Final Report Summary - SOLAIROS (Solubility of Aerosol Iron in Open-ocean Seawater)
Iron (Fe) is as an essential element for marine organisms and its availability in seawater can limit the growth of phytoplankton in large regions of the ocean. Hence knowledge of Fe supply to the ocean is needed to derstand the relationship between phytoplankton growth and global biogeochemical cycles. One of the main routes of Fe into the ocean is via deposition from aerosol particles carried in the atmosphere but there are few studies that examine the process of Fe dissolution into seawater under different conditions. The aim of the SOLAIROS (SOLubility of Aerosol Iron in Open-ocean Seawater) project was to test the hypothesis that'the dissolution of aerosol iron is regulated by variations in the physico-chemical composition of surface seawater'.
The specific research objectives to test the principal hypothesis were:
Objective 1. Quantify the effects of seawater pCO2 and temperature on the fractional dissolution of aerosol iron.
Objective 2. Assess the effects of natural dissolved organic ligands (molecules that bind strongly to Fe) present in seawater on the fractional dissolution of aerosol iron.
Objective 3. Assess the effects of the concentration and size-distribution of dissolved iron (< 0. 2microm) in seawater on the fractional dissolution of aerosol iron.
Work Completed:
The 24 month outgoing phase of the project was conducted at the Bermuda Institute of Ocean Sciences (BIOS) where the researcher was made a Faculty Member and supervised 3 postgraduate/undergraduate research projects. During this phase, the researcher conducted a detailed literature survey and received training in analytical techniques at BIOS and US universities (University of Delaware and Old Dominion University). These techniques included flow injection analysis (FIA), graphite furnace atomic absorption spectrometry, cathodic stripping voltammetry (CSV) and inductively coupled plasma mass spectrometry (ICP-MS). The researcher also received training and use of the HYSPLIT meteorological model and aerosol sampling and processing including nitric acid/HF particle digestion.
A sampling campaign was completed and included collection of (i) monthly bulk aerosol and weekly rain samples from the Tudor Hill Atmospheric sampling tower (Jan 2010-March 2011), (10 samples) and (ii) bulk'low iron'seawater samples (6 x 50L) in the Bermuda Atlantic Time-series Study (BATS) region (31°40'N, 64°10'W) aboard the Atlantic Explorer research ship (April 2010, April and May 2011). In the laboratory, an aerosol dissolution simulator was set-up and several 1 month continuous aerosol leaching experiments were completed using the aerosol and seawater samples collected. The experiments assessed the effects of seawater pCO2, temperature (Objective 1.), natural dissolved organic ligands (Objective 2.) and size-distribution (Objective
3) on the fractional dissolution of aerosol iron. Approximately 600 samples were processed, acidified and stored ready for analysis.
The 12 month return phase was conducted at the University of Plymouth (UK). During this phase, dissolved iron was determined in the seawater samples from the dissolution experiments using FIA. A focus was also made on transferring knowledge acquired in Bermuda and the USA, including sampling, methods used in the sample processing study and analytical techniques for DFe determination in different matrices. Four oral presentations were given throughout the year, three to the Plymouth marine research community and one at an international conference. Research techniques were also transferred during research group meetings and via undergraduate and graduate student supervision.
The researcher was also given the opportunity to follow a career development plan, which included (i) lecturing experience, e. g. teaching inorganic chemistry workshops, (ii) attending professional training courses (including postgraduate supervision training) and (iii) applied as a co-investigator and principle investigator for NERC (UK) and European Commission research grants to continue work on trace metal biogeochemistry in the oceans.
Main results and
Conclusions:
The aerosols collected in Bermuda were found to have well defined seasonal characteristics and were separated as'winter'and'summer'samples. For the winter months (September-April), arriving air masses came from predominantly, an easterly direction. Aerosols were characterised by grey colouring and enriched elemental ratios (e. g. high cadmium to aluminium ratios of > 0. 0001), which indicated the entrainment of anthropogenic (man-made) emissions. In summer months, aerosols were dominated by a high deposition of'Saharan type'aerosols, characterised by red-brown colour and elemental ratios more typical to the average ratios of crustal material. Four of the best quality aerosol samples (2 summer and 2 winter) were chosen for the seawater dissolution experiments. The 50L bulk low iron seawater samples used for the leaches were successfully collected from ~100m depth in the BATS region (near the chlorophyll maximum) and processed without contamination. The seawaters were characterised by low iron concentrations (< 0. 4 nmol/L) and low concentrations of iron binding ligands (< 1 nmol/L).
Objective 1 was completed and small but significant changes (> 1 %) in the solubility of aerosol iron in seawater were found to occur due to changes in pCO2 within current global and future predicted ranges. By comparison, the dominant variable affecting Fe solubility was found to be the presence of strong iron binding organic ligands (Objective 2), which were added at environmentally relevant concentrations (ca. 10 nmol/L). Objective 3 revealed that in most cases the major part of dissolved iron that was released from all aerosols was found to be in the small colloidal range (0. 02 0. 2 microm), which is consistent with field observations in BATS surface seawater following dust events (Ussher et al. in preparation).
The predicted solubility of iron in seawater in the < 0. 02 microm size fraction (often termed soluble iron) is very low (~0. 1 nmol/L). However, when different naturally occurring ligands were added, certain ligands (e. g. aerobactin) were found to increase iron solubility greatly (i. e. > 100 %). Interestingly, this was a fast process and analysis of size fractionated dissolved Fe in seawater leaches showed that much of this additional Fe that was solubilised by the ligands was found in the < 0. 02 microm size range, rather than the colloidal range. This is highly relevant information for two reasons: (i) the soluble < 0. 02 microm size range of Fe is deemed to be the more bioavailable size of iron and hence could provide more iron for phytoplankton uptake and (ii) soluble complexed Fe may have a higher residence time in surface seawater than colloidal Fe which tends to aggregate and be removed.
