Novel approaches to the evaluation of iron and phosphorus availability in dust deposited to the oceans

Lead Research Organisation: University of Birmingham
Department Name: Sch of Geography, Earth & Env Sciences

Abstract

Ocean is a major sink for atmospheric CO2. The carbon uptake capacity of a large part of the global ocean is however limited by the amount of nutrients iron and/or phosphorus in surface waters. Therefore, understanding the origins and fate of iron and phosphorus in surface oceans is important in modeling the climate system and therefore predicting climate change. One of the primary external sources of iron and phosphorus found in the surface waters in the open ocean is through atmospheric dust. Such iron and phosphorus from the dust can increase ocean carbon uptake and alter ocean biogeochemistry, thus affecting climate. The magnitude of the impact of dust input on oceanic carbon uptake and climate is dependent on total dust deposition fluxes as well as the bioavailability of iron and phosphorus in the dust. Global models seem able to simulate the former reasonably well but not the latter. One important reason is that most iron and phosphorus in desert dust are unreactive and thus not bioavailable but they can become much more reactive after being transported in the atmosphere. For example, the fraction of dissolved (<200nm) to total iron and phosphorus (defined as solubility) are orders of magnitude higher in dust over more remote oceans than over desert regions. It is now understood that solubility of iron, and probably phosphorus, in dust is controlled to a large extent by processes in the source area and in the atmosphere. Mechanistic understanding of some of the processes have been developed and/or parameterized into global models. However, major gaps remain on the solubility, lability and bioavailability of iron and phosphorus in dust deposited to the ocean, particularly from the wet deposition. These gaps significantly affects the ability of global models to represent the current climate system and therefore to predict climate change. The aim of this work is to evaluate the solubility and lability (availability) of iron and phosphorus in dust, particularly in rainwater deposited to the oceans. The objectives are: (1) To elucidate the fundamental parameters and processes controlling the concentration and partition of labile, soluble (<1nm), dissolved, and total iron and phosphorus in dust from rainwater; (2) To clarify how iron interact with phosphorus and trace metals in rainwater and affect their solubilities; and (3) To quantify the labile and/or soluble, dissolved and total iron and phosphorus deposition fluxes at five sites downwind of dust source regions for model constraining. These objectives will be met by field measurements and laboratory simulations employing state-of-the-art separation techniques (e.g., flow field flow fractionation, FFF) and in-situ labile trace metal speciation techniques (i.e., diffusive gradients in thin Films, DGT) coupled with high resolution ICP-MS and Electron and Atomic Microscopy. The scientific results will be fed to global models to improve the estimation of atmospheric nutrient deposition fluxes, and can be readily fed to Met Office and NERC Earth system models to predict the impact of atmospheric nutrient input on ocean productivity and climate in the present and the future.

Publications

10 25 50
publication icon
Baker AR (2016) Trace element and isotope deposition across the air-sea interface: progress and research needs. in Philosophical transactions. Series A, Mathematical, physical, and engineering sciences

publication icon
Shi Z (2015) Atmospheric processing outside clouds increases soluble iron in mineral dust. in Environmental science & technology

 
Description Air pollution-aerosol interactions produce more bioavailable iron for ocean ecosystems: It has long been hypothesized that acids formed from anthropogenic pollutants and natural emissions dissolve iron (Fe) in airborne particles, enhancing the supply of bioavailable Fe to the oceans. However, field observations have yet to provide indisputable evidence to confirm this hypothesis. Single-particle chemical analysis for hundreds of individual atmospheric particles collected over the East China Sea shows that Fe-rich particles from coal combustion and steel industries were coated with thick layers of sulfate after 1 to 2 days of atmospheric residence. The Fe in aged particles was present as a "hotspot" of (insoluble) iron oxides and throughout the acidic sulfate coating in the form of (soluble) Fe sulfate, which increases with degree of aging (thickness of coating). This provides the "smoking gun" for acid iron dissolution, because iron sulfate was not detected in the freshly emitted particles and there is no other source or mechanism of iron sulfate formation in the atmosphere.

