect of powdery mildew development on photosynthesis, chlorophyll fluorescence, leaf chlorophyll and carotenoid concentrations on three woody plants frequently planted in urban environments was studied. Rates of photosynthetic CO2 fixation were rapidly reduced in two of the three genotypes tested prior to visible signs of infection. Effects on chlorophyll fluorescence (Fo, Fv/Fo, Fv/Fm), leaf chlorophyll and carotenoid content were not manifest until >25 per cent of the leaf area was observed to be covered by mycelial growth indicating reduced photo-synthetic rates during the early stages of infection were not due to degradation of the leaf chloroplast structure. Observation of the fluorescence transient (OJIP curves) showed powdery mildew infection impairs photosynthetic electron transport system by reducing the size but not heterogeneity of the plastoquninone pool, effecting both the acceptor and donor side of photosystem II. Impairment of the photosynthetic electron transport system was reflected by reduced values of a performance index used in this investigation as a measure of photochemical events within photosystem II electron transport. In addition interpretation of the fluorescence data indicated powdery mildew infection may impair the photo-protective process that facilitates the dissipation of excess energy within leaf tissue.

Studies on Covalently Linked Porphyrin−C60 Dyads: Stabilization of Charge-Separated States by Axial Coordination

The effect of axial ligation on the photoinduced charge separation and charge recombination of a series of covalently linked porphyrin−C60 dyads is investigated. Toward this, meso-tetraphenylporphyrin and its zinc(II) derivative are functionalized at the ortho or para positions of one of the aryl groups to bear a fulleropyrrolidine entity through a flexible ethylene dioxide bridge to probe the effect of intramolecular electron transfer phenomena. In o-dichlorobenzene, 0.1 M (TBA)ClO4, the synthesized dyads exhibit seven one electron reversible redox reactions within in the potential window of the solvent and the measured redox potentials and UV−visible absorption spectra reveal charge−transfer interactions between the electron donor, porphyrin, and the electron acceptor, fullerene entities. The geometric and electronic structures of the dyads probed by ab initio B3LYP/3-21G(*) methods also revealed the existence of charge−transfer interactions. The excited state photochemical events are monitored by both steady state and time-resolved emission as well as transient absorption techniques. In o-dichlorobenzene or in benzonitrile, the main quenching pathway involves charge separation from the excited porphyrin to the moiety. The kcs and kcr are found to depend on the type of substitution (ortho or para) and the metal ion in the porphyrin cavity. Relatively long-lived charge-separated states are observed upon coordinating pyridine axial ligands to the central metal ion of the zinc porphyrin−C60 dyads, and this has been attributed to the electronic and geometric effects caused by the axial coordinated ligand.

The effect of chilling on the photosynthetic activity in coffee (Coffea arabica L.) seedlings: The protective action of chloroplastid pigments

Coffea arabica is considered to be sensitive to low temperatures, being affected throughout its entire life cycle. Injury caused by chilling (low temperatures above zero degree centigrade) is characterized primarily by inhibition of the photosynthetic process. The objective of this work was to evaluate the role of photosynthetic pigments in the tolerance of coffee (C. arabica L.) seedlings to chilling. The evaluation the photosynthetic activity was made by emission of Chl a fluorescence at room temperature (25 ºC) in vivo and in situ, using a portable fluorometer. The pigment content was obtained by extraction with 80 % acetone, while estimation of membrane lipid peroxidation was determined by measuring the MDA content in leaf tissue extracts. The results indicated a generalized reduction in the quantum yield of PSII when the seedlings were maintained in the dark. The reduction occurred in the seedlings submitted to chilling treatment as well as in the control ones. This demonstrates that not only chilling acts to cause an alteration in PSII. It is possible that the tissue storage reserves had been totally exhausted, with the respiratory rate exceeding the photosynthetic rate; the later was nil, since the seedlings were kept in the dark. The efficiency in the capture, transfer and utilization of light energy in PS II photochemical reactions requires a sequence of photochemical, biochemical and biophysical events which depend on the structural integrity of the photosynthetic apparatus. However, this efficiency was found to be related to the protective action of chloroplastid pigments, rather than to the concentration of these pigments.

