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Tropospheric Halogens and Their Impact on Air Pollution

Postdoctoral Fellowship | 17/01/2018

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This project was aimed at investigating the role of sea salt aerosol and the ocean surface as a source of tropospheric halogens, and at unraveling the impact of halogens on tropospheric ozone and NOx. The project research activities included both further implementation of halogen chemistry in a state-of-the-art global chemical transport model and laboratory chemical kinetics studies targeting the activation of aqueous phase halogens by gas phase oxidants.

By Postdoc Johan Albrecht Schmidt, Department of Chemistry, University of Copenhagen

Tropospheric halogen chemistry has become a hot topic in atmospheric research. Recent studies have revealed that halogens (chlorine, bromine, and iodine) are present at significant concentrations in wide regions of the troposphere and impact on the composition of the troposphere including key air pollutants and green house gasses such as ozone, nitrogen oxides (NOx), methane, and mercury. 

The Carlsberg Foundation supported the Postdoctoral Fellowship-project Halogen emissions from ocean and sea salt aerosol: Global implications and opportunities. This project was aimed at investigating the role of sea salt aerosol and the ocean surface as a source of tropospheric halogens, and at unraveling the impact of halogens on tropospheric ozone and NOx. 

The project research activities included both further implementation of halogen chemistry in a state-of-the-art global chemical transport model and laboratory chemical kinetics studies targeting the activation of aqueous phase halogens by gas phase oxidants. 

The outcome of the project included a detailed assessment of halogens impact on ozone and NOx in the troposphere and the unexpected discovery that sulfites dissolved in sea salt aerosol and cloud droplets provide a critical sink for reactive halogens in the lower troposphere.

Simulated distribution of inorganic chlorine, bromine, and iodine (Cly, Bry, Iy) in the troposphere. Figure taken from Sherwen et al (2016).

Unraveling the Role of Halogens in the Troposphere

The recent efforts to unravel the role of halogens in the troposphere has been driven by innovations in field measurements, satellite remote sensing, laboratory experiments, and global chemical modeling. 

Recent advances in field measurement techniques such as Long-Path and Multi-Axial Differential Optical Absorption Spectroscopy, have facilitated the detection and quantification of key reactive species, bromine oxide (BrO) and iodine oxide (IO), at part-per-trillion level (Wang et al., 2015).

Fact box 1: Halogens in the Troposphere and Stratosphere

Ozone is found throughout Earth's atmosphere. The concentration of ozone peaks in the stratosphere (approximately 10 to 50 km above sea level) where it forms the ozone layer, a protective layer that shields Earth's surface from harmful UV radiation. Because of this, stratospheric ozone is sometimes called the “good ozone”. Ozone is formed in the stratosphere when short wavelength UV radiation (only present at high altitudes) splits molecular oxygen into two oxygen atoms. The oxygen atoms react with molecular oxygen thereby forming ozone.

At the same time, new techniques for analyzing satellite data have revealed a global background of inorganic halogens in the troposphere. 

These observations facilitate the implementation of halogen chemistry in new global chemical transport models which provide a means for assessing the impact of halogens on the composition of the troposphere. 

Accurate laboratory measurement of key chemical kinectics parameters provide a basis for global chemical models. 

This Carlsberg Foundation post doctoral project was concerned with developing the representation of halogens in the GEOS-Chem, a state-of-the-art global chemical transport model, and building a laboratory setup for studying the chemical activation of dissolved halogens by ozone and other gas phase oxidants.


"Recent work using models to explore global halogen chemistry have shown that, even at very small concentrations, halogens can change the concentration and lifetimes of key climate and air-quality gases. This is primarily through decreasing ozone, a key atmospheric oxidant. The effects of these halogen species have increased through time, and this has lead to a buffering of human-driven global increases in ozone since the preindustrial" – Dr. Tomás Sherwen, Postdoctoral Research Associate, Wolfson Atmospheric Chemistry Laboratories, University of York, UK.

Halogens in Earth's Atmosphere

The presence of halogens such as chlorine and bromine in Earth's stratosphere has been well known for decades. The sources of stratospheric halogens include man-made Chloro-Fluoro-Carbons (CFCs) and “Halons”, compounds that are chemically inert enough to avoid breakdown and washout in the troposphere, but release their halogen atoms when exposed to stratospheric UV radiation.

Once in the stratosphere, the halogen atoms will react and form a number of reactive inorganic species destroying stratospheric ozone in the process. These inorganic halogens are only very slowly removed from the stratosphere.

In the troposphere, the inorganic halogens rapidly react with abundant volatile organic carbon forming acid halides that are taken up by rain and clouds and thereby removed from the troposphere. However, recent observations in the marine troposphere and upper troposphere have revealed levels of halogen radicals BrO and IO high enough to have significant impact on the composition of wider troposphere.

The sources of halogens in the lower atmosphere are predominantly natural and include short lived halocarbons and sea salt aerosols. Short lived halocarbons such as bromoform and methyl iodide are emitted from the ocean by algae, seaweed and other organisms. 

In addition, Earth's oceans emit enormous amounts of sea salt aerosol. These aerosols are rich in halides (Cl⁻, Br⁻, I⁻), and chemical processing of the aerosols by ozone and other oxidants can oxidize the halides to volatile inorganic halogens that are rapidly released the gas phase (c.f. Artiglia et al., 2017). The level of ozone in the the lower troposphere has increased by about a factor of 2 due to anthropogenic activities. 

