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GAW Aerosol Program
It is the goal of GAW to ensure long-term measurements in order to detect trends and reasons for them (WMO, 2001a). With respect to aerosols, the objective of GAW is to determine the spatio-temporal distribution of aerosol properties related to climate forcing and air quality up to multi-decadal time scales. While much of the recent international focus has been directed at assessing the impact of human activities on regional and global scales, there is growing recognition that the management of urban environments requires special attention, and has resulted in an increasing interest in regional air quality within WMO.
Airborne aerosols affect many aspects of human health and the environment. Aerosol mass and its toxicity are known to have links to chronic respiratory and acute cardio-vascular problems. Aerosols are also tightly linked to problems of visibility reduction, acid rain, and urban smog in many locations of the world. Furthermore, aerosols influence the atmospheric energy budget through direct and indirect effects. Direct effects refer to the scattering and absorption of radiation and their subsequent influence on planetary albedo and the climate system. Indirect effects refer to the increase in available cloud condensation nuclei (CCN) due to an increase in anthropogenic aerosol concentration. This is believed to change the cloud droplet number concentration for a constant cloud liquid water content (LWC), and the resulting increase in cloud albedo influences the Earth’s radiation budget. Cloud lifetimes and precipitation frequencies are also thought to be affected. Despite the uncertainty, it is believed that in regions with high anthropogenic aerosol concentrations, aerosol forcing may be of the same magnitude, but opposite in sign to the combined effect of all greenhouse gases. More information may be found in IPCC (2001).
The scale of these impacts depends on the aerosol particle sizes. On a mass basis, the tropospheric aerosol is generally dominated by two populations of particles, with characteristically different sources, sinks, sizes and chemical compositions. The submicrometer fraction (''fine particles'') generally originates from condensation sources (both high and low temperature) and from atmospheric gas-to-particle conversion processes, and is primarily removed by precipitation. In contrast, the supermicrometer fraction (''coarse particles'') is produced by mechanical processes (soil erosion, seawater bubble-bursting), and is mainly removed by sedimentation and has a considerably shorter atmospheric residence time than the submicrometer fraction. Size-segregation therefore allows the determination of the aerosol properties of each of these two populations so that their atmospheric effects can be evaluated separately. It also provides data that can be used to validate models used as the scientific basis for policies dealing with related issues.
Since the atmospheric residence time of aerosol particles is relatively short, a large number of measuring stations are needed. GAW consists of 22 Global stations which cover different types of aerosols: Clean and polluted continental, marine, arctic, dust, biomass burning, and free troposphere. Recently, the Scientific Advisory Group (SAG) for Aerosols and Optical Depth within GAW made additional recommendations, mainly concerning better coverage of the polluted continental aerosol type. While Global stations are expected to measure as many of the key variables as possible, the approximately 300 GAW Regional stations generally carry out a smaller set of observations.
According to the recommendations of the SAG, Regional stations should measure the optical depth, mass concentration and major chemical components in two size fractions, as well as the light scattering and absorption coefficient. Those stations wanting to add aerosol number concentrations for health effects are advised to do so. At Global stations, a larger number of measurements are envisaged. These include the Regional parameters list and in addition, the light scattering and hemispheric backscattering coefficients at various wavelengths, aerosol number concentration, cloud condensation nuclei (CCN) concentration at 0.5% supersaturation, and diffuse, global and direct solar radiation. Additional parameters such as the aerosol size distribution, detailed size fractionated chemical composition, dependence of aerosol properties on relative humidity, CCN concentration at various supersaturations, and the vertical distribution of aerosol properties should be measured intermittently at Global stations (WMO, 2003). A set of recommendations has been prepared by the SAG, covering the following subtopics: General sampling considerations, aerosol mass and chemistry, in-situ measurements of aerosol radiative properties, condensation nuclei, cloud condensation nuclei, aerosol optical depth, and aerosol lidar (WMO, 22003). Potential providers of aerosol data are advised to consult this report for planning of their aerosol measurements.
