J. C. McConnell

Atmospheric Chemistry Experiment (ACE): Mission overview

SCISAT-1, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite mission for remote sensing of the Earths atmosphere. It was launched into low Earth circular orbit (altitude 650 km, inclination 74°) on 12 Aug. 2003. The primary ACE instrument is a high spectral resolution (0.02 cm-1) Fourier Transform Spectrometer (FTS) operating from 2.2 to 13.3 μm (750-4400 cm-1). The satellite also features a dual spectrophotometer known as MAESTRO with wavelength coverage of 285-1030 nm and spectral resolution of 1-2 nm. A pair of filtered CMOS detector arrays records images of the Sun at 0.525 and 1.02 μm. Working primarily in solar occultation, the satellite provides altitude profile information (typically 10-100 km) for temperature, pressure, and the volume mixing ratios for several dozen molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S. This paper presents a mission overview and some of the first scientific results. Copyright 2005 by the American Geophysical Union.

Ozone climatology using interactive chemistry: Results from the Canadian Middle Atmosphere Model

The climatology of ozone produced by the Canadian Middle Atmosphere Model (CMAM) is presented. This three-dimensional global model incorporates the radiative feedbacks of ozone and water vapor calculated on-line with a photochemical module. This module includes a comprehensive gas-phase reaction set and a limited set of heterogeneous reactions to account for processes occurring on background sulphate aerosols. While transport is global, photochemistry is solved from about 400 hPa to the top of the model at ∼95 km. This approach provides a complete and comprehensive representation of transport, emission, and photochemistry of various constituents from the surface to the mesopause region. A comparison of model results with observations indicates that the ozone distribution and variability are in agreement with observations throughout most of the model domain. Column ozone annual variation is represented to within 5–10% of the observations except in the Southern Hemisphere for springtime high latitudes. The vertical ozone distribution is generally well represented by the model up to the mesopause region. Nevertheless, in the upper stratosphere, the model generally underestimates the amount of ozone as well as the latitudinal tilting of ozone isopleths at high latitude. Ozone variability is analyzed and compared with measurements. The comparison shows that the phase and amplitude of the seasonal variation as well as shorter timescale variations are well represented by the model at various latitudes and heights. Finally, the impact of incorporating ozone radiative feedback on the model climatology is isolated. It is found that the incorporation of ozone radiative feedback results in a cooling of ∼8 K in the summer stratopause region, which corrects a warm bias that results when climatological ozone is used.

Autocatalytic release of bromine from Arctic snow pack during polar sunrise

Measurements and modeling studies strongly suggest that spring time depletion of ozone in the Arctic planetary boundary layer (PBL) is due to catalytic destruction by bromine atoms. However, the source of the bromine is uncertain. In this note, we propose that the source of the bromine at polar sunrise is the snow pack on the ice covering Arctic ocean and that it is released auto-catalytically, stimulated by a bromine seed from one of the brominated organic compounds, such as CHBr{sub 3}, by photolysis. In this manner {approximately}100 pptv of bromine can be transferred to the atmosphere where it can reside in the gas phase or, by scavenging, be partitioned in the aerosol or ice crystal phase. Moreover, it appears that heterogeneous recycling of bromine may be a process that self-terminates as ozone depletes to low levels. We also have included chlorine chemistry in the model in order to simulate inferred levels of chlorine atoms. This is important as it results in the production of HCHO which acts to convert post ozone depletion active bromine into HBr which is then returned to the snow pack or scavenged by aerosols or ice crystals. {copyright} American Geophysical Union 1996

Evidence for bromine monoxide in the free troposphere during the Arctic polar sunrise

