Periodic Reporting for period 4 - CLIMAHAL (Climate dimension of natural halogens in the Earth system: Past, present, future)
Berichtszeitraum: 2022-03-01 bis 2023-06-30
Summary:
Naturally-emitted very short-lived halogens (VSLH) have a profound impact on the chemistry and composition of the atmosphere, destroying greenhouse gases and altering aerosol production, which together can change the Earth´s radiative balance. Therefore, natural halogens possess leverage to influence climate, although their contribution to climate change is not well established and most climate models have yet to consider their effects. Also, there is increasing evidence that natural halogens i) impact on the air quality of coastal cities, ii) accelerates the atmospheric deposition of mercury (a toxic heavy metal) and iii) that their natural ocean and ice emissions are controlled by biological and photochemical mechanisms that may respond to climate changes. Motivated by the above, this project aims to quantify the so far unrecognized natural halogen-climate feedbacks and the impact of these feedbacks on global atmospheric oxidizing capacity (AOC) and radiative forcing (RF) across pre-industrial, present and future climates. Answering these questions is essential to predict if these climate-mediated feedbacks can reduce or amplify future climate change. To this end we are developing a multidisciplinary research approach using laboratory and field observations and models interactively that allows us to peel apart the detailed physical processes behind the contribution of natural halogens to global climate change. Furthermore, the work also involves examining past-future climate impacts of natural halogens within a holistic Earth System model, where we develop the multidirectional halogen interactions in the land-ocean-ice-biosphere-atmosphere coupled system. This provides a breakthrough in our understanding of the importance of these natural processes for the composition and oxidation capacity of the Earth´s atmosphere and climate, both in the presence and absence of human influence.
Overall objectives of the project:
As the main objective, CLIMAHAL aims to study the so far unrecognized interplay between natural emissions of climate active halogens and their impact on AOC and RF in pre-industrial, present and future climates, using an interdisciplinary approach. To fulfill this overall objective the project consists of three main subprojects. The specific objectives are as follows:
A) Laboratory work and data collection.
A1) Specific objectives
a) Generation of experimental data on photochemistry and polymerization rate parameters of the higher iodine oxides (I2O2, I2O3, I2O4) monomers.
b) Compilation of available data on atmospheric gases, ocean and ice biological and abiotic parameters from previous and on-going studies and monitoring networks.
B) Development of an integrated model of the multidirectional halogen interactions within the land-ocean-ice-biosphere-atmosphere coupled system
B1) Specific objectives
a) Generation of a novel holistic Earth-system approach, by developing a state-of-the-art integrated model to connect, in a single model, the multidirectional fluxes of VSLH within different Earth system components to climate processes.
b) Validation of the new model performance in the present-day atmosphere against all available data (from subproject A) including emissions of natural halogens from the different components of the Earth system (e.g. ocean, ice, biosphere) along with their concentrations and variability throughout the global atmosphere. To this end, a number of model-measurement comparisons will be performed to evaluate the new developed model.
C) Earth System modelling of natural halogen-climate interactions across past/present/future climates.
C1) Specific objectives
Use the implemented model developed in subproject B to:
a) determine the contribution of each individual Earth system component to the natural halogen loading of the atmosphere.
b) analyze the impact of predicted sea-ice retreat on the emissions and budget of halogens in the polar regions.
c) determine the key mechanisms by which natural halogens influence climate.
d) discover the contribution, if any, of natural halogen processes to the climate of the Anthropocene.
f) find the possible biogeochemistry-halogen-climate feedbacks that modulate (diminish or amplify) climate change.
