PICO
Tropospheric chlorine has the potential to perturb atmospheric oxidation capacity, which plays an important role in climate change and air quality. Although sea-salt is the predominant source of tropospheric chlorine, oxidation of chlorocarbons could prove to be a more important source of tropospheric chlorine than previously thought due to their increasing abundances over the last 2 decades. The most abundant chlorocarbon, methyl chloride (CH3Cl) is predominantly emitted from natural sources and has stayed relatively stable over the last 20 years. However, the concentrations of a range of chlorine-containing very short-lived substances (Cl-VSLS) have varied substantially over the same time period, particularly anthropogenically emitted Cl-VSLS: dichloromethane (CH2Cl2), 1,2-dichloroethane (C2H4Cl2), perchloroethylene (C2Cl4), trichloromethane (CHCl3) and trichloroethylene (C2HCl3). Additionally, there are a number of bromine and iodine containing Cl-VSLS (e.g. CH2BrCl) that are released from natural sources.
Here, the Frontier Research System for Global Change version of the University of California Irvine Chemical Transport Model is used to explore the impact of Cl-VSLS in the troposphere. A tropospheric chlorine chemistry scheme including appropriate sources, reactions and sinks of chlorine species was incorporated into the model and multi-year simulations were used to assess the spatio-temporal trends in Cl-VSLS. Two approaches were compared to assess the impact of Cl-VSLS. The first method involves constraining the chlorocarbons using latitude- and time-varying surface concentrations generated from measurement data, whilst the second method used fully geographically varying emissions. We consider a more comprehensive set of chlorocarbons than previous studies and explore how their abundances have changed over time. We find that the contribution of chlorocarbons to tropospheric inorganic chlorine has increased; from ~4100 Gg/year in 2000 to ~4600 Gg/year in 2022. The impact of chlorocarbons on tropospheric composition will also be presented.
How to cite: Vest, K., Hossaini, R., Wild, O., Mazzeo, A., Hou, X., and O'Connor, F.: The impact of chlorocarbons on tropospheric composition: a global model study, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3074, https://doi.org/10.5194/egusphere-egu25-3074, 2025.
Estimating the emissions of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) is of great significance for assessing global ozone depletion and climate change, where the emissions from fluorochemical plants play an important role. However, no research has been conducted on the HCFC and HFC emissions from fluorochemical plants based on observation data and diffusion model. This study observed the concentration of two HCFCs and six HFCs around three typical fluorochemical plants in China. It used the Gaussian plume diffusion model to explore their emissions. The results showed that the concentration difference between the downwind and upwind sites (from now on referred to as down-up difference) of each substance is ranked. Only in plant A, the substance with the largest down-up difference is HCFC. The total HFC down-up differences of the three plants were higher than that of HCFCs, and the total emissions of six HFCs accounted for 46% of three plant’s emission, suggesting that the HFC production of the three typical fluorochemical plants in China had reached a large scale with the phase-out of ODSs (ozone-depleting substances). The total emissions of HCFCs and HFCs from the three plants are 56.62 Mt (million ton) CO2-equiv yr−1. The emissions from the three plants are approximately 20–76% of the bottom-up national emissions estimated using IPCC 2019 emission factors. On the contrary, the emissions of the three plants are 2–6 times higher than the national emissions (contained 20 fluorochemical plants) based on the IPCC 2006 emission factor. This revealed that using the default emission factors for fluorochemical production recommended by IPCC 2006 to estimate the emissions of HCFCs and HFCs from fluorochemical plants in China may lead to underestimation.
How to cite: Wu, J., Liu, Z., Ma, T., An, M., Ye, T., Zhao, X., Li, M., Wang, F., Yuan, M., Hu, D., Zhang, Y., and Peng, L.: Concentration characteristics and emission estimates ofmajor HCFCs and HFCs at three typical fluorochemical plants in China based on a Gaussian diffusion model, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3966, https://doi.org/10.5194/egusphere-egu25-3966, 2025.
