scholarly journals Estimation of Volcanic Ash Emissions Using Trajectory-Based 4D-Var Data Assimilation

2016 ◽  
Vol 144 (2) ◽  
pp. 575-589 ◽  
Author(s):  
S. Lu ◽  
H. X. Lin ◽  
A. W. Heemink ◽  
G. Fu ◽  
A. J. Segers
Keyword(s):  
Data Assimilation ◽  
Vertical Distribution ◽  
Volcanic Ash ◽  
Transport Model ◽  
Volcanic Eruptions ◽  
Emission Rates ◽  
Plume Height ◽  
Ill Posed ◽  
Total Difference ◽  
The Vertical Distribution

Abstract Volcanic ash forecasting is a crucial tool in hazard assessment and operational volcano monitoring. Emission parameters such as plume height, total emission mass, and vertical distribution of the emission plume rate are essential and important in the implementation of volcanic ash models. Therefore, estimation of emission parameters using available observations through data assimilation could help to increase the accuracy of forecasts and provide reliable advisory information. This paper focuses on the use of satellite total-ash-column data in 4D-Var based assimilations. Experiments show that it is very difficult to estimate the vertical distribution of effective volcanic ash injection rates from satellite-observed ash columns using a standard 4D-Var assimilation approach. This paper addresses the ill-posed nature of the assimilation problem from the perspective of a spurious relationship. To reduce the influence of a spurious relationship created by a radiate observation operator, an adjoint-free trajectory-based 4D-Var assimilation method is proposed, which is more accurate to estimate the vertical profile of volcanic ash from volcanic eruptions. The method seeks the optimal vertical distribution of emission rates of a reformulated cost function that computes the total difference between simulated and observed ash columns. A 3D simplified aerosol transport model and synthetic satellite observations are used to compare the results of both the standard method and the new method.

Ensemble-based data assimilation of volcanic aerosols using FALL3D+PDAF

2021 ◽  
Author(s):  
Leonardo Mingari ◽  
Andrew Prata ◽  
Federica Pardini
Keyword(s):  
Data Assimilation ◽  
Information Transfer ◽  
High Performance ◽  
Volcanic Ash ◽  
Numerical Models ◽  
Volcanic Eruptions ◽  
Column Height ◽  
Eruption Column ◽  
Operational Environment ◽  
Assimilation System

<p>Modelling atmospheric dispersion and deposition of volcanic ash is becoming increasingly valuable for understanding the potential impacts of explosive volcanic eruptions on infrastructures, air quality and aviation. The generation of high-resolution forecasts depends on the accuracy and reliability of the input data for models. Uncertainties in key parameters such as eruption column height injection, physical properties of particles or meteorological fields, represent a major source of error in forecasting airborne volcanic ash. The availability of nearly real time geostationary satellite observations with high spatial and temporal resolutions provides the opportunity to improve forecasts in an operational context. Data assimilation (DA) is one of the most effective ways to reduce the error associated with the forecasts through the incorporation of available observations into numerical models. Here we present a new implementation of an ensemble-based data assimilation system based on the coupling between the FALL3D dispersal model and the Parallel Data Assimilation Framework (PDAF). The implementation is based on the last version release of FALL3D (versions 8.x) tailored to the extreme-scale computing requirements, which has been redesigned and rewritten from scratch in the framework of the EU Center of Excellence for Exascale in Solid Earth (ChEESE). The proposed methodology can be efficiently implemented in an operational environment by exploiting high-performance computing (HPC) resources. The FALL3D+PDAF system can be run in parallel and supports online-coupled DA, which allows an efficient information transfer through parallel communication. Satellite-retrieved data from recent volcanic eruptions were considered as input observations for the assimilation system.</p>

scholarly journals Data Assimilation of Volcanic Aerosols using FALL3D+PDAF

2021 ◽  
Author(s):  
Leonardo Mingari ◽  
Arnau Folch ◽  
Andrew T. Prata ◽  
Federica Pardini ◽  
Giovanni Macedonio ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Volcanic Ash ◽  
Volcanic Eruptions ◽  
Column Height ◽  
Model Parameters ◽  
Eruption Column ◽  
Mass Loading ◽  
Multiple Model ◽  
Volcanic Aerosols ◽  
Forecast Quality

