Over the last decade it has been established that medium- to long-range weather patterns are significantly affected by
stratospheric events, and it is well known that the severe storms are critically dependent on the winds aloft. However,
existing observations of the dynamical atmosphere above the cloud tops are sparse and irregular, and remote
measurements of upper atmospheric winds have historically proved challenging. Current numerical models must rely on
data from widely separated land-based instruments and satellite observations with modest precision and coverage. Even
less plentiful are observations of the neutral atmosphere above 100 km, despite the high potential impact of space
weather on global navigational and electrical systems. We present here a new instrument concept, the Doppler Wind
and Temperature Sounder (DWTS) that will enable global daily measurements of winds and temperature from 15 to 250
km with fine vertical and horizontal sampling. The measurement concept leverages the high spectral resolution
inherently available with gas-filter correlation radiometry. By exploiting the Doppler shifts resulting from a limbviewing
low Earth orbit satellite, DWTS spectrally resolves large ensembles of atmospheric emission features. From
this, we are able to extract horizontal wind vectors and kinetic temperature with unprecedented precision. Here we
review the DWTS measurement concept, present simulation results, and conclude by describing a low-cost operational
system that would quantify atmospheric dynamics from the lower stratosphere into the mid thermosphere for the first
One of the gases which potentially can interfere with the remote sensing of gases of military interest is carbon tetrachloride (CCL4). Carbon tetrachloride is also a strong greenhouse gas with a global warming potential of 4,000. Ground-based, thermal emission measurements of the cold, clear sky have been made showing the v3 fundamental emission band of carbon tetrachloride (CCL4) which is located in the 786-806 cm-1 region. A spectrum of the non-CCl4 background emission features has been simulated using the FASCD3P line-by-line radiation code with measured radiosonde parameters of pressure, temperature and humidity. The simulated spectrum has been used to extract the CCl4 thermal emission band from the atmospheric emission spectrum. Troposhperic CCl4 mixing ratios of 120±20 in 1995 and 135±10 pptv in 2003 were determined from these measurements. In addition, the downward long-wave flux associated with the v3 emission band of CCl4 measured at the surface has been estimated to be 0.046 W/m2 ± 17%. This flux is about one third and one fifth of that corresponding to the chlorofluorocarbons CFC-11 and CFC-12, respectively.
There has been little experimental verification of the radiative forcing from air pollutants under cloudy conditions. This paper reports on the progress which has been made towards validating the predictions of the climate forcing associated with air pollutants. Measurements have been taken over the last three years with a new technique which was developed to measure the greenhouse radiative fluxes from greenhouse gases beneath clouds. These measurements are valuable since there are large spatial and temporal variations in some gases which make it difficult to quantify their climate forcing. As a result of the poor state of knowledge of the radiative forcing associated with prime constituents of smog such as nitric acid or PAN are omitted in the Kyoto protocol for the reduction of greenhouse gases. In our technique, measurements of the surface radiative forcing from the gases below the cloud are taken against the cold black body background of the cloudy sky. Radiative fluxes from ozone, carbon monoxide, nitrous oxide, nitric acid and aerosols have been measured. This technique may have applications to battlefield remote sensing of gases.
Our measurements have been made at 44N over all four seasons. In order to further decrease the uncertainty of the tropospheric forcing, many more measurements need to be made at different latitudes and climates.
The primary objective of the Canadian SciSat-1 mission is to investigate the processes that control the distribution of ozone in the stratosphere. The SciSat-1 satellite consists of two major science instruments; an Atmospheric Chemistry Experiment (ACE) high-resolution Fourier-transform spectrometer (FTS) and an ultraviolet/visible/near-infrared spectrograph. These instruments primarily function in occultation mode; however, during the dark portion of the orbit the Earth passes between the sun and the satellite. This configuration provides the opportunity of acquiring some nadir-view FTIR spectra of the Earth. Preliminary nadir spectra obtained with the ACE FTS are presented and analyzed for methane, ozone and nitrous oxide. Applications of these measurements to the study of global warming and air pollution monitoring are discussed.
The Atmospheric Chemistry Experiment (ACE) was launched in August 2003 on board the Canadian scientific satellite SciSat-1. The ACE payload consists of two instruments: ACE-FTS, a high resolution (0.02 cm-1) Fourier transform infrared spectrometer and MAESTRO (Measurement of Aerosol Extinction in the Stratosphere and Troposphere Retrieved by Occultation), a dual UV-visible-NIR spectrograph. Primarily, the two instruments use a solar occultation technique to make measurements of trace gases, temperature, pressure and atmospheric extinction. It will also be possible to make near-nadir observations with the ACE instruments.
The on-orbit commissioning of the instruments and spacecraft were undertaken in the months following launch. At the end of this period, a series of science-oriented commissioning activities were undertaken. These activities had two aims: the first was to verify and extend the measurement results obtained during the pre-launch Science Calibration Test campaign and the second was to confirm appropriate parameters and establish procedures for operational measurements (occultation and near-nadir observations and exo-atmospheric calibration measurements). One of the most important activities was to determine the relative location of each instrument field of view and optimize the pointing of the sun-tracker to provide the best viewing for both instruments.
