Recent findings of NOx production in snow interstitial air suggest that photochemical production of NOx in or above snow surfaces is sufficient to alter the composition of the overlying atmosphere. Diurnal cycles of NOx of up to 35-pmol/mol magnitudes were observed above snow surfaces. Polar regions are snow covered year-round, while much of the continental mid-latitudes is snow-covered during portions of the year (> 50% of the land surface north of 20ºN is snow covered during winter). If this NOx release significantly altered NOx levels in the overlying atmosphere, the cumulative impact on global NOx and O3 budget could be substantial. Although the observation of NOx release was made at the surface, it is likely that similar processes also occur in the upper troposphere, upon cirrus ice particles.
Problems to be solved
This project is concerned with the mechanism of this NOx release. The current understanding of this process points towards the absorption of some NOy species by aerosols in or above snow surfaces, the presence and subsequent reduction of NO3- in a surface phase followed by photochemical release of NOx. At this moment it is unknown what the reactants or the reaction medium are. Substantial amounts of nitrous acid were found in polar arctic atmospheres, possibly as intermediate of the above process. Simple calculations show that this species is most responsible for the formation of OH radicals in the troposphere. Thus, while at low latitudes the OH production via ozone photo-dissociation seems to be prevailing, at higher latitude a different mechanism seems to be highly probable. NICE is concerned with the re-activation of nitrate in aerosols close to or in snow surfaces and the subsequent photochemical production of NOx. Nitric acid is apparently not the final sink of N-species in the atmosphere, but recycled into the atmosphere. This re-activation of nitrate essentially enhances the lifetime of active nitrogen far beyond the few days that are generally accepted as NOx lifetime. Thus increasing by far the range that NOx may impact atmospheric photochemistry. As a result, ozone tropospheric production may increase considerably. Through the production of HONO a catalytic cycle is possible which produces O3. It is thus conceivable that the nitrate re-activation process may contribute to the springtime O3 maximum in northern latitudes. Protocols under the Convention of Long-Range Transport of Air Pollutants (CLRTAP), are now reviewed and extended (acidification and photo-oxidant protocols regulating NOx, NH3, and VOC emissions; SO2, POP, and heavy metal emissions). In this context the understanding of nitrogen chemistry, especially in relation to the role of heterogeneous interactions with aerosols and snow, is still very primitive. Nitrogen chemistry is closely linked to acidification/eutrophication, photochemical oxidant cycles, and, in high northern latitudes, also to heavy metals. Indications to heterogeneous cycles under investigation here were only discovered during the last year. The NICE project plans to generate data and information that will also be of significance for EU policies on climate (aerosols and ozone as radiatively active species). It is essential to address the life cycles of aerosols in relation to chemical cycles.
Scientific Objectives and approach
Using a combination of field experiments, statistical analysis and conceptual modelling, we will address the following main scientific objectives:
To resolve on which active medium in which phase the reactions occur;
To identify the role of aerosol composition and properties on the re-activation of nitrate in snow-covered tropospheric surfaces;
To identify the source terms of NOx and HONO above snow surfaces;
To quantify the effective lifetime of NOx produced in this reaction cycle;
The overall objective of the fieldwork is to determine exchange mechanisms of atmospheric nitrogen species between the atmosphere and snow, as well as their chemical and physical interaction with surfaces. While the recycling of nitric acid in aerosols / snow surface seems to occur everywhere where snow is present, clean background atmospheric and snow conditions are best found in the Arctic. The experimental work will consist of a three-month field campaign during the winter-spring transition of 2001 at Ny-Ålesund, Svalbard. Chemical and aerosol key parameters such as the HONO mixing ratio or the aerosol scattering coefficient and condensation particle count will be measured at two different altitudes above the snow surface to quantify gradients and identify which species are involved in the reactions.
The questions posed here receive currently very high international attention. A number of other international experimental are planned at this time; (e.g. PSE2000, at Alert, Nunavut, Canada in spring 2000), with which NICE will co-evolve. Dissemination of the results and communication of the conclusions is vital for achieving the objectives, both scientifically and socio-economic. The results from this work will be disseminated on three levels; data archiving with public access to enliven an international scientific debate, peer-reviewed scientific publications, and the final report to the EU together with a press release to inform the European public of our findings. The anticipated results of this work will be the quantification of the atmospheric nitrogen budget and composition in relation to the presence of aerosol and/ or snow surfaces. The mechanism of NOx liberation through nitrate re-activation has profound consequences on the ozone budget. Nitrate is usually the end product of the nitrogen cycle. Re-activation to NOx may multiply the ozone formation potential of each nitrogen atom emitted in the atmosphere. This process may be the key to the observed spring maximum in tropospheric ozone in northern latitudes. The results of this work will enable us to determine how the global O3 budget and distribution is altered by the re-activation of HNO3 or other NOy species.
Funding SchemeCSC - Cost-sharing contracts