Denis G. Dufour

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.

The ACE-MAESTRO instrument on SCISAT: description, performance, and preliminary results

The Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument on the SCISAT satellite is a simple, compact spectrophotometer for the measurement of atmospheric extinction, ozone, nitrogen dioxide, and other trace gases in the stratosphere and upper troposphere as part of the Atmospheric Chemistry Experiment (ACE) mission. We provide an overview of the instrument from requirements to realization, including optical design, prelaunch and on-orbit performance, and a preliminary examination of retrievals of ozone and NO(2).

Initial comparison of ozone and NO2 profiles from ACE‐MAESTRO with balloon and satellite data

[1] Atmospheric retrievals of ozone and NO 2 by the Measurements of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation (MAESTRO) instrument which is part of the Atmospheric Chemistry Experiment (ACE) satellite aboard SCISAT are compared statistically with coincident measurements by ozonesondes, the Fourier Transform Spectrometer (ACE-FTS) also aboard SCISAT, the SAGE III and the POAM III instruments. The ozone mixing ratio profiles from MAESTRO and ozonesondes agree within about 5-10% from 16-30 km in the northern middle and high latitudes. Further, ACE-FTS and MAESTRO ozone profiles agree within ∼5-15% from 16-50 km. MAESTRO ozone profiles show a systematic bias which is opposite for sunrise (SR) and sunset (SS) events and was also seen in comparisons with SAGE III and POAM III ozone. MAESTRO SS ozone profiles mostly agree within 5-10% from 16-40 km with either SAGE III or POAM III SR or SS retrievals, but show a significant high bias from 40-55 km, reaching a maximum of ∼20-30%. MAESTRO SR ozone profiles show a low bias of ∼5-15% from 20-50 km, as compared to SAGE III and POAM III SR or SS measurements. The NO 2 profiles agree within about 10-15% between ACE-FTS and MAESTRO from 15-40 km for the SR and 22-35 km for the SS measurements. Further, MAESTRO NO 2 profiles agree with SAGE III NO 2 mostly within 10% from 25-40 km. MAESTRO NO 2 profiles agree with POAM III SR profiles within 5-10% from 25-42 km. However, compared to POAM III SS profiles, MAESTRO NO 2 profiles show a low bias between 20 and 25 km (∼30-50%), a high MAESTRO bias between 25 and 32 km (10-30%), and again a low bias above 33 km that increases with altitude to 50-60%.

Calibration of instruments for atmospheric ozone measurements II: the ACE FTS and MAESTRO spectrograph

A novel and simple technique is described for the calibration of satellite instruments for the measurement of atmospheric ozone. Ozone is generated in a gas cell and spectral measurements of the ozone absorption are measured with a standard Fourier-transform spectrometer (FTS) in order to determine the amount of ozone in the cell. The satellite instrument then views the cell using an appropriate illumination source. In this presentation the preliminary results from the ozone calibration procedure are presented for the ACE FTS and MAESTRO instruments to show how consistently both instruments measure ozone. The thermal infrared band of ozone at 4.7 microns was used to provide the calibration of the ACE interferometer, whereas the Chappuis band at 600 nm was used to characterize the response of the MAESTRO instrument. The ozone transmission spectra that were derived from the ACE FTS and MAESTRO spectrograph measurements were found to be in good agreement with the simulated spectra of known amounts of ozone from a radiative transfer model. All of the results yielded column ozone amounts that were within 10% of each other. These calibration measurements were taken at the University of Toronto in March 2003, before the expected launch date of the SciSat-1 satellite in August 2003.

Laboratory procedure for simulating nadir measurements with the ACE-FTS

The Atmospheric Chemistry Experiment (ACE), or SCISAT mission, is a Canadian science satellite designed to investigate the chemical and dynamical processes that control the distribution of ozone in the stratosphere and upper troposphere. The ACE mission payload consists of two science instruments: a high-resolution infrared Fourier-Transform Spectrometer (FTS) and an ultraviolet/visible/near-infrared spectrometer. These instruments primarily function in occultation mode. However, during the dark portion of the orbit, the Earth passes between the Sun and the satellite. In this configuration, the ACE-FTS has the opportunity to acquire some nadir-view spectra of the Earth. Since the ACE-FTS was designed to view a hot source (i.e., the Sun) at high resolution using a single scan, it was necessary to determine if the ACE-FTS could also provide nadir spectra of the relatively cold atmosphere and surface with sufficient signal-to-noise ratio (SNR). As part of the pre-launch test program, laboratory measurements were performed to investigate this possibility. This paper reports the laboratory spectra of methane, ozone, and carbon monoxide gases measured with the ACE-FTS to determine the performance characteristics of the instrument when viewing a low-intensity blackbody source. From these results, it was shown that the ACE-FTS may be capable of measuring the column amounts of several abundant trace gases, such as methane, ozone, and nitrous oxide, in the atmosphere with sufficient SNR.

Evaluation of the ACE FTS for obtaining nadir measurements

The SciSat-1 mission is a dedicated Canadian science satellite that will investigate processes that control the distribution of ozone in the stratosphere. The SciSat-1 satellite consists of primarily two science instruments; an Atmospheric Chemistry Experiment (ACE) high-resolution Fourier-transform spectrometer (FTS) and an ultraviolet-visible-near-infrared spectrograph. These instruments will primarily function in occultation mode; however, during the dark portion of the orbit the Earth will pass between the sun and the satellite. This configuration will give rise to the opportunity of acquiring some nadir-view FTIR spectra of the Earth. Since the ACE FTS was designed to view a hot source (i.e., the Sun) at high resolution using a single scan, it is necessary to determine if the FTS will provide nadir spectra of the relatively cold atmosphere and surface with a sufficient signal-to-noise ratio. Methane, ozone and carbon monoxide gases were used in the cell for the purpose of determining the measurement characteristics of the ACE FTS instrument for a low-intensity source. These measurements were compared with data obtained from the Interferometric Monitor for Greenhouse (IMG) gases onboard the ADEOS satellite. The results show that the ACE FTS should be able to measure the abundant trace gases in the atmosphere with sufficient signal-to-noise ratio.

Characterization of simultaneous ozone measurements by the MAESTRO and FTS space instruments

Scisat-1 is a Canadian satellite scheduled for launch in December 2002. The mission objective is to study stratospheric ozone trends, with a particular focus on the polar regions where significant ozone loss has been observed in the past two decades. Measurements of solar attenuation by ozone and other stratospheric constituents are to be taken during occultation events (sunrises and sunsets) by the two instruments on board Scisat-1, ACE-MAESTRO (Atmospheric Chemistry Experiment - Measurement of Aerosol Extinction in The Stratosphere and Troposphere Retrieved by Occultation) and the ACE-FTS (ACE Fourier Transform Spectrometer). These instruments are designed to measure absorption spectra in the visible/near-ultraviolet and infrared regions, respectively; MAESTRO is a dual concave grating spectrometer while the FTS is a Michelson interferometer. One of the principal scientific objectives for both instruments is the retrieval of vertical ozone profiles. An important question to ask when characterizing the instruments prior to launch is: Do we measure the same amount from both instruments? In order to answer this question, an experimental apparatus that allows simultaneous instrument characterization, consisting of a simulated solar source and gas cells, has been assembled. The experimental apparatus and its use in the characterization of MAESTRO and FTS are described.