Mike Harvey

A mesoscale phytoplankton bloom in the polar Southern Ocean stimulated by iron fertilization

Changes in iron supply to oceanic plankton are thought to have a significant effect on concentrations of atmospheric carbon dioxide by altering rates of carbon sequestration, a theory known as the ‘iron hypothesis’. For this reason, it is important to understand the response of pelagic biota to increased iron supply. Here we report the results of a mesoscale iron fertilization experiment in the polar Southern Ocean, where the potential to sequester iron-elevated algal carbon is probably greatest. Increased iron supply led to elevated phytoplankton biomass and rates of photosynthesis in surface waters, causing a large drawdown of carbon dioxide and macronutrients, and elevated dimethyl sulphide levels after 13 days. This drawdown was mostly due to the proliferation of diatom stocks. But downward export of biogenic carbon was not increased. Moreover, satellite observations of this massive bloom 30 days later, suggest that a sufficient proportion of the added iron was retained in surface waters. Our findings demonstrate that iron supply controls phytoplankton growth and community composition during summer in these polar Southern Ocean waters, but the fate of algal carbon remains unknown and depends on the interplay between the processes controlling export, remineralisation and timescales of water mass subduction.

Measurements of air‐sea gas exchange at high wind speeds in the Southern Ocean: Implications for global parameterizations

velocity is proposed, which is consistent with previou s 3 He/ SF 6 dual tracer result sf rom the coastal and open ocean obtained at lower wind speeds. This suggests that factors controlling air-sea gas exchange in this region are similar to those in other parts of the world ocean, and that the parameterization presented here should be applicable to the global ocean. Citation: Ho, D. T., C. S. Law, M. J. Smith, P. Schlosser, M. Harvey, and P. Hill (2006), Measurements of airsea gas exchange at high wind speeds in the Southern Ocean: Implications for global parameterizations, Geophys. Res. Lett., 33, L16611, doi:10.1029/2006GL026817.

Maritime Aerosol Network as a component of Aerosol Robotic Network

The paper presents the current status of the Maritime Aerosol Network (MAN), which has been developed as a component of the Aerosol Robotic Network (AERONET). MAN deploys Microtops handheld Sun photometers and utilizes the calibration procedure and data processing (Version 2) traceable to AERONET. A web site dedicated to the MAN activity is described. A brief historical perspective is given to aerosol optical depth (AOD) measurements over the oceans. A short summary of the existing data, collected on board ships of opportunity during the NASA Sensor Intercomparison and Merger for Biological and Interdisciplinary Oceanic Studies (SIMBIOS) Project is presented. Globally averaged oceanic aerosol optical depth (derived from island-based AERONET measurements) at 500 nm is similar to 0.11 and Angstrom parameter (computed within spectral range 440-870 nm) is calculated to be similar to 0.6. First results from the cruises contributing to the Maritime Aerosol Network are shown. MAN ship-based aerosol optical depth compares well to simultaneous island and near-coastal AERONET site AOD.

Hygroscopic and volatile properties of marine aerosol observed at Cape Grim during the P2P campaign

Environmental context. The marine environment covers 71% of the Earth’s surface, and accounts for most of the planet’s cloud cover. Water droplets in these clouds all form on pre-existing marine aerosol particles. The number, size and composition of these particles determine the cloud droplet size and consequently, the cloud’s light scattering and precipitation behaviour. Marine aerosols, therefore, have a major influence on the planet’s radiation balance and climate. The origin of marine aerosols is still not completely resolved. The newly developed VH-TDMA technique has been applied to marine aerosols that come from the Southern Ocean. The technique enabled new insights into the composition and structure of these aerosols. It has been found that organic matter constitutes 20–40% of these particles, which suppresses their hygroscopic growth.

7Be deposition in a high-rainfall area of New Zealand

Concentrations of natural 7Be in air and rainwater were monitored for one year at Hokitika, New Zealand. The mean airborne concentration was 3.1±1.3 mBq m−3, the mean Hokitika, New Zealand. The mean airborne concentration was 3.1±1.3 mBq m−3, the mean concentration in rainwater was 2600±1200 Bq m−3, and the mean total deposition was estimated to be 130±99 Bq m−2 wk−1. Most of the 7Be was wet deposited and the washout ratio was independent of precipitation amount. A significant linear relationship exists between the weekly wet deposition flux and weekly precipitation at this high-rainfall site.

