Air-snow exchange investigations at Summit, Greenland: An overview

The Greenland Ice Sheet Project 2 (GISP2) and Greenland Ice Core Project (GRIP) deep drilling programs at Summit, Greenland included support (both logistical and scientific) of extensive investigation of atmospheric transport and air-snow exchange processes of gases and particles relevant to the interpretation of the ice-core records. Much of the sampling for the air-snow exchange investigations was conducted at a unique solar-powered camp 30 km southwest of the GISP2 drill camp (even further from the GRIP camp) and was characterized by a high degree of international collaboration and cooperation. The wide range of expertise and analytical capabilities of the 20-plus investigators participating in these studies has provided important insight into the meteorological, physical, and chemical processes which interact to determine the composition of snow and firn at Summit. Evolving understanding of this system will allow improved reconstruction of the composition of the atmosphere over Greenland in the past from the detailed Summit ice-core records. This paper provides an overview of air-snow exchange investigations at Summit, including their development through the course of the drilling programs (1989–1993), significant findings related to both air-snow exchange issues and the present state of the Arctic free troposphere, as well as the major outstanding questions which are being addressed in ongoing experiments at Summit

Temporal and spatial variability of snow accumulation in central Greenland

Snow accumulation records from central Greenland are explored to improve the understanding of the accumulation signal in Greenland ice core records. Results from a “forest” of 100 bamboo poles and automated accumulation monitors in the vicinity of Summit as well as shallow cores collected in the Summit and Crete areas are presented. Based on these accumulation data, a regression has been calculated to quantify the signal-to-noise variance ratio of ice core accumulation signals on a variety of temporal (1 week to 2 years) and spatial (20 m to 200 km) scales. Results are consistent with data obtained from year-round automated accumulation measurements deployed at Summit which suggest that it is impossible to obtain regional snow accumulation data with seasonal resolution using four accumulation monitors positioned over a length scale of ∼30 km. Given this understanding of the temporal and spatial dependence of noise in the ice core accumulation signal, the accumulation records from 17 shallow cores are revisited. Each core spans the time period from 1964 to 1983. By combining the accumulation records, the regional snow accumulation record has been obtained for this period. The results show that 9 of the 20 years can be identified as having an accumulation different from the 20 year mean with 99% confidence. The signal-to-noise variance ratio for the average accumulation signal sampled at annual intervals is 5.8±0.5. The averaged accumulation time series may be useful to climate modelers attempting to validate their models with accurate regional hydrologic data sets

Sulfate and MSA in the air and snow on the Greenland Ice Sheet

Sulfate and methanesulfonic acid (MSA) concentrations in aerosol, surface snow, and snowpit samples have been measured at two sites on the Greenland Ice Sheet. Seasonal variations of the concentrations observed for these chemical species in the atmosphere are reproduced in the surface snow and preserved in the snowpit sequence. The amplitude of the variations over a year are smaller in the snow than in the air, but the ratios of the concentrations are comparable. The seasonal variations for sulfate are different at the altitude of the Ice Sheet compared to those observed at sea level, with low concentrations in winter and short episodes of elevated concentrations in spring. In contrast, the variations in concentrations of MSA are similar to those measured at sea level, with a first sequence of elevated concentrations in spring and another one during summer, and a winter low resulting from low biogenic production. The ratio MSA/sulfate clearly indicates the influence of high-latitude sources for the summer maximum of MSA, but the large impact of anthropogenic sulfate precludes any conclusion for the spring maximum. The seasonal pattern observed for these species in a snowpit sampled according to stratigraphy indicates a deficit in the accumulation of winter snow at the summit of the Greenland Ice Sheet, in agreement with some direct observations. A deeper snowpit covering the years 1985–1992 indicates the consistency of the seasonal pattern for MSA over the years, which may be linked to transport and deposition processes

Seasonal variations of concentrations and optical properties of water soluble HULIS collected in urban environments

