Radiation and chemistry in the stratosphere: Sensitivity to O_2 absorption cross sections in the Herzberg continuum

We propose that a significant overestimate of the molecular oxygen absorption cross sections in the important spectral window from 200-220 nm is in large part responsible for the discrepancy between observed and modeled vertical profiles of some halocarbons (CFCl_3 in particular), as well as for the long-standing problem of simultaneously fitting N_2O, CH_4, CF_2Cl_2 and CFCl_3 profiles with a single eddy diffusion model. Recent measurements of the direct solar flux in the stratosphere by J.R. Herman and co-workers seem to support this idea. Replacing our current O_2 cross sections in the 200-220 nm range by values in better agreement with the results of the above group leads to a reduction in N_2O, CF_2Cl_2 and CFCl_3 concentrations (by factors of 0.70, 0.62 and 0.19, respectively, at 30 km), while CH_4, H_2 and CO profiles are essentially unchanged. Moreover, the predicted concentration of HNO_3 above 30 km is reduced by ∼50%, yielding better agreement with observations. The reduction in O_2 cross sections produces a 10-20% decrease in ozone above about 35 km, but a fairly large increase (∼30%) near the peak around 20-25 km. The changes in other stratospheric species are also briefly discussed

Temperatures and optical depths of Saturn's rings and a brightness temperature for Titan

The Pioneer Saturn infrared radiometer viewed Saturn's rings at 20- and 45-µm wavelength under several conditions of illumination. The data are analyzed to infer radial locations of major ring boundaries, temperatures and temperature gradients, and normal optical depths. Error bounds on the above inferred quantities are given. Most ring boundaries are defined to ±0.01 R_s(1 R_s≡6 × 10^4 km) and are in good agreement with those inferred from the imaging photopolarimeter experiment. Temperatures generally decrease with radial distance from the planet. A significant temperature gradient exists from the colder north (unilluminated) side of the rings to the warmer south side. The gradient appears to be steepest on the south side. Ring optical depths are greater than some previously published values and are approximately 0.1 for the Cassini division and the C ring. In addition, the C ring optical depth decreases towards the planet. The temperature drop during eclipse is ≳10 K, implying low thermal inertia for the ring particles. Titan's 45-µm brightness temperature is 75±5 K, in good agreement with earth-based observations

A critical analysis of ClO and O_3 in the mid-latitude stratosphere

In the upper stratosphere, an altitude range in which ozone should be in photochemical steady-state, calculated ozone abundances that are derived from a one-dimensional photochemical model with updated chemistry are up to 60% smaller than mean observed values. On the other hand, the model results for the key free radicals (HO_x, NO_x, and ClO_x species) in the catalytic destruction of ozone are shown to be in reasonable agreement with available measurements. The general validity of the model simulation of ClO_x chemistry is confirmed through a detailed intercomparison between the computed ClO diurnal variation and recently published ground-based microwave observations. Since many field measurements are performed near sunrise or sunset, the uncertainties in the model results arising from the details of the radiation field calculations at large zenith angles are discussed. Although the calculated ozone discrepancy could be the result of a number of errors in adopted photochemical parameters, a sensitivity analysis shows that no reasonable change in any one or two parameters can resolve this problem. The limited available observations regarding the ratio of atomic oxygen to ozone suggest a possible discrepancy in that quantity, which could be responsible for a large part of the ozone problem

