S. P. Smyshlyaev

On the formation of HNO3 in the Antarctic mid to upper stratosphere in winter

We address the previously unresolved puzzle of nitric acid formation in the polar winter mid to upper stratosphere, first indicated by Limb Infrared Monitor of the Stratosphere observations in the Arctic winter of 1978–1979. Several theoretical studies over the past 2 decades have tried to reproduce these observations with varying success. More recently, the onset, altitude range, and duration of the formation process have been clarified by the Cryogenic Limb Array Etalon Spectrometer onboard UARS and by a series of ground-based observations taken at the South Pole during the Antarctic winters of 1993, 1995, and 1999. Using the Stony Brook-SSt. Petersburg two-dimensional photochemical model, we have reexplored HNO3 formation via both the ion cluster chemistry and heterogeneous chemistry on sulfate aerosols considered by earlier investigators. By including what we believe to be a realistic flux of NOy from the mesosphere, we find that the model can generate observed mixing ratios through a combination of ion-clusterenhanced chemistry in the upper to mid stratosphere, augmented by heterogeneous chemistry on sulfate aerosol below ∼40 km. Results are presented which clarify the relative role of various processes and assumed NOy fluxes. These also point up the need to incorporate more accurate downward NOy fluxes in models being used to simulate the polar stratosphere. Finally, we emphasize the need to consider the influence of repartitioning of NOy or NOx into HNO3 before observed variations in amounts of the former reaching the mid to lower stratosphere in winter and early spring can properly be used as tracers to reflect variations in thermospheric-mesospheric NOy production or transport.

A two-dimensional model with input parameters from a general circulation model : Ozone sensitivity to different formulations for the longitudinal temperature variation

Net heating and temperature derived from the middle atmospheric version of the National Center for Atmospheric Researchs Community Climate Model (MA CCM2) history tapes are used to evaluate three different approaches to account for zonal temperature asymmetries in the calculation of gas phase and heterogeneous chemical reaction rate constants and polar stratospheric cloud (PSC) surface area in a two-dimensional chemistry transport model (2-D CTM). The first method uses the daily (and monthly) averaged three-dimensional (3-D) temperature distribution derived from the MA CCM2 to calculate chemical and heterogeneous reaction rates at each 3-D grid point, followed by zonal averaging (pseudo-3-D method). The second method uses 3-D daily temperature statistics from the MA CCM2 to calculate the monthly averaged probability function (stochastic approach). The third method is based on a planetary wave superposition on the zonally averaged temperature (wave approach). The sensitivity of the gas phase reactions to the longitudinal temperature asymmetry is small, while the sensitivity of the heterogeneous reaction rates is comparable to the ozone response to aircraft emissions. All three methods of accounting for longitude temperature asymmetry give similar PSC morphologies in the southern hemisphere, in good agreement with climatological data and independent model calculations. In the northern hemisphere, where the CCM2 winter temperatures at high latitudes are known to be warmer than those observed, the PSCs predicted by the pseudo-3-D and wave methods are much scarcer than those observed or calculated by other authors using climatological temperatures. For the same reason, all other methods employed in the present study failed to predict any PSCs in the northern hemisphere.

Comparison of recent modeled and observed trends in total column ozone

We present a comparison of trends in total column ozone from 10 two-dimensional and 4 three-dimensional models and solar backscatter ultraviolet–2 (SBUV/2) satellite observations from the period 1979–2003. Trends for the past (1979–2000), the recent 7 years (1996–2003), and the future (2000–2050) are compared. We have analyzed the data using both simple linear trends and linear trends derived with a hockey stick method including a turnaround point in 1996. If the last 7 years, 1996–2003, are analyzed in isolation, the SBUV/2 observations show no increase in ozone, and most of the models predict continued depletion, although at a lesser rate. In sharp contrast to this, the recent data show positive trends for the Northern and the Southern Hemispheres if the hockey stick method with a turnaround point in 1996 is employed for the models and observations. The analysis shows that the observed positive trends in both hemispheres in the recent 7-year period are much larger than what is predicted by the models. The trends derived with the hockey stick method are very dependent on the values just before the turnaround point. The analysis of the recent data therefore depends greatly on these years being representative of the overall trend. Most models underestimate the past trends at middle and high latitudes. This is particularly pronounced in the Northern Hemisphere. Quantitatively, there is much disagreement among the models concerning future trends. However, the models agree that future trends are expected to be positive and less than half the magnitude of the past downward trends. Examination of the model projections shows that there is virtually no correlation between the past and future trends from the individual models.