A technique for the remote sensing of the forcing function of global warming (i.e., the increase in the surface radiative forcing) is described. Climate models predict that the emission of greenhouse gases into the atmosphere has altered the radiative energy balance at the earth's surface through increasing the greenhouse radiation from the atmosphere. With measurements at high spectral resolution, this increase can be unambiguously attributed to each of several anthropogenic gases. Calibrated spectra of the greenhouse radiation from the atmosphere have been measured at ground level from Peterborough, Ontario using an FTIR spectrometer with a resolution of 0.1 cm-1. This long-wave radiation consists of the thermal emission from naturally occurring gases, such as CO2, H2O and O3, as well as from many trace gases, such as CH4, CFC11, CFC12, CFC22 and HNO3. The forcing radiative fluxes from CFC11, CFC12, CCl4, N2O, CH4, HNO3 and CO2 have been quantitatively measured. Measurements of the fluxes associated with each gas are presented. Models indicate that an energy flux imbalance of about 3 W/m2 has been created by anthropogenic emissions of greenhouse gases of which we have measured over 1.0 W/m2. Much of the remaining flux change is due to CO2 and CH4 increases which are difficult to measure without a long baseline data series. Overall, it has been demonstrated that the greenhouse radiation is measurable and it should be monitored on a worldwide basis over the long term since the predicted increase in this energy flux is the fundamental forcing of global warming.
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.
The SciSat-1 satellite 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. Hence, preliminary tests were performed on the ACE FTS instrument using a background source that provided a radiative contrast of about 100 K with the gas in a cell, thereby approximately simulating the atmospheric temperature conditions of the Earth. Methane, ozone and carbon monoxide gases were used in the cell for the purpose of determining the measurement characteristics of the ACE FTS instrument with respect to the nadir radiation emanating from the planet’s surface and atmosphere over most of the thermal infrared region. The signal-to-noise ratio from the laboratory test measurements is used to estimate the error on column measurements of carbon monoxide and other gases.
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.
The feasibility of using nadir observations to make column measurements of several stratospheric gases will be evaluated for the Atmospheric Chemistry Experiment (ACE) Fourier-transform spectrometer (FTS), which is scheduled for launch in 2002 on the SCISAT-1 platform. The measurement technique is based on using FTIR spectroscopy to measure the atmospheric absorption of cold gases below the satellite against the thermal emission background from the warm earth. The FASCOD3 and MODTRAN4 transmission codes are used to simulate the background emission spectra above the earth; the measured spectra are processed to yield the column concentration of a particular gas in the atmosphere. The gases that can be successfully measured with this technique include ozone, carbon dioxide, carbon monoxide, methane and nitrous oxide. This technique will be demonstrated for nadir IMG spectra obtained in 1997 at a resolution of 0.1 cm-1. Since ACE points at the sun throughout its orbit, even when the earth is in the way, nadir FTS measurements will automatically be taken in addition to the occultation measurements, if enough power is available. A nadir observation will consist of 100 co-added scans at a resolution of 0.4 cm-1, which will require a total time of 16 seconds and achieve a signal-to-noise ratio of 80:1. Using the spectroscopic structure obtained from nadir IMG spectra at a resolution of 0.1 cm-1 over an extended spectral interval, it will be demonstrated that a signal-to-noise ratio of 50:1 gives ozone columns with an error of less than 2%.
A new technique for measuring air pollutants in the lower part of the troposphere has been developed. The technique is based on Fourier-transform infrared (FTIR) spectroscopy, which is used to measure the atmospheric thermal emission from gases beneath uniform cloud cover. The cloud acts as a cold background emission source against which the emission from gases in the warmer atmosphere beneath the cloud may be detected. The region of the infrared spectrum near 2400 cm-1, which is nearly void of significant atmospheric water vapour emission, is used to infer the cloud base temperature. The FASCOD3 atmospheric transmission code is used to simulate the background emission spectrum below the cloud, which is then subtracted from the measured spectrum to yield the thermal emission band of a particular gas. Based on the band intensity, the average concentration of the gas in the lower atmosphere may be determined. In order to have sufficient detection sensitivity, the cloud base must exceed an altitude of about 1 km. The gases that have been successfully measured with this technique include tropospheric ozone, carbon dioxide, carbon monoxide and nitrous oxide. By comparing the tropospheric ozone amounts to the surface amounts measured with an ozone analyser over the past two summers, it was discovered that the ozone residing in the lower troposphere sometimes has a concentration that is nearly twice the value recorded at the surface. This result has important implications concerning air pollution models, which normally incorporate ozone amounts from meteorological stations at the surface.
The Stratospheric Wind Interferometer For Transport studies (SWIFT) is a passive sensor designed to measure winds in the stratosphere from a satellite. It is a field-widened Michelson interferometer very similar to the WINDII instrument on UARS but operates in the mid-IR, where it detects the Doppler shifts of atmospheric thermal emission lines of ozone. SWIFT uses a HgCdTe array detector to view the emission at the Earth's limb. Measurements are subsequently inverted by computer to obtain true vertical profiles of the stratospheric wind in the altitude range 20 to 40 km. Two orthogonal fields of view allow wind vectors to be obtained by combining the components observed from different directions a few minutes apart. Prototype Ge wafer etalon filters and a field-widened Michelson interferometer for the Mid-IR have been built and tested, with good results. Modeling studies indicate that a measurement precision of 5 m/s can be obtained throughout the altitude range of interest. In addition to the winds, SWIFT will measure ozone densities in the stratosphere. SWIFT has been selected for flight on NASDA's GCOM-A1 satellite and a Phase A study is being supported by ESA and the Canadian Space Agency.