COMPILED REPORTS OF THE
U.S. ICE CORE RESEARCH WORKSHOP

1.7 SPECIALTY GROUP REPORT: ATMOSPHERIC TRANSPORT/ POST-DEPOSITION STUDIES

This report contains three sections. First, a brief scientific justification for atmospheric studies of importance to glacial record research is presented. A list of three specific objectives related to the justification is given next. Finally, detailed research methods to achieve the objectives are presented. 

I. Justification

The purpose of the atmospheric studies summarized here is to provide a better understanding of how glacial snow, firn, and ice can be used to determine characteristics of the atmosphere during previous times. 

II. Objectives

The objectives of the atmospheric work summarized here can be divided into three categories:

      A. To determine the source regions and transport pathways for contaminants 
reaching the atmosphere over Greenland.

      B. To determine rates and mechanisms of deposition from the atmosphere 
onto the Ice Sheet.

      C. To develop an understanding of processes which may change the 
distribution of contaminants in the Ice Sheet after deposition.
Research methods to achieve each of these three objectives will now be discussed.

III. Research Methods to Achieve the Objectives

      A. Source Regions and Transport Pathways

            1. Sampling at the surface of the Ice Sheet

One of the primary objectives of the atmospheric component of GISP II will be identifying source regions and transport pathways relevant to deposition on to the ice sheet. We will attempt to identify geographical source areas, the relative importance of statospheric and troposheric inputs, and the roles of natural (marine, continental) and anthropogenic sources. We hope to characterize variations in transport processes on seasonal and shorter time scales.

Sources will be characterized using a variety of chemical and meteorological approaches. Regional scale apportionments can be estimated using signatures of pollution tracers from North America and Eurasia. Stable Pb isotopic ratios can also be used to resolve regional and smaller sources. The relative importance of statospheric and tropospheric inputs can be deduced from 7Be/21OPb and ozone measurements. Distributions of various elements can be used to identify marine, crustal and anthropogenic sources. For example, size fractionated aerosol samples can be used to distinguish fine particulate, anthropogenically derived Cd from Cd in volcanic emissions. In addition, it may be possible to use the concentration of Bi and T1 to identify specific volcanic events.

Meteorological analysis will be used to complement the chemical approaches. Synoptic analysis can be used to describe large scale transport pathways. Forward and backward trajectories will be correlated with chemical data. More sophisticated meteorological models may also be compared with the chemical data.

Measurements will be made at a ground station near the GISP II site during each summer season and by aircraft at selected times through the project. Total and size fractionated aerosol samples will be collected daily at the ground station for the determination of trace metals, radionuclides and stable isotopes of Pb. Continuous monitoring of 03 will also be conducted at the sampling site. The aircraft sampling will include as many of these types of samples as the constraints of the platform allow.

            2. Aircraft Sampling

The variability of chemical species in ice cores is currently being used to infer both natural (e.g., volcanic) and anthropogenic impacts on regional and global scale atmospheric chemical composition. The transport of aerosol and gas species to Greenland summit occurs at various altitudes ranging from the atmospheric boundary layer (e.g., sea salt) to the stratosphere (e.g., volcanic debris, 1OBe). In other words, the deposition to the Greenland summit surface is influenced, to variable degrees at specific times, by the entire atmospheric column. Ground-based atmospheric sampling during GISP II, while very important, cannot address a number of questions related to sources and transport.

We strongly recommend that the atmospheric chemistry community be encouraged to develop a series of aircraft experiments to define the structure and composition of the atmospheric column over Greenland. Such studies would include remote sensing of aerosol vertical distribution, mixed layer height, tropopause height, and ozone vertical distribution along flight lines designed to characterize the range of meteorological conditions which occur over Greenland and adjacent regions. The aircraft would also include in situ sampling capabilities similar to those planned for the ground-based atmospheric chemistry campaign.

An aircraft sampling program offers the opportunity for seasonal sampling and detailed chemical characterization of air masses from specific source regions (e.g., central Europe, North Atlantic, etc.).

Finally, it should be noted that the aircraft studies proposed above will require funding levels of $1-2M per expedition. However, there are currently several major atmospheric chemistry projects being proposed and developed for the North Atlantic region (e.g., EUROTRAC, NASA/ABLE-3, NSF/AEROCE) which could provide much of the data required for GISP II atmospheric chemical study objectives. It is important to initiate communication and coordination between these efforts as soon as possible.

      B. Deposition Processes

            1. Wet Deposition

Included in this category are the mechanisms of nucleation scavenging, in-cloud scavenging by existing cloud droplets and ice crystals, and below-cloud scavenging. The first process is believed to be dominant in the polar regions. Developing an understanding of wet deposition rates and mechanisms requires simultaneous measurement of contaminant concentrations in precipitation and in the air passing through clouds. Characteristics of the contaminant particles and gases as well as characteristics of the clouds must also be determined. This is extremely important since the aerosol population to a large degree determine the microphysical characteristics of cloud/precipitation systems and thus what mechanisms are dominant in determining deposition rates and efficiencies, especially in remote regions. For instance, increased concentrations of CCN may shift the snow crystals growth process away from riming dominant (more efficient aerosol scavenging) to diffusional growth dominant (less efficient aerosol scavenging).

The sampling should be conducted on an event basis. An aerosol sampler (H1 VOL + filterpack) should be operated simultaneously with collection of falling precipitation. Cloudwater should also be sampled. The precipitation, cloudwater, and filter samples should be analyzed for the following species: PH, anions (Cl-, S042-, N03- cations (NH4+, Na+, K+ Ca2+; Mg2+), trace metals (analysis by INAA, AA, XRF, etc.), radionuclides (e.g., 7Be, 1OBe, 21OPb), elemental and organic carbon, and stable isotopes (e.g.,d180, dD). In addition, air monitoring should be conducted for several species which may be incorporated into the Ice Sheet, such as HN03, H202, S02, CH4, C02, CO, and N20.