Halogen chemistry is a central element of tropospheric ozone depletion events (ODEs) during polar spring. Key processes such as source mechanisms that produce reactive halogen species, their transport, and interhalogen interactions as well as the influence of the quickly changing climate, however, remain in the centre of Arctic research.
We deployed a Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) instrument in Utqiagvik (formerly Barrow), Alaska, in December 2023. First results from measurements performed between March and May 2024 show that this period exhibits active halogen chemistry with many episodes of enhanced bromine monoxide coinciding with strongly reduced ozone concentrations. Further, analysis results of chlorine monoxide are presented. Additional Multi-AXis (MAX-) DOAS observations have been conducted since the beginning of April 2024.
Comparison to data from the instrument’s previous deployment at the German research station Neumayer, Antarctica (Nasse, 2019), indicates differences in the prevailing atmospheric conditions and trace gas amounts between both hemispheres which will be discussed in detail.
How to cite: Lauster, B., Donner, S., Frieß, U., Platt, U., Reischmann, L., Simpson, W., Ziegler, S., and Wagner, T.: Long-path DOAS observations of halogen oxides at Utqiagvik, Alaska, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4239, https://doi.org/10.5194/egusphere-egu25-4239, 2025.
The cycles of the toxic element mercury (Hg) and bromine (Br) are inextricably linked, since Br radicals are a major oxidant for elemental mercury, Hg(0). Global dispersion of Hg occurs through transport of Hg(0), due to its atmospheric lifetime of 6 months. Upon oxidation of Hg(0) to soluble divalent mercury, Hg(II), deposition will occur on timescales of approximately 1 week. There are many uncertainties associated with atmospheric Hg chemistry, leading to uncertain predictions of its fate and impacts on ecosystems. Here we assess the role of Br in the oxidation of Hg(0) using a chemistry-climate model WACCM and quantify sources of uncertainty due to different factors. Oxidation of Hg(0) by Br is found to dominate near the surface in the Southern Hemisphere midlatitudes, as well as throughout the upper troposphere and lower stratosphere. Elsewhere in the troposphere, the reaction of Hg(0) by hydroxyl (OH) radicals is the primary oxidation pathway. However, these results are highly dependent on the model’s lower troposphere bromine concentrations. Comparing different model versions of GEOS-Chem and WACCM, the chemical lifetime of Hg(0) can vary by a factor of more than 20 in the Southern Hemisphere midlatitudes due to differences in simulated Br concentrations. The models show much closer agreement in their simulated OH concentrations, highlighting the higher uncertainties in Br chemistry. We also explored the uncertainty in Hg reaction rates using global sensitivity analysis in a box model representing WACCM chemistry. Uncertainties in the OH-driven oxidation reactions of Hg(0) dominate uncertainties in the Hg(0) lifetime in the Northern Hemisphere, while the reaction rate of Br with Hg(0) is the key uncertainty over much of the Southern Ocean. We identify ~10 reactions out of the full chemical mechanism of 72 Hg-related reactions that contribute almost all of the variability in outputs, indicating the potential for constructing a simplified mechanism for Hg chemistry. Overall, our results emphasize that predictions of Hg deposition are highly impacted by uncertainties in lower troposphere Br radical concentrations, suggesting that more observational constraints on Br are necessary to improve the accuracy of Hg models.
How to cite: Feinberg, A., Sonke, J., and Saiz-Lopez, A.: Quantifying the role of bromine in the atmospheric oxidation of mercury (Hg), EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-6802, https://doi.org/10.5194/egusphere-egu25-6802, 2025.
