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Remote Sensing

Satellite ozone profiles checked using ground-based laser radars

Laser radar observations from 13 widely distributed sites confirm the high data quality of GOMOS, the first operational satellite instrument that measures ozone profiles using stellar occultations.
28 June 2006, SPIE Newsroom. DOI: 10.1117/2.1200605.0263

Public concerns about human impacts on Earth's atmosphere have been growing because of ozone layer decline, climate change, and air pollution increase. Beginning in the late 1980s, clear evidence of this impact led to the Montreal protocol for protecting the ozone layer. The further monitoring of human impacts and the effectiveness of this agreement and its amendments critically relies on the availability of key atmospheric state parameters.

Ozone is a key atmospheric trace gas species. Although its presence at high altitudes provides protection against harmful ultraviolet radiation, too much ozone in the boundary layer—better known as ‘smog’—poses a threat to human health. Therefore, it is important to observe its vertical distribution on a global scale. From the ground, the ozone layer can be monitored very accurately with a high vertical resolution using balloon sondes and laser radars. However, extending these high-quality data to a global scale requires space-borne instruments.

A wide variety of remote sensing techniques has been applied to measure ozone in the stratosphere (about 10–50km altitiude; see Figure 1). Solar and lunar occultation-viewing instruments, such as the Stratospheric Aerosol and Gas Experiment (SAGE)1 instrument, provide a high vertical resolution, but with limited global coverage. Nadir-viewing spectrometers, such as the Solar Backscatter Ultraviolet (SBUV)2 instrument provide data with much better global coverage, but with a much lower vertical resolution. Spectrometers viewing the scattered sunlight in the atmospheric limb, such as the SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY)3 instrument, provide good coverage and resolution, but their observations are very sensitive to instrument calibration and pointing of the satellite platform.

Figure 1. Different viewing geometries are used for detecting ozone in the atmosphere from space.

The Global Ozone Monitoring by Occultation of Stars (GOMOS)4 instrument observes stars above and subsequently through the atmosphere, providing spectra with ozone absorption features. Specific advantages of the stellar occultation principle are its self-calibrating property, well-known pointing information, high vertical resolution, and good global coverage due to the multitude of available stars.

Since its launch in March 2002 aboard the European Space Agency's (ESA) polar-orbiting environmental satellite Envisat, GOMOS has measured over 350,000 occultations. Such an operational exploitation of the stellar occultation technique makes the GOMOS observations unique. The measurements have been subjected to thorough quality assessments to identify any dependencies on geophysical (e.g., latitude) and observational (e.g., star characteristics) parameters. This assessment is especially important since GOMOS continuously observes stars that have different visual magnitudes and (equivalent blackbody) temperatures.

The Envisat QUality Assessment with Lidar (EQUAL) project provides long-term validation support to Envisat's three atmospheric chemistry instruments, including GOMOS. This ESA-funded project involves 13 LIght Detection and Ranging (LIDAR) stations around the world (see Figure 2) measuring ozone and temperature profiles. DIfferential Absorption Lidar (DIAL) systems emit high-energy (∼200mJ) ultraviolet laser pulses at two different wavelengths (usually around 308 and 353nm). The differential absorption effect of ozone is exploited to derive ozone profiles between 10- and 50-km altitude, with a vertical resolution of ∼1–4km. Most systems are operated manually and generally at nighttime (see Figure 3). Between 2002 and 2006, more than 4,000 LIDAR profiles were generated for quality assessment studies.

Figure 2. This logo of the Envisat Quality Assessment with Lidar project shows the locations of the 13 Light Detection and Ranging (LIDAR) stations.

Figure 3. Laser beams probe the night sky above Lauder, New Zealand, taking LIght Detection and Ranging LIDAR measurements. The photo was taken using a 60 second exposure time during a full moon.

A comprehensive quality assessment was performed on GOMOS data collected during the first year after launch5. The processing has since been improved, and the whole mission data set has been reprocessed. GOMOS ozone profiles (version IPF 5.0) show excellent agreement with LIDAR (see Figure 4). Furthermore, the complete assessment confirms that the high data quality is independent of star characteristics, interannual effects, and latitude region, with only a slight (5%) negative bias in Polar regions.

Figure 4. Global Ozone Monitoring by Occultation of Stars (GOMOS) ozone profiles (measured in a dark atmospheric limb) are compared to collocated high-quality LIDAR measurements. Left panel shows mean ozone profiles. Right panel shows their relative differences.

The high data quality and the prospect of the Envisat mission lasting until 2010 offer the potential to use GOMOS measurements for a wide variety of research purposes. This includes studies for ozone monitoring, as well as in numerical weather forecast models. Envisat data are freely available to scientific users, who may make inquiries at ESA's Earth Observation Helpdesk (eohelp@esa.int) or its web portal (http://eopi.esa.int/). The EQUAL project will continue to provide up-to-date quality assessment studies.

The author is grateful for the contributions and support of ESA, and the GOMOS and EQUAL teams.

Yasjka Meijer
Laboratory for Environmental Monitoring (LVM), RIVM
Bilthoven, Utrecht
The Netherlands
Yasjka Meijer is a project leader and research scientist at the National Institute for Public Health and the Environment (RIVM) in Bilthoven, The Netherlands, where he manages the EQUAL project and the RIVM stratospheric ozone LIDAR system, which is based in Lauder, New Zealand. He obtained his PhD in Physics at Eindhoven University of Technology and his Master's degree in Physics at the Free University in Amsterdam.