Periodic Reporting for period 2 - O3-SML (Ozone dry deposition to the sea surface microlayer)
Période du rapport: 2021-04-01 au 2022-09-30
• What is the problem/issue being addressed?
Some of the largest uncertainties in atmospheric science arise because of poorly understood chemistry that takes place on the many different interfaces and substrates that interact with the atmosphere. The sea surface microlayer (SML) – the top few millimetres of the ocean surface containing large chemical, physical and biological gradients that separate it from the underlying seawater - is a critical yet little understood interface which covers more than 70% of the Earth’s surface. A key interaction of the SML with the atmosphere is via ozone (O3), which is reactive with chemicals present at the surface of the ocean. Estimates of O3 deposition velocities (vD) vary widely, in large part from a lack of direct field measurements of oceanic O3 fluxes, and translate into large differences in the predicted ocean dry deposition flux. Crucially, the lack of co-measured oceanic biogeochemical information means that current observations cannot be used to disaggregate the variability in vD that arises from chemical, physical or biological processes.
The deposition velocity of O3 is significantly enhanced by chemical reactions in seawater, particularly reactions involve iodide (I-) and organic, and these reactions in combination can substantially modify vD depending on the abundance and reactivity of substrates in the SML. In the absence of more definitive observed values, most atmospheric chemistry models use a constant or near-constant vD. More sophisticated deposition parameterisations are available but either neglect the role of organics or contain crude approximations of their impacts. The choice of parameterisation has a large influence on the range and spatial variability of vD and on predicted ozone mixing ratios.
While there is a growing body of work on surface iodide and its interactions with O3 and progress on including this process in global models, the nature and reactivity of the organic material in seawater which interacts with O3 is unknown. Dissolved organic carbon (DOC) is a mixture of many chemical species and represents the largest organic carbon pool in the marine environment. Previous laboratory experiments have shown that DOC contributes to the enhancement of vD over seawater, however these studies did not use representative natural oceanic DOC, contain any mechanistic interpretation, or attempt to use DOC representative of the SML. Nevertheless, extrapolation of such results in a global model shows that O3-organic interactions have the potential to make an equally large global impact on surface ozone concentrations as iodide.
Recently developed state of the art models of oceanic O3 deposition that include chemical reaction with iodide implicitly assume that reactivity is driven by reactions in the aqueous phase of the SML, as appears to be the case for iodide, rather than directly at the surface. Our recent experimental work suggests however this may not be correct, showing that O3 uptake was controlled by a surface Langmuir-Hinshelwood mechanism rather than by liquid phase O3 + I- reactions. Such kinetics are typical of surface reactions of O3 with organic compounds. This finding implies that current model parameterizations – even the more sophisticated recent versions - are incorrect not only in their assumed chemical reactivity to O3, but in their entire framework, since they assume reactivity is driven by reactions in the bulk aqueous phase.
A further motivation for fully characterising ozone-surface interactions in the SML is that these not only determine how quickly O3 may be irreversibly taken up at the ocean surface, but also influence the re-emissions of other reactive trace gases out of the ocean. This includes emissions that go on and affect gas phase tropospheric O3 processes and chemicals that may constitute a source of secondary organic aerosols (SOA) to the marine atmosphere. Our previous work has shown that the ozonolysis of iodide in the SML appears to be the dominant source of reactive iodine – namely HOI and I2 - to the atmosphere and this leads to significant gas phase chemical loss of tropospheric O3. There is a need however to verify these studies at much lower iodine concentrations, especially for HOI where no direct field observations exist. Laboratory studies also demonstrate that chemistry of surface-active organics gives rise to a number of atmospherically relevant compounds including oxidized volatile organic compounds (VOCs). These are potentially highly significant findings that have the potential to transform our understanding of the marine atmosphere and particularly that of marine climate-active SOA. Currently however, it is unknown how these laboratory results may apply to the atmosphere.
• Why is it important for society?
