Trends Recorded in Greenland in Relation with Northern Hemisphere Anthropogenic Pollution

Contributed by R.J. Delmas and M. Legrand, Laboratoire de Glaciologie et Géophysique de l'Environnement du CNRS, France
Reprinted from IGACtivities Newsletter No. 14, September 1998.

1. Introduction
The composition of the Northern hemisphere atmosphere has changed significantly over the last century. Emissions by fossil fuel burning of significant amounts of carbon dioxide, sulfur dioxide, nitrogen oxides, and other species are relatively well quantified, but the impact of these pollutants on the composition of the background atmosphere, in particular over the large areas located at high northern latitudes, is much more poorly documented. Anthropogenic pollution can modify radiative forcing, aerosol composition, and oxidation capacity of the atmosphere. Polar snow does not record all the parameters involved in these processes, but ice core studies are invaluable for documenting the evolution of several key atmospheric parameters since the beginning of human activities.

Over recent years, several ice cores have been recovered in Greenland and the trends of most pollutants over several centuries are now satisfactorily depicted. However, the validity of a few records may be questionable, in particular for species affected by postdepositional effects (see paper on transfer functions, this issue).

2. Aerosol composition changes: Sulfate and ammonium
The extent of acid pollution to high Arctic regions was detected early, but high elevation areas were poorly documented. A preindustrial sulfate figure in the range 30-50 ng g-1 has been determined recently for ice at various Northern Greenland sites, with highest values in the north (Fischer et al., in press a). At Summit, mean non-sea-salt sulfate (nss- SO4=) concentration was 26 ng g-1 until about 1900 (see Table 1). Just after the turn of the century, SO2 emissions by fossil fuel burning began to impact sulfate deposition in Greenland markedly. The detailed Eurocore-GRIP nss-SO4= profile is shown Figure 1. In conjunction with several volcanic eruptions which blur the pure anthropogenic signal, nss-SO4= concentrations start to rise during the first decade of the century (Legrand et al., 1997). A break is observed in the 1930s probably in relation to the economic crisis. After World War II, a much sharper increase (5 ng g-1 /a) is found which culminates at 110 ng g-1 at the end of the sixties. At Summit, the mean value for the time period 1950-1989 is 85 ng g-1 (Table 1). Sulfur pollution abatement measures are clearly reflected in Greenland sulfate profiles which show a steady decreasing trend since about 1980.

Table 1.
Comparison of pre- and post-industrial levels of various compounds determined at Summit, central Greenland. Data are from Mayewski et al., 1993, De Angelis and Legrand, 1994, Legrand and De Angelis, 1996, Legrand et al., 1997, Anklin and Bales, 1997. Concentrations are in ng g-1. (*): mean value for the most recent 3 years (1992- 94).

  Pre-Industrial Last Decade
Non-sea-salt sulfate 26 85
Ammonium 5.3 +/- 9.6 --
Nitrate 68 120
Fluoride 0.06 0.19
Formaldehyde 2-3 5
Hydrogen peroxide 125 255(*)
Formate 7.7 +/- 2.3 6.3 +/- 2.4
Acetate 6.5 +/- 1.9 9.8 +/- 3.2
Methanesulfonate 2.9 +/- 1.9 1.9 +/- 1.05


Figure 1.
Upper part of the Eurocore- GRIP nss- sulfate profile (402 values) obtained at Summit (central Greenland) covering the two last centuries. The gray continuous curve is a running average on 3 values. In the 19th century, variability was natural: spikes were caused by large volcanic eruptions (e.g., Tambora, 1815 and Krakatoa, 1883). Note the sudden increase in the concentrations observed in 1903, linked partly to a volcanic event (most likely Santa Maria, 1902). The continuous line is a polynomial average of degree 10 including 200 values. The first "bump" on this curve is most probably linked to increasing pollution, but also to sporadic volcanism. A marked decrease of pollution is observed in the last two decades.
Detailed measurements show that the anthropogenic impact affects mostly the dry- deposited fraction of sulfate (Fischer et al., in press b). North America is proposed as the major source of pollution sulfate in Greenland for the first half of the century, Eurasia for post- war decades.

Interestingly, sulfate concentrations during the last glacial maximum, i.e., some 25,000 years ago (Legrand et al., 1997), reached significantly higher levels than in the 1960s when pollution was at its maximum. However, during the ice age, sulfate deposition was mostly in the form of large CaSO4 particles, rather than pollution sulfate which is deposited in the form of H2SO4 (assumed to be fine aerosol). Ca and NH4+ in ice cores do not show trends similar to sulfate. The former is not affected by pollution. For the latter, a 100% increase occurred during recent decades, but it is limited and affects only springtime deposition (Fuhrer et al., 1996). NH4+ background concentration in the ice (5.3 ng g-1) is contributed mainly by soil and vegetation emissions. Forest fires at high northern latitudes are a sporadic source (in the form of ammonium formate) to Greenland snow, but this contribution does not show a definite temporal trend over the last century (Legrand et al., 1995). It is worth noting that in European glaciers, closer to potential NH3 sources, ammonium concentrations started to grow already some 100 years ago, before sulfate and nitrate (Döscher et al., 1996).