During the Arctic polar springtime, dramatic ozone losses occur not only in the stratosphere but also in the underlying troposphere. These tropospheric ozone loss events have been observed over large areas, in the planetary boundary layer (PBL) throughout the Arctic. They are associated with enhanced concentrations of halogen species and are probably caused by catalytic reactions involving bromine monoxide (BrO) and perhaps also chlorine monoxide (ClO). The origin of the BrO, the principle species driving the ozone destruction, is thought to be the autocatalytic release of bromine from sea salt accumulated on the Arctic snow pack, followed by photolytic and heterogeneous reactions which produce and recycle the oxide. Satellite observations have shown the horizontal and temporal extent of large BrO enhancements in the Arctic troposphere, but the vertical distribution of the BrO has remained uncertain. Here we report BrO observations obtained from a high-altitude aircraft that suggest the presence of significant amounts of BrO not only in the PBL but also in the free troposphere above it. We believe that the BrO is transported from the PBL into the free troposphere through convection over large Arctic ice leads (openings in the pack ice). The convective transport also lifts ice crystals and water droplets well above the PBL, thus providing surfaces for heterogeneous reactions that can recycle BrO from less-reactive forms and thereby maintain its ability to affect the chemistry of the free troposphere.

Doubled CO2-induced cooling in the middle atmosphere : Photochemical analysis of the ozone radiative feedback

Doubled CO2-induced cooling in the middle atmosphere : Photochemical analysis of the ozone radiative feedback

Polar vortex evolution during the 2002 Antarctic major warming as observed by the Odin satellite

In September 2002 the Antarctic polar vortex split in two under the influence of a sudden warming. During this event, the Odin satellite was able to measure both ozone (O3) and chlorine monoxide (ClO), a key constituent responsible for the so-called “ozone hole”, together with nitrous oxide (N2O), a dynamical tracer, and nitric acid (HNO3) and nitrogen dioxide (NO2), tracers of denitrification. The submillimeter radiometer (SMR) microwave instrument and the Optical Spectrograph and Infrared Imager System (OSIRIS) UV-visible light spectrometer (VIS) and IR instrument on board Odin have sounded the polar vortex during three different periods: before (19–20 September), during (24–25 September), and after (1–2 and 4–5 October) the vortex split. Odin observations coupled with the Reactive Processes Ruling the Ozone Budget in the Stratosphere (REPROBUS) chemical transport model at and above 500 K isentropic surfaces (heights above 18 km) reveal that on 19–20 September the Antarctic vortex was dynamically stable and chemically nominal: denitrified, with a nearly complete chlorine activation, and a 70% O3 loss at 500 K. On 25–26 September the unusual morphology of the vortex is monitored by the N2O observations. The measured ClO decay is consistent with other observations performed in 2002 and in the past. The vortex split episode is followed by a nearly complete deactivation of the ClO radicals on 1–2 October, leading to the end of the chemical O3 loss, while HNO3 and NO2 fields start increasing. This acceleration of the chlorine deactivation results from the warming of the Antarctic vortex in 2002, putting an early end to the polar stratospheric cloud season. The model simulation suggests that the vortex elongation toward regions of strong solar irradiance also favored the rapid reformation of ClONO2. The observed dynamical and chemical evolution of the 2002 polar vortex is qualitatively well reproduced by REPROBUS. Quantitative differences are mainly attributable to the too weak amounts of HNO3 in the model, which do not produce enough NO2 in presence of sunlight to deactivate chlorine as fast as observed by Odin.

Stratospheric profiles of nitrogen dioxide observed by Optical Spectrograph and Infrared Imager System on the Odin satellite