Photochemical Modeling of the Earth's Stratosphere

We have helped develop a one-dimensional photochemical model of the Earth's stratosphere, in order to provide an up-to-date comparison with mid-latitude observations. This work focuses on the present state of the stratosphere, and includes studies of the radiation field (absorption and scattering), the important partitioning and vertical distribution of halo-carbons and their products, as well as certain intriguing discrepancies related to light and heavy ozone. We briefly comment on the detection by J. R. Herman and J. E. Mentall of a 10% ratio of total scattered flux to direct solar flux at a wavelength of about 200 nm and an altitude of 40 km. This ratio is over a factor of two higher than our theoretical results and cannot be explained without the existence of a scattering component not included in the model. We also explicitly demonstrate the first-order effects of the inclusion of sphericity (spherical shell atmosphere) on the stratospheric photochemistry at solar zenith angles close to 90°. The resulting changes in model concentrations for short-lived radicals such as O, OH, ClO, and NO are largest in the lower stratosphere, but relatively small compared to current observational uncertainties. We propose that a significant overestimate of the molecular oxygen absorption cross sections in the important spectral window from about 200 to 220 nm is in large part responsible for the discrepancy between observed and modeled vertical profiles of some halocarbons (CFCl3 in particular), as well as for the long-standing problem of simultaneously fitting N2O, CH4, CF2Cl2, and CFCl3 profiles with a single eddy diffusion model. Recent measurements of atmospheric transmission by J. R. Herman and coworkers seem to support this idea. The use of their proposed reduction in O2 cross sections leads to significant decreases in the CFCl3 concentration above about 20 km, with smaller reductions in N2O, CF2Cl2 and HNO3. The concentrations of CH4, H2, and CO are not significantly altered. Changes in other gases (including ozone) are also discussed, as well as the effect on eddy diffusion coefficients obtained from measurements of N2O or CH4 profiles in the stratosphere. Accurate determinations of these small O2 absorption cross sections are needed, since they affect the vertical distribution of halo-carbons in the stratosphere, and the lifetime of these species has an impact on ozone depletion estimates. In terms of the halocarbon decomposition products in the stratosphere, our model vertical distribution of ClO is shown to provide a reasonably good fit to the mean of available observations. As discussed by others, changes in certain rate constants affecting HOx in the lower stratosphere have led to decreases in model ClO concentrations by over a factor of three in the lower stratosphere, thus improving the shape of the vertical profile. In addition, the amount of upper stratospheric ClO has increased due to recent changes in the kinetics (reactions O + HO2, O + ClO, and possibly OH + HCl). The diurnal variation of ClO observed from the ground (microwave emission) by P. Solomon and coworkers is consistent with our model results in terms of the maximum day-to-night decrease in column abundance above about 30 km. However, the observed mid-morning increase is slower than theoretical values, while the predicted afternoon decrease might be too slow, even if one considers the uncertainties in photochemical data. This could indicate the existence of missing chemistry in the models. Although the different observations show somewhat contradictory results. Other observations (balloon-borne microwave spectroscopy and infrared laser radiometry) are also discussed in relation to our model. To first-order, indirect evidence for the breathing cycle between ClO and ClONO2 seems to have been established. The mean observed HCl mixing ratio profile decreases somewhat faster towards the lower stratosphere than model profiles, a discrepancy which has previously been noted, particularly at high latitudes. Measurements of ethane in the lower stratosphere seemed to indicate that the atomic chlorine concentration was three to five times lower than predicted, but more recent data do not show such a discrepancy. The fluorine products consist mostly of HF and COF2. We show that the main uncertainty for this system is the value of the quantum yield (as a function of wavelength) for COF2 photodissociation, which translates into a factor of three or more uncertainty in the ratio of HF to COF2 concentrations in the upper stratosphere. If this quantum yield has an average value close to 0.25, a better model fit to observations of HF and [HF]/[HCl] is obtained than if the value is close to unity. Simultaneous stratospheric measurements of COF2 and HF, as well as ClO and HCl, would greatly enhance our ability to test photochemical models of these halocarbon products. Finally, we stress that, although generally good agreement is found between our model and observations of HOx, NOx, and ClOx species (involved in catalytic cycles destroying ozone), the mean observed mid-latitude ozone abundance from about 35 to 50 km is up to 50 or 60% greater than current model results. Certain observations of a 10 to 15% daytime increase in ozone concentration in the 30 to 40 km region are also puzzling, if real. We explore the model sensitivity to various input parameters and point out that, given the present uncertainties in photochemical laboratory data, no reasonable change in one or even three or four of these parameters can eliminate the ozone discrepancy. There might well be some missing chemistry in relation to the effectiveness of the loss processes for odd oxygen, or a (less likely) unknown significant O3 source. We have to understand the present upper stratospheric ozone distribution, before estimates of possible future ozone depletion can be made with confidence. We also discuss our understanding of heavy ozone photochemistry, which might be related to a light ozone photochemical source. Fast isotopic exchange processes between O and O2 will dominate the heavy odd oxygen chemistry, and we do not find any significant heavy ozone enhancement possibilities in the stratosphere, unless unusually large fractionation processes exist. The in situ mass spectrometer observations of a 40% enhancement in 18O32O2 near 30 km by K. Mauersberger remain a mystery, and further data collection -- possibly via infrared or microwave spectroscopy as well -- should be undertaken if this potentially significant discrepancy is to be understood.</p

Observed changes in stratospheric circulation: decreasing lifetime of N2O, 2005–2021

