Chemistry of the Mesosphere I
Presiding: A K Smith, NCAR Atmospheric Chemistry Division; D Marsh, NCAR Atmospheric Chemistry Division
SA33A-01 13:35h
Causes and Consequences of Mesospheric Water Vapor Layers
Mesospheric observations from the UARS HALOE experiment have shown the existence of distinct water vapor layers both at low and at high latitudes. It has been speculated that the former are a consequence of heterogeneous chemistry acting on meteoric dust, whereas the latter are apparently associated with phase changes associated with the formation and evolution of polar mesospheric clouds. Further analysis of the HALOE water vapor observations suggest that there are two separate layers of enhanced water vapor at high latitudes and that there is a physical and/or chemical connection between the low latitude layer and the two high latitude layers. In this talk we will 1) use HALOE observations to characterize these mesospheric water vapor layers and their annual variation, 2) discuss the role of heterogeneous chemistry and surface/gas exchange on meteoric dust as a source of the lower altitude water vapor layer, and 3) explore the possibility that this altitude layer is causally connected to the formation and evolution of polar mesospheric clouds.
SA33A-02 13:55h
Modeling of Mesospheric ice Particles:Challenges and Assessment of Current Capabilities
Considerable progress has been made in the past years concerning the experimental capabilities to observe mesospheric ice particles from space, from the ground, and in situ. These observations have provided a wealth of experimental data defining the properties of mesospheric ice particles like their global spatial and temporal occurrence patterns, their light scattering signatures, as well as their interaction with the charged part of the atmosphere, i.e., the ionosphere. Despite this progress regarding the observational data base, a quantitative description of related physical and chemical processes is still a challenging task due to uncertainties of several microphysical aspects concerning ice evolution in the harsh environment of the polar summer mesopause region. In the current paper, we discuss the key uncertainties regarding the microphysics of mesospheric ice particles including issues like current knowledge of the water vapor saturation pressure at mesopause temperatures, particle sedimentation characteristics, ice particle nucleation, and the effect of turbulent transport on the ice particle size distribution. The sensitivity of ice particle properties towards these uncertainties is assessed using the CARMA (=Community Aerosol and Radiation Model for Atmospheres) microphysical model. Mie scattering calculations using the simulated size distributions are compared to published characteristics of mesospheric ice particles observed from space and from the ground.
SA33A-03 14:10h
The Seasonal and Latitudinal Variation of the OH (v = 0) Concentration in the Mesosphere
The OSIRIS instrument on the Odin satellite makes limb measurements of both the scattered sunlight and the airglow. While the intensity of the 300 nm scattered spectrum is large in the mesosphere it is possible to extract the signal from the NO-g and the OH (A X) bands, both systems arise from solar fluorescence although the driving wavelengths are quite disparate. For the OH emission the line-of-sight limb radiances are retrieved and converted to OH (v = 0) concentration profiles. These profiles exhibit a strong daytime variation dependent on altitude, latitude and season. In this paper some of these new findings are presented and their association with variations in the mesospheric water content discussed.
SA33A-04 INVITED 14:25h
Mesospheric influence on stratospheric ozone: energetic particle precipitation
In the stratosphere between about 25 and 40 km, the NOx (NO + NO2) catalytic cycle is the main ozone loss mechanism. Crucial to our attribution and prediction of stratospheric ozone trends, therefore, is understanding stratospheric NOx variability. The mesosphere plays a critical, but still poorly quantified, role in controlling stratospheric NOx. The primary source of stratospheric NOx is oxidation of N2O. A second source is energetic particle production of NO. Highly energetic protons or electrons are required to produce NO directly in the stratosphere, and since these occur relatively rarely, they are unlikely to play a major role in determining ozone trends. Energetic particle production of NO in the mesosphere or thermosphere, however, is much more routine, and descent of the NO so produced can significantly perturb stratospheric ozone distributions. This presentation focuses on the role that NOx descent in the mesosphere plays in influencing stratospheric ozone variability. The long-term effect of energetic particles on stratospheric ozone depends on the energy (deposition altitude) and flux (NOx production) of energetic particles, the timing and location of the impacts (e.g., in sunlight or not), and on the efficiency with which the NOx is transported downward. Using satellite observations and modeling, all of these different criteria are investigated with a view toward understanding long-term effects of mesospheric processes on stratospheric ozone. Particular emphasis is placed on the contribution of medium energy electrons (30-300 keV), which deposit their energy in the mesosphere, to observed NOx variability.
SA33A-05 14:45h
Seasonal and Solar Cycle Atmospheric Meteoric Metal Content Changes
High temporal resolution vertical column contents of the most dominant meteoric metal species, Mg+, Mg, Fe+, Fe, and Si have been measured pole-to pole at ~ 10:30 AM using the nadir-viewing Global Ozone Measuring Experiment (GOME) UV/VIS spectrometer on the ERS-2 satellite. Emphasis initially was placed on the month of November and latitudes between 10 and 30 degrees, which encompass the Leonid meteor shower period and radiant location. Metals were more abundant in 2000 than in 1996 showing a solar cycle dependence. This reflects atmospheric control of the ablation and the metal ion and neutral chemical processes. This atmospheric control, rather than changes in the meteoroid influxes is also demonstrated by the changes in the metal abundances for equinox winter and summer months. To compare the solar activity influence to that of the seasonal atmospheric changes, the variations of the metal species vertical contents were extracted from the GOME measurements for monthly periods centered on the June and December solstices and on the March and September equinoxes for both 1996 and 2000, as in the first study.