Abstract. Modelling atmospheric dispersal of volcanic ash and aerosols is becoming increasingly valuable for assessing the potential impacts of explosive volcanic eruptions on infrastructures, air quality, and aviation. Management of volcanic risk and reduction of aviation impacts can strongly benefit from quantitative forecasting of volcanic ash. However, an accurate prediction of volcanic aerosol concentrations using numerical modelling relies on proper estimations of multiple model parameters which are prone to errors. Uncertainties in key parameters such as eruption column height, physical properties of particles or meteorological fields, represent a major source of error affecting the forecast quality. The availability of near-real-time geostationary satellite observations with high spatial and temporal resolutions provides the opportunity to improve forecasts in an operational context by incorporating observations into numerical models. Specifically, ensemble-based filters aim at converting a prior ensemble of system states into an analysis ensemble by assimilating a set of noisy observations. Previous studies dealing with volcanic ash transport have demonstrated that a significant improvement of forecast skill can be achieved by this approach. In this work, we present a new implementation of an ensemble-based Data Assimilation (DA) method coupling the FALL3D dispersal model and the Parallel Data Assimilation Framework (PDAF). The FALL3D+PDAF system runs in parallel, supports online-coupled DA and can be efficiently integrated into operational workflows by exploiting high-performance computing (HPC) resources. Two numerical experiments are considered: (i) a twin experiment using an incomplete dataset of synthetic observations of volcanic ash and, (ii) an experiment based on the 2019 Raikoke eruption using real observations of SO2 mass loading. An ensemble-based Kalman filtering technique based on the Local Ensemble Transform Kalman Filter (LETKF) is used to assimilate satellite-retrieved data of column mass loading. We show that this procedure may lead to nonphysical solutions and, consequently, conclude that LETKF is not the best approach for the assimilation of volcanic aerosols. However, we find that a truncated state constructed from the LETKF solution approaches the real solution after a few assimilation cycles, yielding a dramatic improvement of forecast quality when compared to simulations without assimilation.

Ensemble-Based Data Assimilation of volcanic ash clouds from satellite observations: application to the 24 December 2018 Mt.Etna explosive eruption.

2021 ◽  
Author(s):  
Federica Pardini ◽  
Stefano Corradini ◽  
Antonio Costa ◽  
Lorenzo Guerrieri ◽  
Tomaso Esposti Ongaro ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Kalman Filters ◽  
Volcanic Ash ◽  
Explosive Eruption ◽  
Numerical Models ◽  
Numerical Procedure ◽  
Optimal Solution ◽  
Volcanic Eruptions ◽  
Satellite Observations ◽  
Mt Etna

<p>Explosive volcanic eruptions release high amounts of ash into the atmosphere. Accurate tracking and forecasting of ash dispersal into the atmosphere and quantification of its uncertainty is of fundamental importance for volcanic hazard mitigation. Numerical models represent a powerful tool to monitor ash clouds in real-time, but limits and uncertainties affect numerical results. A way to improve numerical forecasts is by assimilating satellite observations of ash clouds through Data Assimilation algorithms, such as Ensemble-based Kalman Filters. In this study, we present the implementation of the so-called Local Ensemble Transform Kalman Filters inside a numerical procedure which simulates the release and transport of volcanic ash during explosive eruptions. The numerical procedure consists of the eruptive column model PLUME-MoM coupled with the tephra transport and dispersal model HYSPLIT. When satellite observations are available, ash maps supplied by PLUME-MoM/HYSPLIT are sequentially corrected/modified using ash column loading as retrieved from space. The new volcanic ash state represents the optimal solution with minimized uncertainties with respect to numerical estimates and observations. To test the Data Assimilation procedure, we used satellite observations of the volcanic cloud released during the explosive eruption that occurred at Mt. Etna (Italy) on 24 December 2018. Satellite observations have been carried out by the Spinning Enhanced Visible and InfraRed Imager (SEVIRI) instrument, on board the Meteosat Second Generation (MSG) geostationary satellite. Results show that the assimilation procedure significantly improves the current ash state and the forecast. In addition, numerical tests show that the use of sequential Kalman Filters does not require a precise initialization of the numerical model, being able to improve the forecasts as the assimilation cycles are performed.</p>

scholarly journals Estimation of the vertical profile of sulfur dioxide injection into the atmosphere by a volcanic eruption using satellite column measurements and inverse transport modeling

