The Atmospheric Chemistry Experiment (ACE) is a Canadian satellite mission for remote sensing of the Earth's atmosphere that was launched into a high-inclination (74°), circular low-earth (650 km from the surface) orbit on August 13, 2003. This orbit gives SCISAT coverage of tropical, mid-latitude, and polar regions, allowing it to study a range of atmospheric processes. The main goal of ACE is to study the atmospheric chemistry and dynamics that affect stratospheric ozone depletion, but ACE measurements are also being used to study ozone depletion in the Antarctic, the relationship between chemistry and climate change, the atmospheric effects of biomass burning, the effects of aerosols and clouds on the global energy balance, and many other areas of atmospheric science.

The primary instrument on SCISAT is the ACE-FTS, a high spectral resolution infrared Fourier Transform Spectrometer (FTS). SCISAT-1 also features a UV-visible-NIR spectrophotometer known as MAESTRO. Working primarily in solar occultation mode, the satellite provides altitude profile information (typically 10-100 km) for temperature, pressure, and the volume mixing ratios (VMRs) of dozens of molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S.

As SCISAT orbits the Earth, the FTS measures sequences of atmospheric absorption spectra during sunrise and sunset. These spectra, measured in the limb viewing geometry with different slant paths and tangent heights are inverted to obtain vertical profiles of temperature, pressure and many atmospheric trace gases with a vertical resolution of about 4 km from the cloud tops up to about 150 km. In addition, a pair of filtered imagers make high signal-to-noise ratio (SNR) measurements of atmospheric extinction for the monitoring of aerosols and clouds and provide an important diagnostic for the variation of the flux over the distorted solar disk in this region. MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation) is a dual optical spectrophotometer that covers the 285-1030 nm spectral region. It has a vertical resolution of 1-2 km and measures primarily ozone, nitrogen dioxide, and aerosol/cloud extinction.

The ACE-FTS mission concept is based on the ATMOS (Atmospheric Trace MOlecule Spectroscopy) instrument that NASA flew four times (1985, 1992, 1993, and 1994) on the Space Shuttle but the ACE-FTS has been miniaturized by nearly a factor of 10 in mass, power, and volume as compared to ATMOS. With fewer than 400 occultations, ATMOS has made and continues to make valuable contributions to atmospheric science. The ACE-FTS will measure well over 20,000 occultations over its mission lifetime and is augmented by measurements from the two imagers and MAESTRO. In combination, these instruments are now providing a wealth of scientific data that will be used to help improve our understanding of processes in the Earth's atmosphere for years to come.

This information is a combination of text from the following papers:

Bernath, Peter (2017), The Atmospheric Chemistry Experiment (ACE), Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 186, pp. 3-16

Bernath, Peter (2006), Atmospheric Chemistry Experiment (ACE): Analytical Chemistry from Orbit, Trends in Analytical Chemistry, Vol. 25, No. 7., pp. 647-654

Bernath, Peter (2005), Atmospheric Chemistry Experiment: Spectroscopy from Orbit, Optics & Photonics News, Optical Society of America, pp. 24-27.

Bernath, Peter (2001), The Atmospheric Chemistry Experiment (ACE): An Overview, Spectroscopy from Space, Kluwer Academic Publishers, pp. 147-161.