Major contributors to the organic aerosol include water-soluble macromolecular compounds (e.g. HULIS<sub>WS</sub>: Water Soluble Humic LIke Substances). The nature and sources of HULIS<sub>WS</sub> are still largely unknown. This work is based on a monitoring in six different French cities performed during summer and winter seasons. HULIS<sub>WS</sub> analysis was performed with a selective method of extraction complemented by carbon quantification. UV spectroscopy was also applied for their chemical characterisation. HULIS<sub>WS</sub> carbon represent an important contribution to the organic aerosol mass in summer and winter, as it accounts for 12–22% of Organic Carbon and 34–40% of Water Soluble Organic Carbon. We found strong differences in the optical properties (specific absorbance at 250, 272, 280 nm and E2/E3 ratio) and therefore in the chemical structure between HULIS<sub>WS</sub> from samples of summer- and wintertime. These differences highlight different processes responsible for emissions and formation of HULIS<sub>WS</sub> according to the season, namely biomass burning in winter, and secondary processes in summer. Specific absorbance can also be considered as a rapid and useful indicator of the origin of HULIS<sub>WS</sub> in urban environment

Size distribution of EC and OC in the aerosol of Alpine valleys during summer and winter

International audienceCollections of samples were conducted for the determination of the size distributions of EC and OC during the intensive sampling campaigns of the POVA program, in two Alpine valleys of the French Alps, in summer and in winter. The comparison of concentrations obtained for samples collected in parallel with impactor- and filter-based methods indicates that the correction of pyrolysis seems to work for the impactor samples despite non even deposits. The size distributions of the concentrations of EC and OC present large evolutions between winter and summer, and between a suburban and a rural site. In winter, an overwhelming proportion of the mass fraction of both species is found in the droplet and accumulation modes, often (but not always) in association with sulfate and other chemical species resulting from secondary formation processes. Some indications of gas/particles exchanges can be found for the other parts of the size spectrum (the Aitken and super micron modes) in the case of the rural site. In summer, the changes are more drastic with, according to the case, a dominant droplet or accumulation mode. Particularly at the rural site, the large extent of processing of the aerosol due to gas/particles exchanges is evident for the Aitken and super micron modes, with increasing of the OC mass fractions in these size ranges. All of these observations give indications on the degree of internal vs. external mixing of the species investigated in the different modes

A simple model to estimate atmospheric concentrations of aerosol chemical species based on snow core chemistry at Summit, Greenland

A simple model is presented to estimate atmospheric concentrations of chemical species that exist primarily as aerosols based on snow core/ice core chemistry at Summit, Greenland. The model considers the processes of snow, fog, and dry deposition. The deposition parameters for each of the processes are estimated for SO42− and Ca2+ and are based on experiments conducted during the 1993 and 1994 summer field seasons. The seasonal mean atmospheric concentrations are estimated based on the deposition parameters and snow cores obtained during the field seasons. The ratios of the estimated seasonal mean airborne concentration divided by the measured mean concentration ( ) for SO42− over the 1993 and 1994 field seasons are 0.85 and 0.95, respectively. The ratios for Ca2+ are 0.45 and 0.90 for the 1993 and 1994 field seasons. The uncertainties in the estimated atmospheric concentrations range from 30% to 40% and are due to variability in the input parameters. The model estimates the seasonal mean atmospheric SO42− and Ca2+ concentrations to within 15% and 55%, respectively. Although the model is not directly applied to ice cores, the application of the model to ice core chemical signals is briefly discussed

Modeling of the processing and removal of trace gas and aerosol species by Arctic radiation fogs and comparison with measurements

A Lagrangian radiation fog model is applied to a fog event at Summit, Greenland. The model simulates the formation and dissipation of fog. Included in the model are detailed gas and aqueous phase chemistry, and deposition of chemical species with fog droplets. Model predictions of the gas phase concentrations of H2O2, HCOOH, SO2, and HNO3 as well as the fog fluxes of S(VI), N(V), H2O2, and water are compared with measurements. The predicted fluxes of S(VI), N(V), H2O2, and fog water generally agree with measured values. Model results show that heterogeneous SO2 oxidation contributes to approximately 40% of the flux of S(VI) for the modeled fog event, with the other 60% coming from preexisting sulfate aerosol. The deposition of N(V) with fog includes contributions from HNO3 and NO2 initially present in the air mass. HNO3 directly partitions into the aqueous phase to create N(V), and NO2 forms N(V) through reaction with OH and the nighttime chemistry set of reactions which involves N2O5 and water vapor. PAN contributes to N(V) by gas phase decomposition to NO2, and also by direct aqueous phase decomposition. The quantitative contributions from each path are uncertain since direct measurements of PAN and NO2 are not available for the fog event. The relative contributions are discussed based on realistic ranges of atmospheric concentrations. Model results suggest that in addition to the aqueous phase partitioning of the initial HNO3 present in the air mass, the gas phase decomposition of PAN and subsequent reactions of NO2 with OH as well as nighttime nitrate chemistry may play significant roles in depositing N(V) with fog. If a quasi-liquid layer exists on snow crystals, it is possible that the reactions taking place in fog droplets also occur to some extent in clouds as well as at the snow surface

