Our predictive capability for the future state of the Arctic atmosphere relies in part on our ability to quantify air-snow-ice interactions, understand chemical processes that influence the oxidation capacity and monitor pollutant levels in polar locations. The chemistry that leads to the release of reactive gases from the snowpack is largely uncharacterized. Photolysis of nitrate ions contained in the snow appears to play an important role in leading to emissions of nitrogen species. Possible effects are in-situ production of ground-level ozone and re-deposition of nitrogen emitted from snow-covered surfaces. Nitrate re-activation may also lead to the production of chemically active organics such as HCHO and CH3CHO. In fact, OH formed in the snowpack can also oxidize organic matter and halide ions in the snow, producing carbonyl compounds and halogens that are released to the atmosphere or incorporated into snow crystals. These reactions modify the composition of the snow, of the interstitial air and of the overlying atmosphere. Thus, there is a need for long-term and continuous measurements of reactive nitrogen (NOx, HONO and all NOy), carbonyl compounds (formaldehyde, acetaldehyde, acetone etc.), reactive halogens and mercury compounds in the Arctic.
NOx and HONO formation and release from the snowpack can impact OH radical levels. In addition, the chemistry of nitrogen oxides is strongly linked to ozone formation. The various chemical mechanisms lead to a significant influence of NOx levels on the oxidation power of the Arctic troposphere. The release of NOx from the snowpack also is a process that reverses the removal of NOx from the atmosphere through its conversion to HNO3 and the deposition of NO3- having an impact on the NOx budget on larger scales. In addition, there are uncertainties surrounding the partitioning of NOy as they are transported into the Arctic and the mechanism for the conversion and cycling between NOx and NOy. The amount of NOy that reaches the polar regions depends strongly on the form of NOy that is transported, such as PAN. The PAN decomposition in the snowpack may account for the high nitrate levels in surface snow, but this has not been proven experimentally.
Sea ice appears to be the major source for halogen release via the bromine and chlorine explosion mechanism, with a likely role of frost flowers in providing sea salt surfaces and with the importance of deposition on the snow which can release and recycle the halogen compounds. In addition, halogen compounds can impact tropospheric ozone and mercury levels, as is obvious in sudden polar ozone depletion events. The monitoring of halogens in the polar troposphere over an extended period of time is thus important to answer whether halogen-catalyzed ozone loss is indeed important and whether they are also able to convert Hg( 0) into more soluble Hg(II) and, thus, their observations will also shed new light on the fate of mercury in polar environments.
Mercury in its elemental form Hg(0), is generally speaking long-lived in the atmosphere and because of this can be transported on intercontinental and hemispheric scales. Several experimental and modeling studies have concentrated on the Arctic because of the elevated concentrations of Hg found in the tissues of predatory mammals, and also because Mercury Depletion Events (MDEs) occurring during polar sunrise would appear to provide a mechanism by which elevated amounts of Hg may be rapidly introduced into the ecosystem. Mercury measurements in the Arctic showed the depletion of atmospheric elemental Hg concentrations with the simultaneous formation of a reactive form of Hg (RGM) through reaction sequences in which bromine, chlorine and compounds like BrO and ClO play prominent roles. The exact mechanism of the transformation of Hg(0) to RGM is as yet undetermined. The combined measurements of O3 and Hg provide a laboratory to study the response of gaseous Hg(0) to the release of halogen-containing compounds, the rate of release of which can be constrained by the comparatively well-known chemistry of ODEs.
The presence of lower carbonyls in the Arctic troposphere may have a great influence on radical chemistry since they represent a primary source of HOx radicals by photo-dissociation and a sink of species such as Br, to form relatively inert HBr, so terminating the chain leading to O3 depletion. Updated studies report that formaldehyde is likely emitted from the snow pack. The strength of this source might be comparable to that of the reaction between CH4 and OH in the Arctic region. Measurement data indicate a significant impact of snow pack emissions also for acetaldehyde and acetone. Increasing evidence indicates that reactive gases, such as HCHO are emitted from the snow-pack to the atmosphere. These remarks partly explain why atmospheric chemistry over snow- covered surface cannot be simulated by models using gas-phase processes only. Chemical models tend to underestimate generally HCHO concentrations in the Arctic, implying that an additional HCHO source not included in the models should be responsible for the high levels observed.