Naturally-emitted very short-lived halogens (VSLH) have a profound impact on the chemistry and composition of the atmosphere, destroying greenhouse gases and altering aerosol production, which together can change the Earth´s radiative balance. Therefore, natural halogens possess leverage to influence climate, although their contribution to climate change is not well established and most climate models have yet to consider their effects. Also, there is increasing evidence that natural halogens i) impact on the air quality of coastal cities, ii) accelerates the atmospheric deposition of mercury (a toxic heavy metal) and iii) that their natural ocean and ice emissions are controlled by biological and photochemical mechanisms that may respond to climate changes. Motivated by the above, this project aims to quantify the so far unrecognized natural halogen-climate feedbacks and the impact of these feedbacks on global atmospheric oxidizing capacity (AOC) and radiative forcing (RF) across pre-industrial, present and future climates. Answering these questions is essential to predict if these climate-mediated feedbacks can reduce or amplify future climate change. To this end we are developing a multidisciplinary research approach using laboratory and field observations and models interactively that allows us to peel apart the detailed physical processes behind the contribution of natural halogens to global climate change. Furthermore, the work also involves examining past-future climate impacts of natural halogens within a holistic Earth System model, where we develop the multidirectional halogen interactions in the land-ocean-ice-biosphere-atmosphere coupled system. This provides a breakthrough in our understanding of the importance of these natural processes for the composition and oxidation capacity of the Earth´s atmosphere and climate, both in the presence and absence of human influence.
Overall objectives of the project:
As the main objective, CLIMAHAL aims to study the so far unrecognized interplay between natural emissions of climate active halogens and their impact on AOC and RF in pre-industrial, present and future climates, using an interdisciplinary approach. To fulfill this overall objective the project consists of three main subprojects. The specific objectives are as follows:
A) Laboratory work and data collection.
A1) Specific objectives
a) Generation of experimental data on photochemistry and polymerization rate parameters of the higher iodine oxides (I2O2, I2O3, I2O4) monomers.
b) Compilation of available data on atmospheric gases, ocean and ice biological and abiotic parameters from previous and on-going studies and monitoring networks.
B) Development of an integrated model of the multidirectional halogen interactions within the land-ocean-ice-biosphere-atmosphere coupled system
B1) Specific objectives
a) Generation of a novel holistic Earth-system approach, by developing a state-of-the-art integrated model to connect, in a single model, the multidirectional fluxes of VSLH within different Earth system components to climate processes.
b) Validation of the new model performance in the present-day atmosphere against all available data (from subproject A) including emissions of natural halogens from the different components of the Earth system (e.g. ocean, ice, biosphere) along with their concentrations and variability throughout the global atmosphere. To this end, a number of model-measurement comparisons will be performed to evaluate the new developed model.
C) Earth System modelling of natural halogen-climate interactions across past/present/future climates.
C1) Specific objectives
Use the implemented model developed in subproject B to:
a) determine the contribution of each individual Earth system component to the natural halogen loading of the atmosphere.
b) analyze the impact of predicted sea-ice retreat on the emissions and budget of halogens in the polar regions.
c) determine the key mechanisms by which natural halogens influence climate.
d) discover the contribution, if any, of natural halogen processes to the climate of the Anthropocene.
f) find the possible biogeochemistry-halogen-climate feedbacks that modulate (diminish or amplify) climate change.
During this mid-term reporting period, most of the project´s objectives have been initiated and some of them have already been achieved. These are described in sequence:
- We have completed the compilation of available data on atmospheric gases, ocean and ice biological and abiotic parameters from previous and on-going studies and monitoring networks. The implementation of these databases in the CLIMAHAL’s Earth System Model allows the comparison and testing of the ability of the model to reproduce present day levels of emissions and atmospheric concentrations of VSLH.
- The laboratory determination of the photochemistry and polymerization rate parameters of the higher order iodine oxides (I2O2, I2O3, I2O4) monomers is complete. The determination of the photolysis cross section of these iodine oxides monomers, at atmospheric relevant wavelengths, has already been finished.
- We have started the development of a state-of-the-art integrated model to connect, in a single model, land-ocean-ice-biosphere-atmosphere chemistry and multidirectional fluxes of VSLH to climate processes. Initial results have been obtained on the impact of VSLH on atmospheric ozone during the
21st century (Iglesias-Suarez et al., 2020).
