Stratospheric Photochemistry Research Articles

Solar FTIR measurements of NOx vertical distributions – Part 1: First observational evidence of a seasonal variation in the diurnal increasing rates of stratospheric NO2 and NO

Abstract. Observations of nitrogen dioxide (NO2) and nitrogen oxide (NO) in the stratosphere are relevant to understand long-term changes and variabilities in stratospheric nitrogen oxide (NOx) and ozone (O3) concentrations. Due to the versatile role of NO2 and NO in stratospheric O3 photochemistry, they are important for recovery and build-up of O3 holes in the stratosphere and therefore can indirectly affect human life. Thus, we present in this work the evaluation of NO2 and NO stratospheric partial columns (> 16 km altitude) retrieved from ground-based Fourier-transform infrared (FTIR) measurements of over 25 years at Zugspitze (47.42° N, 10.98° E; 2964 m a.s.l.) and 18 years at Garmisch (47.47° N, 11.06° E; 745 m a.s.l.), Germany. The obtained stratospheric columns are only weakly influenced by tropospheric pollution and show only a very small bias of 2.5 ± 0.2 % when comparing NO2 above Zugspitze and Garmisch. Stratospheric columns of both NO2 and NO show a diurnal increase that depends on local solar time (LST). We quantified this behavior by calculating diurnal increasing rates. Here, we find mean values for the NO2 diurnal increasing rate of (0.89 ± 0.14) × 1014 and (0.94 ± 0.14) × 1014 cm−2 h−1 at Zugspitze and Garmisch, respectively. The mean NO morning diurnal increasing rate above Zugspitze is found to be (1.42 ± 0.12) × 1014 cm−2 h−1. Regarding the seasonal dependency of these increasing rates, for the first time, we were able to experimentally detect a significant seasonal variation in both NO2 diurnal increasing rates and NO morning diurnal increasing rates with a maximum of (1.13 ± 0.04) × 1014 cm−1 h−1 for NO2 and (1.76 ± 0.25) × 1014 cm−1 h−1 for NO in September and a minimum of (0.71 ± 0.18) × 1014 cm−1 h−1 in December for NO2 and a minimum of (1.18 ± 0.41) × 1014 cm−1 h−1 in November for NO. This similar behavior may be explained by the interconnection of both species in stratospheric photochemistry. The outcome of this work is a retrieval and analysis strategy of FTIR data for NOx stratospheric columns, which can help to further validate photochemical models or improve satellite validations. The first use of this data set is shown in the companion paper (Nürnberg et al., 2023) wherein experiment-based NOx scaling factors describing the diurnal increase in the retrieved partial columns are extracted and recently published model-based scaling factors are validated.

Nitrogen dioxide line shift coefficients induced by air pressure

Theoretical line shifts of nitrogen dioxide lines induced by air are reported at the room temperature (296 K). Calculations are performed for ∼31500 lines in the spectral range of 5700–7500 cm−1 for the bands including rotational quantum numbers up to = 87 and = 20. The data have been evaluated theoretically by a semi-empirical method which is based on the impact theory of broadening and includes the correction factors whose parameters can be determined by fitting the shift coefficients to the experimental data. Nitrogen dioxide plays an important role in the Earth’s tropospheric and stratospheric photochemistry, it is one of the most common anthropogenic substances emitted into the atmosphere. Nitrogen dioxide is involved in the production and destruction of ozone. The infra-red high resolution spectra of NO2 are the subject of intense interest during recent years. Obtained results are implemented in spectroscopic databases aimed at atmospheric applications such as HITRAN and GEISA. As to the line shift parameters, there are few papers. In this work, theoretical NO2-line shifts induced by air are reported for many lines.