Modeling of the formation of short-chain acids in propane flames

International audienceIn order to better understand their potential formation in combustion systems, a detailed kinetic mechanism for the formation of short-chain monocarboxylic acids, formic (HCOOH), acetic (CH3COOH), propionic (C2H5COOH) and propenic (C2H3COOH)) acids, has been developed. Simulations of lean (equivalence ratios from 0.9 to 0.48) laminar premixed flames of propane stabilized at atmospheric pressure with nitrogen as diluent have been performed. It was found that amounts up to 25 ppm of acetic acid, 15 ppm of formic acid and 1 ppm of C3 acid can be formed for some positions in the flames. Simulations showed that the more abundant C3 acid formed is propenic acid. A quite acceptable agreement has been obtained with the scarce results from the literature concerning oxygenated compounds, including aldehydes (CH2O, CH3CHO) and acids. A reaction pathways analysis demonstrated that each acid is mainly derived from the aldehyde of similar structure

To better understand the formation of short-chain acids in combustion systems

International audienceOur study aims at a better control and understanding of the transfer of a complex [DNA supercoiled plasmid - dodecyltrimethylammonium surfactant] layer from a liquid-vapour water interface onto a silicon surface without any additional cross-linker. The production of the complexed layer and its transfer from the aqueous subphase to the substrate is achieved with a Langmuir-Blodgett device. The substrate consists of a reconstructed boron doped silicon substrate with a nanometer-scale roughness. Using X-ray photoelectron spectroscopy and atomic force microscopy measurements, it is shown that the DNA complexes are stretched in a disorderly manner throughout a 2-4 nm high net-like structure. The mechanism of transfer of this layer onto the planar surface of the semi-conductor and the parameters of the process are analysed and illustrated by atomic force microscopy snapshots. The molecular layer exhibits the typical characteristics of a spinodal decomposition pattern or dewetting features. Plasmid molecules appear like long flattened fibers covering the surface, forming holes of various shapes and areas. The cluster-cluster aggregation of the complex structure gets very much denser on the substrate edge. The supercoiled DNA plasmids undergo conformational changes and a high degree of condensation and aggregation is observed

Size distribution of EC and OC in the aerosol of Alpine valleys during summer and winter

Collections of samples were conducted for the determination of the size distributions of EC and OC during the intensive sampling campaigns of the POVA program, in two Alpine valleys of the French Alps, in summer and in winter. The comparison of concentrations obtained for samples collected in parallel with impactor- and filter-based methods is rather positive with slopes of 0.95 and 0.76 for OC and EC, respectively and correlations close to 1 (0.92 and 0.90 for OC and EC, respectively, n=26). This is an indication that the correction of pyrolysis seems to work for the impactor samples despite non even deposits. The size distributions of the concentrations of EC and OC present large evolutions between winter and summer, and between a suburban and a rural site. In winter, an overwhelming proportion of the mass fraction of both species is found in the droplet and accumulation modes, often (but not always) in association with sulfate and other chemical species resulting from secondary formation processes. Some indications of gas/particles exchanges can be found for the other parts of the size spectrum (the Aitken and super micron modes) in the case of the rural site. In summer, the changes are more drastic with, according to the case, a dominant droplet or accumulation mode. Particularly at the rural site, the large extent of processing of the aerosol due to gas/particles exchanges is evident for the Aitken and super micron modes, with increasing of the OC mass fractions in these size ranges. All of these observations give indications on the degree of internal vs. external mixing of the species investigated in the different modes