CLIMAHAL is progressing as anticipated in the project planning and so far has already led to some major achievements, which are highlighted here:
- For the first time in a global chemistry-climate model, we have implemented an interactive polar module into the very short-lived (VSL) halogen version of the CAM-Chem model (Fernandez et al., Journal of Advances in Modeling Earth Systems, 2019). Therefore, within CLIMAHAL, we have now a chemistry-climate model with a complete representation of halogen sources and chemistry from pole to pole and from the Earth's surface up to the stratopause.
- We have recently led an ice core study in Greenland discovering that atmospheric iodine levels have tripled since 1950 due to anthropogenic ozone and the retreat of Arctic sea ice (Cuevas et al., Nature Communications, 2018). From the iodine concentrations in the RECAP ice-core (coastal East Greenland), we have investigated the evolution of atmospheric iodine levels in the North Atlantic Ocean from 1750 to 2011. This is the longest record of atmospheric iodine in the Northern Hemisphere, which confirms an anthropogenically-driven enhancement in iodine emissions that was already anticipated by our global 3D chemistry-climate model (CAM-Chem).
- We have led a study on computational chemistry that demonstrated a new photochemical mechanism (gas phase photoreduction of halogenated gaseous oxidized mercury) of atmospheric mercury that changes the global chemistry of mercury and its deposition to the surface environment (Saiz-Lopez et al., Nature Communications, 2018). We have found that experimental rainfall Hg(II) photoreduction rates are much slower that modelled rates in global Hg models. Computing absorption cross section of Hg(II) species (HgCl2, HgBr2, HgBrOCl, HgBrI, HgBrOBr, HgBrOI, HgBrNO2, HgBrONO, HgBrOH, HgBrOOH, and HgO) we show that fast gas-phase Hg(II) photolysis can dominate atmospheric mercury reduction and lead to a substantial increase in the modelled, global atmospheric Hg lifetime by a factor two.
- We have then studied the photolysis of key Hg(I) species with the mercury cycling in the atmosphere and its deposition to the Earth’s surface (Saiz-Lopez et al., JACS, 2019). High-level quantum-chemical methods are employed to calculate UV-Vis absorption spectra and cross-section of HgCl, HgBr, HgI and HgO influence H. The results show that Hg(I) photo-reduction can occur at time scales that eventually could have a considerable impact on atmospheric mercury chemistry.
- After reporting the efficient gas-phase photolysis of Hg(II) and Hg(I) species, we continue this work studying its competition with thermal reactivity and whether the photolysis of Hg(II) leads to other stable Hg(II) species, to Hg(I), or to Hg0 and its competition with thermal reactivity (Francés-Monerris et al., Angewandte Chemie, 2020). Our results. Based on non-adiabatic dynamics simulations, show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg(0) by photolysis. The last three works mentioned above have developed new chemical schemes that are key in the determining role of halogen-mercury interactions in the global atmospheric cycle of Hg, its chemistry and surface deposition.
- In collaboration with the Department of Chemistry of the University of Colorado, we have discovered that significant levels of reactive iodine can reach the stratosphere (0.77 ± 0.10 pptv) (Koening et al., PNAS, 2020). These iodine levels, measured in the lower stratosphere and upper troposphere, would be responsible for up to 32% of the halogen-induced ozone loss, exceeding the relative contribution of chlorine (28%) and almost reaching the contribution of bromine (40%).
- Several long-term simulations using our state of the art Earth System Model have allowed us to project varying natural halogen emissions and investigate their impact on tropospheric ozone levels over the 21st century (Iglesias-Suarez et al., Nature Climate Change, 2020). Our results show that emissions of natural halogens (chlorine, bromine and iodine) buffer the increase in global tropospheric ozone, as climate changes during the 21st century. At present, natural halogens (dominated by ocean emissions) are estimated to destroy ~13 % of global tropospheric ozone. However, this fraction remains stable through the twenty-first century, due to compensation from hemispheric, regional and vertical heterogeneity in tropospheric ozone.
Bibliography
- Photoreduction of gaseous oxidized mercury changes global atmospheric mercury speciation, transport and deposition. A. Saiz-Lopez et al., Nature Communications 9:4796, doi: 10.1038/s41467-018-07075-3 2018.