Delivery of anthropogenic bioavailable iron from mineral dust and combustion aerosols to the ocean: we, for the first time, we interactively combined laboratory kinetic experiments with global aerosol modeling to more accurately quantify anthropogenic soluble Fe due to air pollution. Firstly, we determined Fe dissolution kinetics of African dust samples at acidic pH values with and without ionic species commonly found in aerosol water (i.e., sulfate and oxalate). Then, by using acidity as a master variable, we constructed a new empirical scheme for Fe release from mineral dust due to inorganic and organic anions in aerosol water. We implemented this new scheme and applied an updated mineralogical emission database in a global atmospheric chemistry transport model to estimate the atmospheric concentration and deposition flux of soluble Fe under preindustrial and modern conditions. Our improved model successfully captured the inverse relationship of Fe solubility and total Fe loading measured over the North Atlantic Ocean (i.e., 1-2 orders of magnitude lower Fe solubility in northern- African- than combustion-influenced aerosols). The model results show a positive relationship between Fe solubility and water-soluble organic carbon (WSOC) = Fe molar ratio, which is consistent with previous field measurements. We estimated that deposition of soluble Fe to the ocean increased from 0.05-0.07 Tg Fe yr1 in the preindustrial era to 0.11-0.12 Tg Fe yr1 in the present day, due to air pollution. Over the high-nitrate, low-chlorophyll (HNLC) regions of the ocean, the modeled Fe solubility remains low for mineral dust (<1 %) in a base simulation but is substantially enhanced in a sensitivity simulation, which permits the Fe dissolution for mineral aerosols in the presence of excess oxalate under low acidity during daytime. Our model results suggest that human activities contribute to about half of the soluble Fe supply to a significant portion of the oceans in the Northern Hemisphere, while their contribution to oceans in high latitudes remains uncertain due to limited understanding of Fe source and its dissolution under pristine conditions.

Atmospheric Processing Outside Clouds Increases Soluble Iron in Mineral Dust: we experimentally simulate and model the cycling of Fe-bearing dust between wet aerosol and cloud droplets. Our results show that insoluble Fe in dust particles readily dissolves under acidic conditions relevant to wet aerosols. By contrast, under the higher pH conditions generally relevant to clouds, Fe dissolution tends to stop, and dissolved Fe precipitates as poorly crystalline nanoparticles. If the dust-bearing cloud droplets evaporated again (returning to the wet aerosol stage with low pH), those neo-formed Fe nanoparticles quickly redissolve, while the refractory Fe-bearing phases continue to dissolve gradually. Overall, the duration of the acidic, wet aerosol stage ultimately increases the amount of potentially bioavailable Fe delivered to oceans, while conditions in clouds favor the formation of Fe-rich nanoparticles in the atmosphere.

Understanding the nature of atmospheric acid processing of mineral dusts in supplying bioavailable phosphorus to the oceans: Acid dissolution occurs rapidly (seconds to minutes) and is controlled by the amount of H+ ions present. For H+ < 10-4 mol/g of dust, 1-10% of the total P is dissolved, largely as a result of dissolution of surface-bound forms. At H+ > 10-4 mol/g of dust, the amount of P (and calcium) released has a direct proportionality to the amount of H+ consumed until all inorganic P minerals are exhausted and the final pH remains acidic. Once dissolved, P will stay in solution due to slow precipitation kinetics. Dissolution of apatite-P (Ap-P), the major mineral phase in dust (79-96%), occurs whether calcium carbonate (calcite) is present or not, although the increase in dissolved P is greater if calcite is absent or if the particles are externally mixed. The system was modeled adequately as a simple mixture of Ap-P and calcite. P dissolves readily by acid processes in the atmosphere in contrast to iron, which dissolves more slowly and is subject to reprecipitation at cloud water pH. We show that acidification can increase bioavailable P deposition over large areas of the globe, and may explain much of the previously observed patterns of variability in leachable P in oceanic areas where primary productivity is limited by this nutrient (e.g., Mediterranean).
Exploitation Route We expect that our data will be used by earth system models to test the impact of soluble iron on ecosystems and to estimate the amount of extra carbon that has been absorbed due to anthropogenic activities and the impact of projected decrease in soluble iron (due to air pollution control) on ocean carbon uptake; this will have potentially important implications for climate treaties if we have to further cut the CO2 emissions as a result of air pollution.