A large portion of the Arctic is covered by ocean and sea ice, from which reactive halogen species can be emitted to the atmosphere. Springtime ozone depletion events (ODEs) have been primarily attributed to catalytic destruction of ozone by reactive bromine released from snowpacks and blowing snow over sea ice and cycled through heterogeneous reactions on aerosol surfaces. Mechanisms to represent polar springtime bromine explosions and ODEs have been developed and tested in various atmospheric models, by considering both blowing snow and snowpacks, with varying degrees of success when compared with observations of reactive bromine and ozone in polar regions. In this study, two independent chemical transport models (CTMs), DEHM (Danish Eulerian Hemispheric Model) and GEM-MACH (Global Environmental Multi-scale – Modelling Air quality and Chemistry), were used to simulate Arctic lower tropospheric ozone for the year 2015. Both models include bromine chemistry and a representation of snow-sourced bromine mechanism: a blowing-snow bromine source mechanism in DEHM and a snowpack bromine source mechanism in GEM-MACH.
The comparison of model simulation results with available observations in the Arctic showed that the model with the snowpack bromine source mechanism (GEM-MACH) was able to capture most of the observed springtime ODEs in the Arctic, while the model considering blowing-snow sourced bromine alone (DEHM) simulated much fewer ODEs. The snowpack-sourced mechanism is seen to be essential in sustaining the continued bromine production under a variety of meteorological conditions, while the blowing-snow bromine source mechanism triggered by high wind conditions tends to be more episodic. This is consistent with observational evidence that the ODEs observed in the Arctic tend to occur during calm wind conditions favouring the snowpack bromine source mechanism to take effect in the surface air with ODEs at high wind speed conditions to occur sporadically. The study demonstrated that the springtime ozone depletion process plays a central role in driving the surface ozone seasonal cycle in the Central Arctic, and that the bromine-mediated ODEs, while occurring most notably within the lowest few hundred metres of air above the Arctic Ocean, can induce a 5-7% of loss in the total pan-Arctic tropospheric ozone burden during springtime. The study also demonstrated that atmospheric aerosols play an integral role in the Arctic springtime bromine explosions and ODEs through heterogeneous cycling of reactive bromine, particularly over a deeper vertical layer and at distance from the snowpack bromine source area, which has implications for the potential role of Arctic haze aerosols that may play in the springtime ODEs. The uncertainty in parameterising the Arctic bromine source mechanism will also be discussed.
How to cite: Gong, W., Beagley, S., Toyota, K., Skov, H., Christensen, J., Lupu, A., Pendlebury, D., Zhang, J., Im, U., Kanaya, Y., Saiz-Lopez, A., Sommariva, R., Effertz, P., Halfacre, J., Jepsen, N., Kivi, R., Koenig, T., Müller, K., Nordstrøm, C., and Petropavlovski, I. and the TOAR-II Ozone Over the Oceans Focus Working Group - Arctic: Modelling Arctic springtime ozone depletion events: role of snow sourced bromine chemistry and its impact on Arctic tropospheric ozone budget, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7049, https://doi.org/10.5194/egusphere-egu25-7049, 2025.
While the dominant role of halogens in Arctic ozone loss during spring has been widely studied in the last decades, the impact of sea-ice halogens on surface ozone abundance over the northern hemisphere (NH) mid-latitudes remains unquantified. Here, we use a state-of-the-art global chemistry-climate model including polar halogens (Cl, Br, and I), which reproduces Arctic ozone seasonality, to show that Arctic sea-ice halo- gens reduce surface ozone in the NH mid-latitudes (47°N to 60°N) by ~11% during spring. This background ozone reduction follows the southward export of ozone-poor and halogen-rich air masses from the Arctic through polar front intrusions toward lower latitudes, reducing the springtime tropospheric ozone column within the NH mid-latitudes by ~4%. Our results also show that the present-day influence of Arctic halogens on surface ozone destruction is comparatively smaller than in preindustrial times driven by changes in the chemical interplay between anthropogenic pollution and natural halogens. We conclude that the impact of Arctic sea-ice halogens on NH mid-latitude ozone abundance should be incorporated into global models to improve the representation of ozone seasonality.