GAW participants endeavor to provide precise, accurate and timely observations of the aerosol parameters detailed below. In order to achieve this and as important information for the user community, the data available from the WDCA (located in Ispra, Italy) should have certain properties. They should be traceable to the original observation signals, they should be of a known quality, they should include all the information required by a user to permit the sensible use of the data, and they should include a contact point for the participant submitting the data. The construction of a template for data submission is considered to be a major step towards reaching this goal. More information about this topic is found in the contribution by J. Wilson in these Proceedings.
The establishment of a World Calibration Centre (WCC) for the long list of aerosol parameters is an important task in ensuring a high quality of data. It was therefore decided to distribute this task between two different institutions. The Institute for Tropospheric Research in Leipzig has agreed to host the WCC for the physical parameters, and more information about this activity is found in the contribution by A. Wiedensohler in these Proceedings. A host for the WCC for the chemical parameters still must be located.
It is also the task of the SAG to oversee the deployment and operation of 12 Precision Filter Radiometers (PFRs) provided by the Swiss Government for measuring aerosol optical depth (AOD). The first three instruments were delivered to Mauna Loa (Hawaii), Hohenpeissenberg (Germany), Mace Head (Ireland), Bratt's Lake (Canada), Izana (Tenerife), Ryori (Japan), Alice Springs (Australia), and Ny Alesund (Spitsbergen), and are currently in operation or will start their operation in the near future. First results confirm the high stability of the instruments. Negotiations for further placement of instruments are being conducted with, Bukit Koto Tabang (Indonesia) and Irkutsk (Russia). No final decision has been made for siting the final two instruments, but locations in Africa and South America are being considered.
Collaboration with partner programs (such as EMEP or other organisations) is of high importance. A joint EMEP/WMO workshop was therefore held in Interlaken in November 1999, where a closer collaboration between the two organisations was welcomed by both sides (EMEP, 2000). The newly founded Task Force on Measurements and Modeling (TFMM), co-chaired by EMEP and WMO, was assigned to take the lead in the further development of these joint activities, e.g. to plan on 5-10 "supersites" where joint measurements should be performed.
None of the Global sites perform the full suite of measurements listed above, with only very few stations performing more than half of the measurements. In addition, there are no generally accepted procedures available for some of the parameters mentioned above. Research is therefore urgently needed in order to fill this gap. Capacity building and fund raising are therefore important priorities in order to achieve the goals of the GAW aerosol programme.
All future activities should contribute to the strategic vision of the aerosol programme over the next 5-10 years as prepared by SAG:
All of the key aerosol parameters / geographical regimes will be represented within the network of stations that are carrying out a comprehensive list of aerosol parameters, according to the GAW aerosol protocols.
We are well on the way to understanding the spatial and temporal variability of the aerosol in the regions represented by GAW sites.
We will have established new sites/stations in regions that are not currently represented by GAW aerosol global stations
We will have strong links established between GAW aerosol and data user communities involving climate modeling, air quality modeling, satellite observation programs
Vision for regional air quality
a. GAW data will be used for regional air quality model validation and studies
b. Partnerships will have been established with other interested organizations such as WHO for defining and implementing regional programs
References
EMEP (2000) Proc. EMEP-WMO Workshop on Fine Particles – Emissions, Modelling and Measurements, (eds. J.E. Hanssen, R. Ballaman, and R. Gehrig), Interlaken, 22-25 November, 1999, EMEP/CCC-Report 9/2000, Norwegian Institute for Air Research, Kjeller.IPCC (2001) Climate Change 2001: The Scientific Basis, (eds. J. T. Houghton, Y. Ding, D. J. Griggs, M. Noguer, P. J. van der Linden, X. Dai, K. Maskell and C. A. Johnson). Cambridge University Press, New York.
WMO (2001) Strategy for the Implementation of the Global Atmosphere Watch Programme (2001-2007), WMO No. 142. World Meteorological Organization, Geneva.
WMO (2003) WMO/GAW Aerosol Measurement Procedures Guidelines and Recommendations, (WMO TD No. 1178) - GAW Report No. 153, World Meteorological Organization, Geneva.