Study of the acid-diffusion effect on line edge roughness using the edge roughness evaluation method

The acid-diffusion effect on line edge roughness (LER) in a negative-type chemically amplified resist of electron beam lithography was evaluated experimentally. The edge roughness evaluation method was utilized to quantify the acid-diffusion length. Adjusting the prebaking temperature controlled the acid-diffusion coefficient in the range from 8.1 nm2/s at 70 °C to 0.8 nm2/s at 130 °C. Acid diffusion reduced the LER of a 100 nm isolated line from 6.9 to 4.0 nm while increasing the diffusion length from 15 to 44 nm, but a too-long diffusion length degraded line and space patterns. These results suggest that acid diffusion has two opposing effects on LER: one is the smoothing effect reducing LER and the other is the degrading image contrast effect increasing LER. It is concluded that LER occurs as a consequence of the amplification of the so-called uncertainty band by the slope of the quasibeam profile. The bandwidth signifies the effect of molecular structure, the spatial dispersion of photochemical events caused by shot noise, the smoothing effect of acid diffusion, and development uncertainty at the pattern edge. The magnitude of the amplification is determined by the slope of the quasibeam profile that comprises aerial-image quality, acid-diffusion length, and resist contrast.

Photoinduced charge separation at polymer–solution interface

Photoinduced charge separation using electrostatic repulsion between an anionic polymer film (Nafion) and an anionic sensitizer aqueous solution has been studied for the photoproduction of dihydrogen from proton. It was found that only anionic sensitizers were effective to photoproduce cation radical (MV +) in the anionic polymer film, while cationic sensitizers were not effective. The initial formation rate of MV + by tris(bathophenanthroline)ruthenium (Ru(bpds) 3 4−) sensitizer was faster than that by tris(4,4′-dicarboxy-2,2′-bipyridine)ruthenium (Ru(dcbpy) 3 4−). The presence of anthracene in the Nafion increased the total amount of MV + species because the charge transfer complex formed between anthracene and methylviologen (MV 2+) prevents the dimerization of MV +. The formation of MV + dimer is depressed by the coexisting anthracene. The action spectrum for the formation rate of one-electron reduction species agreed with the absorption spectrum of the sensitizer suggesting that an electron transfer from the sensitizer in its MLCT excited state to the MV 2+ induces the photochemical event.

Dendritic multiporphyrin arrays as light-harvesting antennae: effects of generation number and morphology on intramolecular energy transfer.

A series of star- and cone-shaped dendritic multiporphyrin arrays, (nPZn)4PFB and (nPZn)1PFB, respectively, that contain energy-donating dendritic zinc porphyrin (PZn) wedges of different numbers (n = 1, 3, and 7) of the PZn units, attached to an energy-accepting free-base porphyrin (PFB) core, were synthesized by a convergent growth approach. For the cone-shaped series ((nPZn)1PFB), the efficiency of energy transfer (phi ENT) from the photoexcited PZn units to the focal PFB core, as evaluated from the fluorescence lifetimes of the PZn units, considerably decreased as the generation number increased: (1PZn)1PFB (86%), (3PZn)1PFB (66%), and (7PZn)1PFB (19%). In sharp contrast, the star-shaped series ((nPZn)4PFB) all showed high phi ENT values: (1PZn)4PFB (87%), (3PZn)4PFB (80%), and (7PZn)4PFB (71%). Energy transfer efficiencies of (3PZn)4-ester-PFB, (1PZn)4-ester-PFB, and (3PZn)1-ester-PFB, whose dendritic PZn wedges are connected by an ester linkage to the PFB core, were almost comparable to those of the corresponding ether-linked versions. Fluorescence depolarization (P) studies showed much lower P values for star-shaped (7PZn)4PFB and (3PZn)4PFB than cone-shaped (7PZn)1PFB and (3PZn)1PFB, respectively, indicating a highly efficient energy migration among the PZn units in the star-shaped series. Such a morphology-assisted photochemical event is probably responsible for the excellent light-harvesting activity of large (7PZn)4PFB molecules.

