A C T In order to clarify the CO 2 exchange between the seawater and the overlying air during the sea-ice formation, we have carried out tank experiments in a low-temperature room.CO 2 concentration above the sea-ice began to increase since the beginning of the sea-ice formation, and increased at larger rates with time and the decrease in air temperature.This increase of CO 2 concentration in air was mainly caused by the increase in dissolved inorganic carbon concentration in the brine of the upper part of sea-ice, changes in CO 2 solubility and dissociation constants of carbonic acid.The CO 2 flux increased logarithmically with time, and reached a level of 2 × 10 -4 to 5 × 10 -4 g-C m -2 hr -1 at 50 mm ice thickness.We found that the CO 2 flux was correlated well with the salinity and negatively with the volume of the brine in the upper part of the sea-ice.These suggested the larger role of the difference in partial pressure of CO 2 between brine and air as compared to that of competitive change in the brine volume.Present results suggest the necessity to examine the CO 2 exchange between the seawater and air in seasonal sea-ice areas.

A global ocean biogeochemical model is used to quantify the sensitivity of marine biogeochemistry and air‐sea CO2 exchange to variations in dust deposition over decadal timescales. Estimates of dust deposition generated under four climate states provide a large range in total deposition with spatially realistic patterns; transient ocean model experiments are conducted by applying a step-function change in deposition from a current climate control. Relative to current conditions, higher dust deposition increases diatom and export production, nitrogen fixation and oceanic net CO2 uptake from the atmosphere, while reduced dust deposition has the opposite effects. Over timescales less than a decade, dust modulation of marine productivity and export is dominated by direct effects in high-nutrient, low-chlorophyll regions, where iron is the primary limiting nutrient. On longer timescales, an indirect nitrogen fixation pathway has increased importance, significantly amplifying the ocean biogeochemical response. Because dust iron input decouples carbon cycling from subsurface macronutrient supply, the ratio of the change in net ocean CO2 uptake to change in export flux is large, 0.45‐0.6. Decreasing dust deposition and reduced oceanic CO2 uptake over the next century could provide a positive feedback to global warming, distinct from feedbacks associated with changes in stratification and circulation.

Air pollution in Hong Kong has close relations to synoptic systems, especially continental high pressure in cold season and tropical storms in the warm season. Here, a case study was performed of the characteristics of a photochemical pollution episode during March 27‐31, 2000, in which the maximum concentrations of ozone (O3), nitrogen oxides (NOx), carbon monoxide (CO), respirable suspended particulates (RSP) and sulphur dioxide (SO2) reached 265 μgm −3 , 2166 μgm −3 (roadside), 5405 μgm −3 , 349.3 μgm −3 and 221 μgm −3 , respectively. The O3/CO and O3/NOx ratios were found to be 0.1 and 6.29, respectively, at rural site without significant correlation. At an urban site, moderate negative correlation was observed for O3, CO, NOx, where the ratios of O3/CO and O3/NOx were relatively lower (0.03 and 0.21, respectively). The episode lasted 2 d in the Hong Kong territory and the Pearl River Delta (PRD) region. Synchronous meteorological and air monitoring data were adopted for analysis with focus on concentration variability, weather pattern, mechanisms of local photochemical production and regional transport. Our results indicate that clear skies with high temperature and low humidity caused by the strong high-pressure system over South China as well as local emissions were attributable to the high O3 level observed. In addition, backward trajectories and the O3/NOx, O3/CO ratios provided evidence that regional transport did play an important role in the formation of the episode. The MM5/STEM-2K1 and TAPM model systems were used to simulate the regional and urban O3 pattern in PRD and Hong Kong. The modeling surface concentrations of O3 were compared with the measurement from Hong Kong air quality monitoring network, which showed that the ground O3 level was underestimated 10‐20% due to the uncertainty in emission data, simple chemical mechanism as well as MM5 meteorological prediction. Sensitivity study revealed the strong cross-border transport over PRD region, which accounted for 50‐90% of the observed O3 level at nighttime and 40‐70% at daytime in Hong Kong territory depending on site locations.

Using about 138 000 measurements of surface pCO2 in the Atlantic subpolar gyre (50–70°N, 60–10°W) during 1995–1997, we compare two methods of interpolation in space and time: a monthly distribution of surface pCO2 constructed using multiple linear regressions on position and temperature, and a self-organizing neural network approach. Both methods confirm characteristics of the region found in previous work, i.e. the subpolar gyre is a sink for atmospheric CO2 throughout the year, and exhibits a strong seasonal variability with the highest undersaturations occurring in spring and summer due to biological activity. As an annual average the surface pCO2 is higher than estimates based on available syntheses of surface pCO2. This supports earlier suggestions that the sink of CO2 in the Atlantic subpolar gyre has decreased over the last decade instead of increasing as previously assumed. The neural network is able to capture a more complex distribution than can be well represented by linear regressions, but both techniques agree relatively well on the average values of pCO2 and derived fluxes. However, when both techniques are used with a subset of the data, the neural network predicts the remaining data to a much better accuracy than the regressions, with a residual standard deviation ranging from 3 to 11 μatm. The subpolar gyre is a net sink of CO2 of 0.13 Gt-C yr−1 using the multiple linear regressions and 0.15 Gt-C yr−1 using the neural network, on average between 1995 and 1997. Both calculations were made with the NCEP monthly wind speeds converted to 10 m height and averaged between 1995 and 1997, and using the gas exchange coefficient of Wanninkhof.