Tropospheric ozone is a significant climate gas in addition to having a major influence on air quality, public health, on the photochemical processing of atmospheric chemicals, and on food security and ecosystem viability. Dry deposition of O3 to the Earth’s surface is estimated to account for about 30% of overall tropospheric O3 removal, however losses to the ocean SML, which is believed to be the largest single sink, are subject to much greater uncertainties than deposition to land. Model calculations show that ocean dry deposition has the potential to reduce surface O3 mixing ratios by several ppb, which is of a magnitude where it can influence human exposure and impact on ecosystems and agricultural crop yields.
• What are the overall objectives?
The major aim of “O3-SML” is to obtain substantial new insight, via lab and field observations, into O3 fluxes over the Earth’s oceans coupled with new fundamental understanding of its major biogeochemical controls and to translate these into policy relevant conclusions through the use of numerical models. This work will provide the first definitive constraint on O3 dry deposition over the ocean and an improved numerical representation for chemistry-transport models. A secondary objective is to better quantify the trace gas fluxes out of the ocean that arise from oceanic O3-SML interactions and assess their atmospheric importance, in comparison to other mechanisms for oceanic VOC production.
Some of the largest uncertainties in atmospheric science arise because of poorly understood chemistry that takes place on the many different interfaces and substrates that interact with the atmosphere. The sea surface microlayer (SML) – the top few millimetres of the ocean surface containing large chemical, physical and biological gradients that separate it from the underlying seawater - is a critical yet little understood interface which covers more than 70% of the Earth’s surface. A key interaction of the SML with the atmosphere is via ozone (O3), which is reactive with chemicals present at the surface of the ocean. Estimates of O3 deposition velocities (vD) vary widely, in large part from a lack of direct field measurements of oceanic O3 fluxes, and translate into large differences in the predicted ocean dry deposition flux. Crucially, the lack of co-measured oceanic biogeochemical information means that current observations cannot be used to disaggregate the variability in vD that arises from chemical, physical or biological processes.
The deposition velocity of O3 is significantly enhanced by chemical reactions in seawater, particularly reactions involve iodide (I-) and organic, and these reactions in combination can substantially modify vD depending on the abundance and reactivity of substrates in the SML. In the absence of more definitive observed values, most atmospheric chemistry models use a constant or near-constant vD. More sophisticated deposition parameterisations are available but either neglect the role of organics or contain crude approximations of their impacts. The choice of parameterisation has a large influence on the range and spatial variability of vD and on predicted ozone mixing ratios.
While there is a growing body of work on surface iodide and its interactions with O3 and progress on including this process in global models, the nature and reactivity of the organic material in seawater which interacts with O3 is unknown. Dissolved organic carbon (DOC) is a mixture of many chemical species and represents the largest organic carbon pool in the marine environment. Previous laboratory experiments have shown that DOC contributes to the enhancement of vD over seawater, however these studies did not use representative natural oceanic DOC, contain any mechanistic interpretation, or attempt to use DOC representative of the SML. Nevertheless, extrapolation of such results in a global model shows that O3-organic interactions have the potential to make an equally large global impact on surface ozone concentrations as iodide.
Recently developed state of the art models of oceanic O3 deposition that include chemical reaction with iodide implicitly assume that reactivity is driven by reactions in the aqueous phase of the SML, as appears to be the case for iodide, rather than directly at the surface. Our recent experimental work suggests however this may not be correct, showing that O3 uptake was controlled by a surface Langmuir-Hinshelwood mechanism rather than by liquid phase O3 + I- reactions. Such kinetics are typical of surface reactions of O3 with organic compounds. This finding implies that current model parameterizations – even the more sophisticated recent versions - are incorrect not only in their assumed chemical reactivity to O3, but in their entire framework, since they assume reactivity is driven by reactions in the bulk aqueous phase.