2008 ◽  
Vol 8 (14) ◽  
pp. 3881-3897 ◽  
Author(s):  
S. Eckhardt ◽  
A. J. Prata ◽  
P. Seibert ◽  
K. Stebel ◽  
A. Stohl
Keyword(s):  
Vertical Profile ◽  
Volcanic Ash ◽  
Transport Model ◽  
A Priori ◽  
Volcanic Eruptions ◽  
Inversion Method ◽  
Volcanic Plume ◽  
Operational Environment ◽  
Data Set ◽  
Emission Profile

Abstract. An analytical inversion method has been developed to estimate the vertical profile of SO2 emissions from volcanic eruptions. The method uses satellite-observed total SO2 columns and an atmospheric transport model (FLEXPART) to exploit the fact that winds change with altitude – thus, the position and shape of the volcanic plume bear information on its emission altitude. The method finds the vertical emission distribution which minimizes the total difference between simulated and observed SO2 columns while also considering a priori information. We have tested the method with the eruption of Jebel at Tair, Yemen, on 30 September 2007 for which a comprehensive observational data set from various satellite instruments (AIRS, OMI, SEVIRI, CALIPSO) is available. Using satellite data from the first 24 h after the eruption for the inversion, we found an emission maximum near 16 km above sea level (a.s.l.), and secondary maxima near 5, 9, 12 and 14 km a.s.l. 60% of the emission occurred above the tropopause. The emission profile obtained in the inversion was then used to simulate the transport of the plume over the following week. The modeled plume agrees very well with SO2 total columns observed by OMI, and its altitude agrees with CALIPSO aerosol observations to within 1–2 km. The inversion result is robust against various changes in both the a priori and the observations. Even when using only SEVIRI data from the first 15 h after the eruption, the emission profile was reasonably well estimated. The method is computationally very fast. It is therefore suitable for implementation within an operational environment, such as the Volcanic Ash Advisory Centers, to predict the threat posed by volcanic ash for air traffic. It could also be helpful for assessing the sulfur input into the stratosphere, be it in the context of volcanic processes or also for proposed geo-engineering techniques to counteract global warming.

scholarly journals Estimation of the vertical profile of sulfur dioxide injection into the atmosphere by a volcanic eruption using satellite column measurements and inverse transport modeling

2008 ◽  
Vol 8 (1) ◽  
pp. 3761-3805 ◽  
Author(s):  
S. Eckhardt ◽  
A. J. Prata ◽  
P. Seibert ◽  
K. Stebel ◽  
A. Stohl
Keyword(s):  
Vertical Profile ◽  
Volcanic Ash ◽  
Transport Model ◽  
A Priori ◽  
Volcanic Eruptions ◽  
Inversion Method ◽  
Volcanic Plume ◽  
Operational Environment ◽  
Data Set ◽  
Emission Profile

Abstract. An analytical inversion method has been developed to estimate the vertical profile of SO2 emissions from volcanic eruptions. The method uses satellite-observed total SO2 columns and an atmospheric transport model (FLEXPART) to exploit the fact that winds change with altitude – thus, the position and shape of the volcanic plume bear information on its emission altitude. The method finds the vertical emission distribution which minimizes the total difference between simulated and observed SO2 columns while also considering a priori information. We have tested the method with the eruption of Jebel at Tair on 30 September 2007 for which a comprehensive observational data set from various satellite instruments (AIRS, OMI, SEVIRI, CALIPSO) is available. Using satellite data from the first 24 h after the eruption for the inversion, we found an emission maximum near 16 km above sea level (asl), and secondary maxima near 5, 9, 12 and 14 km a.s.l. 60% of the emission occurred above the tropopause. The emission profile obtained in the inversion was then used to simulate the transport of the plume over the following week. The modeled plume agrees very well with SO2 total columns observed by OMI, and its altitude and width agree mostly within 1–2 km with CALIPSO observations of stratospheric aerosol produced from the SO2. The inversion result is robust against various changes in both the a priori and the observations. Even when using only SEVIRI data from the first 15 h after the eruption, the emission profile was reasonably well estimated. The method is computationally very fast. It is therefore suitable for implementation within an operational environment, such as the Volcanic Ash Advisory Centers, to predict the threat posed by volcanic ash for air traffic. It could also be helpful for assessing the sulfur input into the stratosphere, be it in the context of volcanic processes or also for proposed geo-engineering techniques to counteract global warming.

