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AGACC (Advanced exploitation of Ground-based measurements for Atmospheric Chemistry and Climate applications)

Veerle De Bock, Roeland Van Malderen, Hugo De Backer
Royal Meteorological Institute
Ringlaan 3,
B-1180 Brussels, Belgium

Contact: Veerle De Bock, Roeland Van Malderen

 

AGACC - Subject and objectives

In brief, the subject of the project is an advanced exploitation of existing and current ground-based remote-sensing measurements for the study of a selection of atmospheric species that are related to climate and atmospheric chemistry.

The general scientific objectives of the proposal are to derive new and improved datasets for the target geophysical parameters and to carefully characterize the data products, thereby exploiting possible synergies and complementarities between different instruments operated at the same site. The target parameters are:

  • lower tropospheric aerosols, water vapor (H2O) and its HDO isotopologue in the troposphere and lower stratosphere,
  • methane (CH4) and HCFCs that have a direct impact on climate,
  • carbon monoxide (CO), non-methane hydrocarbons, hydrogen cyanide (HCN), formaldehyde (H2CO), which are source gases that influence the oxidizing capacity of the atmosphere, and therefore indirectly also the climate.

A more detailed description of the project is available on the AGACC-website.
The project is funded by the Belgian Federal Public Planning Service for Science Policy under the theme "Climate and Atmosphere" of the "Science for a Sustainable Development (SSD)" Research Program. AGACC is a collaboration between the Belgian Institute for Space Aeronomy (BIRA-IASB), the RMIB, the Université Libre de Bruxelles (ULB) - Service de Chimie Quantique et Photophysique, and the Institute of Astrophysics and Geophysics of the University of Liège (ULg) - Groupe Infra-Rouge de Physique Atmosphérique et Solaire (GIRPAS).

 

AGACC - Water vapor

The abundance and vertical distribution of water vapor in the atmosphere influences very strongly convection and cloudiness, therefore the albedo of the planet as well as the infrared opacity of the atmosphere.

Of particular interest is the upper troposphere - lower stratosphere (UTLS) layer, because of the dynamical coupling existing between these two regions of the atmosphere. The humidity field in the UTLS is of fundamental importance for all aspects of the chemistry and physics, in particular with respect to radiation, cloud dynamics, energy transport or aerosol processing. Thus, it is directly linked to a sound assessment of the future climate. A recent increase in stratospheric water abundance has been demonstrated, and possible effects on ozone and climate have been emphasized. The exact cause of this trend is still poorly understood.

 

AGACC - Aerosols

Much of the current research in the field of atmospheric aerosols arises because of the interrelations between aerosols, clouds, radiation, and climate. The influence of aerosols on atmospheric chemistry and physics, and in particular on the Earth's radiation budget, is complex as it depends on the composition, number and size distribution, and shape of the aerosol particles. Aerosols influence the Earth's radiation budget via the so-called direct effect (scattering, absorbance, and emittance of solar and thermal radiation). Additionally, aerosols have also an indirect impact on the Earth's radiation budget. As they act as cloud condensation nuclei, an increased levels of aerosols can enlarge the amount and reduce the size of water cloud droplets, increase the reflectance for solar radiation, increase the absorption of UV-B radiation, reduce precipitation, and change the cloud's lifetime. Clouds in turn are an important regulator of the Earth's radiation budget.

Overall there exists a great lack of detailed and comprehensive knowledge on the optical properties of aerosols and the respective feedback mechanisms on clouds and the radiative budget. This causes one of the largest uncertainties in radiative forcing and climate change assessments.

 

RMIB Contribution

Water vapor

At Ukkel, the RMIB has measured Vaisala radiosonde profiles of atmospheric relative humidity since January 1990. These humidity profiles have not been exploited up to now, because radiosondes are known to yield no reliable results at low temperatures and low absolute humidity. Recently, two groups have independently developed correction methods to retrieve unbiased and improved relative humidity data from the soundings. These correction algorithms will be applied to both new and old radiosonde data. Nevertheless, also a careful assessment of the used Vaisala radiosondes (RS80-A, RS90-H, and RS92-H) is required, as the types changed over the years and the corrections have to be adapted accordingly. An intercomparison of radiosondes and Fourier Transform Infrared (FTIR) spectrometer measurements of water vapor is also planned for validation purposes and improvement of the data products during simultaneous measurements at the Ukkel Plateau.

As water vapor is highly variable in the upper troposphere and tropopause region, the high temporal and vertical resolution of the radiosondes (one measurement every 10 seconds, i.e. ~100 m resolution) up to altitudes of 30-35 km, will be ideal for assessing the UTLS relative humidity field. The long duration of over 15 years of radiosonde data at Ukkel constitutes a unique data set, which will also allow for trend and climatological analyses.


Aerosols and UV index

During the project, an intercomparison campaign is set up at Ukkel between three instruments:

  • an UV/visible spectrometer using the Multi-Axis Differential Optical Absorption Spectroscopy method (MAXDOAS), which will be used to infer vertically resolved information on aerosol properties in the 0-3 km altitude range,
  • the Brewer ozone spectrophotometer of the RMIB, which measures the total Aerosol Optical Depth (AOD) in the UV-B range,
  • a CIMEL sun photometer, which yields very precise (1%) values for AOD, and information on the scattering phase function.

This unique combination of instruments will provide an unprecedented data set for a more comprehensive characterization of the tropospheric aerosol optical properties (AOD, single scattering albedo, aerosol type, size, refractive index). Additionally, this intercomparison provides a unique opportunity to assess the performance of the individual instruments, and in particular of the Brewer AOD retrieval.

Using the AOD and aerosol type information determined during the project, it will be possible to investigate the impact of aerosols on the UV index and to improve the operational UV index prediction at the RMIB. This will in turn provide better information to the general public on the risk of being exposed to solar UV-B radiation (e.g. development of skin cancer).