Air Pollution as a Climate Forcing: A Workshop
Day 2 Presentations
What Relevant Information is Provided by GAW Stations, Ice Cores?
Laboratory of Atmospheric Chemistry, Paul Scherrer Institute, Villigen PSI, Switzerland
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It is the goal of GAW (Global Atmospheric Watch) to ensure long-term measurements in order to detect trends and determine reasons for them (WMO, 2001a). Aerosols are an important aspect in this program, due to their negative forcing of climate (IPCC, 2001). 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. 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. According to recommendations of the Scientific Advisory Group (SAG) for Aerosols, Regional stations should measure the optical depth, mass concentration and major chemical components in two size fractions, and the light scattering coefficient. 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, the light absorption coefficient, 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, 2001b). 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, no long-term series since the beginning of industrialization are available, and all kinds of archives therefore must be investigated in order to close this gap. Ice cores are considered to be particularly important archives in this context.
For greenhouse gases, it is common practice to plot concentrations from ice cores and atmospheric measurements in the same graph, illustrating the good agreement between the two different types of data (see, e.g. IPCC, 2001, p. 201). This is not the case for aerosol components. Even though ice core data clearly show the increase in concentrations due to anthropogenic activities for aerosol components (IPCC, 2001, p. 307), the situation is more complicated, for several reasons.
First, the link between ice core data and atmospheric concentrations is not as straightforward as for greenhouse gases, where the ice directly samples air aliquots within small bubbles. For aerosol particles a large variation of the so-called scavenging ratio W is found:
W = raCs/Ca
where Cs = concentration in snow or ice (ng/g), Ca = concentration in air (ng m-3) and ra = density of air (g m-3). The reason for this finding is that the transfer of aerosol particles into snow involves several steps (activation of aerosol particles to cloud droplets followed by formation of ice crystals and subsequent transfer of water to the ice phase). In the latter step, this transfer may occur as mere water vapor transport (resulting in rather pure ice crystals) or in the direct accretion of cloud droplets to ice crystals (so-called riming which results in more polluted ice crystals, since here the aerosol material within the cloud droplet is also transferred); see Baltensperger et al. (1998) and references therein. The processes greatly vary with atmospheric conditions, gas/aerosol part and aerosol properties, resulting in a high variability of W, e.g. with season.
Second, the shorter lifetime of aerosol components results in a much higher global variability of atmospheric concentrations than for greenhouse gases. Therefore, simultaneous probing of the atmosphere and the corresponding snow/ice is required to transform ice core data into atmospheric concentrations. Very few data are, however, available in this direction, since long-term data of atmospheric concentrations at sites where ice cores are available are scarce. As an exception, long-term sulfate data from the Jungfraujoch (3580 m asl, Swiss Alps) can be related to data from near-by ice from the Fiescherhorn (Schwikowski et al., 1999a).
Despite these difficulties, a few statements are possible: in Alpine ice coress sulfate concentrations increased roughly by a factor of 10 since the beginning of the industrial period (about 1870), with a maximum around 1975 (Schwikowski et al., 1999a) or 1980 (Preunkert et al., 2001), followed by a significant decrease after that. The evolution of the sulfate signal is consistent with the history of the SO2 emissions in Central Europe (Schwikowski et al., 1999b, Preunkert et al., 2001). An intermediate depression in the sulfate concentration appears to be found in several ice cores, and is also consistent with SO2 emissions (Schwikowski et al., 1999b, Preunkert et al., 2001). In Arctic ice cores, the increase is also very clear, but seems to be slightly less pronounced than in the Alpine ice cores (Fischer et al., 1998), and Antarctic ice cores do not show this trend at all (Dai et al., 1995). The evolution of the black carbon concentration as determined in an Alpine ice core is also consistent with the history of fossil fuel consumption in Central Europe. The relative increase is, however, less pronounced than for sulfate, mainly due to a significant BC concentration already before the beginning of the industrialisation (Lavanchy et al., 1999).
Furthermore, ratios of two different aerosol components of interest (e.g. sulfate and black carbon) are no longer subject to scavenging uncertainties, if the two components are found within the same particle (i.e. the particles are internally mixed). In this case, the ratio of the two components should not be modified by the scavenging processes. At remote sites, aerosol particles are indeed internally mixed, as shown by their hygroscopic growth behavior (Weingartner et al., 2002). Relative changes of this ratio since the beginning of the industrialization should therefore provide valuable information on the history of these components. There are, however, only very few cases, where black carbon and sulfate measurements have been performed on the same ice core, such as at the Colle Gnifetti (4450 m asl, Swiss Alps), see Fig. 1 (IPCC, 2001, Lavanchy et al., 1999, Döscher et al., 1995).
- Baltensperger, U., M. Schwikowski, D.T. Jost, S. Nyeki, H.W. Gäggeler, O. Poulida (1998) Scavenging of atmospheric constituents in mixed phase clouds at the high-alpine site Jungfraujoch; Part I: Basic concept and cloud scavenging, Atmos. Environ., 32, 3975-3983.
- Dai, J. C., L. G. Thompson, E. Mosley-Thompson (1995) A 485 year record of atmospheric chloride, nitrate and sulphate: results of chemical analysis of ice cores from Dyer Plateau, Antarctic Peninsula, Annals of Glaciology, 21, 182-188.
- Döscher A., H.W. Gäggeler, U. Schotterer, M. Schwikowski (1995) A 130 years deposition record of sulphate, nitrate, and chloride from a high-alpine glacier. Water Air Soil Pollut., 85, 603-609.
- Fischer, H., D. Wagenbach, J. Kipfstuhl (1998) Sulphate and nitrate firn concentrations on the Greenland ice sheet 2. Temporal anthropogenic deposition changes. J. Geophys. Res., 103, 21935-21942.
- 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.
- Lavanchy, V.M.H., H.W. Gäggeler, U. Schotterer, M. Schwikowski, U. Baltensperger (1999) Historical record of carbonaceous particle concentrations from a European high-alpine glacier (Colle Gnifetti, Switzerland), J. Geophys. Res., 104, 21227-21236.
- Preunkert, S., M. Legrand, D. Wagenbach (2001) Sulfate trends in a col du Dôme (French Alps) ice core: A record of antrhopogenic sulfate levels in the European midtroposhere over the twentieth century, J. Geophys. Res., 106, 31991-32004.
- Schwikowski M., S. Brütsch, H.W. Gäggeler, U. Schotterer (1999a) A high resolution air chemistry record from an Alpine ice core (Fiescherhorn glacier, Swiss Alps), J. Geophys. Res., 104, 13709-13720.
- Schwikowski, M., A. Döscher, H.W. Gäggeler, U. Schotterer (1999b) Anthropogenic versus natural sources of atmospheric sulphate from an Alpine ice core, Tellus, 51B, 938-951.
- Weingartner, E. M. Gysel, U. Baltensperger (2002) Hygroscopicity of aerosol particles at low temperatures. 1. New Low Temperature H-TDMA instrument: Setup and first applications, Environ. Sci. Technol., 36, 55-62.
- WMO (2001a) Strategy for the Implementation of the Global Atmosphere Watch Programme (2001-2007), WMO No. 142. World Meteorological Organization, Geneva.
- WMO (2001b) Global Atmosphere Watch Measurements Guide, WMO No. 143, World Meteorological Organization, Geneva.