[1] Vertical profiles of nitrogen dioxide in the 19–40 km altitude range are successfully retrieved over the globe from Optical Spectrograph and Infrared Imager System (OSIRIS) limb scatter observations in late 2001 and early 2002. The inclusion of multiple scattering in the radiative transfer model used in the inversion algorithm allows for the retrieval of NO2 down to 19 km. The slant column densities, which represent the observations in the inversion, are obtained by fitting the fine structure in normalized radiance spectra over the 435–449 nm range, where NO2 electronic absorption is readily observable because of long light paths through stratospheric layers rich in this constituent. Details of the spectral fitting and inversion algorithm are discussed, including the discovery of a pseudo-absorber associated with pixelated detectors and a new method to verify altitude registration. Comparisons are made with spatially and temporally coincident profile measurements of this photochemically active trace gas. Better than 20% agreement is obtained with all correlative measurements over the common retrieval altitude range, confirming the validity of OSIRIS NO2 profiles. Systematic biases in the number densities are not observed at any altitude. A ‘‘snapshot’’ meridional cross section between 40� N and 70� S is shown from observations during a fraction of an orbit. INDEX TERMS: 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 0394 Atmospheric Composition and Structure: Instruments and techniques; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); KEYWORDS: optical, Sun-synchronous, polar-orbiting, Fraunhofer, Ring effect, iterative onion peel

A self-consistent evaluation of the rate constants for the production of the OI 6300 Å airglow

Abstract The quenching rate k N 2 of O( 1 D) by N 2 and the specific recombination rate α 1 D of O 2 + leading to O( 1 D) are re-examined in light of available laboratory and satellite data. Use of recent experimental values for the O( 1 D) transition probabilities in a re-analysis of AE-C satellite 6300 A airglow data results in a value for k N 2 of 2.3 × 10 −11 cm 3 s −1 at thermospheric temperatures, in excellent agreement with the laboratory measurements. This implies a value of J O 2 = 1.5 × 10 −6 s −1 for the O 2 photodissociation rate in the Schumann-Runge continuum. The specific recombination coefficient α 1 D = 2.1 × 10 −7 cm 3 s −1 is also in agreement with the laboratory value. Implications for the suggested N ( 2 D) + O 2 → O( 1 D) + NO reaction are discussed.

Canadian middle atmosphere model: Preliminary results from the chemical transport module

Abstract An important objective of middle atmosphere global climate modelling is the development of the capability of predicting the response of the middle atmosphere to natural or anthropogenic perturbations. To achieve this, a comprehensive chemistry package interactively coupled with radiative and dynamical modules is required. This paper presents preliminary results obtained with a photochemistry module which has been incorporated in the Canadian Middle Atmosphere Model (CMAM). The module contains 42 species including necessary oxygen, hydrogen, nitrogen, chlorine, bromine and methane oxidation cycle species. Photochemical balance equations are solved on‐line throughout the middle atmosphere at every dynamical time step. A full diurnal cycle is simulated with photolysis rates provided by a look‐up table. The chemistry solver is a mass conserving, fully implicit, backward difference scheme which currently uses less than 10% of the GCM run time. We present the results obtained from short integrations an...

Simulation of the October-November 2003 solar proton events in the CMAM GCM: Comparison with observations

The FTS instrument on SciSat-I observed a very large NO(x) anomaly in mid February of 2004 near 80 N in the lower mesosphere. It has been proposed that the most likely origin of the lower mesosphere anomaly in February is transport, from the lower thermosphere or upper mesosphere, of high levels of NO(x) associated with high levels of solar activity in 0ct.-Nov. 2003. There was no major solar flare activity during January and February to cause ionization in the mesosphere. Using a middle atmosphere GCM we investigate whether the NO(x) produced directly by the 0ct.-Nov. 2003 solar flares or indirectly via enhanced auroral ionization as a result of magnetospheric precipitation can explain the ACE observations. We find that the solar proton events associated with the solar explosions in 0ct.-Nov. 2003 produce insufficient amounts of NO(x), in the mesosphere and thermosphere (less than 2 ppm at 90 km) to give rise to the observed anomaly. However. there is evidence that intense aurorae caused by the 0ct.-Nov. 2003 solar storms produced thermospheric values of NO(x) reaching hundreds of ppm. The NO(x) created by the auroral particles appears to have lasted much longer than the immediate period of the 0ct.-Nov. 2003 solar storms. It appears that NO(x) rich air experienced confined polar night descent into the middle mesosphere during November and December, prior to the onset of the strong mesospheric vortex in January 2004.