- Rapid increase in atmospheric iodine levels in the North Atlantic since the mid-20th century. C. A. Cuevas et alNature Communications 9, 1452, doi:10.1038/s 41467-018-03756-1, 2018
- Modeling the Sources and Chemistry of Polar Tropospheric Halogens (Cl, Br, and I) Using the CAM-Chem Global Chemistry-Climate Model. R. P. Fernandez et al., Journal of Advances in Modeling Earth Systems, Volume 11, Issue 7, August 2019, Pages 2259-2289, https://doi.org/10.1029/2019MS001655.
- Gas-Phase Photolysis of Hg (I) Radical Species: A New Atmospheric Mercury Reduction Process. A. Saiz-Lopez et al., J. Am. Chem. Soc., https://doi.org/10.1021/jacs.9b02890 2019.
- Quantitative detection of iodine in the stratosphere. T. K. Koenig et al., PNAS, 117 (4) 1860-1866, https://doi.org/10.1073/pnas.1916828117
- Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury. A. Francés-Monerris et al., Angew. Chem. Int. Ed. 2020, 59, 2–8. DOI: 10.1002/anie.201915656
- Natural halogens buffer tropospheric ozone in a changing climate. F. Iglesias-Suarez et al., Nat. Clim. Chang., https:// doi.org/10.1038/s41558-019-0675-6 2020.
- We have completed the compilation of available data on atmospheric gases, ocean and ice biological and abiotic parameters from previous and on-going studies and monitoring networks. The implementation of these databases in the CLIMAHAL’s Earth System Model allows the comparison and testing of the ability of the model to reproduce present day levels of emissions and atmospheric concentrations of VSLH.
- The laboratory determination of the photochemistry and polymerization rate parameters of the higher order iodine oxides (I2O2, I2O3, I2O4) monomers is complete. The determination of the photolysis cross section of these iodine oxides monomers, at atmospheric relevant wavelengths, has already been finished.
- We have started the development of a state-of-the-art integrated model to connect, in a single model, land-ocean-ice-biosphere-atmosphere chemistry and multidirectional fluxes of VSLH to climate processes. Initial results have been obtained on the impact of VSLH on atmospheric ozone during the
21st century (Iglesias-Suarez et al., 2020).
CLIMAHAL is progressing as anticipated in the project planning and so far has already led to some major achievements, which are highlighted here:
- For the first time in a global chemistry-climate model, we have implemented an interactive polar module into the very short-lived (VSL) halogen version of the CAM-Chem model (Fernandez et al., Journal of Advances in Modeling Earth Systems, 2019). Therefore, within CLIMAHAL, we have now a chemistry-climate model with a complete representation of halogen sources and chemistry from pole to pole and from the Earth's surface up to the stratopause.
- We have recently led an ice core study in Greenland discovering that atmospheric iodine levels have tripled since 1950 due to anthropogenic ozone and the retreat of Arctic sea ice (Cuevas et al., Nature Communications, 2018). From the iodine concentrations in the RECAP ice-core (coastal East Greenland), we have investigated the evolution of atmospheric iodine levels in the North Atlantic Ocean from 1750 to 2011. This is the longest record of atmospheric iodine in the Northern Hemisphere, which confirms an anthropogenically-driven enhancement in iodine emissions that was already anticipated by our global 3D chemistry-climate model (CAM-Chem).
- We have led a study on computational chemistry that demonstrated a new photochemical mechanism (gas phase photoreduction of halogenated gaseous oxidized mercury) of atmospheric mercury that changes the global chemistry of mercury and its deposition to the surface environment (Saiz-Lopez et al., Nature Communications, 2018). We have found that experimental rainfall Hg(II) photoreduction rates are much slower that modelled rates in global Hg models. Computing absorption cross section of Hg(II) species (HgCl2, HgBr2, HgBrOCl, HgBrI, HgBrOBr, HgBrOI, HgBrNO2, HgBrONO, HgBrOH, HgBrOOH, and HgO) we show that fast gas-phase Hg(II) photolysis can dominate atmospheric mercury reduction and lead to a substantial increase in the modelled, global atmospheric Hg lifetime by a factor two.