Photo-induced oxidation of a dinuclear Mn2II,II complex to the Mn2III,IV state by inter- and intramolecular electron transfer to RuIIItris-bipyridine

To model the structural and functional parts of the water oxidizing complex in Photosystem II, a dimeric manganese(II,II) complex (1) was linked to a ruthenium(II)tris-bipyridine (RuII(bpy)3) complex via a substituted l-tyrosine, to form the trinuclear complex 2 [J. Inorg. Biochem. 78 (2000) 15]. Flash photolysis of 1 and RuII(bpy)3 in aqueous solution, in the presence of an electron acceptor, resulted in the stepwise extraction of three electrons by RuIII(bpy)3 from the Mn2II,II dimer, which then attained the Mn2III,IV oxidation state. In a similar experiment with compound 2, the dinuclear Mn complex reduced the photo-oxidized Ru moiety via intramolecular electron transfer on each photochemical event. From EPR it was seen that 2 also reached the Mn2III,IV state. Our data indicate that oxidation from the Mn2II,II state proceeds stepwise via intermediate formation of Mn2II,III and Mn2III,III. In the presence of water, cyclic voltammetry showed an additional anodic peak beyond Mn2II,III/III,III oxidation which was significantly lower than in neat acetonitrile. Assuming that this peak is due to oxidation to Mn2III,IV, this suggests that water is essential for the formation of the Mn2III,IV oxidation state. Compound 2 is a structural mimic of the water oxidizing complex, in that it links a Mn complex via a tyrosine to a highly oxidizing photosensitizer. Complex 2 also mimics mechanistic aspects of Photosystem II, in that the electron transfer to the photosensitizer is fast and results in several electron extractions from the Mn moiety.

The origin and day-of-week dependence of photochemical ozone episodes in the UK

Analyses are presented which focus specifically on photochemical ozone events when hourly mean levels of ozone at UK monitoring sites have reached or exceeded the information threshold of 90 ppbv. The geographical origins of such episodes are determined from an analysis of the trajectories associated with elevated ozone levels at 14 rural sites and six urban sites in the UK over the period 1992–1999. The analysis shows that the highest levels of ozone occur under summertime anticyclonic conditions, when the back trajectories have ‘looped’ over mainland Europe and arrive in the UK from a broadly easterly or southeasterly direction. Monitoring data from the same sites over the period 1989–1999 clearly demonstrate that ozone episodes are more prevalent at the end of the week, with the greatest numbers of hours ⩾90 ppbv occurring on Fridays. Relative to Fridays, the number of hours exceedence on the other days are: Sundays 51%, Mondays 50%, Tuesdays 54%, Wednesdays 39%, Thursdays 70% and Saturdays 82%. The observed day-of-week dependence is believed to result from the temporal dependence in the emissions of the precursor VOC and NO x (which are greater on weekdays) and the multi-day timescale required for chemical processing and transport to lead to elevated ozone levels under photochemical episode conditions. To test this hypothesis, a photochemical trajectory model, incorporating temporal variations in precursor emissions, has been used to investigate the formation of ozone along 96 h trajectories arriving at eight southern UK rural sites, for the meteorological conditions of an ozone episode on 31 July 1999. As a direct result of imposing a day-of-week variation in emissions, the model is able to recreate some of the features of the observed general pattern of 90 ppbv exceedences with day of week. In particular, Friday is calculated to be the day with the highest peak ozone mixing ratio at 7 of the 8 sites. The calculated peak mixing ratios on each day at each site are used to infer the number of hours exceedence of the 90 ppbv threshold, using a relationship derived from archived monitoring data. The inferred hours exceedence of the 90 ppbv threshold at all sites combined is calculated to be a factor of two greater on Friday than it is on Monday or Tuesday.