scholarly journals Copernicus atmospheric service for stratospheric ozone: validation and intercomparison of four near real-time analyses, 2009–2012

2014 ◽  
Vol 14 (8) ◽  
pp. 12461-12523 ◽  
Author(s):  
K. Lefever ◽  
R. van der A ◽  
F. Baier ◽  
Y. Christophe ◽  
Q. Errera ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Vertical Distribution ◽  
Real Time ◽  
Total Ozone ◽  
Lower Stratosphere ◽  
Stratospheric Ozone ◽  
The Tropics ◽  
The Vertical Distribution ◽  
Total Column ◽  
Good Agreement

Abstract. This paper evaluates the performance of the stratospheric ozone analyses delivered in near real time by the MACC (Monitoring Atmospheric Composition and Climate) project during the 3 year period between September 2009 and September 2012. Ozone analyses produced by four different chemistry transport models and data assimilation techniques are examined: the ECMWF Integrated Forecast System (IFS) coupled to MOZART-3 (IFS-MOZART), the BIRA-IASB Belgian Assimilation System for Chemical ObsErvations (BASCOE), the DLR/RIU Synoptic Analysis of Chemical Constituents by Advanced Data Assimilation (SACADA), and the KNMI Data Assimilation Model based on Transport Model version 3 (TM3DAM). The assimilated satellite ozone retrievals differed for each system: SACADA and TM3DAM assimilated only total ozone observations, BASCOE assimilated profiles for ozone and some related species, while IFS-MOZART assimilated both types of ozone observations. The stratospheric ozone analyses are compared to independent ozone observations from ground-based instruments, ozone sondes and the ACE-FTS (Atmospheric Chemistry Experiment – Fourier Transform Spectrometer) satellite instrument. All analyses show total column values which are generally in good agreement with groundbased observations (biases <5%) and a realistic seasonal cycle. The only exceptions are found for BASCOE which systematically underestimates total ozone in the Tropics with about 7–10% at Chengkung (Taiwan, 23.1° N/121.365° E), resulting from the fact that BASCOE does not include any tropospheric processes, and for SACADA which overestimates total ozone in the absence of UV observations for the assimilation. Due to the large weight given to column observations in the assimilation procedure, IFS-MOZART is able to reproduce total column observations very well, but alternating positive and negative biases compared to ozonesonde and ACE-FTS satellite data are found in the vertical as well as an overestimation of 30 to 60% in the polar lower stratosphere during ozone depletion events. The assimilation of near real-time (NRT) Microwave Limb Sounder (MLS) profiles which only go down to 68 hPa is not able to correct for the deficiency of the underlying MOZART model, which may be related to the applied meteorological fields. Biases of BASCOE compared to ozonesonde or ACE-FTS ozone profiles do not exceed 10% over the entire vertical stratospheric range, thanks to the good performance of the model in ozone hole conditions and the assimilation of offline MLS profiles going down to 215 hPa. TM3DAM provides very realistic total ozone columns, but is not designed to provide information on the vertical distribution of ozone. Compared to ozonesondes and ACE-FTS satellite data, SACADA performs best in the Arctic, but shows large biases (>50%) for ozone in the lower stratosphere in the Tropics and in the Antarctic, especially during ozone hole conditions. This study shows that ozone analyses with realistic total ozone column densities do not necessarily yield good agreement with the observed ozone profiles. It also shows the large benefit obtained from the assimilation of a single limb-scanning instrument (Aura MLS) with a high density of observations. Hence even state-of-the-art models of stratospheric chemistry still require the assimilation of limb observations for a correct representation of the vertical distribution of ozone in the stratosphere.

Global variations in the vertical distribution of water during Mars Year 34 from multiple spacecraft observations

2021 ◽  
Author(s):  
James Holmes ◽  
Stephen Lewis ◽  
Manish Patel ◽  
Shohei Aoki ◽  
Giuliano Liuzzi ◽  
...  
Keyword(s):  
Data Assimilation ◽  
Vertical Distribution ◽  
Water Vapour ◽  
Hydrological Cycle ◽  
Circulation Model ◽  
Transport Processes ◽  
Temperature Profiles ◽  
Saturation State ◽  
Spacecraft Data ◽  
The Vertical Distribution