3. Hydrogen peroxide
H2O2 is the species most directly related with the oxidation capacity of the atmosphere. It is the most abundant compound found in natural polar snow, but early data were controversial. Atmospheric, snow pit and firn core measurements at Summit using in situ analytical methods have shed a new light on the concentration of this species in Greenland.

Sigg and Neftel (1991) suggested at Summit a 50% increase of H2O2 concentration during the last 200 years, but more recent studies at the same site led to an estimate of the overall increase of 60 +/- 12 % since preindustrial time (Anklin and Bales, 1997), in essential agreement with the expected effect of growing emissions of CH4, NOX and CO. The H2O2 concentration profile departed from natural background fluctuations already in the middle of the last century, i.e., long before nitrate pollution. In most recent years, the increasing trend seems to be more pronounced and the amplitude between winter minima and summer maxima has tripled since 1970.

4. Carbon Monoxide
The determination of CO concentrations in ice cores is difficult due to as yet poorly characterized glaciological artifacts that affect the Greenland ice in particular (Haan and Raynaud, in press). Only a few data are available up to now (Haan et al., 1996). An increase of about 20 ppbv is observed between 1800 and 1950 for this compound at Summit where the preindustrial level is 90 ppbv (55- 60 ppbv in the Antarctic). The figure of 90 ppbv should be regarded as an upper limit.

5. Nitrate
At Summit, nitrate concentration increases from a preindustrial mean value of 68 to 120 ng g-1 for the period 1950- 1989. This trend, valid all over Greenland, starts markedly only since around 1950, i.e., much later than sulfate, and probably has a North American origin. A decline of this pollution is detected only for the most recent years.

This trend observed for nitrate in Greenland firn must be considered with some caution due to the reversibility of gaseous HNO3 deposition onto snow, even in Greenland (Fischer et al., in press a). Measurements carried out at various Greenland sites with different snow accumulation rates, i.e., having a different depth- age relationship, lead to similar temporal changes. This suggests that postdepositional effects have only a limited impact on nitrate trends recorded in Greenland snow. However, it must be kept in mind that N- bearing species deposited in Greenland are not yet formally identified. They may have changed since preindustrial times and in present environmental conditions, as suggested in the discussion of the transfer function (this issue).

6. Formaldehyde
HCHO measurements in Greenland snow are scarce. Data obtained by Staffelbach et al. (1991) at Summit pointed out a doubling of preindustrial concentration (about 2 ng g-1) in recent years, with an acceleration of the increase in the last decades. This observation has must be taken with caution, however, due the reversibility of the deposition of this species on polar snow.

7. Carboxylic acids
Light carboxylic acids in the atmosphere are linked to natural emissions by vegetation and biomass burning. Anthropogenic activities are also potential contributors, directly or indirectly, to their global budgets. Excluding sporadic biomass burning spikes, the non- biomass burning concentration of formate and acetate at Summit over the last two centuries are 7.4 +/- 2.4 and 7.2 +/- 2.7 ng g-1, respectively. Interestingly, a decreasing temporal trend in formate concentration is observed over the last century (Legrand and De Angelis, 1996), suggesting that the anthropogenic source is not dominant for this species. Moreover, it has been calculated that the increasing acidity of the snow caused by anthropogenic sulfate deposition (the incorporation of HCOOH in the snowflakes is acidity- dependant), is the most likely explanation for this decreasing trend. On the contrary, the acetate profile exhibits a well- marked increase over the last three decades (Table 1), a trend presently unexplained, that could be linked with increasing deposition of peroxyacetyl nitrate, which acts as a reservoir of acetic acid.

8. Heavy metals
It is well known that the deposition of several trace metals has increased considerably in Greenland in relation with the development of human activities (Candelone et al., 1995). The ratio between preindustrial level and the last 20 years is found to be significantly higher than for sulfate or nitrate. For example, factors of 2.3, 2.7, and 9 are found for Zn, Cu, and Cd, respectively. The factor is considerably higher for Pb (250) due to the use of organic lead derivates in gasoline. It is worth noting, however, that Summit studies revealed that disturbance of the natural cycle of lead began with Greek and Roman industrial activities (Hong et al., 1994) and that the present level of this metal in Summit snow is just as high as it was during the last glacial maximum (Hong et al., 1996) (Figure 2).


Figure 2.
Lead deposition at Summit (central Greenland) over the last 30,000 years. Included are, from right to left, the decrease of the input of natural lead linked to crustal dust, a marked increase with a climax around 2000 years ago linked to Greek and Roman mining activities (see text), thereafter a nearly continuous growth of anthropogenic pollution culminating about 30 years ago. Since this date, a marked decreasing trend is observed linked to the use of unleaded gasoline. Data are from Hong et al., 1994, 1996 and Candelone et al., 1995. Note the similarity of the figures of modern (some 30 years ago) and pre- industrial (about 25,000 years ago) of lead deposition.

References
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