Albrecht Neftel

Continuous measurements of hydrogen peroxide, formaldehyde, calcium and ammonium concentrations along the new grip ice core from summit, Central Greenland

A new deep core drilling operation started in 1990 in central Greenland and in 1992 reached the bottom at a depth of 3028 m.b. surface. Taking advantage of recent developments in the analytical technique of chemical trace species, continuous high resolution measurements of H20 2, HCHO, NH2 and Ca 2+ concentrations were performed directly on the ice core in the field. During the 1991 season all four components were measured simultaneously between 1300 m.b. surface and 2300 m.b. surface, corresponding to the time interval between 8000 and 38,000 years B.P. In this paper an overview of the results and our first interpretations in terms of climatic changes are given. Key word index: Polar ice cores, glacial interstadials, palaeo climatic changes, flow injection analysis.

High‐resolution ammonium ice core record covering a complete glacial‐interglacial cycle

High-resolution ammonium measurements were performed along the Greenland Ice Core Program (GRIP) deep ice core, covering a complete climatic cycle. No overall anthropogenic increase is observed over the last 300 years; however, springtime concentrations have roughly doubled since 1950. Biomass burning is estimated to be a major source for ammonia emissions for preindustrial times. It contributes between 10% to 40% to the total ammonium deposited on the central Greenland ice sheet during the Holocene. No correlation is found between the ammonium summer concentrations recorded over the last 100 years and the area burned in northern North America, which is considered to be the main source area for ammonium deposited on the central Greenland ice sheet. This suggests that the meteorological factor is predominant for the pattern of ammonium spikes observed in the ice core. If unchanged meteorological conditions are assumed for the Holocene, as indicated by the δ18O ice record, a decreasing biomass burning activity toward present time can be derived from the ammonium ice record. Soil and vegetation emissions are responsible for the ammonium background concentrations in the ice. The record therefore may be used to trace back the biomass history of the North American continent. A pronounced decreasing trend in background ammonium is found during the Holocene, reflecting decreasing temperature and therefore lower NH3 emissions in the source region. Variations in the ammonium concentration during the glacial age are discussed in terms of changes in transport and deposition mechanisms and changes in source strength, which can be related to the extent of the Laurentide ice sheet. The data suggest that the Laurentide ice sheet was built up immediately after the last interglacial and went through several large fluctuations during the last ice age.

The Milan photooxidant plume

In Switzerland, measurement campaigns including aircraft measurements were carried out in the summers of 1992 and 1993 as part of the Pollution and Meteorology (POLLUMET) study. Ozone (O 3 ) concentrations, up to 185 ppb, with a large spatial variability were found south of the Alps in the afternoon. Comparison to measurements north of the Alps shows that these concentration levels are extraordinarily high for central Europe. Backward trajectories reveal that the highest O 3 levels were found 4-5 hours downwind of Milan, Italy. The measurements suggest a reactive organic gas (ROG) sensitive O 3 production regime 1-3 hours downwind in the plume, and a NO x (sum of nitrogen oxide (NO) and nitrogen dioxide (NO 2 )) limitation in air masses not affected by the Milan plume. Air masses originating north of Milan are probably close to the transition zone between the two photochemical regimes. This was found by using measurements of total odd nitrogen (NO y ), NO, NO 2 , formaldehyde (HCHO), and hydrogen peroxide (H 2 0 2 ) yielding indicators for ROG and NO, sensitive O 3 production. The slope of ozone versus NO z (=NO y -NO x : photochemical products of NO x ) were markedly higher in NO x limited conditions (ΔO 3 /ΔNO z = 13.6) than in air masses close to the transition zone (Δ0 3 /ΔNO z = 4.2).

Photochemical oxidant formation over southern Switzerland: 1. Results from summer 1994

We present ground-based and aircraft measurements of photochemically relevant trace gases from the southern part of Switzerland from summer 1994. The region is adjacent to the Po Valley and exhibits the highest ozone concentrations within Switzerland. O3 concentrations of up to 166 parts per billion by volume (ppbv) were measured. Isoprene was 3 ppbv on the ground and 0.3 ppbv at 1000 m altitude on average in the afternoon and dominates the volatile organic compound (VOC) reactivity toward OH at the ground. Measured HONO concentrations of 0.2 ppbv in the afternoon are much higher, as can be explained from gas phase reactions alone. Radical concentrations are derived from steady state calculations using ground-based measurements in the afternoon. The resulting concentrations for OH, HO2, and RO2 are 3.1×106, 1.1×109, and 1.3×109 molecules cm−3, respectively. Isoprene and NO have the largest influence on the estimated OH concentration, followed by O3, photolysis frequency of O3, and HONO. HO2 and RO2 concentrations are most sensitive to changes in HONO, O3, and photolysis frequency of O3. The estimated radical production is larger than the NOx emissions, suggesting low-NOx chemistry. We calculated indicators for NOx or VOC sensitivity and show that reduction of NOx emissions would be more efficient in reducing O3 than VOC reduction upwind of the measurement site.