- We have then studied the photolysis of key Hg(I) species with the mercury cycling in the atmosphere and its deposition to the Earth’s surface (Saiz-Lopez et al., JACS, 2019). High-level quantum-chemical methods are employed to calculate UV-Vis absorption spectra and cross-section of HgCl, HgBr, HgI and HgO influence H. The results show that Hg(I) photo-reduction can occur at time scales that eventually could have a considerable impact on atmospheric mercury chemistry.
- After reporting the efficient gas-phase photolysis of Hg(II) and Hg(I) species, we continue this work studying its competition with thermal reactivity and whether the photolysis of Hg(II) leads to other stable Hg(II) species, to Hg(I), or to Hg0 and its competition with thermal reactivity (Francés-Monerris et al., Angewandte Chemie, 2020). Our results. Based on non-adiabatic dynamics simulations, show that all oxidized forms of mercury rapidly revert directly and indirectly to Hg(0) by photolysis. The last three works mentioned above have developed new chemical schemes that are key in the determining role of halogen-mercury interactions in the global atmospheric cycle of Hg, its chemistry and surface deposition.
- In collaboration with the Department of Chemistry of the University of Colorado, we have discovered that significant levels of reactive iodine can reach the stratosphere (0.77 ± 0.10 pptv) (Koening et al., PNAS, 2020). These iodine levels, measured in the lower stratosphere and upper troposphere, would be responsible for up to 32% of the halogen-induced ozone loss, exceeding the relative contribution of chlorine (28%) and almost reaching the contribution of bromine (40%).
- Several long-term simulations using our state of the art Earth System Model have allowed us to project varying natural halogen emissions and investigate their impact on tropospheric ozone levels over the 21st century (Iglesias-Suarez et al., Nature Climate Change, 2020). Our results show that emissions of natural halogens (chlorine, bromine and iodine) buffer the increase in global tropospheric ozone, as climate changes during the 21st century. At present, natural halogens (dominated by ocean emissions) are estimated to destroy ~13 % of global tropospheric ozone. However, this fraction remains stable through the twenty-first century, due to compensation from hemispheric, regional and vertical heterogeneity in tropospheric ozone.
Bibliography
- Photoreduction of gaseous oxidized mercury changes global atmospheric mercury speciation, transport and deposition. A. Saiz-Lopez et al., Nature Communications 9:4796, doi: 10.1038/s41467-018-07075-3 2018.
- Rapid increase in atmospheric iodine levels in the North Atlantic since the mid-20th century. C. A. Cuevas et alNature Communications 9, 1452, doi:10.1038/s 41467-018-03756-1, 2018
- Modeling the Sources and Chemistry of Polar Tropospheric Halogens (Cl, Br, and I) Using the CAM-Chem Global Chemistry-Climate Model. R. P. Fernandez et al., Journal of Advances in Modeling Earth Systems, Volume 11, Issue 7, August 2019, Pages 2259-2289, https://doi.org/10.1029/2019MS001655.
- Gas-Phase Photolysis of Hg (I) Radical Species: A New Atmospheric Mercury Reduction Process. A. Saiz-Lopez et al., J. Am. Chem. Soc., https://doi.org/10.1021/jacs.9b02890 2019.
- Quantitative detection of iodine in the stratosphere. T. K. Koenig et al., PNAS, 117 (4) 1860-1866, https://doi.org/10.1073/pnas.1916828117
- Photodissociation Mechanisms of Major Mercury(II) Species in the Atmospheric Chemical Cycle of Mercury. A. Francés-Monerris et al., Angew. Chem. Int. Ed. 2020, 59, 2–8. DOI: 10.1002/anie.201915656
- Natural halogens buffer tropospheric ozone in a changing climate. F. Iglesias-Suarez et al., Nat. Clim. Chang., https:// doi.org/10.1038/s41558-019-0675-6 2020.
