Bromine cycle

Bromine in the atmosphere

Bromine is released into the atmosphere through a variety of sources. Once bromine is in a gaseous state in the atmosphere, photolysis and chemical reactions can break apart compounds containing bromine or form new bromine containing compounds. Bromine can quickly cycle between gas and particle phase throughout the atmosphere. Due to the quick cycling the main loss mechanisms include dry and wet deposition. Deposition allows bromine to be taken out of the atmosphere and move to the lithosphere and hydrosphere.

Background concentrations of bromine containing compounds CHBr₃ and CH₂Br₂ are 1-2 ppt.

Tropospheric O3 depletion

Bromine is important to the ozone balance of the atmosphere. Bromine can react with ozone to produce BrO and O₂. BrO has the ability to photolyze back to Br and O₃ or BrO can react to HO₂ to form HOBr. HOBr readily cycles to the aqueous phase. In high NO_x areas, BrO can react with NO to produce Br and NO₂. This reaction releases Br back into the atmosphere where it can continue to destroy ozone. These reactions rely on sunlight, so ozone is depleted at a greater rate in the summer months.

$${\displaystyle \text{Br}+{\text{O}}_3^{}⟶\text{BrO}+{\text{O}}_2^{}}$$$${\displaystyle \text{BrO}+\text{hv}\stackrel{O2}{\to }\text{Br}+{\text{O}}_3^{}}$$$${\displaystyle \text{BrO}+{\text{HO}}_2^{}⟶\text{HBrO}+{\text{O}}_2^{}}$$$${\displaystyle \text{BrO}+\text{NO}⟶\text{Br}+{\text{NO}}_2^{}}$$

The bromine explosion reaction seen below is of concern as two Br radicals are produced through these reactions from one starting Br radical. This increases the amount of Br available to react with ozone. This reaction converts HO_x to BrO_x.

$${\displaystyle \text{Br}+{\text{HO}}_2^{}+\text{Br}-/\text{HBr}(\text{surface})+\text{hv}⟶2\text{Br}+\text{OH}-/{\text{H}}_2^{}\text{O}}$$

Bromine cycling and interactions with ozone are dependent on VOC and NO_x concentrations. Research and models suggest bromine contributes to 5-15% of tropospheric ozone layer losses. These reactions deplete the ozone within the atmosphere, as well as alter the oxidation potential of atmosphere.

Stratospheric O3 depletion

Winter sea ice is a significant atmospheric contribution of bromine, especially as polar regions were a significant location to understand halogen influence in the troposphere. Organic bromine gases such as CH₃Br, CH₂Br₂, CH₂IBr are emitted by microorganisms in sea ice and snow at ten-fold higher rates than from other environments. Additionally, these gases are a source of bromine in the stratosphere. In polar areas, decreasing sea ice due to melting releases bromine and at the Arctic and Antarctic boundary layer. Polar Stratospheric Clouds (PSC) also play a role in the availability of bromine in the stratosphere. PSCs can act as surfaces for reactions to occur on, allowing for bromine to become activated Br radicals, which are reactive with other compounds, including ozone. Br radicals in the stratosphere react with ozone to form BrO, which can further react with HO_x, ClO_x, or O, destroying stratospheric ozone.