A further motivation for fully characterising ozone-surface interactions in the SML is that these not only determine how quickly O3 may be irreversibly taken up at the ocean surface, but also influence the re-emissions of other reactive trace gases out of the ocean. This includes emissions that go on and affect gas phase tropospheric O3 processes and chemicals that may constitute a source of secondary organic aerosols (SOA) to the marine atmosphere. Our previous work has shown that the ozonolysis of iodide in the SML appears to be the dominant source of reactive iodine – namely HOI and I2 - to the atmosphere and this leads to significant gas phase chemical loss of tropospheric O3. There is a need however to verify these studies at much lower iodine concentrations, especially for HOI where no direct field observations exist. Laboratory studies also demonstrate that chemistry of surface-active organics gives rise to a number of atmospherically relevant compounds including oxidized volatile organic compounds (VOCs). These are potentially highly significant findings that have the potential to transform our understanding of the marine atmosphere and particularly that of marine climate-active SOA. Currently however, it is unknown how these laboratory results may apply to the atmosphere.
• Why is it important for society?
Tropospheric ozone is a significant climate gas in addition to having a major influence on air quality, public health, on the photochemical processing of atmospheric chemicals, and on food security and ecosystem viability. Dry deposition of O3 to the Earth’s surface is estimated to account for about 30% of overall tropospheric O3 removal, however losses to the ocean SML, which is believed to be the largest single sink, are subject to much greater uncertainties than deposition to land. Model calculations show that ocean dry deposition has the potential to reduce surface O3 mixing ratios by several ppb, which is of a magnitude where it can influence human exposure and impact on ecosystems and agricultural crop yields.
• What are the overall objectives?
The major aim of “O3-SML” is to obtain substantial new insight, via lab and field observations, into O3 fluxes over the Earth’s oceans coupled with new fundamental understanding of its major biogeochemical controls and to translate these into policy relevant conclusions through the use of numerical models. This work will provide the first definitive constraint on O3 dry deposition over the ocean and an improved numerical representation for chemistry-transport models. A secondary objective is to better quantify the trace gas fluxes out of the ocean that arise from oceanic O3-SML interactions and assess their atmospheric importance, in comparison to other mechanisms for oceanic VOC production.
The main achievements are described below separated by work package.
WP1: Mechanisms of reactive O3 uptake to the SML
We developed a new temperature-controlled kinetic flow reactor and have improved the precision of the measurement of O3 uptake. Measurement of the temperature-dependence of the heterogeneous O3-iodide reaction is ongoing. We deployed the flow reactor on the CONNECT cruise (see below) to determine ambient O3 uptake coefficients to the SML and underlying water, and have at least 100 frozen samples from the PPAO time series which will be analysed for O3 uptake coefficients. All these samples have also been co-sampled for biogeochemical parameters.
WP2: Eddy covariance O3 flux measurements at coastal observatories and on ships
We have obtained a nearly 2 year data set (Nov 2019-Oct 2021) of oceanic O3 flux measurements (by eddy covariance – chemiluminescence) at the Penlee Point Atmospheric Observatory (PPAO) and associated biogeochemical measurements in the seawater footprint of the PPAO. There are gaps in the data due to the first Covid-19 lockdown and to various technical issues; nevertheless, we have gathered a very large amount of unique high quality data which will allow us to determine key biogeophysical drivers (including sea surface iodide, DOC, SPE-DOM, fatty acids, surfactant activity and meteorological parameters) of the oceanic O3 flux. Reporting of the preliminary results and technical set up are reported in paper 3 (Loades et al. 2020).
We were able to secure 3 scientific berths on the GEOMAR SO287-CONNECT pan Atlantic cruise from Gran Canaria (11 Dec 2021) to Ecuador (11 Jan 2022). The overall objectives of the cruise were to study how biogeochemical and ecological processes are interconnected over large distances and how large are the quantities of substances exchanged between the ocean and the atmosphere. We successfully measured oceanic O3 fluxes and O3 uptake coefficients to the SML and underlying water, and have extracted dozens of seawater samples for analyses of iodine, DOC, SPE-DOM, fatty acids, and surfactant activity. Analysis of these samples and data are ongoing.