Coupled Stratospheric Chemistry–Meteorology Data Assimilation. Part I: Physical Background and Coupled Modeling Aspects

A coupled stratospheric chemistry–meteorology model was developed by combining the Canadian operational weather prediction model Global Environmental Multiscale (GEM) with a comprehensive stratospheric photochemistry model from the Belgian Assimilation System for Chemical ObsErvations (BASCOE). The coupled model was called GEM-BACH for GEM-Belgian Atmospheric CHemistry. The coupling was made across a chemical interface that preserves time-splitting while being modular, allowing GEM to run with or without chemistry. An evaluation of the coupling was performed by comparing the coupled model, refreshed by meteorological analyses every 6 h, against the standard offline chemical transport model (CTM) approach. Results show that the dynamical meteorological consistency between meteorological analysis times far outweighs the error created by the jump resulting from the meteorological analysis increments at regular time intervals, irrespective of whether a 3D-Var or 4D-Var meteorological analysis is used. Arguments in favor of using the same horizontal resolution for chemistry, meteorology, and meteorological and chemical analysis increments are also presented. GEM-BACH forecasts refreshed by meteorological analyses every 6 h were compared against independent measurements of temperature, long-lived species, ozone and water vapor. The comparison showed a relatively good agreement throughout the stratosphere except for an upper-level warm temperature bias and an ozone deficit of nearly 15%. In particular, the coupled model simulation during an ozone hole event gives better ozone concentrations than a 4D-Var chemical assimilation at a lower resolution.

Seasonal stratospheric photochemistry on Uranus and Neptune

A time-variable 1D photochemical model is used to study the distribution of stratospheric hydrocarbons as a function of altitude, latitude, and season on Uranus and Neptune. The results for Neptune indicate that in the absence of stratospheric circulation or other meridional transport processes, the hydrocarbon abundances exhibit strong seasonal and meridional variations in the upper stratosphere, but that these variations become increasingly damped with depth due to increasing dynamical and chemical time scales. At high altitudes, hydrocarbon mixing ratios are typically largest where the solar insolation is the greatest, leading to strong hemispheric dichotomies between the summer-to-fall hemisphere and winter-to-spring hemisphere. At mbar pressures and deeper, slower chemistry and diffusion lead to latitude variations that become more symmetric about the equator. On Uranus, the stagnant, poorly mixed stratosphere confines methane and its photochemical products to higher pressures, where chemistry and diffusion time scales remain large. Seasonal variations in hydrocarbons are therefore predicted to be more muted on Uranus, despite the planet’s very large obliquity. Radiative-transfer simulations demonstrate that latitude variations in hydrocarbons on both planets are potentially observable with future JWST mid-infrared spectral imaging. Our seasonal model predictions for Neptune compare well with retrieved C2H2 and C2H6 abundances from spatially resolved ground-based observations (no such observations currently exist for Uranus), suggesting that stratospheric circulation — which was not included in these models — may have little influence on the large-scale meridional hydrocarbon distributions on Neptune, unlike the situation on Jupiter and Saturn.

Oxygen isotope anomaly in tropospheric CO2 and implications for CO2 residence time in the atmosphere and gross primary productivity

The abundance variations of near surface atmospheric CO2 isotopologues (primarily 16O12C16O, 16O13C16O, 17O12C16O, and 18O12C16O) represent an integrated signal from anthropogenic/biogeochemical processes, including fossil fuel burning, biospheric photosynthesis and respiration, hydrospheric isotope exchange with water, and stratospheric photochemistry. Oxygen isotopes, in particular, are affected by the carbon and water cycles. Being a useful tracer that directly probes governing processes in CO2 biogeochemical cycles, Δ17O (=ln(1 + δ17O) − 0.516 × ln(1 + δ18O)) provides an alternative constraint on the strengths of the associated cycles involving CO2. Here, we analyze Δ17O data from four places (Taipei, Taiwan; South China Sea; La Jolla, United States; Jerusalem, Israel) in the northern hemisphere (with a total of 455 measurements) and find a rather narrow range (0.326 ± 0.005‰). A conservative estimate places a lower limit of 345 ± 70 PgC year−1 on the cycling flux between the terrestrial biosphere and atmosphere and infers a residence time of CO2 of 1.9 ± 0.3 years (upper limit) in the atmosphere. A Monte Carlo simulation that takes various plant uptake scenarios into account yields a terrestrial gross primary productivity of 120 ± 30 PgC year−1 and soil invasion of 110 ± 30 PgC year−1, providing a quantitative assessment utilizing the oxygen isotope anomaly for quantifying CO2 cycling.