The Progress beyond the state-of-the-art is shown below according to the state-of-the-art of the main scientific goals addressed in the CLIMAHAL original proposal:
1.3.1 The marine iodine paradox
In the laboratory, we have made the first determination of the absorption cross sections of IxOy, x =
2, 3, 5, y = 1-12 at λ = 355 nm by combining pulsed laser photolysis of I2/O3 gas mixtures in air with time-resolved photo-ionization time-of-flight mass spectrometry, using NO2 actinometry for signal calibration (Lewis et al., 2020). The oxides selected for absorption cross section determinations are those presenting the strongest signals in the mass spectra, where signals containing 4 iodine atoms are absent. The method is validated by measuring the absorption cross section of IO at 355 nm, which is found to be in good agreement with the most recent literature
1.3.2 Biogeochemical emissions of VSLH in the Earth System
The new source of atmospheric bromine we have reported from the winter sea ice in Antarctica (Abrahamsson et al, 2018), represents a so far unaccounted for flux of bromocarbons (CHBr3, CH2Br2, CHBr2Cl, and CHBrCl2) to the atmosphere ranging 0.6–1.8 Gmol Br per month of polar night, which should be compared with the estimated global contribution of 2.4–3.5 Gmol Br y−1 based on CHBr3 and CH2Br2. This new source of polar bromine implies a re-assessment of the bromine-mediated impacts on the global tropospheric ozone budget and mercury deposition.
The iodine record from the RECAP ice core has allowed us to estimate, for the first time, the evolution of iodine emissions from the North Atlantic Ocean during the last 11700 years. The high levels of iodine recorded during the Holocene Thermal Maximum (~11500-5500 years before present) can only be explained by natural drivers influencing iodine emissions to the atmosphere. These results show that at the onset of the Holocene, enhanced ocean primary production coupled with maxima in solar irradiance and open-water conditions in the Arctic Ocean and in the Nordic Seas controlled iodine emissions to the atmosphere (Fig. 3, Corella et al., 2019), resulting in a 4 millennia-long period of high atmospheric iodine concentrations.
Based on the state-of-the-art laboratory and theoretical knowledge of halogen (Cl, Br and I) emissions from sea-ice/snowpack in the polar regions, we have implemented an interactive polar module into the very short-lived (VSL) halogen version of the CAM-Chem model (Fernandez et al., 2019). Therefore we have now in CAM-Chem a complete representation of halogen sources and chemistry from pole to pole and from the Earth's surface up to the stratopause.
1.3.3 Stratospheric injection of natural VSLH and impact on ozone photochemistry in the present and future atmosphere
The natural emissions of VSLH also have the potential to influence stratospheric chemistry through the emissions of very short-lived ozone-depleting substances, affecting the stratospheric ozone layer and therefore the Earth’s energy budget. Although stratospheric ozone depletion was caused by anthropogenic emissions of long-lived halocarbons, which have been addressed under the Montreal Protocol, the future recovery of stratospheric ozone may be influenced by interactions between ocean biogeochemistry production and transport of natural VSLH to the stratosphere.
The iodine injection to the stratosphere and its role in stratospheric ozone is not as well known as the injection of bromine. Previous studies have only reported lower limits of < 0.1pptv in the lower stratosphere (WMO, 2014). We have recently proposed (Saiz- Lopez et al., 2015, based on aircraft observations, that 0.25- 0.7 pptv of reactive iodine (implying 0.15-0.45 pptv IO) could be injected in the lower stratosphere of the Eastern Pacific Ocean via convective outflow. These iodine levels account for up to 30% of the contemporary ozone loss in the tropical lower stratosphere, and can exert a stratospheric ozone depletion potential equivalent to, or even larger than, that of very short-lived bromocarbons. This hypothesis has been very recently confirmed by quantitative detection of iodine monoxide radicals and particulate iodine from aircraft in the stratosphere (Koening et al., 2020). These measurements support our earlier prediction (Saiz- Lopez et al., 2015) that 0.77 ± 0.10 parts per trillion by volume (pptv) total inorganic iodine (Iy) is injected to the stratosphere.