Combined chemistry-climate model of the atmosphere

A combined three-dimensional global model of the chemistry and dynamics of the lower and middle atmosphere (up to 90 km from the Earth’s surface) is described. With the use of this model within the AMIP2 (1979–1995) program, numerical calculations were performed with consideration for the interactive coupling between the ozone content, radiation heating, and atmospheric circulation. Comparisons were made between calculated and observed data on the ozone content and temperature. Heterogeneous processes on the surface of polar stratospheric clouds were shown to be important for a correct simulation of the spatial and temporal distribution of atmospheric ozone.

Simulation of the impact of thunderstorm activity on atmospheric gas composition

A chemistry-climate model of the lower and middle atmosphere has been used to estimate the sensitivity of the atmospheric gas composition to the rate of thunderstorm production of nitrogen oxides at upper tropospheric and lower stratospheric altitudes. The impact that nitrogen oxides produced by lightning have on the atmospheric gas composition is treated as a subgrid-scale process and included in the model parametrically. The natural uncertainty in the global production rate of nitrogen oxides in lightning flashes was specified within limits from 2 to 20 Tg N/year. Results of the model experiments have shown that, due to the variability of thunderstorm-produced nitrogen oxides, their concentration in the upper troposphere and lower stratosphere can vary by a factor of 2 or 3, which, given the influence of nitrogen oxides on ozone and other gases, creates the potential for a strong perturbation of the atmospheric gas composition and thermal regime. Model calculations have shown the strong sensitivity of ozone and the OH hydroxyl to the amount of lightning nitrogen oxides at different atmospheric altitudes. These calculations demonstrate the importance of nitrogen oxides of thunderstorm origin for the balance of atmospheric odd ozone and gases linked to it, such as ozone and hydroxyl radicals. Our results demonstrate that one important task is to raise the accuracy of estimates of the rate of nitrogen oxide production by lightning discharges and to use physical parametrizations that take into account the local lightning effects and feedbacks arising in this case rather than climatological data in models of the gas composition and general circulation of the atmosphere.

Sensitivity of model assessments of high-speed civil transport effects on stratospheric ozone resulting from uncertainties in the NOxproduction from lightning

Lightning NOx production is one of the most important and most uncertain sources of reactive nitrogen in the atmosphere. To examine the role of NOx lightning production uncertainties in supersonic aircraft assessment studies, we have done a number of numerical calculations with the State University of New York at Stony Brook-Russian State Hydrometeorological Institute of Saint-Petersburg two-dimensional model. The amount of nitrogen oxides produced by lightning discharges was varied within its quoted uncertainty from 2 to 12 Tg N/yr. Different latitudinal, altitudinal, and seasonal distributions of lightning NOx production were considered. Results of these model calculations show that the assessment of supersonic aircraft impacts on the ozone layer is very sensitive to the strength of NOx production from lightning. The high-speed civil transport produced NOx leads to positive column ozone changes for lightning NOx production less than 4 Tg N/yr, and to total ozone decrease for lightning NOx production more than 5 Tg N/yr for the same NOx emission scenario. For large lightning production the ozone response is mostly decreasing with increasing emission index, while for low lightning production the ozone response is mostly increasing with increasing emission index. Uncertainties in the global lightning NOx production strength may lead to uncertainties in column ozone up to 4%. The uncertainties due to neglecting the seasonal variations of the lightning NOx production and its simplified latitude distribution are about 2 times less (1.5–2%). The type of altitude distribution for the lightning NOx production does not significally impact the column ozone, but is very important for the assessment studies of aircraft perturbations of atmospheric ozone. Increased global lightning NOx production causes increased total ozone, but for assessment of the column ozone response to supersonic aircraft emissions, the increase of lightning NOx production leads to column ozone decreases in response to aircraft emissions.