How to cite: Cuevas, C. A., Fernandez, R. P., Berná, L., Tomazzeli, O. G., Mahajan, A. S., Li, Q., Kinnison, D. E., Wang, S., Lamarque, J.-F., Tilmes, S., Skov, H., and Saiz-Lopez, A.: Arctic halogens reduce ozone in the northern mid-latitudes, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8956, https://doi.org/10.5194/egusphere-egu25-8956, 2025.
Halogens are known to deplete ozone both in the troposphere and stratosphere. As the source distribution and thus the local contribution of chlorine and bromine containing species to atmospheric ozone depletion is reasonably well known, the respective role of iodine containing species is subject of current research. In contrast to the major sources of chlorine in the stratosphere which derive from man-made chlorinated hydrocarbons, about half of stratospheric bromine stems from bromocarbons of natural origin while iodine predominantly originates from inorganic species (I2, and HOI) emitted from the oceans. It has been indicated previously that tropical cyclones emit elevated amounts of brominated species (e.g. CHBr3 and CH2Br2) which are efficiently transported into the extratropical upper troposphere and eventually into the lower stratosphere. Here we provide evidence that also significant amounts of inorganic iodine emitted from the ocean surface and/or through sea-spray are transported to the extratropical upper troposphere through tropical cyclone driven fast vertical transport.
Our finding is based on the simultaneous detection of elevated amounts of brominated very short-lived substances (VSLS) and iodine oxide (IO, ~0.3 pptv) while simultaneously relatively low mixing ratios of the anthropogenically emitted CH2Cl2 were measured in the upper troposphere (~13 km, θ~360 K) of the mid-Atlantic on October 1st,2017. CLaMS back-trajectory calculations driven by ECMWF ERA-Interim reanalysis suggest that these iodine-rich air masses originate from marine surface air masses being uplifted by the category 5 hurricane Maria.
The measurements were performed from aboard HALO (High Altitude and LOng range Aircraft) during the WISE (Wave-driven ISentropic Exchange) campaign over the mid-Atlantic in September and October 2017. IO was detected in limb scattered skylight using the miniDOAS instrument, while the organic chlorinated and brominated species were detected by the HAGAR-V and GhOST GC/MS instruments.
Our findings suggest that tropical storms lead to elevated emissions of inorganic iodine rapidly transported from the tropical marine boundary layer into the upper troposphere. This mechanism implies a potentially significant role of iodine in ozone destruction in the remnant air of tropical storms and a possible pathway for iodine to enter the lower stratosphere in significant amounts.
How to cite: Voss, K., Weyland, B., Butz, A., Lauther, V., Volk, C. M., Vogel, B., Engel, A., Schuck, T., Keber, T., Rotermund, M., and Pfeilsticker, K.: Enhanced inorganic iodine in the upper troposphere potentially driven by elevated emission and/or fast vertical transport by tropical cyclone, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9344, https://doi.org/10.5194/egusphere-egu25-9344, 2025.
Arctic tropospheric bromine monoxide (BrO) plays a critical role in atmospheric chemistry, particularly during ozone depletion events and the oxidation of gaseous elemental mercury in spring. The contributions of various potential sources, such as sea ice, open ocean, and aerosols, to the production of reactive bromine remain unclear. In this study, we present long-term observations of BrO and aerosol profiles retrieved from Multi-Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements in Ny-Ålesund, Svalbard (78.92°N, 11.93°E), covering the periods from March to May between 2017 and 2023. Retrieved tropospheric BrO partial columns are then compared with BrO observations from the GOME-2 satellite instrument and model results from a global chemistry transport model, p-TOMCAT, respectively. Aerosol extinction exhibits the strongest correlation with BrO (R=0.67 in March, 0.54 in April, 0.47 in May), indicating that airborne particles are associated with the enhancement of reactive bromine.