&lt;div&gt; &lt;p&gt;&lt;span data-contrast=&quot;auto&quot;&gt;Observations of the vertical distribution of water vapour provide a unique snapshot of the vertical transport processes that contribute to the global martian hydrological cycle. While previous datasets have largely been seasonally and spatially sparse, vertical profiles of water retrieved from the Nadir and Occultation for MArs Discovery (NOMAD) and Atmospheric Chemistry Suite (ACS) instruments on the ExoMars Trace Gas Orbiter (TGO) provide the most complete dataset so far. These data are now capable of providing robust constraints on the 4-D distribution of water, especially when also combined with retrievals of additional atmospheric properties (e.g. temperature profiles, dust column) that exert an influence on the evolving global water distribution.&amp;#160;&lt;/span&gt;&lt;span data-ccp-props=&quot;{&amp;quot;335551550&amp;quot;:6,&amp;quot;335551620&amp;quot;:6}&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt; &lt;/div&gt; &lt;div&gt; &lt;p&gt;&lt;span data-contrast=&quot;auto&quot;&gt;A key limitation though is the fact that observations of water profiles are still relatively limited in coverage, in the global sense, and the vertical distribution of water at latitudes and times not regularly probed by NOMAD and ACS remains poorly understood.&amp;#160;&amp;#160;&lt;/span&gt;&lt;span data-ccp-props=&quot;{&amp;quot;335551550&amp;quot;:6,&amp;quot;335551620&amp;quot;:6}&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt; &lt;/div&gt; &lt;div&gt; &lt;p&gt;&lt;span data-contrast=&quot;auto&quot;&gt;To address this, we have created a global reference climatology of water vertical distribution for Mars Year (MY) 34 through a multi-spacecraft data assimilation combining several retrieval datasets with a Mars Global Circulation Model. Retrievals of dust column and temperature profiles from Mars Climate Sounder on the Mars Reconnaissance Orbiter and water vapour and temperature profiles from multiple instruments on the ExoMars TGO during the primary science phase covering the latter half of MY34 are combined through assimilation to create one unified physically consistent global dataset.&lt;/span&gt;&lt;span data-ccp-props=&quot;{&amp;quot;335551550&amp;quot;:6,&amp;quot;335551620&amp;quot;:6}&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt; &lt;/div&gt; &lt;div&gt; &lt;p&gt;&lt;span data-contrast=&quot;auto&quot;&gt;The vertical water vapour distribution is investigated globally. During the initial coverage of TGO observation that covers the dusty season in MY34, northern polar latitudes are largely absent of water vapour below 20 km with variations in abundance above this altitude throughout the dusty season linked to transport from mid-latitudes during a global dust storm, perihelion season and the intense MY34 C storm. The atmosphere is in a supersaturated state above 60 km for most of the time period investigated, with lower altitudes showing more diurnal variation in the saturation state of the atmosphere. A key benefit of the data assimilation technique is that constraints on dynamical transport imposed by the assimilated water vapour and temperature profiles leads to improvements in the simulated water ice distribution even though it is not altered directly by the assimilation process.&lt;/span&gt;&lt;span data-ccp-props=&quot;{&amp;quot;335551550&amp;quot;:6,&amp;quot;335551620&amp;quot;:6}&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt; &lt;/div&gt; &lt;div&gt; &lt;p&gt;&lt;span data-contrast=&quot;auto&quot;&gt;The climatology created, which will become publicly available for wider use by the martian scientific community, has also been independently validated against water vapour profiles from the SPICAM instrument.&lt;/span&gt;&lt;span data-ccp-props=&quot;{&amp;quot;335551550&amp;quot;:6,&amp;quot;335551620&amp;quot;:6}&quot;&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt; &lt;/div&gt;

scholarly journals On the vertical distribution of boundary layer halogens over coastal Antarctica: implications for O<sub>3</sub>, HO<sub>x</sub>, NO<sub>x</sub> and the Hg lifetime

2007 ◽  
Vol 7 (4) ◽  
pp. 9385-9417 ◽  
Author(s):  
A. Saiz-Lopez ◽  
J. M. C. Plane ◽  
A. S. Mahajan ◽  
P. S. Anderson ◽  
S. J.-B. Bauguitte ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Vertical Distribution ◽  
Transport Model ◽  
Current Knowledge ◽  
Sea Salt ◽  
Chemical Transport Model ◽  
Sea Salt Aerosol ◽  
Iodine Chemistry ◽  
The Antarctic ◽  
The Vertical Distribution