Measurements of concentration gradients of HNO2 and HNO3 over a semi-natural ecosystem

Abstract Gradients of HNO2 and HNO3 have been measured over a semi-natural grassland in Central Switzerland. The site was characterised by a high excess of NH3 over HNO3. In such a situation the measured product of NH3 and HNO3 often exceeds the thermodynamic equilibrium product over solid NH4NO3. As a consequence, transient formation and re-evaporation of NH4NO3 can occur leading to a height-dependent flux of HNO3. Consequently, the constant flux approximation, which is a pre-requisite for the determination of HNO3 deposition by the gradient method, is violated. Measured HNO2 concentrations are higher as can be explained by production of homogeneous gas-phase reaction alone. But no evidence for a ground-based source from the vegetation-covered soil has been found.

Experimental assessment of N2O background fluxes in grassland systems

In the absence of, or between, fertilization events in agricultural systems, soils are generally assumed to emit N2O at a small rate, often described as the ‘background’ flux. In contrast, net uptake of N2O by soil has been observed in many field studies, but has not gained much attention. Observations of net uptake of N2O form a large fraction (about half) of all individual flux measurements in a long-term time series at our temperate fertilized grassland site. Individual uptake fluxes from chamber measurements are often not statistically significant but mean values integrated over longer time periods from days to weeks do show a clear uptake. An analysis of semi-continuous chamber flux data in conjunction with continuous measurements of the N2O concentration in the soil profile and eddy covariance measurements suggests that gross production and gross consumption of N2O are of the same order, and as consequence only a minor fraction of N2O molecules produced in the soil reaches the atmosphere.

Stable-Isotope Ratios and Concentration of CO2 in Air from Polar Ice Cores

Analyses of air trapped in an ice core from the South Pole indicate that the CO2 concentration may have increased by about 10 ppm and that the ISC/l2C ratio decreased slightly in the thirteenth century. These changes, if really of atmospheric origin, must be due to a significant input into the atmosphere of CO2, either of biogenic or of oceanic origin. 180/60 ratios in CO2 from different ice cores are much lower than those which have been observed in atmospheric carbon dioxide. A possible explanation is that the CO2 has equilibrated isotopically with the ice. We have calculated equilibrium isotope-fractionation factors between ice and carbon dioxide and found that the observed 180/60 ratios of CO2 are indeed near isotopic equilibrium with the ice. This indicates that an exchange of oxygen atoms probably occurs between ice and included CO2.

A new membrane tube technique (METT) for continuous gas measurements in soils

We present a novel technique for monitoring trace gas concentrations in the air filled pore space of the soil with a high temporal resolution. The method is based on gas diffusing from the air filled pore space into air flowing through an air permeable, hydrophobic, polypropylene tube (Accurel® PP V8/2). Gas permeation efficiencies of the membrane tube were determined for NO, N2O, CO2 and 222Rn in the laboratory. For a length of 1.5 m and flows smaller than 0.8 L min-1, the permeation efficiency was larger than 96%. The effective diffusion coefficients of NO, N2O, CO2 and 222Rn in the membrane are 6.2, 6.6, 5.6 and 6.6 times smaller than the corresponding diffusion coefficients in air, respectively. For tubes shorter than 1.5 m the contribution of pressure gradient induced transport into the membrane tube is below 0.9% of concentration gradient induced transport.Profiles of NO and 222Rn have been measured in the soil of a wheat field. For destructive NO measurements the inlet concentration of the tube was adjusted to the concentrations measured at the outlet whereas for the non-destructive 222Rn measurements the sampled gas was recycled.

Process-based modelling of nitrous oxide emissions from different nitrogen sources in mown grassland

The process-based Pasture Simulation Model (PaSim 2.5) has been extended to simulate N2O production and emission from grassland caused by nitrogen inputs from different sources. The model was used to assess the influence of management on N2O emissions, such as the effect of shifts in the amount and timing of fertilizer application. Model performance has been tested against season-long field measurements at two different field sites. Simulation results agreed favourably with measured N2O emission and soil air concentrations, except during an extremely wet period at one site when grass growth was very poor. The results of short-term and long-term simulation runs demonstrated the potential of the model to estimate N2O emission factors under various conditions. During the first growing season, simulated emissions from organic fertilizers were lower than from synthetic fertilizers because more of the nitrogen was used to build up soil organic matter. The relative difference between the fertilizer types became larger with increasing application rate. The difference between fertilizer types was smaller at steady-state when higher soil organic matter content from repeated application of organic fertilizer over time led to enhanced mineralization and N2O emissions. The dependence of simulated N2O emissions on N input was close to linear at low, but non-linear at high fertilization rates. Emission factors calculated from the linear part of the curve suggested that the factors used in the current IPCC method underestimate the long-term effects of changes in fertilizer management. Furthermore the simulations show that N2O emissions caused by nitrogen inputs from the decomposition of harvest losses and from biological fixation in grassland can be considerable and should not be neglected in national emission inventories.