WP3 Linking ambient O3 fluxes with oceanic chemical and physical variables
The data collected in WP1 and WP2 are currently being analysed to explore these links. Initial analyses confirms our early results that iodide comprises only 20-70% of the ozone reactivity in the SML. We observed depletion of iodide in the SML, which appears to be due to its reaction with atmospheric O3. We also find a weak wind-speed dependence of ozone vD, contrary to the current state-of-the-art O3 deposition model. Finally, we analysed samples from the MOSAiC campaign in the Arctic for iodide speciation - there is currently a gap in iodide observations in this region. Interpretation of these data is ongoing.
WP4 Quantify the major gaseous emissions products of ozone reactions in the sea surface microlayer
We have quantified the production of organic iodine from the O3-iodide interaction and found it to be negligible compared to inorganic iodine emissions (paper 1, Tinel et al. 2020).
Through a UK NERC capital equipment grant, we have acquired a TOFWERK Vocus S CI-APi-TOF with a chemical ionization source capable of generating bromide ions (Br-), alongside a number of other ions, to detect iodine species as their bromide cluster (delivered in Nov 2021). The same instrument (“Br-APi-TOF”) has very recently been deployed to successfully detect HOI (alongside IBr and ICl) for the first time in the marine atmosphere down to low ppt levels by another group. This offers better performance than our original suggested method of SIFT-MS. We have successfully calibrated the Br-APi-TOF for I2 (showing a detection limit of ~ 1 pptv for 5 min averages) and have been working on a calibration method for HOI. Lab experiments to better quantify emissions of HOI and I2 from the O3-iodide interaction will take place once the Br-APi-TOF has returned from field work.
We have not yet started to quantify VOC emission from ozonolysis of SML samples, but we have the samples in the freezer and will perform these experiments in late 2022/early 2023.
WP5 – Develop a framework for linking O3 dry deposition to oceanic trace gas emissions for use in global models
Progress on this front has been very encouraging. We have incorporated a new scheme for oceanic O3 deposition incorporating O3-iodide reactions into the global atmospheric chemistry transport model GEOS-Chem (Paper 2 - Pound et al., 2020). We have successfully tested a box model which interactively links oceanic O3 deposition to subsequent iodine emissions; the results of this are currently being written up for publication. The next step is implementing the box model scheme into GEOS-Chem, and we have a plan for how this will work.
We have also developed methods for determining fatty acids in seawater and for storing samples for quantitative off-line analyses of surface tension (papers in preparation). We have studied fatty acid production of phytoplankton monocultures (which, so far, show no appreciable production) and have just initiated a collaboration with Professor Yin Chen from the University of Warwick to study production of organics (and potentially, concurrent O3 uptake) from interactions between heterotrophic bacteria and diatom cultures.
WP1: Mechanisms of reactive O3 uptake to the SML
We developed a new temperature-controlled kinetic flow reactor and have improved the precision of the measurement of O3 uptake. Measurement of the temperature-dependence of the heterogeneous O3-iodide reaction is ongoing. We deployed the flow reactor on the CONNECT cruise (see below) to determine ambient O3 uptake coefficients to the SML and underlying water, and have at least 100 frozen samples from the PPAO time series which will be analysed for O3 uptake coefficients. All these samples have also been co-sampled for biogeochemical parameters.
WP2: Eddy covariance O3 flux measurements at coastal observatories and on ships
We have obtained a nearly 2 year data set (Nov 2019-Oct 2021) of oceanic O3 flux measurements (by eddy covariance – chemiluminescence) at the Penlee Point Atmospheric Observatory (PPAO) and associated biogeochemical measurements in the seawater footprint of the PPAO. There are gaps in the data due to the first Covid-19 lockdown and to various technical issues; nevertheless, we have gathered a very large amount of unique high quality data which will allow us to determine key biogeophysical drivers (including sea surface iodide, DOC, SPE-DOM, fatty acids, surfactant activity and meteorological parameters) of the oceanic O3 flux. Reporting of the preliminary results and technical set up are reported in paper 3 (Loades et al. 2020).