Abstract. A one-dimensional chemical transport model has been developed to investigate the vertical gradients of bromine and iodine compounds in the Antarctic coastal boundary layer. The model has been applied to interpret recent year-round observations of iodine and bromine monoxides (IO and BrO) at Halley Station, Antarctica. The model requires an equivalent I atom flux of ~109 molecule cm−2 s−1 from the snowpack in order to account for the measured IO levels, which are up to 20 ppt during spring. Using the current knowledge of gas-phase iodine chemistry, the model predicts significant gradients in the vertical distribution of iodine species. However, recent ground-based and satellite observations of IO imply that the radical is well-mixed in the boundary layer, indicating a longer than expected atmospheric lifetime for the radical. This can be modelled by including photolysis of the higher iodine oxides (I2O2, I2O3, I2O4 and I2O5), and rapid recycling of HOI and INO3 through sea-salt aerosol. The model also predicts significant concentrations (up to 25 ppt) of I2O5 in the lowest 10 m of the boundary layer, which could lead to the formation of ultrafine iodine oxide aerosols. Heterogeneous chemistry involving sea-salt aerosol is also necessary to account for the vertical profile of BrO. Iodine chemistry causes a large increase (typically more than 3-fold) in the rate of O3 depletion in the BL, compared with bromine chemistry alone. Rapid entrainment of O3 from the free troposphere is required to account for the observation that on occasion there is little O3 depletion at the surface in the presence of high concentrations of IO and BrO. The halogens also cause significant changes to the vertical profiles of HO and HO2 and the NO2/NO ratio. The average Hg0 lifetime against oxidation is also predicted to be about 10 h during springtime. Overall, our results show that halogens profoundly influence the oxidizing capacity of the Antarctic troposphere.

scholarly journals On the vertical distribution of boundary layer halogens over coastal Antarctica: implications for O<sub>3</sub>, HO<sub>x</sub>, NO<sub>x</sub> and the Hg lifetime

2008 ◽  
Vol 8 (4) ◽  
pp. 887-900 ◽  
Author(s):  
A. Saiz-Lopez ◽  
J. M. C. Plane ◽  
A. S. Mahajan ◽  
P. S. Anderson ◽  
S. J.-B. Bauguitte ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Vertical Distribution ◽  
Transport Model ◽  
Current Knowledge ◽  
Sea Salt ◽  
Chemical Transport Model ◽  
Sea Salt Aerosol ◽  
Iodine Chemistry ◽  
The Antarctic ◽  
The Vertical Distribution

Abstract. A one-dimensional chemical transport model has been developed to investigate the vertical gradients of bromine and iodine compounds in the Antarctic coastal boundary layer (BL). The model has been applied to interpret recent year-round observations of iodine and bromine monoxides (IO and BrO) at Halley Station, Antarctica. The model requires an equivalent I atom flux of ~1010 molecule cm−2 s−1 from the snowpack in order to account for the measured IO levels, which are up to 20 ppt during spring. Using the current knowledge of gas-phase iodine chemistry, the model predicts significant gradients in the vertical distribution of iodine species. However, recent ground-based and satellite observations of IO imply that the radical is well-mixed in the Antarctic boundary layer, indicating a longer than expected atmospheric lifetime for the radical. This can be modelled by including photolysis of the higher iodine oxides (I2O2, I2O3, I2O4 and I2O5), and rapid recycling of HOI and INO3 through sea-salt aerosol. The model also predicts significant concentrations (up to 25 ppt) of I2O5 in the lowest 10 m of the boundary layer. Heterogeneous chemistry involving sea-salt aerosol is also necessary to account for the vertical profile of BrO. Iodine chemistry causes a large increase (typically more than 3-fold) in the rate of O3 depletion in the BL, compared with bromine chemistry alone. Rapid entrainment of O3 from the free troposphere appears to be required to account for the observation that on occasion there is little O3 depletion at the surface in the presence of high concentrations of IO and BrO. The halogens also cause significant changes to the vertical profiles of OH and HO2 and the NO2/NO ratio. The average Hg0 lifetime against oxidation is also predicted to be about 10 h during springtime. An important result from the model is that very large fluxes of iodine precursors into the boundary layer are required to account for the observed levels of IO. The mechanisms which cause these emissions are unknown. Overall, our results show that halogens profoundly influence the oxidizing capacity of the Antarctic troposphere.

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