Transient absorption spectroscopy in biology using the Super-ACO storage ring FEL and the synchrotron radiation combination

The Super-ACO storage ring FEL, covering the UV range down to 300 nm with a high average power (300 mW at 350 nm) together with a high stability and long lifetime, is a unique tool for the performance of users applications. We present here the first pump-probe two color experiments on biological species using a storage ring FEL coupled to the synchrotron radiation. The intense UV pulse of the Super-ACO FEL is used to prepare a high initial concentration of chromophores in their first singlet electronic excited state. The nearby bending magnet synchrotron radiation provides, on the other hand a pulsed, white light continuum (UV–IR), naturally synchronized with the FEL pulses and used to probe the photochemical subsequent events and the associated transient species. We have demonstrated the feasibility with a dye molecule (POPOP) observing a two-color effect, signature of excited state absorption and a temporal signature with Acridine. Applications on various chromophores of biological interest are carried out, such as the time-resolved absorption study of the first excited state of Acridine.

Photophysical and photochemical events during the photosensitization of Hypocrellin A on a colloidal CdS semiconductor

UV–visible spectroscopy, fluorescence quenching measurements, and single-photon counting have been employed to study electron transfer from excited Hypocrellin A (HA) into the conduction band of a colloidal CdS semiconductor. It has been determined that HA dye adsorbed on colloidal CdS with an apparent association constant of 940 (mol/l) −1, and fluorescence lifetime measurements gave a rate constant of 5.16×10 9 s −1 for the electron transfer process. The transferred electrons could be used to reduce the substrate 2,2,6,6-tetramethyl-1-piperdinyloxy (TEMPO). In this regard electron paramagnetic resonance spectroscopy was used to study the photoreduction of TEMPO by electrons in the CdS semiconductor. The results suggested that CdS photoreduction efficiency was enhanced by photosensitization of HA in visible region.

Nature of the Surface Crossing Process in Bacteriorhodopsin: Computer Simulations of the Quantum Dynamics of the Primary Photochemical Event

The quantum dynamics of the primary photoisomerization event in bacteriorhodopsin is studied by a semiclassical trajectory approach. The relevant surface crossing probability is evaluated from the wave functions and potential surfaces of a hybrid quantum mechanical/molecular mechanics (QM/MM) Hamiltonian of the complete chromophore−protein−solvent system. The QM/MM model combines consistently the quantum mechanical Hamiltonian of the chromophore with the microscopic electric field of the ionized groups and induced dipoles of the protein−solvent system. The QCFF/PI Hamiltonian of the chromophore is adjusted to reproduce relevant ab initio results. The nonadiabatic coupling term is calculated numerically from the corresponding wave functions. The simulations are performed by combining the ENZYMIX and QCFF/PI molecular modeling programs. The effect of the protein on the absorption spectrum of the chromophore is examined. It is found that this spectrum reflects the effect of the protein permanent ...

Photochemical fragmentation processes in laser ablation of organic solids

Studies on ultraviolet (UV) laser ablation of molecular solids have received considerable attention due to its proven and potential applications. Despite its active practical use the mechanisms of laser ablation are still being studied and debated. One crucial mechanistic discussion is on the relative importance of direct photodissociation of chemical bonds versus thermal ejection following rapid conversion of light energy into heat in the ablation processes. It is generally believed that these two processes are coupled in UV ablation resulting in difficulty in analyzing the relative importance of the two mechanisms. In the simulations presented here the breathing sphere model is enhanced allowing the photon absorption event to break a bond in the molecule and then have subsequent abstraction and recombination reactions. The initial system to model is chlorobenzene. Chlorobenzene is chosen because of simplicity of its fragmentation, entailing exclusively scission of the carbon–chlorine bond to yield phenyl and chlorine radicals. The results from the simulations allow us to study the photochemical events and their coupling with the thermal processes.

Primary photoreaction of photoactive yellow protein studied by subpicosecond-nanosecond spectroscopy.