The overall objective of this research activity is to determine exchange mechanisms (adsorption/desorption) of atmospheric relevant species between the atmosphere and snow, by combining them with 3-D meteorological measurements. In particular, the scientific objectives of this research activity are:
- To quantify atmospheric fluxes of NOx, HNO3, HONO, particle nitrates and organic nitrates such as peroxyacetyl nitrate (PAN) above snow surfaces.
- To characterize the individual NOy component species and their coupling to carbonyl compounds in order to overcome ambiguities concerning the source region and budget of reactive nitrogen oxides in polar regions.
- To determine how the global O3 budget and distribution is altered by the re-activation of HNO3 or other NOy species by means of snow photochemistry.
- To quantify atmospheric fluxes of halogen compounds above snow surfaces in order to determine processes governing their deposition and redistribution.
- To assess the variations in the global mercury cycle between atmospheric and snow/ice interface over time that can occur with climate change which is believed to represent the major driving mechanism that may influence the redistribution of mercury on global and regional scales.
- To study the role of halogens and radical chemistry (OH) involving Hg compounds in the Arctic Boundary Layer for better assessing deposition estimates of Hg compounds.
- To study the gaseous Hg exchange processes related to the Hg chemistry in the lower troposphere and at the air-snow/ice interfaces to assess the chemical/physical mechanisms in the arctic ecosystem.
- To assess re-emission processes of previously deposited mercury. The balance between deposition and re-emission would in many areas determine the time scale over which intervention on anthropogenic emissions would start to be noticeable at remote sites.
- To investigate the spatial and temporal variability of carbonyl compounds during the different seasons related to ozone trends.
- To quantify the atmospheric HCHO and other aldehydes fluxes.
- To separate the contribute of atmospheric emissions from those coming from the snow pack.
- To better understand the radical chemistry through the evaluation of the formaldehyde fluxes.
The chemical measurements will be determined and investigated at two different heights above the snow surface during winter and spring periods to quantify and evaluate the atmospheric fluxes and the seasonal and temporal variations of all species. Real-time measurements of all species will be carried out over very short sampling times by means high sensibility, accurate and suitable automatic instruments. Some of them has been built by CNR-IIA. In addition, the diffusion denuder technique will be used for non conventional species. Measurements will be carried out near the sea on the coast of the fiord (ground level) and the on Mount Zeppelin (474m asl), in order to collect at the two altitudes both air samples and snow-to-air emission fluxes of Hg, interstitial air in polar snow and ice cores and surface snow based. A number of different sampling techniques will be used to assess the level of mercury species in the Arctic troposphere as well as a number of additional meteorological measurements will be also performed during the field studies. The emission of HCHO by snow affects the atmospheric lifetime of numerous species, and determining the physical and chemical processes involved is necessary for an adequate representation of polar tropospheric chemistry. During measurements campaigns, sampling of these compounds will be carried out by 2,4-DNPH cartridges with the use of active pumps for a sampling time of 8 hours. The samples will be analysed in our laboratories with HPLC-UV detector technique, after the campaigns. Formaldehyde and other carbonyl concentrations surrounding Ny-Ålesund (in different sites) will be investigated with passive devices, in order to create an air pollution map concerning this kind of compounds. In order to better understand the radical chemistry, formaldehyde fluxes measurements will be carried out at different heights up to 30 meters, at the top of the Climate-Change Tower, using active pump. It is, however, clear that measurements made in the near surface environment are significantly impacted by air-snow exchange processes and therefore these processes need to be better characterized.