Five days of backward trajectories in an altitude range of 0–3 km (at 200 m intervals) were used to calculate the contact time of air masses with various surface types (sea ice, open ocean, land, and the free troposphere). Along the trajectories, whenever the air mass meets open ocean or sea ice surface (e.g. < 500 m), the corresponding bromine emission flux from sea salt aerosols generated from open ocean (Gong et al., 2003) and blowing snow (Yang et al., 2008) is calculated and accumulated. Results show that, in March, MAX-DOAS BrO is in a positive correlation with sea ice contact time (R=0.29) and bromine emission flux from blowing snow on sea ice (R=0.33), suggesting that sea-ice-sourced sea salt aerosols generated by blowing snow could represent a significant source of reactive bromine. Throughout the entire spring (March-May), the contact time with sea ice accounts for 52.41% of all bromine explosion events (BEEs) observed in Ny-Ålesund, whereas the contact time with open ocean accounts for only 2.85%. This indicates that, in comparison to sea ice, the contribution of open ocean is less significant in Ny-Ålesund. These results confirm the critical role of sea ice-related processes in the production of reactive bromine during spring.
How to cite: Li, Q., Luo, Y., Yang, X., Zilker, B., and Richter, A.: Source mechanisms of tropospheric bromine monoxide in Ny-Ålesund between 2017 and 2023, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9442, https://doi.org/10.5194/egusphere-egu25-9442, 2025.
The observation of bromine monoxide (BrO) in the polar regions, in particular the study of tropospheric bromine explosion events (BEEs) during the polar spring, has been an ongoing task since the late 1980s. Since the mid-1990s, BrO has also been monitored from satellites, allowing global observation of BrO and the large-scale tropospheric BrO plumes resulting from BEEs in polar regions. With the launch of the TROPOspheric Monitoring Instrument (TROPOMI) in October 2017, there is an instrument that enables daily high-resolution measurements of BrO. From the satellite measurements, total BrO columns are obtained. However, the total column consists mainly of stratospheric BrO and usually only a small amount of BrO is located in the lower troposphere. In order to investigate the tropospheric BEEs, a stratospheric separation method must be applied to subtract the stratospheric contribution from the total BrO column and thereby estimate the amount of tropospheric BrO.
In this study, five different stratospheric separation methods are applied to the TROPOMI BrO dataset to calculate the amount of tropospheric BrO: (1) a constant stratospheric BrO value, (2) a high pass filtering method applied in near real time processing, (3) an empirical multiple linear regression model, (4) a climatology-based method developed by Theys et al. (2011), and (5) a recently developed method for the OMPS satellite by Chong et al. (2024). The different separation methods are compared to each other and the results of all five methods are validated using airborne tropospheric BrO measurements from the Heidelberg Airborne Imaging DOAS Instrument (HAIDI) during the CHemistry in the Arctic: Clouds, Halogens, and Aerosols (CHACHA) campaign, which took place in Alaska in spring 2022.
This work was supported by the DFG funded Transregio-project TR 172 “Arctic Amplification (AC)³“ in subproject C03.
How to cite: Zilker, B., Richter, A., Brockway, N., Peterson, P., Bigge, K., Simpson, W. R., Chong, H., Theys, N., Seo, S., Bösch, H., and Burrows, J. P.: Validation of TROPOMI tropospheric BrO columns employing CHACHA airborne campaign measurements, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16206, https://doi.org/10.5194/egusphere-egu25-16206, 2025.
Halogens (chlorine, bromine and iodine) have been studied extensively in the stratosphere, where they catalytically destroy ozone, but more recently there has been growing interest in their effect on tropospheric composition and oxidative capacity. Of particular interest is how they affect ozone, a powerful greenhouse gas and air pollutant at the Earth’s surface. Reactive halogen species such as BrO and IO can impact ozone concentrations directly through catalytic cycles and indirectly via affecting the partitioning of HOX and NOX, resulting in important effects on tropospheric composition that are not yet fully understood.
