TEMPO was selected in 2012 by NASA as the first Earth Venture Instrument, for launch circa 2018. It will measure atmospheric pollution for greater North America from space using ultraviolet and visible spectroscopy. TEMPO measures from Mexico City to the Canadian tar sands, and from the Atlantic to the Pacific, hourly and at high spatial resolution (~2 km N/S×4.5 km E/W at 36.5°N, 100°W). TEMPO provides a tropospheric measurement suite that includes the key elements of tropospheric air pollution chemistry. Measurements are from geostationary (GEO) orbit, to capture the inherent high variability in the diurnal cycle of emissions and chemistry. The small product spatial footprint resolves pollution sources at sub-urban scale. Together, this temporal and spatial resolution improves emission inventories, monitors population exposure, and enables effective emission-control strategies.
TEMPO takes advantage of a commercial GEO host spacecraft to provide a modest cost mission that measures the spectra required to retrieve O3, NO2, SO2, H2CO, C2H2O2, H2O, aerosols, cloud parameters, and UVB radiation. TEMPO thus measures the major elements, directly or by proxy, in the tropospheric O3 chemistry cycle. Multi-spectral observations provide sensitivity to O3 in the lowermost troposphere, substantially reducing uncertainty in air quality predictions. TEMPO quantifies and tracks the evolution of aerosol loading. It provides near-real-time air quality products that will be made widely, publicly available. TEMPO will launch at a prime time to be the North American component of the global geostationary constellation of pollution monitoring together with European Sentinel-4 and Korean GEMS.
Following the successful launch of the Ozone Mapping and Profiler Suite (OMPS) aboard the Suomi National Polar-orbiting
Partnership (NPP) spacecraft, the NASA OMPS Limb team began an evaluation of sensor and data product
performance in relation to the original goals for this instrument. Does the sensor design work as well as expected, and
can limb scatter measurements by NPP OMPS and successor instruments form the basis for accurate long-term
monitoring of ozone vertical profiles? While this paper does not address the latter question, the answer to the former is a
qualified Yes given this early stage of the mission.
In the area of aerosol remote sensing, one of the more noteworthy points of the last decade has been the realization that dust and smoke can be sensed from space over land and ocean by utilizing observations of scattered ultraviolet light [Torres, et al. 1998]. The spectral contrast ratio available from the Total Ozone Mapping Spectrometer (TOMS) backscatter ultraviolet (buv) data does provide a wealth of qualitative information, such as the ability to track the global dispersion of dust and smoke from regional sources. Quantitative information, e.g. total optical depth, single scattering albedo, however, is more difficult to extract from buv data. Assumptions must be made concerning various parameters that influence buv observations, e.g. the height of the aerosol layer, surface albedo, aerosol size distribution and index of refraction. While the necessity of assumptions is due in part to the availability of only two wavelengths from historical TOMS data, these assumptions may not truly be needed for future sensors. We examine what can be gained from making measurements of polarization in addition to those of radiance (as is currently done by TOMS and its successor the Ozone Measuring Instrument, OMI, on EOS-AURA) in the TOMS spectral coverage range free from ozone absorption (340-380 nm). Measurements of the degree of linear polarization and the plane of polarization with an uncertainty of less than 0.005 would help to determine the aerosol layer height to within less than 1 km. Multi-angle measurements would also help to better utilize the polarization data by defining the particle effective radius.
The Ozone Mapping Profiler Suite will produce ozone profiles using the limb scatter technique. While this technique has been used in the 1980s for mesospheric retrievals with data from the Solar Mesospheric Explorer, its use for the stratosphere and upper troposphere is relatively recent. To increase the scientific experience with this method, the Limb Ozone Retrieval Experiment LORE was flown on-board STS107 in 2003. A significant amount of data from
thirteen orbits was down-linked during the mission and exists for analysis. LORE was an imaging filter radiometer, consisting of a linear diode array, five interference filters (plus a blank for dark current) and a simple telescope with color correcting optics. The wavelengths for the channels were 322, 350, 602, 675 & 1000 nm and can be viewed as a minimum set of measurements needed for ozone profiling from 50 km to 10 km. The temporal sampling of the channels, along with the shuttle orbital and attitude (e.g. pitch) motions present a challenge in retrieving precise ozone profiles. Presented are the retrieval algorithms for determination of the channel's altitude scale, cloud top height and aerosol extinction. Also shown are a sub-set of flight data and the corresponding retrieved ozone profiles.