1.3.4 Recent development in natural tropospheric halogens and climate connections
Natural halogens (Br, Cl, or I) play important roles in the chemistry and composition of the troposphere (Simpson et al., 2015). Several new recent developments point to the potential of these halogens to exert an impact upon the lifetime of climate forcing agents. Halogen-driven ozone loss in the marine troposphere contributes approximately -0.10 Wm-2 to the radiative flux at the tropical tropopause (Saiz-Lopez et al., 2012). This negative flux is of similar magnitude to the ~0.33 Wm-2 contribution of tropospheric ozone to present-day radiative balance as recently estimated from satellite observations. This work originally outlined the potential key implications of natural halogen-driven ozone loss on the radiation balance of the Earth´s atmosphere, leading to a negative radiative effect (or cooling effect) on climate (Saiz-Lopez et al., 2012).
In a recent work (Iglesias-Suarez et al., 2020) we have shown that emissions of natural halogens (chlorine, bromine and iodine) buffer the increase in global tropospheric ozone (O3), as climate changes during the 21st century. At present, natural halogens (mainly emitted by the oceans) are estimated to destroy ~13% of global tropospheric O3 burden. This fraction remains stable through the 21st century, despite increased levels of natural halogen emissions. These findings reveal a strong buffering capacity of natural halogens controlling tropospheric O3, as the climate warms.
Bibliography
- Abrahamsson, K. et al.: Nat. Commun., 9(1), doi:10.1038/ s41467-018-07062-8, 2018.
- Cuevas, C. A. et al.: Nat. Commun., 9(1), doi:10.1038/s41467-018-03756-1 2018.
- Fernandez, R. P. et al.: J. Adv. Model. Earth Syst., 11(7), 2259–2289, doi:10.1029/2019MS001655 2019.
- Iglesias-Suarez, F. et al.: Nat. Clim. Chang., doi:10.1038/s41558-019-0675-6 2020.
- Koenig, T. K. et al.: Proc. Natl. Acad. Sci. U. S. A., 117(4), doi:10.1073/pnas.1916828117 2020.
-Lewis et al., 2020: Atmos. Chem. Phys., 20, 10865–10887, doi.org/10.5194/acp-20-10865-2020 2020.
- Saiz-Lopez, A. et al.: Atmos. Chem. Phys., 12(9), 3939–3949, doi:10.5194/acp-12-3939-2012 2012.
- Saiz-Lopez, A. et al.: Geophys. Res. Lett., 42(16), doi:10.1002/2015GL064796 2015.
- Simpson, W. R. et al.: Chem. Rev., 115(10), 4035–4062, doi:10.1021/ cr5006638, 2015.
- Unep: Global Mercury Assessment 2013: Sources, Emissions, Releases, and Environmental Transport, UNEP Chem. Branch, Geneva, Switz., 2013.
- World Meteorological Organization (WMO): Scientific Assessment of Ozone Depletion: 2014., 2014.
1.3.1 The marine iodine paradox
In the laboratory, we have made the first determination of the absorption cross sections of IxOy, x =
2, 3, 5, y = 1-12 at λ = 355 nm by combining pulsed laser photolysis of I2/O3 gas mixtures in air with time-resolved photo-ionization time-of-flight mass spectrometry, using NO2 actinometry for signal calibration (Lewis et al., 2020). The oxides selected for absorption cross section determinations are those presenting the strongest signals in the mass spectra, where signals containing 4 iodine atoms are absent. The method is validated by measuring the absorption cross section of IO at 355 nm, which is found to be in good agreement with the most recent literature
1.3.2 Biogeochemical emissions of VSLH in the Earth System
The new source of atmospheric bromine we have reported from the winter sea ice in Antarctica (Abrahamsson et al, 2018), represents a so far unaccounted for flux of bromocarbons (CHBr3, CH2Br2, CHBr2Cl, and CHBrCl2) to the atmosphere ranging 0.6–1.8 Gmol Br per month of polar night, which should be compared with the estimated global contribution of 2.4–3.5 Gmol Br y−1 based on CHBr3 and CH2Br2. This new source of polar bromine implies a re-assessment of the bromine-mediated impacts on the global tropospheric ozone budget and mercury deposition.