We were able to secure 3 scientific berths on the GEOMAR SO287-CONNECT pan Atlantic cruise from Gran Canaria (11 Dec 2021) to Ecuador (11 Jan 2022). The overall objectives of the cruise were to study how biogeochemical and ecological processes are interconnected over large distances and how large are the quantities of substances exchanged between the ocean and the atmosphere. We successfully measured oceanic O3 fluxes and O3 uptake coefficients to the SML and underlying water, and have extracted dozens of seawater samples for analyses of iodine, DOC, SPE-DOM, fatty acids, and surfactant activity. Analysis of these samples and data are ongoing.
WP3 Linking ambient O3 fluxes with oceanic chemical and physical variables
The data collected in WP1 and WP2 are currently being analysed to explore these links. Initial analyses confirms our early results that iodide comprises only 20-70% of the ozone reactivity in the SML. We observed depletion of iodide in the SML, which appears to be due to its reaction with atmospheric O3. We also find a weak wind-speed dependence of ozone vD, contrary to the current state-of-the-art O3 deposition model. Finally, we analysed samples from the MOSAiC campaign in the Arctic for iodide speciation - there is currently a gap in iodide observations in this region. Interpretation of these data is ongoing.
WP4 Quantify the major gaseous emissions products of ozone reactions in the sea surface microlayer
We have quantified the production of organic iodine from the O3-iodide interaction and found it to be negligible compared to inorganic iodine emissions (paper 1, Tinel et al. 2020).
Through a UK NERC capital equipment grant, we have acquired a TOFWERK Vocus S CI-APi-TOF with a chemical ionization source capable of generating bromide ions (Br-), alongside a number of other ions, to detect iodine species as their bromide cluster (delivered in Nov 2021). The same instrument (“Br-APi-TOF”) has very recently been deployed to successfully detect HOI (alongside IBr and ICl) for the first time in the marine atmosphere down to low ppt levels by another group. This offers better performance than our original suggested method of SIFT-MS. We have successfully calibrated the Br-APi-TOF for I2 (showing a detection limit of ~ 1 pptv for 5 min averages) and have been working on a calibration method for HOI. Lab experiments to better quantify emissions of HOI and I2 from the O3-iodide interaction will take place once the Br-APi-TOF has returned from field work.
We have not yet started to quantify VOC emission from ozonolysis of SML samples, but we have the samples in the freezer and will perform these experiments in late 2022/early 2023.
WP5 – Develop a framework for linking O3 dry deposition to oceanic trace gas emissions for use in global models
Progress on this front has been very encouraging. We have incorporated a new scheme for oceanic O3 deposition incorporating O3-iodide reactions into the global atmospheric chemistry transport model GEOS-Chem (Paper 2 - Pound et al., 2020). We have successfully tested a box model which interactively links oceanic O3 deposition to subsequent iodine emissions; the results of this are currently being written up for publication. The next step is implementing the box model scheme into GEOS-Chem, and we have a plan for how this will work.
We have also developed methods for determining fatty acids in seawater and for storing samples for quantitative off-line analyses of surface tension (papers in preparation). We have studied fatty acid production of phytoplankton monocultures (which, so far, show no appreciable production) and have just initiated a collaboration with Professor Yin Chen from the University of Warwick to study production of organics (and potentially, concurrent O3 uptake) from interactions between heterotrophic bacteria and diatom cultures.
By the end of the project, we are confident we will:
• Quantitatively understand at a process level how O3 dry deposition is affected by the combined contributions of organic material and iodide in the SML
• Construct a definitive framework for calculating O3 dry deposition over the world’s oceans, and interactively link this to re-emissions of reactive trace gases (iodine and VOCs) to the atmosphere
• Assess the significance of O3 deposition-induced emissions on global and regional atmospheric chemistry
These expected results all go significantly beyond the state of the art.
• Quantitatively understand at a process level how O3 dry deposition is affected by the combined contributions of organic material and iodide in the SML
• Construct a definitive framework for calculating O3 dry deposition over the world’s oceans, and interactively link this to re-emissions of reactive trace gases (iodine and VOCs) to the atmosphere
• Assess the significance of O3 deposition-induced emissions on global and regional atmospheric chemistry
These expected results all go significantly beyond the state of the art.