Seasonal Variations in Hydrogen Peroxide in Polar Ice Cores

Hydrogen peroxide is present in polar snow and ice in remarkably high concentrations. With values up to 300 ppb, H20 2 is one of the most concentrated impurities in polar ice. We present a continuous H20 2 firn record from Siple Station (Antarctica); it covers the last 83 years with a resolution of 10-20 samples per year. A very strong seasonality is present in this record. This seasonality is also observed in a Greenland ice core from Dye 3, where we have continuously measured the top 10 m with the same resolution. The maximum concentrations correspond to summer snow layers and can exceed winter snow concentrations by a factor of 10. This property makes H20 2 a useful tracer for dating suitable cores by counting annual layers. The different steps needed to relate the atmospheric to the ice-core H20 2 concentration are discussed . As with isotopic tracers, diffusion in the firn smooths the original H 20 2 concentration profile. INTRODUCTION More than lOO years ago, Schiine (1874) found that H20 2 is present in hoar frost. Nevertheless, only during the past few years has it been appreciated that H 2 0 2 is one of the most important species in atmospheric chemistry. Because of its intermediate position between extreme reactive species such as OH or H02 radicals and the long-living species, it is a useful tool for testing atmospheric-chemistry model calculations. Reliable measurements of the atmospheric concentrations have existed only for a few years. Because of the reactivity of H 20 2 in the liquid phase and its high solubility, it is used up quickly in clouds over mid-latitudes (Kelly and others 1985) and the interpretation of the measured data in precipitation samples is extremely difficult. In 1984 it was realized that H20 2 is present in relatively large amounts in polar snow and ice samples (Neftel and others 1984). It is quite astonishing that hydrogen peroxide, a rather unstable and reactive species, survives the firnification process and is still detectable in ice several thousand years old. The observed long-term trend in Greenland deep ice cores points to a very slow disintegration of H20 2 with a half-life of the order of several thousand years (Neftel and others 1986). The suppressed chemical reactivity in snow which is well below the freezing point and the ease with which H20 2 is built into the ice matrix favour the preservation of records of H 20 2 in the polar ice sheets. It seems, therefore, that the polar precipitation archives could be used to trace back in time the atmospheric H 20 2 for high latitudes, if the transfer function from the atmospheric concentration to the ice concentration is fully understood. In this paper we present two continuous H 20 2 measuring series: from Siple Station, West Antarctica, covering the last 83 years, and from Dye 3, south Greenland, covering the last 10 years . The different steps required to evaluate the atmospheric concentration from the measured ice data are discussed. MEASURING TECHNIQUE The data presented in this paper are based on discrete samples. The samples are cut with a band saw to give a cross-section of about 2 cm2. The sample length is chosen according to the desired vertical resolution . Saw-dust is removed as far as possible, with a Teflon brush. The samples are put into polyacrylic vessels and melted in a moderately warm-water bath. Immediately afte r melting, aliquots of these samples are inserted into the measuring system. It is important to keep the time between melting and measurement as short as possible, because H20 2 tends to decay in aqueous solutions. Typical values are 1-5% in 15 min. We have two independent systems to measure H20 2 concentrations in aqueous solutions at the ppb (= ILg/ kg) level. Both methods are fast and need only about 200 ILl of sample per measurement. The first method is based on the chemiluminescent reaction of hydrogen peroxide with TCPO, i.e. bis(2,4,6-trichlorophenyl) oxalate (Klockow and Jacob 1986). The emitted light during the reaction is proportional to the H20 2 concentration. The solubility of TCPO in water is insufficient; therefore this reagent has to be dissolved in acetone. Perylene is used as a f1uorescer. It can be added to the acetone solution. Because the reaction is pH-dependent, the samples have to be adjusted to pH = 7.8 with a borate buffer solution. In the second method, called the enzymatic method, hydrogen peroxide reacts with 4-ethylphenol and builds dimers of this compound quantitatively. This reaction needs the presence of the enzyme peroxidase. The concentration of this dimer is proportional to the H20 2 concentration and can be determined f1uorometrically. Addition of borate buffer is necessary also in this method. A similar method is described in Lazrus and others (1985). The two systems are presented schematically in Figure I. Sample input is exactly the same for both devices. The input tube is moved from sample to sample whi le the peristaltic pump is running. The sampling time is about half a minute. While the tube is moved, an air segment is inserted automatically between two sample segments. This air bubble is necessary to separate the samples. Without these air segments, mixing in the thin tubes would be stronger. Ten samples are measured in one series between two blanks. The blank water is bi-distilled and a platinum-coated catalyst in the water container removes any remaining H 2 0 2 • From time to time, the systems have to be calibrated with standard solutions, diluted from a 1%0 H 20 2 stock solution. This stock solution is more stable than the diluted standards, but at least every month it has to be calibrated by titration with KMnO •. Both methods need only small samples, and are fast and very sensitive; both methods, however, exhibit interferences with other trace species: The enzymatic method is sensitive to organic peroxides, such as methylhydroperoxide and peroxyacetic acid, too (Lazrus and others 1985). This interference can be eliminated: the enzyme catalase destroys H20 2 much faster than other peroxides. The presence of organic peroxides can therefore be tested by adding catalase to the sample before measurement. Until now, we have never fou nd organic peroxides in polar ice.