Dust ablation on the giant planets: Consequences for stratospheric photochemistry

Ablation of interplanetary dust supplies oxygen to the upper atmospheres of Jupiter, Saturn, Uranus, and Neptune. Using recent dynamical model predictions for the dust influx rates to the giant planets (Poppe et al., 2016), we calculate the ablation profiles and investigate the subsequent coupled oxygen–hydrocarbon neutral photochemistry in the stratospheres of these planets. We find that dust grains from the Edgeworth–Kuiper Belt, Jupiter-family comets, and Oort-cloud comets supply an effective oxygen influx rate of 1.0−0.7+2.2×107 O atoms cm−2−1 to Jupiter, 7.4−5.1+16×104 cm−2 s−1 to Saturn, 8.9−6.1+19×104 cm−2 s−1 to Uranus, and 7.5−5.1+16×105 cm−2 s−1 to Neptune. The fate of the ablated oxygen depends in part on the molecular/atomic form of the initially delivered products, and on the altitude at which it was deposited. The dominant stratospheric products are CO, H2O, and CO2, which are relatively stable photochemically. Model-data comparisons suggest that interplanetary dust grains deliver an important component of the external oxygen to Jupiter and Uranus but fall far short of the amount needed to explain the CO abundance currently seen in the middle stratospheres of Saturn and Neptune. Our results are consistent with the theory that all of the giant planets have experienced large cometary impacts within the last few hundred years. Our results also suggest that the low background H2O abundance in Jupiter’s stratosphere is indicative of effective conversion of meteoric oxygen to CO during or immediately after the ablation process — photochemistry alone cannot efficiently convert the H2O into CO on the giant planets.

Reduction of Climate Sensitivity to Solar Forcing due to Stratospheric Ozone Feedback

Abstract An accurate assessment of the role of solar variability is a key step toward a proper quantification of natural and anthropogenic climate change. To this end, climate models have been extensively used to quantify the solar contribution to climate variability. However, owing to the large computational cost, the bulk of modeling studies to date have been performed without interactive stratospheric photochemistry: the impact of this simplification on the modeled climate system response to solar forcing remains largely unknown. Here this impact is quantified by comparing the response of two model configurations, with and without interactive ozone chemistry. Using long integrations, robust surface temperature and precipitation responses to an idealized irradiance increase are obtained. Then, it is shown that the inclusion of interactive stratospheric chemistry significantly reduces the surface warming (by about one-third) and the accompanying precipitation response. This behavior is linked to photochemically induced stratospheric ozone changes, and their modulation of the surface solar radiation. The results herein suggest that neglecting stratospheric photochemistry leads to a sizable overestimate of the surface response to changes in solar irradiance. This has implications for simulations of the climate in the last millennium and geoengineering applications employing irradiance changes larger than those observed over the 11-yr sunspot cycle, where models often use simplified treatments of stratospheric ozone that are inconsistent with the imposed solar forcing.

SO<sub>2</sub> photolysis as a source for sulfur mass-independent isotope signatures in stratospehric aerosols