This research is of interest to the general public and has been reported in BBC Radio 4 Inside the Science programme.
Sectors Environment

 
Description The findings of the work are used to sparkle the interests of your people on mineral dust, nutrient and marine phyotoplankton (and climate change). The main mechanism is through the Community Day at the University of Birmingham. My research team hosted events for each year from 2012 to 2014 to explain how mineral dust generated from the deserts can affect ocean productivity. We have also interacted with international media to introduce our new results that air pollution can generate new sources of iron nutrients which may fertilize the ocean. This could be potentially beneficial by helping to slow down global warming.
First Year Of Impact 2014
Sector Environment
Impact Types Cultural,Societal

 
Description ASSEMBLE-EU
Amount € 1,500 (EUR)
Organisation Interuniversity Institute for Marine Science at Eilat 
Sector Academic/University
Country Israel
Start 01/2014 
End 08/2014
 
Description Birmingham Fellowship
Amount £15,000 (GBP)
Organisation University of Birmingham 
Sector Academic/University
Country United Kingdom
Start 07/2011 
End 07/2014
 
Description Directed - International
Amount £841,656 (GBP)
Funding ID NE/P016499/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 11/2016 
End 11/2021
 
Description Directed International
Amount £275,345 (GBP)
Funding ID NE/S006699/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 04/2019 
End 01/2021
 
Description Directed International Programme
Amount £6,500,000 (GBP)
Funding ID NE/N007190/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 01/2016 
End 01/2019
 
Description International Opportunities Fund
Amount £40,294 (GBP)
Funding ID NE/R005281/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 04/2018 
End 03/2019
 
Description JSPS London Japan academic visit funding
Amount £2,000 (GBP)
Organisation Japan Society for the Promotion of Science (JSPS) 
Sector Learned Society
Country Japan
Start 01/2012 
End 03/2012
 
Description Royal Society - NSFC International Travel Grant
Amount £11,900 (GBP)
Organisation The Royal Society 
Sector Academic/University
Country United Kingdom
Start 05/2015 
End 03/2017
 
Description Japan Akinori Ito 
Organisation Japan Agency for Marine-Earth Science and Technology
Country Japan 
Sector Public 
PI Contribution We have provided the iron dissolution kinetics under different relevant atmospheric conditions to our partner
Collaborator Contribution Partner Akinori Ito has used our data to develop a new dust iron dissolution scheme and then updated his global model. He then carried out global modelling of iron cycling in the atmosphere.
Impact Ito, A., Shi, Z., 2015. Atmospheric delivery of anthropogenic bioavailable iron from mineral dust and combustion aerosols. Atmospheric Chemistry Physics, 16, 85-99 23051-23088, doi:10.5194/acp-16-85-2016.
Start Year 2014
 
Description Shandong University: Li Weijun 
Organisation Shandong University
Country China 
Sector Academic/University 
PI Contribution Expertise in iron biogeochemistry; provide supervision on research
Collaborator Contribution Collected samples Carried out STEM/NanoSIMS analysis
Impact Li, W., Xu, L., Liu, X., Zhang, J. Lin, Y., Yao, X., Gao, H., Zhang, D., Chen, J., Wang, W., Harrison, R.M., Zhang, X., Shao, L., Fu, P., Nenes, A., Shi, Z., 2016. Aerosol - pollution interaction produces more soluble iron for the ocean ecosystems. Science Advances, 3, e1601749
Start Year 2012
 
Description Tianjin University 
Organisation Tianjin University
Country China 
Sector Academic/University 
PI Contribution We designed the experiments.
Collaborator Contribution Professor Pingqing Fu's group supported the field sampling in both summer and winter.
Impact The project is ongoing with no direct output yet.
Start Year 2018
 
Description Acid iron dissolution theory Press release 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Media (as a channel to the public)
Results and Impact Press release

Further interviews by Voice of America and Smithsonian; further reports in 29 international outlets
Year(s) Of Engagement Activity 2017
URL http://www.birmingham.ac.uk/news/latest/2017/03/iron-dissolved-by-air-pollution.aspx
 
Description BBC Radio 4 Inside the Science 
Form Of Engagement Activity A broadcast e.g. TV/radio/film/podcast (other than news/press)
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Interviewed by Dr Adam Rutherford regarding our findings of pollution solubilising iron, which fertilizes the ocean.

http://www.bbc.co.uk/programmes/b08g2tvp
Year(s) Of Engagement Activity 2017
URL http://www.bbc.co.uk/programmes/b08g2tvp
 
Description University of Birmingham Community Day 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Around 150 primary and secondary school students and their family attended for a univeristy visit to Birmingham University. They are fascinated by the fact that there is a teleconnection between dust and marine organisms.
Year(s) Of Engagement Activity 2014
 
Description University of Birmingham Community Day public engagement event 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact The event was carried out during Community Days at the University of Birmingham. Three team members participated in this event and we were all busy from 11 to 4pm. Even when we are closing, these are still children and family coming to our stand. There were discussions regarding dust, nutrient and ocean productivity and climate change.

Intensive interests from the children and families; the printed posters (over 100 copies) were taken away completely every time.
Year(s) Of Engagement Activity 2012,2013,2014