The primary photochemical event of photoactive yellow protein (PYP) was studied by laser flash photolysis experiments on a subpicosecond-nanosecond time scale. PYP was excited by a 390-nm pulse, and the transient difference absorption spectra were recorded by a multichannel spectrometer for a more reliable spectral analysis than previously possible. Just after excitation, an absorbance decrease due to the stimulated emission at 500 nm and photoconversion of PYP at 450 nm were observed. The stimulated emission gradually shifted to 520 nm and was retained up to 4 ps. Then, the formation of a red-shifted intermediate with a broad absorption spectrum was observed from 20 ps to 1 ns. Another red-shifted intermediate with a narrow absorption spectrum was formed after 2 ns and was stable for at least 5 ns. The latter is therefore believed to correspond to I1 (PYP(L)), which has been detected on a nanosecond time scale or trapped at -80 degrees C. Singular value decomposition analysis demonstrated that the spectral shifts observed from 0.5 ps to 5 ns could be explained by two-component decay of excited state(s) and conversion from PYP(B) to PYP(L). The amount of PYP(L) at 5 ns was less than that of photoconverted PYP, suggesting the formation of another intermediate, PYP(H). In addition, the absorption spectra of these intermediates were calculated based on the proposed reaction scheme. Together, these results indicate that the photocycle of PYP at room temperature has a branched pathway in the early stage and is essentially similar to that observed under low-temperature spectroscopy.

Thermoluminescence in Chloroplasts as an Indicator of Alterations in Photosystem 2 Reaction Centre by Biotic and Abiotic Stresses

Thermoluminescence (TL) in green plants arises from charge recombination of charged molecules in the reaction centre (RC) of photosystem 2 (PS2) in chloroplasts. The TL technique is used for detection of alterations in the architecture of PS2 RCs. The donor side 'S-states' and the acceptor side quinone molecules (QA and QB) are involved the charge recombination processes of PS2. High temperature (70–75 °C) glow peaks are also used to detect non-photosynthetic peroxidation processes in thylakoid membranes. The TL peaks with their characteristic charge recombination can be utilised for the study of chloroplast development, ageing, chemical, biotic, and abiotic stress induced alterations in the PS2 RC and for the study of the primary photochemical events of photosynthesis. The technique has been used successfully in the characterisation of transgenic plants in the study of genetically engineered organisms.

Wavelength-selective photodenitrogenation of azoalkanes to high-spin polyradicals with cyclopentane-1,3-diyl spin-carrying units and their photobleaching: EPR/UV spectroscopy and product studies of the matrix-isolated species.

The photolysis of the mono-, bis-, and trisazoalkanes 1, 2, and 3 in a toluene matrix at 77 K has been studied by EPR and UV spectroscopy. The purpose was to find the optimal conditions for the generation of the corresponding organic high-spin polyradicals (the triplet diradicals D-1, D-2, and D-3, the tetraradicals T-2 and T-3, and the hexaradical H-3) all with localized cyclopentane-1,3-diyl spin-carrying units, connected by m-phenylene (except D-1) as ferromagnetic coupler. Irradiation of these azoalkanes at 333, 351, or 364 nm gave different polyradical compositions. This observed wavelength dependence is due to the secondary photoreaction (photobleaching) of the polyradical intermediate. The photobleaching process has been examined in detail for the triplet diradical D-1, for which pi,pi excitation affords the cyclopentenes 5 instead of the housane 4 (the usual product of the diradical D-1 on warm-up of the matrix). The pi,pi-excited diradical D-1 fragments into a pair of allyl and methyl radicals (the latter was observed by EPR spectroscopy of a photobleached sample), and recombination affords the cyclopentene. Similar photochemical events are proposed for the photobleaching of the tetraradical T-2 and hexaradical H-3, derived from the respective azoalkanes 2 and 3. Thus, photobleaching of the polyradicals competes effectively with their photogeneration from the azoalkane. This unavoidable event is the consequence of spectral overlap between the cumyl-radical pi,pi chromophore of the polyradical and the n,pi chromophore of the azoalkane at the wavelength (364 nm), at which the latter is photoactive for the required extrusion of molecular nitrogen.

Blue-fluorescent antibodies.