The specific research objectives to test the principal hypothesis were:
Objective 1. Quantify the effects of seawater pCO2 and temperature on the fractional dissolution of aerosol iron.
Objective 2. Assess the effects of natural dissolved organic ligands (molecules that bind strongly to Fe) present in seawater on the fractional dissolution of aerosol iron.
Objective 3. Assess the effects of the concentration and size-distribution of dissolved iron (< 0. 2microm) in seawater on the fractional dissolution of aerosol iron.
Work Completed:
The 24 month outgoing phase of the project was conducted at the Bermuda Institute of Ocean Sciences (BIOS) where the researcher was made a Faculty Member and supervised 3 postgraduate/undergraduate research projects. During this phase, the researcher conducted a detailed literature survey and received training in analytical techniques at BIOS and US universities (University of Delaware and Old Dominion University). These techniques included flow injection analysis (FIA), graphite furnace atomic absorption spectrometry, cathodic stripping voltammetry (CSV) and inductively coupled plasma mass spectrometry (ICP-MS). The researcher also received training and use of the HYSPLIT meteorological model and aerosol sampling and processing including nitric acid/HF particle digestion.
A sampling campaign was completed and included collection of (i) monthly bulk aerosol and weekly rain samples from the Tudor Hill Atmospheric sampling tower (Jan 2010-March 2011), (10 samples) and (ii) bulk'low iron'seawater samples (6 x 50L) in the Bermuda Atlantic Time-series Study (BATS) region (31°40'N, 64°10'W) aboard the Atlantic Explorer research ship (April 2010, April and May 2011). In the laboratory, an aerosol dissolution simulator was set-up and several 1 month continuous aerosol leaching experiments were completed using the aerosol and seawater samples collected. The experiments assessed the effects of seawater pCO2, temperature (Objective 1.), natural dissolved organic ligands (Objective 2.) and size-distribution (Objective
3) on the fractional dissolution of aerosol iron. Approximately 600 samples were processed, acidified and stored ready for analysis.
The 12 month return phase was conducted at the University of Plymouth (UK). During this phase, dissolved iron was determined in the seawater samples from the dissolution experiments using FIA. A focus was also made on transferring knowledge acquired in Bermuda and the USA, including sampling, methods used in the sample processing study and analytical techniques for DFe determination in different matrices. Four oral presentations were given throughout the year, three to the Plymouth marine research community and one at an international conference. Research techniques were also transferred during research group meetings and via undergraduate and graduate student supervision.
The researcher was also given the opportunity to follow a career development plan, which included (i) lecturing experience, e. g. teaching inorganic chemistry workshops, (ii) attending professional training courses (including postgraduate supervision training) and (iii) applied as a co-investigator and principle investigator for NERC (UK) and European Commission research grants to continue work on trace metal biogeochemistry in the oceans.
Main results and
Conclusions:
The aerosols collected in Bermuda were found to have well defined seasonal characteristics and were separated as'winter'and'summer'samples. For the winter months (September-April), arriving air masses came from predominantly, an easterly direction. Aerosols were characterised by grey colouring and enriched elemental ratios (e. g. high cadmium to aluminium ratios of > 0. 0001), which indicated the entrainment of anthropogenic (man-made) emissions. In summer months, aerosols were dominated by a high deposition of'Saharan type'aerosols, characterised by red-brown colour and elemental ratios more typical to the average ratios of crustal material. Four of the best quality aerosol samples (2 summer and 2 winter) were chosen for the seawater dissolution experiments. The 50L bulk low iron seawater samples used for the leaches were successfully collected from ~100m depth in the BATS region (near the chlorophyll maximum) and processed without contamination. The seawaters were characterised by low iron concentrations (< 0. 4 nmol/L) and low concentrations of iron binding ligands (< 1 nmol/L).
Objective 1 was completed and small but significant changes (> 1 %) in the solubility of aerosol iron in seawater were found to occur due to changes in pCO2 within current global and future predicted ranges. By comparison, the dominant variable affecting Fe solubility was found to be the presence of strong iron binding organic ligands (Objective 2), which were added at environmentally relevant concentrations (ca. 10 nmol/L). Objective 3 revealed that in most cases the major part of dissolved iron that was released from all aerosols was found to be in the small colloidal range (0. 02 0. 2 microm), which is consistent with field observations in BATS surface seawater following dust events (Ussher et al. in preparation).
The predicted solubility of iron in seawater in the < 0. 02 microm size fraction (often termed soluble iron) is very low (~0. 1 nmol/L). However, when different naturally occurring ligands were added, certain ligands (e. g. aerobactin) were found to increase iron solubility greatly (i. e. > 100 %). Interestingly, this was a fast process and analysis of size fractionated dissolved Fe in seawater leaches showed that much of this additional Fe that was solubilised by the ligands was found in the < 0. 02 microm size range, rather than the colloidal range. This is highly relevant information for two reasons: (i) the soluble < 0. 02 microm size range of Fe is deemed to be the more bioavailable size of iron and hence could provide more iron for phytoplankton uptake and (ii) soluble complexed Fe may have a higher residence time in surface seawater than colloidal Fe which tends to aggregate and be removed.