Using Aura Microwave Limb Sounder satellite observations of stratospheric nitrous oxide (N2O), ozone, and temperature from 2005 through 2021, we calculate the atmospheric lifetime of N2O to be decreasing at a rate of −2.1 ± 1.2 %/decade. This decrease is occurring because the N2O abundances in the middle tropical stratosphere, where N2O is photochemically destroyed, are increasing at a faster rate than the bulk N2O in the lower atmosphere. The cause appears to be a more vigorous stratospheric circulation, which models predict to be a result of climate change. If the observed trends in lifetime and implied emissions continue, then the change in N2O over the 21st century will be 27 % less than those projected with a fixed lifetime, and the impact on global warming and ozone depletion will be proportionately lessened. Because global warming is caused in part by N2O, this finding is an example of a negative climate–chemistry feedback.</p

Solar Occultation Satellite Data and Derived Meteorological Products: Sampling Issues and Comparisons with Aura MLS

Derived Meteorological Products (DMPs, including potential temperature (theta), potential vorticity, equivalent latitude (EqL), horizontal winds and tropopause locations) have been produced for the locations and times of measurements by several solar occultation (SO) instruments and the Aura Microwave Limb Sounder (MLS). DMPs are calculated from several meteorological analyses for the Atmospheric Chemistry Experiment-Fourier Transform Spectrometer, Stratospheric Aerosol and Gas Experiment II and III, Halogen Occultation Experiment, and Polar Ozone and Aerosol Measurement II and III SO instruments and MLS. Time-series comparisons of MLS version 1.5 and SO data using DMPs show good qualitative agreement in time evolution of O3, N2O, H20, CO, HNO3, HCl and temperature; quantitative agreement is good in most cases. EqL-coordinate comparisons of MLS version 2.2 and SO data show good quantitative agreement throughout the stratosphere for most of these species, with significant biases for a few species in localized regions. Comparisons in EqL coordinates of MLS and SO data, and of SO data with geographically coincident MLS data provide insight into where and how sampling effects are important in interpretation of the sparse SO data, thus assisting in fully utilizing the SO data in scientific studies and comparisons with other sparse datasets. The DMPs are valuable for scientific studies and to facilitate validation of non-coincident measurements

ACE-FTS and HALOE Observations of Hydrogen Fluoride (HF) and Their Comparison with SLIMCAT Chemical Transport Model Calculations

The majority of fluorine in the atmosphere has resulted from the anthropogenic emission of chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs). Most tropospheric fluorine is present in its emitted \u27organic\u27 form due to the molecules having long lifetimes (up to a decade or longer). Thus they are able to reach the stratosphere where they are broken down, liberating fluorine. The principal degradation products are carbonyl fluoride (COF2), carbonyl chloride fluoride (COClF), and hydrogen fluoride (HF); of these HF is the most abundant. In fact at the top of the stratosphere most of the fluorine is present as HF, which, due to its extreme stability, is an almost permanent reservoir of stratospheric fluorine. Since anthropogenic emissions of fluorine continue unabated, the amount of HF in the atmosphere continues to increase. The use of satellite remote-sensing techniques allows the measurement of HF atmospheric abundances with impressive global coverage, and the investigation of HF trends, and seasonal and latitudinal variability. This work presents global distributions and trends of HF using data from two satellite limb instruments: the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS), onboard the SCISAT satellite, which has been recording atmospheric spectra since 2004, and the HALogen Occultation Experiment, onboard the Upper Atmosphere Research Satellite (UARS), which recorded atmospheric spectra between 1991 and 2005. These observations are compared with the output of SLIMCAT, a state-of-the-art three-dimensional chemical transport model (CTM). The model aids in the interpretation of the HF satellite observations, and the comparison provides a validation of emission inventories and the atmospheric degradation reaction schemes used in the model

Validation of ozone data from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES)

The Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) onboard the International Space Station provided global measurements of ozone profiles in the middle atmosphere from 12 October 2009 to 21 April 2010. We present validation studies of the SMILES version 2.1 ozone product based on coincidence statistics with satellite observations and outputs of chemistry and transport models (CTMs). Comparisons of the stratospheric ozone with correlative data show agreements that are generally within 10%. In the mesosphere, the agreement is also good and better than 30% even at a high altitude of 73km, and the SMILES measurements with their local time coverage also capture the diurnal variability very well. The recommended altitude range for scientific use is from 16 to 73km. We note that the SMILES ozone values for altitude above 26km are smaller than some of the correlative satellite datasets; conversely the SMILES values in the lower stratosphere tend to be larger than correlative data, particularly in the tropics, with less than 8% difference below similar to 24km. The larger values in the lower stratosphere are probably due to departure of retrieval results between two detection bands at altitudes below 28km; it is similar to 3% at 24km and is increasing rapidly down below