Abstract. Signatures of sulfur isotope mass-independent fractionation (S-MIF) have been observed in stratospheric sulfate aerosols deposited in polar ice. The S-MIF signatures are thought to be associated with stratospheric photochemistry following stratospheric volcanic eruptions, but the exact mechanism responsible for the production and preservation of these signatures is debated. In order to identify the origin and the mechanism of preservation for these signatures, a series of laboratory photochemical experiments were carried out to investigate the effect of temperature and added O2 on the S-MIF produced by two absorption band systems of SO2: photolysis in the 190 to 220 nm region and photoexcitation in the 250 to 350 nm region. The SO2 photolysis (SO2 + hν → SO + O) experiments showed S-MIF signals with large 34S/34S fractionations, which increases with decreasing temperature. The overall S-MIF pattern observed for photolysis experiments, including high 34S/34S fractionations, positive mass-independent anomalies in 33S, and negative anomalies in 36S, is consistent with a major contribution from optical isotopologue screening effects and data for stratospheric sulfate aerosols. In contrast, SO2 photoexcitation produced products with positive S-MIF anomalies in both 33S and 36S, which is different from stratospheric sulfate aerosols. SO2 photolysis in the presence of O2 produced SO3 with S-MIF signals, suggesting the transfer of the S-MIF anomalies from SO to SO3 by the SO + O2 + M → SO3 + M reaction. This is supported with energy calculations of stationary points on the SO3 potential energy surfaces, which indicate that this reaction occurs slowly on a single adiabatic surface, but that it can occur more rapidly through intersystem crossing. Based on our experimental results, we estimate a termolecular rate constant on the order of 10−37 cm6 molecule−2 s−1. This rate can explain the preservation of mass independent isotope signatures in stratospheric sulfate aerosols and provides a minor, but important, oxidation pathway for stratospheric SO2. The production and preservation of S-MIF signals requires a high SO2 column density to allow for optical isotopologue screening effects to occur and to generate a large enough signature that it can be preserved. In addition, the SO2 plume must reach an altitude of around 20 to 25 km, where SO2 photolysis becomes a dominant process. These experiments are the first step towards understanding the origin of the sulfur isotope anomalies in stratospheric sulfate aerosols.

Source characteristics of O3 and CO2 at Mt. Waliguan Observatory, Tibetan Plateau implied by using 7Be and 210Pb

The weekly averages of near-surface 7Be, 210Pb, O3, and CO2 concentrations at the Global Atmospheric Watch Observatory, Mt. Waliguan (101.98°E, 36.287°N, 3810 m a.s.l.), from October 2002 to January 2004 are presented. With the establishment of the new datasets of DCCW (Differential Concentrations in Contiguous Weeks) of 7Be, 210Pb, and O3, CO2 (Δ7Be, Δ210Pb, ΔO3, ΔCO2, respectively, the impacts of upper-level downward transports and land-surface emissions on O3 and CO2 concentrations are implied by 7Be and 210Pb being as independent tracers. The relations among Δ7Be, Δ210Pb, and ΔO3, ΔCO2 are examined statistically and compared. The results indicate that with the DCCWs, the interferences with the tracing significance of 7Be and 210Pb from the seasonal wet scavenging of atmospheric aerosol are greatly reduced, and the weighting sources of O3 or CO2 variations are more pronounced. Basically, the variability of surface O3 is controlled predominately by air mass transported from the upper atmosphere levels while the emission from the Continent Boundary Layer (CBL) has an obvious input for CO2. The relation between Δ210Pb and ΔO3 reflects that influences of CBL emission are generally positive/negative for surface O3 budget in summer/winter, and the relation of Δ7Be and ΔCO2 also reveals that upper level downward transport has positive/negative inputs for CO2 in summer/winter. With the highly correlated relations between 7Be and O3, a quantitative estimation is made of the stratospheric contributions to the budget of surface O3 at WLG: the monthly averages of stratospheric O3 range from 6 × 10−9 to 8 × 10−9 (volume mixing ratio) in April and from June to August, and 2×10−9 to 4×10−9 in the remaining months. For the ultimate sources of the baseline concentration of surface O3, which consist of only stratospheric transport and tropospheric photochemistry production, the contribution from stratospheric transport is estimated to be about 20 × 10−9 from May to July, and (12−15)×10−9 in the remaining months, and the total relative contribution rate is about 35% to 40%.