Here, we explore how bromine and iodine chemistry can impact tropospheric ozone concentrations. Using the FRSGC/UCI CTM, we expand the existing tropospheric chemistry scheme (that includes a comprehensive description of iodine chemistry) by including a detailed bromine chemistry scheme. This encompasses major gas-phase and heterogeneous reactions, including reactions occurring on ice particles, alongside physical processes like wet and dry deposition. Sources of bromine include (1) CH3Br and emissions of five short-lived bromocarbons (CHBr3, CH2Br2, CH2BrCl, CHBr2Cl and CHBrCl2), (2) debromination of sea-salt aerosol from the open ocean and from blowing snow in polar regions, and (3) transport from the stratosphere. We explore different approaches to representing open ocean sea-salt debromination, comparing a ‘depletion factor’ based parameterisation with sea-salt debromination arising from a series of heterogeneous reactions. The impact of these different parameterisations on the resulting bromine budget is shown.
A detailed evaluation of the model performance is presented using ground-based, aircraft and satellite observations of key radicals (e.g. halogen oxides) with the results comparing reasonably well with observations. We also explore the spatial and temporal variation of natural halogens in the troposphere and the importance of different sources, and further quantify the impact of reactive halogens on present-day ozone concentrations and tropospheric oxidising capacity.
How to cite: Randell, J., Hossaini, R., Wild, O., Mazzeo, A., and Hou, X.: Assessing the Impact of Bromine and Iodine Chemistry on Tropospheric Composition, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16865, https://doi.org/10.5194/egusphere-egu25-16865, 2025.
Reactive chlorine radicals are known to efficiently react with ambient hydrocarbons, thereby affecting boundary layer oxidation capacity and pollutant lifetimes. HCl is the most abundant and long-lived inorganic chlorine reservoir species in the troposphere, yet high frequency, in situ observations are limited due to sampling challenges. In this work, we report HCl field observations using a Tunable Infrared Laser Direct Absorption Spectrometer (TILDAS), deployed in Manchester, England, during the 2021-2022 Integrated Research Observation System for Clean Air campaign. Instrument precision was estimated as 1.1 pptv (Allan Werle minimum of 1.4 minute), with 3σ limits of detection of 3.3 pptv.
Observations obtained during June and July 2021 generally exhibited a diurnal profile on clear days, peaking at midafternoon (mean daily mixing ratios ranging between 15 – 89 pptv). Conversely, observations from February 2022 displayed no obvious profile with mixing ratios remaining muted throughout the observation period (mean daily mixing ratios ranging between 7-13 pptv), suggesting suppression of Cl-liberation mechanisms. Despite observations occurring in an inland polluted urban environment, particle dispersion analysis for both seasons shows air masses spend most of their time passing over the ocean in the 72-hours preceding arrival at the observation site. The thermodynamic equilibrium model ISORROPIA II will be used to explore the role of partitioning between particulate phase Cl- and gas phase HCl, with model inputs supplied from observed non-refactory, submicron particulate SO42-, NO3-, NH4+, and Cl- ions, as well as gas phase observations of HCl, HNO3 and NH3. Data will be further interpreted using gas phase box modelling, further incorporating co-located CIMS observations of other inorganic Cl-species, including ClNO2 andCl2, to gain a greater understanding of seasonal chlorine chemistry mechanisms at an inland, urban measurement site.
How to cite: Halfacre, J. W., Stewart, J., Rowlinson, M., Matthews, E., Bannan, T., Green, J. R., Vizuete, W., Evans, M. J., Herndon, S. C., Roscioli, J. R., Dyroff, C., Yacovitch, T. I., Allan, J., Coe, H., Andrews, S. J., Brown, S. S., and Edwards, P. M.: Comparison of seasonal differences in hydrogen chloride observations from an inland urban site, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17490, https://doi.org/10.5194/egusphere-egu25-17490, 2025.