            2. Dry Deposition

Although wet deposition is often assumed to dominate in removing atmospheric contaminants, previous work has shown t hat dry deposition may be important in locations where cold weather is frequent and where amounts of precipitation are low. It is likely that dry deposition dominates for at least part of the year in central Greenland.

Two methods of estimating dry deposition should be attempted. First, direct measurements of dry deposition should be possible by sampling aging surface snow at various time intervals after snowfall. Work at Dye 3 suggests that sampling at intervals of at least 4 days are needed to provide measurable accumulation on the surface. Second, characteristics of the atmosphere, the surface, and the depositing species can be used to model dry deposition. Characteristics of the atmosphere include wind speed and temperature profiles, while characteristics of the surface include the geometry of the roughness elements. Characteristics ofthe depositing species include particle size distributions, particle density, and reactivity of gases. It is proposed that measurements be conducted at the GISP-2 site to enable dry deposition modeling.

Data from aircraft sampling and from ground-based sampling at other locations (e.g. Dye 3, coastal Greenland, Canadian Arctic, and Scandinavian Arctic) can be used to estimate airborne concentrations at other times of the year to allow modeling of dry deposition at Summit on a year-round basis.

Direct dry deposition monitoring should be conducted during dry periods in each of the five summer field seasons. Information should also be collected to permit dry deposition modeling for those species where sufficient data from summit exist. In addition, data from aircraft sampling and from ground-level sites at other Arctic locations can be used to estimate airborne concentrations throughout model dry deposition on a year-round basis.

Chemical species of interest would be the same as those of interest in wet deposition.

            3. Occult (Fog) Deposition

Occult deposition is defined here in context to ice core related studies as any wet deposition process other than that due to precipitation in the form of snow. This includes deposition of cloud (fog) droplets to the snow surface by direct impaction due to near surface eddy motion. It also includes deposition associated with surface hoar frost formation. Cloud droplet impactions can occur with or without precipitation. Hoar frost deposition occurs primarily during clear sky conditions. In some desert climates (which many high altitude and or high latitude regions are classified) occult deposition may be a significant part of the total annual precipitation.

Occult deposition may be studied in two ways, direct measurements or modeling studies. However, in order for modeling studies of eddy flux deposition of cloud droplets to the surface to be successful it is necessary to know the droplet size distribution. In addition to adequately model chemical wet deposition by cloud droplets it is necessary to know the composition of the cloud droplets as a function of their size. This may be dependent on the aerosol source region, chemical transformations along the transport pathways and the dynamics of the cloud formation process. Therefore it is recommended that at a minimum, measurements of the cloud droplet size distribution and chemical composition versus size be included as a priority measurement at ice core retrieval site in order to make a direct comparison to wet deposition by snow.

Wet deposition due to frost formation (water mass and chemical) can also be measured on site and modeled using knowledge of the vertical gradient of temperature, absolute humidity and wind speed. On site measurements are needed to ascertain the relative magnitude of thedeposition process. Modeling studies can be used to extend the results of the measurements to other locations or at other times if the appropriate micrometeorological measurements are made.

Measurements of cloud droplet size distributions can be made instrumentally and continuously during cloud events while an ice core camp is manned. At the same time, discrete collection of cloud droplets versus droplet size should be made over intervals of about a few hours, to determine what, if any, dependency exists between cloud composition and droplet size. These measurements should be made throughout a precipitation event. The ideal situation would be continuous collections. Such a method does not exist. Discrete collection methods are available.

Hoar frost deposition should be measured on an event basis, intra. storm. Surrogate surfaces may be used or the surface deposit removed from the natural snow surface. The period between frost collection should be tied to changes in the meteorologic conditions (i.e., the lack of suitable frost forming conditions). This is where the distribution between dry depositional process may be made. Measurements of deposition rates and chemical composition of cloud droplets and frost should be conducted as part of any wet chemical deposition program at the GISP and other ice core sites.

Cloud droplet and frost deposition of anions, cations, trace elements, radionuclides, and stable isotopes at a minimum should be characterized at the GISP H core site during all summer seasons that coring activities are being conducted. Additionally, the frequency of potential occult depositions during the winter, unmanned season should be addressed by the installation of suitable remote meteorological station with a cloud occurrence measurement device such as a simple nephelometer.

      C. Post-Deposition Processes

      A number of investigators have shown that contaminant concentrations in snow may change with time as the snow ages, even in the absence of dry deposition. Examples of process affecting these concentrations include snow sublimation, meltwater percolation, and diffusion of contaminants through the snowpack. Possible research methods to explore post-deposition changes in the Arctic include the following:

      1. Measurement of contaminant concentrations in surface snow during dry periods of varying length between storms is needed to assess sublimation. Comparing time-varying concentrations for species with and without appreciable dry deposition may help separate the effects of sublimation.

      2. Measurement of contaminant concentrations in shallow snowpits are needed for comparison with previous measurements of concentrations in fresh snow corresponding to the same set of storms. This will provide an indication of changes in concentration between the original fresh snow and the older snow in the pits.

      3. Statistical analysis of concentrations in intermediate depth cores is needed for species whose airborne concentrations and deposition rates are believed to have been constant over the period of the cores. This may identify longer term post-deposition changes in concentration.

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