The forte of catalytic antibodies has resided in the control of the ground-state reaction coordinate. A principle and method are now described in which antibodies can direct the outcome of photophysical and photochemical events that take place on excited-state potential energy surfaces. The key component is a chemically reactive optical sensor that provides a direct report of the dynamic interplay between protein and ligand at the active site. To illustrate the concept, we used a trans-stilbene hapten to elicit a panel of monoclonal antibodies that displayed a range of fluorescent spectral behavior when bound to a trans-stilbene substrate. Several antibodies yielded a blue fluorescence indicative of an excited-state complex or "exciplex" between trans-stilbene and the antibody. The antibodies controlled the isomerization coordinate of trans-stilbene and dynamically coupled this manifold with an active-site residue. A step was taken toward the use of antibody-based photochemical sensors for diagnostic and clinical applications.

Effect of irradiance spectra on the photoinduced toxicity of three polycyclic aromatic hydrocarbons

AbstractPhotoinduced toxicity of polycyclic aromatic hydrocarbons (PAHs) is dependent on the concentration of compounds present and the dose of light received. Of the light present, only those wavelengths absorbed by the compound have the potential to initiate the photochemical events underlying phototoxicity. This suggests that variation in light spectra present in natural waters, arising from variation in dissolved organic carbon composition, is an important determinant of phototoxicity risk in specific, PAH‐contaminated waterbodies. To quantify the effect of environmentally realistic variation in light spectra on toxicity, brine shrimp (Artemia salina) assays were conducted under various light spectra and with three PAHs (pyrene, fluoranthene, and anthracene) of known phototoxicity potential. In these spectral assays, the total ultraviolet light present was equivalent; only the spectral characteristics varied. Based on the absorbance spectra of these PAHs, it was predicted that toxicity, quantified using immobilization as the endpoint, would vary significantly among light spectra in pyrene assays, but not in anthracene assays, and that variation in toxicity in fluoranthene assays would be intermediate. The results supported these assumptions. In the pyrene exposures, the glass filter time to 50% population immobilization (IT50) (39.5 min) was 117% longer than the KCr filter IT50 (18.2 min). In the fluoranthene exposures, the glass filter IT50 (49.5 min) was 27% longer than the KCr filter IT50 (39.1 min). In the anthracene exposures, the glass filter IT50 (62.2 min) was not statistically different from the KCr filter IT50 (63.8 min). Comparison of these results with the results of assays conducted under neutral‐density filters (that change intensity but not spectral distribution) demonstrate that multiplying spectral intensity by wavelength‐specific absorbance accurately predicts relative photoinduced toxicity among the experimental treatments. These results indicate that quantifying the spectral characteristics of PAH‐contaminated aquatic environments may be an important component of risk assessment at these sites.

Effect of Sea Breeze on Air Pollution in the Greater Athens Area. Part II: Analysis of Different Emission Scenarios

The Mediterranean Campaign of Photochemical Tracers–Transport and Chemical Evolution that took place in the greater Athens area from 20 August to 20 September 1994 has confirmed the role of sea-breeze circulation in photochemical smog episodes that had been suggested already by a number of experiments and numerical studies. The meteorological and photochemical modeling of this campaign were discussed in Part I. Part II focuses on the study of the 14 September photochemical smog event associated with a sea-breeze circulation. The objective of the study is to identify and to understand better the nonlinear processes that produce high ozone concentrations. In particular, the effect of land and sea breezes is investigated by isolating the effect of nighttime and daytime emissions on ozone concentrations. The same principle then is used to isolate the effect on ozone concentrations of the two main sources of emissions in the greater Athens area: the industrial area around Elefsis and the Athens urban area. Last, the buildup of ozone from one day to another is investigated. From this study, it comes out that ozone production in the Athens area is mainly a 1-day phenomenon. The increased values of photochemical pollutant (up to 130 ppb at ground level) reached during summertime late afternoons on mountain slopes to the north and northeast of the city are related mainly to the current-day emissions. Nevertheless, the recirculation of old pollutants can have an important effect on ozone concentrations in downtown Athens, the southern part of the peninsula, and over the sea, especially near Aigina Island.