Ozone and NO2 variations measured during the 1 August 2008 solar eclipse above Eureka, Canada with a UV‐visible spectrometer

On 1 August 2008, a solar eclipse of 98% totality passed over the Polar Environment Atmospheric Research Laboratory at Eureka, Canada (80.05°N, 86.42°W), which is run by the Canadian Network for the Detection of Atmospheric Change. During the eclipse, a zenith‐sky UV‐visible spectrometer measured slant column densities (SCDs) and vertical column densities (VCDs) of ozone up to 82% occultation and NO2 up to 96% occultation, beyond which low light intensities and changes in the solar spectrum due to limb darkening compromised data quality. Ozone VCDs during the eclipse remained within natural variability, and this study is inconclusive regarding ozone oscillations due to limited temporal resolution and measurement errors toward eclipse maximum. Measured NO2 SCDs increased and decreased symmetrically around the eclipse maximum. NO2 SCDs were also calculated using a photochemical box model and a one‐dimensional radiative transfer model. The modeled ratio of eclipse day SCDs to the previous day's SCDs was compared to the measurements. They agreed within error bars leading up to maximum occultation, but the model ratio was systematically larger than the measured ratio for the second half of the eclipse, perhaps due to changing cloud conditions throughout the eclipse. The measured NO2 SCD ratio of 1.84−0.43+0.12 at 96% totality is larger than observed in past studies and agrees with modeled ratio of 1.91. Therefore our current understanding of stratospheric photochemistry is sufficient to predict the evolution of NOx chemistry through a solar eclipse.

Effect of local and regional sources on the isotopic composition of nitrous oxide in the tropical free troposphere and tropopause layer

Measurements and models of the spatiotemporal variability of surface N2O mixing ratios and isotopic compositions are increasingly used to constrain the global N2O budget. However, large variability observed on the small spatial scales of soil chambers and shipboard sampling, which appears to be very sensitive to local environmental conditions, has made extrapolation to the global scale difficult. In this study, we present measurements of the isotopic composition of N2O (δ15Nbulk, δ15Nα, δ15Nβ, and δ18O) from whole‐air samples collected at altitudes of 0.5 to 19km by the NASA DC‐8 and WB‐57 aircraft during the Costa Rica‐Aura Validation Experiment (CR‐AVE) and the Tropical Composition, Cloud and Climate Coupling Experiment (TC4) campaigns in January–February 2006 and July–August 2007, respectively. The vertical profiles of isotopic composition showed predictable, repeating patterns consistent with the influence of a surface source at lower altitudes and the influence of stratospheric photochemistry in the lower stratosphere. Their correlations with marine tracers at lower altitudes are consistent with a predominantly oceanic source, although a soil source cannot be ruled out. Measurements in a combustion plume revealed a strong depletion in 15N at the central nitrogen atom (i.e., low δ15Nα values), providing new information on N2O isotopic compositions from combustion. This new data set demonstrates that a coherent picture of the isotopic composition of tropospheric N2O is possible at currently attainable precisions and that its variations from 0.5 km to the lower stratosphere are a useful tool in investigating the sources and distributions of this important greenhouse gas.

A system of models of a short-lived rocket-induced “Ozone Hole”

We studied the mechanisms of perturbation of the stratosphere by jet exhausts of rocket vehicles and regularities of the recovery of the stratospheric parameters. Asymptotic methods are used to analyze different scenarios of noncatastrophic ozone destruction, one of which is identified as a real mechanism of perturbation of the stratospheric photochemistry during launches of liquid-fuel rockets. Models of the turbulent diffusion of rocket plume and parameters of the initial perturbation are refined. The results of the work markedly increase the realism of the corresponding computer models.

Constraints on inorganic gaseous iodine in the tropical upper troposphere and stratosphere inferred from balloon-borne solar occultation observations

Abstract. We report upper limits of IO and OIO in the tropical upper troposphere and stratosphere inferred from solar occultation spectra recorded by the LPMA/DOAS (Limb Profile Monitor of the Atmosphere/Differential Optical Absorption Spectroscopy) payload during two stratospheric balloon flights from a station in Northern Brazil (5.1° S, 42.9° W). In the tropical upper troposphere and lower stratosphere, upper limits for both, IO and OIO, are below 0.1 ppt. Photochemical modelling is used to estimate the compatible upper limits for the total gaseous inorganic iodine burden (Iy) amounting to 0.09 to 0.16 (+0.10/−0.04) ppt in the tropical lower stratosphere (21.0 km to 16.5 km) and 0.17 to 0.35 (+0.20/−0.08) ppt in the tropical upper troposphere (16.5 km to 13.5 km). In the middle stratosphere, upper limits increase with altitude as sampling sensitivity decreases. Our findings imply that the amount of gaseous iodine transported into the stratosphere through the tropical tropopause layer is small. Thus, iodine-mediated ozone loss plays a minor role for contemporary stratospheric photochemistry but might become significant in the future if source gas emissions or injection efficiency into the upper atmosphere are enhanced. However, photochemical modelling uncertainties are large and iodine might be transported into the stratosphere in particulate form.