One retrieval technique of ozone profiles using scattered light from the limb of the atmosphere utilizes measurements made high in the atmosphere as a reference. While this procedure relaxes the radiometric accuracy required, it accentuates the need for stray light characterization. In addition, when the entire limb (all altitudes of interest) is imaged simultaneously, as done by the Limb Ozone Retrieval Experiment (LORE) with a linear diode array, the stray light must be characterized for the reference altitude to within 1.0e-04 of the maximum signal in the field of regard (typically at the lowest altitudes). For this system this further translates into the need to know the spatial point-spread function over 5-6 orders of magnitude. We demonstrate the use of pre-flight laboratory instrument characterization, in flight observations and radiative transfer modeling to characterize the stray light of LORE during STS107.
The background and an approach for deriving tropospheric ozone, water vapor and aerosols from direct sun observations, zenith sky radiances and the ratio of radiances at large zenith (near horizon) to the corresponding zenith sky radiances is described. Surface based remote sensing measurements of zenith sky and large zenith angle atmospheric radiance measurements have been made in the wavelength range of 305 nm to 923 nm near local noon from a mountain site at an altitude of 2.6 km in the vicinity of Boulder, Colorado. The wavelength dependence of the limb/zenith radiance ratios indicate significant limb darkening in the ultraviolet that changes to limb brightening with increasing wavelengths that is a maximum in the blue. Model calculations of a molecular atmosphere with absorption by ozone for wavelengths between 300 and 400 nm indicates that this ratio is sensitive to tropospheric ozone. Results from measurements and model calculations of the sensitivity of these ratios to tropospheric ozone are presented.
A new instrument has been developed to measure spacecraft attitude which utilizes ultraviolet radiation scattered in the Earth's limb. The sensor consists of a very stable UV bandpass filter with a center wavelength at 355 nm, imaging optics, and a linear diode array detector. The radiance of the limb at this wavelength is dominated by Rayleigh scattering and typically decreases by 15% per kilometer above 20 km. The theoretical resolution at the limb of this device is 0.39 km per pixel for a nominal orbital altitude of 306 km (approximately equals 0.012 degree(s)) and represents a significant improvement over typical infrared-based attitude sensors which have an accuracy of approximately equals 0.1 degree(s). This system was integrated with the Shuttle Solar Backscatter Ultraviolet experiment and flown on STS-72 in January of 1996. The calibration and optical characterization of the device will be presented. Results from the first flight of this instrument, showing an agreement with available shuttle pointing data of +/- 0.05 degree(s), will also be discussed.
The possibility of utilizing limb scattering to monitor stratospheric ozone from space has been recognized as a possible remote sensing technique for many years. Unfortunately, due to the complexity of the radiative transfer problem associated with the spherical shell geometry and associated multiple scattering, the problem has received relatively little attention. While the complexity of the problem remains, the development of newer codes allows for the problem to now be tractable. Utilizing one such recently developed code, the authors have made a series of sensitivity studies relating changes in the limb radiances to changes in the ozone profiles. The calculations allow for the spherical geometry, include all orders of multiple scattering, and may include aerosols and other absorbing gases when relevant. Calculations of the changes are presented for a series of standard ozone profiles and for perturbed profiles. The results of these calculations indicate that measurement accuracies on the order of plus or minus 1% should provide adequate sensitivity to determine the significant structure of the ozone profile. Due to the complexity of the spherical, multiple scattering problem, direct inversion of the measured radiances is probably not feasible at this time. However, table look-ups provide a reasonable alternative, much as is currently done in the TOMS/SBUV retrieval algorithms. Results of the sensitivity studies for a range of conditions are presented.
The calibration method reported here makes use of the reflectances of several large, uniform areas determined from calibrated and atmospherically corrected SPOT Haute Resolution Visible (HRV) scenes of White Sands, New Mexico. These reflectances were used to predict the radiances in the first two channels of the NOAA-11 Advanced Very High Resolution Radiometer (AVHRR). The digital counts in the AVHRR image corresponding to these known reflectance areas were determined by the use of two image registration techniques. The plots of digital counts versus pixel radiance provided the calibration gains and offsets for the AVHRR. A reduction in the gains of 4 and 13 percent in channels 1 and 2 respectively was found during the period 1988-11-19 to 1990-6-21. An error budget is presented for the method and is extended to the case of cross-calibrating sensors on the same orbital platform in the Earth Observing System (EOS) era.