Simulation of the indirect impact that the 11-year solar cycle has on the gas composition of the atmosphere

An interactive three-dimensional chemistry-climate model combining models of the gas composition and general circulation of the lower and middle atmosphere is used to study the impact of changes in extra-atmospheric solar radiative fluxes induced by solar activity on the stratospheric heating and subsequent temperature and ozone variations in the stratosphere and troposphere. The results have shown that a change in the atmospheric radiative heating resulting from variations in solar activity has a direct effect on the temperature and circulation of the atmosphere. Atmospheric temperature variations affect the rates of temperature-dependent chemical reactions, and this is considered the first type of indirect impact of solar activity on the atmospheric gas composition. On the other hand, as a result of the variation in atmospheric heating, its circulation changes, thus affecting the transport of minor gases into the atmosphere. This effect is considered the second type of indirect impact of solar activity on atmospheric gases. The results of our calculations have shown that both types of indirect impact of the variation in solar activity on the atmospheric gas composition are comparable in order of magnitude to the direct impact of solar activity on atmospheric gases.

Modeling the variability of gas and aerosol components in the stratosphere of polar regions

A thermodynamics-microphysics model of the formation and evolution of stratospheric clouds is developed. This model was integrated into the global chemistry-climate model of the lower and middle atmosphere. Model experiments on the study of the evolution of the gas and aerosol compositions of the Arctic and Antarctic atmosphere were performed. The results of an investigation into the observed differences of changes in the contents of gaseous impurities and aerosol in polar regions showed that the presence of nitrification in the Antarctic and its absence in the Arctic are the main factors controlling distinctions between the formation of a full-value ozone hole in the Antarctic and only occasional “mini-holes” in the Arctic.

Evaluation of simulated photolysis rates and their response to solar irradiance variability

The state of the stratospheric ozone layer and the temperature structure of the atmosphere are largely controlled by the solar spectral irradiance (SSI) through its influence on heating and photolysis rates. This study focuses on the uncertainties in the photolysis rate response to solar irradiance variability related to the choice of SSI data set and to the performance of the photolysis codes used in global chemistry-climate models. To estimate the impact of SSI uncertainties, we compared several photolysis rates calculated with the radiative transfer model libRadtran, using SSI calculated with two models and observed during the Solar Radiation and Climate Experiment (SORCE) satellite mission. The importance of the calculated differences in the photolysis rate response for ozone and temperature changes has been estimated using 1D radiative-convective-photochemical model. We demonstrate that the main photolysis reactions, responsible for the solar signal in the stratosphere, are highly sensitive to the spectral distribution of SSI variations. Accordingly, the ozone changes and related ozone-temperature feedback are shown to depend substantially on the SSI dataset being used, which highlights the necessity of obtaining accurate SSI variations. To evaluate the performance of photolysis codes, we compared the results of eight, widely used, photolysis codes against two reference schemes. We show that, in most cases, absolute values of the photolysis rates and their response to applied SSI changes agree within 30%. However, larger errors may appear in specific atmospheric regions because of differences, for instance, in the treatment of Rayleigh scattering, quantum yields or absorption cross-sections.

Influence of wave activity on the composition of the polar stratosphere

The planetary wave impact on the polar vortex stability, polar stratosphere temperature, and content of ozone and other gases was simulated with the global chemical–climatic model of the lower and middle atmosphere. It was found that the planetary waves propagating from the troposphere into the stratosphere differently affect the gas content of the Arctic and Antarctic stratosphere. In the Arctic region, the degree of wave activity critically affects the polar vortex formation, the appearance of polar stratospheric clouds, the halogen activation on their surface, and ozone anomaly formation. Ozone anomalies in the Arctic region as a rule are not formed at high wave activity and can be registered at low activity. In the Antarctic Regions, wave activity affects the stability of polar vortex and the depth of ozone holes, which are formed at almost any wave activity, and the minimal ozone values depend on the strong or weak wave activity that is registered in specific years.