The iodine record from the RECAP ice core has allowed us to estimate, for the first time, the evolution of iodine emissions from the North Atlantic Ocean during the last 11700 years. The high levels of iodine recorded during the Holocene Thermal Maximum (~11500-5500 years before present) can only be explained by natural drivers influencing iodine emissions to the atmosphere. These results show that at the onset of the Holocene, enhanced ocean primary production coupled with maxima in solar irradiance and open-water conditions in the Arctic Ocean and in the Nordic Seas controlled iodine emissions to the atmosphere (Fig. 3, Corella et al., 2019), resulting in a 4 millennia-long period of high atmospheric iodine concentrations.
Based on the state-of-the-art laboratory and theoretical knowledge of halogen (Cl, Br and I) emissions from sea-ice/snowpack in the polar regions, we have implemented an interactive polar module into the very short-lived (VSL) halogen version of the CAM-Chem model (Fernandez et al., 2019). Therefore we have now in CAM-Chem a complete representation of halogen sources and chemistry from pole to pole and from the Earth's surface up to the stratopause.
1.3.3 Stratospheric injection of natural VSLH and impact on ozone photochemistry in the present and future atmosphere
The natural emissions of VSLH also have the potential to influence stratospheric chemistry through the emissions of very short-lived ozone-depleting substances, affecting the stratospheric ozone layer and therefore the Earth’s energy budget. Although stratospheric ozone depletion was caused by anthropogenic emissions of long-lived halocarbons, which have been addressed under the Montreal Protocol, the future recovery of stratospheric ozone may be influenced by interactions between ocean biogeochemistry production and transport of natural VSLH to the stratosphere.
The iodine injection to the stratosphere and its role in stratospheric ozone is not as well known as the injection of bromine. Previous studies have only reported lower limits of < 0.1pptv in the lower stratosphere (WMO, 2014). We have recently proposed (Saiz- Lopez et al., 2015, based on aircraft observations, that 0.25- 0.7 pptv of reactive iodine (implying 0.15-0.45 pptv IO) could be injected in the lower stratosphere of the Eastern Pacific Ocean via convective outflow. These iodine levels account for up to 30% of the contemporary ozone loss in the tropical lower stratosphere, and can exert a stratospheric ozone depletion potential equivalent to, or even larger than, that of very short-lived bromocarbons. This hypothesis has been very recently confirmed by quantitative detection of iodine monoxide radicals and particulate iodine from aircraft in the stratosphere (Koening et al., 2020). These measurements support our earlier prediction (Saiz- Lopez et al., 2015) that 0.77 ± 0.10 parts per trillion by volume (pptv) total inorganic iodine (Iy) is injected to the stratosphere.
1.3.4 Recent development in natural tropospheric halogens and climate connections
Natural halogens (Br, Cl, or I) play important roles in the chemistry and composition of the troposphere (Simpson et al., 2015). Several new recent developments point to the potential of these halogens to exert an impact upon the lifetime of climate forcing agents. Halogen-driven ozone loss in the marine troposphere contributes approximately -0.10 Wm-2 to the radiative flux at the tropical tropopause (Saiz-Lopez et al., 2012). This negative flux is of similar magnitude to the ~0.33 Wm-2 contribution of tropospheric ozone to present-day radiative balance as recently estimated from satellite observations. This work originally outlined the potential key implications of natural halogen-driven ozone loss on the radiation balance of the Earth´s atmosphere, leading to a negative radiative effect (or cooling effect) on climate (Saiz-Lopez et al., 2012).
In a recent work (Iglesias-Suarez et al., 2020) we have shown that emissions of natural halogens (chlorine, bromine and iodine) buffer the increase in global tropospheric ozone (O3), as climate changes during the 21st century. At present, natural halogens (mainly emitted by the oceans) are estimated to destroy ~13% of global tropospheric O3 burden. This fraction remains stable through the 21st century, despite increased levels of natural halogen emissions. These findings reveal a strong buffering capacity of natural halogens controlling tropospheric O3, as the climate warms.
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