This research supported by the Carlsberg Foundation shows that man-made activities have indirectly (via ozone) increased the emission of volatile inorganic halogens from ocean and sea salt aerosols creating a negative feedback loop: Levels of tropospheric ozone increase, increasing emissions of halogens, that in turn suppress ozone via catalytic ozone destroying reactions (Sherwen et al, 2017).

Reduction in tropospheric ozone due to halogen chemistry. Figure taken from Sherwen et al (2016).

Halogens Remove Air Pollution

Stratospheric halogens contribute to the depletion of stratospheric ozone (“good ozone”, see Fact box 1), and as a result the dominant anthropogenic sources of stratospheric halogens have been banned by the Montreal protocol.
Fact box 2: Ozone in the Troposphere and Stratosphere

Ozone is also present in the troposphere - the lowest part of Earth's atmosphere. The concentration of ozone at Earth's surface, although much lower than in the stratosphere, can be high enough in some regions to have significant negative health and economic impact. Because of this, tropospheric ozone is also called the “bad ozone”. Human exposure to ozone is linked to premature death, asthma, bronchitis, and other illnesses. 

Ozone also causes agricultural damage, with crops such as cotton, soybeans, and peanut being especially susceptible. Increases of a few ppb ozone can lead to significantly lower crop yields. Tropospheric ozone is formed when volatile organic carbon is oxidized in the presence of NOx, and its production is there closely coupled to air pollution.

In the troposphere, halogens suppress ozone at ground level (“bad ozone”, see Fact box 2), via catalytic chemical cycles that convert ozone to molecular oxygen (O2) without consuming any halogens. 

Halogens also drive oxidation of toxic NOx (NO and NO2), by formation of halonitrates (e.g. BrONO2) that are rapidly consumed by rain, clouds, and others forms of water. NOx is a critical catalyst for ozone production in the troposphere.

Research supported by the Carlsberg Foundation has shown that halogens lower the level of tropospheric ozone by about 18 % and this suppression is dominated by reduced ozone production due to halogen enhanced NOx loss. Halogens decrease ozone at Earth's surface by up to ~10 ppb in some populated coastal environments (Sherwen et al., 2016). 

The halogen bromine also plays a critical role in the removal of mercury from the atmosphere as bromine atoms react rapidly with gaseous elemental mercury (Hg(0)) thereby initiating mercury oxidation and deposition. With these impacts in mind, tropospheric and stratospheric halogens can be described as “good”  and “bad” halogens, respectively.

Coupling to Sulfur Chemistry in the Marine Environment

Volatile halogens such as HOBr react with sulfites present in cloud droplets and aerosols producing sulfate and non-volatile halide ions. 

A recent study supported by the Carlsberg Foundation suggests that this coupling is an important sink of inorganic bromine in the lower troposphere. 

Simulations indicate that this newly proposed sink mechanism has a large impact on the budget of tropospheric bromine and helps reconcile the relatively low levels of inorganic bromine observed in many regions of the marine boundary layer (the lowest 0.5 to 2 km over the oceans).

“Sulfur-halogen interactions in clouds represent an important source of sulfate aerosol and an important sink of reactive halogen in the troposphere.” – Qianjie Chen, PhD candidate, University of Washington, USA.

Johan Albrecht Schmidt about his Career and his Grant from the Carlsberg Foundation

“The research I did with support from the Carlsberg Foundation with the Postdoctoral Fellowship indeed stimulated my interest in development of novel gas sensor technologies. Furthermore, I am now Project Manager and Head of Sensors at Airlabs. My current work involves developing new methods for monitoring air pollution in urban environments,” says Johan Albrecht Schmidt.

My References

[1] T. Sherwen, J. A. Schmidt, M. J. Evans, L. J. Carpenter, K. Großmann, S. D. Eastham, D. J. Jacob, B. Dix, T. K. Koenig, R. Sinreich, I. Ortega, R. Volkamer, A. Saiz-Lopez, C. Prados-Roman, A. S. Mahajan, C. Ordóñez: “Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem”, Atmospheric Chemistry and Physics, 16, 12239–12271, (2016).

[2] Q. Chen, J. A. Schmidt, V. Shah, L. Jaeglé, T. Sherwen, B. Alexander: “Sulfate production by reactive bromine: Implications for the global sulfur and reactive bromine budgets”, Geophysical Research Letters, 44, 7069–7078, (2017).

[3] T. Sherwen, M. J. Evans, L. J. Carpenter, J. A. Schmidt, L. J. Mickley: “Halogen chemistry reduces tropospheric O3 radiative forcing”, Atmospheric Chemistry and Physics, 17, 1557–1569, (2017).

Other References

[4] S. Wang, et al.: “Active and widespread halogen chemistry in the tropical and subtropical free troposphere”, Proceedings of the National Academy of Sciences of the USA, 112, 9281–9286, (2015).

[5] L. Artiglia, et al.: “A surface-stabilized ozonide triggers bromide oxidation at the aqueous solution-vapour interface”, Nature Communications, 8, 700, (2017).

[6] L. Geng, L. T. Murray, L. J. Mickley, P. Lin, Q. Fu, A. J. Schauer,  B. Alexander: “Isotopic evidence of multiple controls on atmospheric oxidants over climate transitions”, Nature, 546, 133–