DMS and SO2 at Baring Head, New Zealand: Implications for the Yield of SO2 from DMS

Atmospheric dimethyl sulfide (DMS) and sulfur dioxide (SO2) concentrations were measured at Baring Head, New Zealandduring February and March 2000. Anti-correlated DMS and SO2 diurnalcycles, consistent with the photochemical production of SO2 from DMS, were observed in clean southerly air off the ocean. The data is used to infer a yield of SO2 from DMS oxidation. The estimated yields are highly dependent on assumptions about the DMS oxidation rate. Fitting the measured data in a photochemical box model using model-generated OH levels and the Hynes et al. (1986) DMS + OH rate constant suggests that theSO2 yield is 50–100%, similar to current estimates for the tropical Pacific.However, the observed amplitude of the DMS diurnal cycle suggests that the oxidation rate is higher than that used by the model, and therefore, that theSO2 yield is lower in the range of 20–40%.

Summertime aerosol measurements in the ross sea region of Antarctica

Abstract The physical and chemical characteristics of atmospheric aerosol were determined at a site remote from anthropogenic influences, on the edge of the Antarctic continent. The number concentration (0.12–3.12 μm diameter) ranged between 9 and 90 cm −3 and the corresponding mass between 0.1 and 3.7 μg m −3 . The concentration of sulphate in two filter samples was 0.29 and 0.48 μg m −3 . Size distributions at the site were remarkably invariant. The two major factors affecting the size distribution and concentration were occurrence of precipitation and atmospheric stability, respectively. Modes in the volume distribution occurred at about 0.2 and 2.0 μm diameter. The smallest particles

Aerosol variability in the vertical in the southwest Pacific

Vertical variations of the tropospheric aerosol concentration and size distribution, for sizes between 0.12-μm and 8-μm diameter, in the Southwest Pacific have been examined. The data were obtained from flights by an instrumented aircraft over a period of more than 3 years. The data show that 44% of the variance in the concentration of the accumulation mode aerosol (0.12 μm to 0.5 μm) can be accounted for when it is modeled by an exponential decrease with height to 2400 m and a constant concentration above. The coarse mode aerosol concentration (greater than 0.5μm) also decreases exponentially with height to 2400 m (46% of the variance accounted for) at a rate only slightly greater than for the accumulation mode aerosol. More of the variance can be accounted for by fitting the exponential decrease multiplied by a different constant term to each vertical profile separately. This implies an improvement in the predictability of the aerosol concentration in the vertical if the column amount or the concentration at some height is known. Examination of the shape of the size distributions in the free troposphere shows no coarse mode, although the tail of the accumulation mode distribution extends to sizes greater than 0.5 μm. At low altitudes the concentration of accumulation mode particles is greater and a coarse mode is apparent. The shapes of the distributions, particularly above the boundary layer, are similar and independent of the particle concentration.

A seasonal cycle in the Southwest Pacific free tropospheric aerosol concentration

Measurements made of the concentration of aerosol particles in the diameter range 0.15 − 3μm in the free troposphere over the southwest Pacific show evidence of a seasonal cycle. The maximum concentration occurs in spring and the minimum in autumn. The amplitude of the cycle is greatest at 30°S where, in early spring, the mean aerosol concentration is 18 cm−3, more than 10 times the mean autumn value. In the vicinity of the equator the seasonality disappears and concentrations throughout the year apparently remain close to the autumn levels at other latitudes. A number of different mechanisms could account for the observed seasonality but it is probable that the peak in early spring is due to biomass burning through the tropical southern hemisphere dry season. Continued high values through late spring and early summer are thought to be the result of the seasonal production of aerosol precursors, such as dimethyl sulfide (DMS), at middle and high latitudes.

Dimethylsulfide, a limited contributor to new particle formation in the clean marine boundary layer

[1] Condensation sinks for sulfuric acid are predicted using measured aerosol number distributions for five sites, Baring Head and Cape Grim (clean marine), Tarawa (equatorial clean marine), the East Antarctic Plateau and the free troposphere (ultra clean). The condensation sink determined at each site allowed an estimate of the minimum level of sulfur dioxide required for new sulfate particle formation at that site. The sulfur dioxide concentration required for new particle production is nearly two orders of magnitude larger than the observed average at the sites assessed, except the free troposphere. In the marine boundary layer any available sulfur dioxide is converted heterogeneously to sulfate via the condensation sink. This implies that a significant proportion of the sub-micron sulfate aerosol in the marine boundary layer is likely to have been entrained from the free troposphere, where a limited condensation sink and sufficiently high levels of sulfur species can support homogeneous nucleation.