Photochemistry of ethyl chloride caged in amorphous solid water

Caging and photo-induced decomposition of ethyl chloride molecules (EC) within a layer of amorphous solid water (ASW) on top of clean and oxygen-covered Ru(001) under ultra-high vacuum (UHV) conditions are presented. The caged molecules were estimated to reside 1.5 +/- 0.2 nm above the solid surface, based on parent molecule thermal decomposition on the clean ruthenium. Dissociative electron attachment (DEA) of the caged molecules following 193 nm laser irradiation, result in initial fragmentation to ethyl radical and chloride anion. It was found that photoreactivity on top of the clean ruthenium surface (Ru) is twenty times faster than on the oxygen-covered surface (O/Ru), with DEA cross sections: sigma(Ru) = (3.8 +/- 1) x 10(-19) cm(2) and sigma(O/Ru) = (2.1 +/- 0.3) x 10(-20) cm(2). This difference is attributed to the higher work function of oxygen-covered ruthenium, leading to smaller electron attachment probability due to mismatch of the ruthenium photo-electron energy with the adsorbed EC excited electron affinity levels. EC molecules fragmented within the cage, result in post-irradiation TPD spectra that reveal primarily C(4)H(8), C(3)H(5) and C(3)H(3), without any oxygen-containing molecules. Unique stabilization of the photoproducts has been observed with the first layer of water molecules in direct contact with the substrate, desorbing near 180 K, a significantly higher temperature than the desorption of fully caged molecules. This study may contribute for understanding stratospheric photochemistry and processes in interstellar space.

“Clumped-isotope” geochemistry—The study of naturally-occurring, multiply-substituted isotopologues

Clumped isotope geochemistry is concerned with the state of ordering of rare isotopes in natural materials. That is, it examines the extent to which rare isotopes (D, 13C, 15N, 18O, etc.) bond with or near each other rather than with the sea of light isotopes in which they swim. Abundances of isotopic ‘clumps’ in natural materials are influenced by a wide variety of factors. In most cases, their concentrations approach (within ca. 1%, relative) the amount expected for a random distribution of isotopes. Deviations from this stochastic distribution result from: enhanced thermodynamic stability of heavy-isotope ‘clumps’; slower kinetics of reactions requiring the breakage of bonds between heavy isotopes; the mass dependence of diffusive and thermo-gravitational fractionations; mixing between components that differ from one another in bulk isotopic composition; biochemical and photochemical fractionations that may reflect combinations of these simpler physical mechanisms; and, in some cases, other processes we do not yet understand. Although clumped isotope geochemistry is a young field, several seemingly promising applications have already emerged. Most importantly, it appears that proportions of 13C– 18O bonds in carbonate minerals are sensitive to their growth temperatures, independent of bulk isotopic composition. Thus, ‘clumped isotope’ analysis of ancient carbonates can be used as a quantitative paleothermometer that requires no assumptions about the δ 18O of waters from which carbonates grew. This approach has been used to reconstruct marine temperatures across the Phanerozoic (reaching back to the Silurian), terrestrial ground temperatures across the Cenozoic, thermal histories of aqueously altered meteorites, among other applications. Clumped isotope geochemistry is also placing new constraints on the atmospheric budget and stratospheric photochemistry of CO 2, and should be capable of placing analogous new constraints on the budgets of other atmospheric gases. Finally, this field could be extended to encompass sulfates, volatile hydrocarbons, organic moieties and other materials.

