Abstract.It has been shown that NOs is produced photochemically within the snowpack of polar regions. If emitted to the atmosphere, this process could be a major source of NOs in remote snowcovered regions. We report here on measurements made at the German Antarctic station, Neumayer, during austral summer 1999, aimed at detecting and quantifying emissions of NO• from the surface snow. Gradients of NOs were measured, and fluxes calculated using local meteorology measurements. On the 2 days of flux measurements, the derived fluxes showed continual release from the snow surface, varying between ~0 and 3x108 molecs/cm2/s. When not subject to turbulence, the variation was coincident with the uv diurnal cycle, suggesting rapid release once photochemically produced. Scaling the diurnal average of Feb. 7th(1.3x108 molecs/cm2/s) suggests an annual emission over Antarctica of the order 0.0076TgN.

Tower-based measurements of hydrogen peroxide (H 2 O 2 ) and formaldehyde (HCHO) exchange were performed above the snowpack of the Greenland ice sheet. H 2 O 2 and HCHO fluxes were measured continuously between 16 June and 7 July 2000, at the Summit Environmental Observatory. The fluxes were determined using coil scrubber-aqueous phase fluorometry systems together with micrometeorological techniques. Both compounds exhibit strong diel cycles in the observed concentrations as well as in the fluxes with emission from the snow during the day and the evening and deposition during the night. The averaged diel variations of the observed fluxes were in the range of +1.3 Â 10 13 molecules m À2 s À1 (deposition) and À1.6 Â 10 13 molecules m À2 s À1 (emission) for H 2 O 2 and +1.1 Â 10 12 and À4.2 Â 10 12 molecules m À2 s À1 for HCHO, while the net exchange per day for both compounds were much smaller. During the study period of 22 days on average ð0:8 þ4:6 À4:3 Þ Â 10 17 molecules m À2 of H 2 O 2 were deposited and ð7:0 þ12:6 À12:2 Þ Â 10 16 molecules m À2 of HCHO were emitted from the snow per day. A comparison with the inventory in the gas phase demonstrates that the exchange influences the diel variations in the boundary layer above snow covered areas. Flux measurements during and after the precipitation of new snow shows that o16% of the H 2 O 2 and more than 25% of the HCHO originally present in the new snow were available for fast release to the atmospheric boundary layer within hours after precipitation. This release can effectively disturb the normally observed diel variations of the exchange between the surface snow and the atmosphere, thus perturbing also the diel variations of corresponding gas-phase concentrations. r

Hexachlorocyclohexanes (HCHs) were determined simultaneously in air and seawater during two cruises across the Atlantic Ocean between the Arctic Ocean (NyA°lesund/ Svalbard, 79°N; 12°E) and the Antarctic Continent (Neumayer Station/ Ekstroem Ice Shelf, 70°S; 8.2°W) in 1999/ 2000. The concentrations of R-HCH and γ-HCH in air and surface waters of the Arctic exceeded those in Antarctica by 1-2 orders of magnitude. The gaseous concentrations of γ-HCH were highest above the North Sea and between 20°N and 30°S. Fugacity fractions were used to estimate the direction of the air-sea gas exchange. These showed for R-HCH that the measured concentrations in both phases were close to equilibrium in the North Atlantic (78°N-40°N), slightly undersaturated between 30°N and 10°S and again close to equilibrium between 20°S and 50°S. γ-HCH has reached phase equilibrium in the North Atlantic as R-HCH, but the surface waters of the tropical and southern Atlantic were strongly undersaturated with γ-HCH, especially between 30°N and 20°S. These findings are significantly different from two earlier estimates around 1990 as a result of global emission changes within the past decade. Therefore, we investigated the time trend of the HCHs in the surface waters of the Atlantic between 50°N and 60°S on the basis of archived samples taken in 1987-1997 and those from 1999. A decrease of R-HCH by a factor of approximately 4 is observed at all sampling locations. No decrease of γ-HCH occurred between 30°N and 30°S, but there was a decrease in the North Atlantic, North Sea, and in the South Atlantic south of 40°S. The constant level of γ-HCH in the tropical Atlantic confirms the conclusion that the tropical Atlantic acts as a sink for γ-HCH at present time. The measured R-HCH seawater concentrations were compared with results from a global multimedia fate and transport model. Whereas the time trend over 13 years and the latitudinal gradient were well reproduced by the model, the absolute levels were too high by a factor of 4.5. This may be explained by the zonal averaging employed in the model as well as uncertain emissions and degradation rates.

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Models and observational strategies of carbon exchange need to take into account synoptic and mesoscale transport for correct interpretation of the relation between surface fluxes and atmospheric concentration gradients.A dequate quantification of the geographical distribution of sources and sinks of C02 is still a major task with considerable implications for both our understanding of the global climate and the possible opportunities to mitigate climate change. Atmospheric measurements of C02 mixing ratios at a number of locations around the globe have helped significantly to quantify the source-sink distribu-AFFILIATIONS: DOLMAN, TOLK, AND