Correction to “Latitudinal and seasonal models of stratospheric photochemistry on Saturn: Comparison with infrared data from IRTF/TEXES”

Correction to “Latitudinal and seasonal models of stratospheric photochemistry on Saturn: Comparison with infrared data from IRTF/TEXES”

Latitudinal and seasonal models of stratospheric photochemistry on Saturn: Comparison with infrared data from IRTF/TEXES

The variation of hydrocarbon abundances with altitude, latitude, and season in Saturn's stratosphere is investigated using a one‐dimensional, time‐variable, photochemical model. The results indicate that hydrocarbon abundances at pressures less than ∼0.01 mbar are extremely sensitive to solar flux variations due to changes in season, latitude, orbital radius, solar cycle, and ring shadowing. Long vertical diffusion times in the 0.01–1 mbar region introduce phase lags in the response to insolation changes. At pressures greater than 1 mbar, vertical diffusion timescales are longer than a Saturnian year. Therefore, relatively long‐lived hydrocarbons like C2H2, C2H6, and C3H8 exhibit little seasonal variation in the lower stratosphere, and the yearly averaged solar insolation combined with vertical diffusion control species abundances in this region. Species with short photochemical lifetimes (e.g., CH3, C2H4) continue to experience seasonal variations, even at low altitudes. The model results are compared with infrared observations from the TEXES spectrometer at NASA's Infrared Telescope Facility. We find that the observed C2H2 mixing‐ratio variation with latitude is reasonably well predicted, whereas the C2H6 distribution is poorly predicted by the models. Meridional transport is likely affecting the distribution of C2H6, which has a long photochemical lifetime, whereas the distribution of shorter‐lived C2H2 is controlled by photochemistry and vertical diffusion. Given the different observed behavior of C2H2 and C2H6, we constrain meridional transport timescales at 2 mbar on Saturn to be in the range ∼100–700 years, corresponding to meridional wind or diffusion speeds of ∼0.4–2 cm s−1.

Triple oxygen isotope composition of tropospheric carbon dioxide as a tracer of terrestrial gross carbon fluxes

Stratospheric photochemistry leads to anomalous oxygen isotope enrichments in CO2 (for which Δ17O = δ17O − 0.516 × δ18O ≠ 0). This isotope anomaly is not lost until air returns to the troposphere and CO2 undergoes isotope exchange with water primarily in the terrestrial biosphere and oceans. A two‐box model is used to investigate the contribution of stratospheric production and contemporary surface carbon fluxes to tropospheric Δ17OCO2. The predicted magnitude of ∼0.15‰ is large enough that measurement of a globally averaged tropospheric Δ17OCO2 should provide a new constraint for gross carbon exchanges between the biosphere and atmosphere in terrestrial carbon cycle models. Importantly, Δ17OCO2 should be complementary to the primary isotopic tracer of gross carbon exchanges, δ18OCO2, but is not dependent on numerous hydrologic variables. Furthermore, with improved measurement precision, Δ17OCO2 could serve as a direct tracer of gross carbon exchanges and their variations.

Ab initio and RRKM studies of decomposition and interconversion pathways of ClCH2OH and CH3OCl isomeric species

The photofragmentation and photointerconversion pathways of the CH3OCl and ClCH2OH species are studied using quantum mechanical and RRKM theories. Ten possible channels are investigated in total and the relative importance of the various pathways in stratospheric photochemistry is discussed.

Tropospheric Ozone, Chemistry and Man-induced Trends

This paper presents an overview of the global large-scale aspects of tropospheric ozone. It describes and quantifies the major sources of tropospheric ozone, namely stratospheric influx and tropospheric photochemistry, which today contribute on the order of 500 Tg O3/yr and 4000 Tg O3/yr, respectively. Photochemistry also provides the largest sink of tropospheric ozone. The tropospheric production of ozone is regulated by the presence of nitrogen oxides. The ensuing impact of anthropogenic emissions on the tropospheric burden, distribution and temporal trend of O3 is discussed.