Atmospheric Chemistry of Reactive Halogen Compounds

Reactive halogen species including chlorine, bromine and iodine radicals play a pivotal role in atmospheric processes from the marine boundary layer to urban pollution. These radicals catalyse ozone destruction cycles and modulate the oxidising capacity of the troposphere by interacting with nitrogen oxides, volatile organic compounds and other oxidants such as hydroxyl radicals. Sources encompass sea‐salt aerosol reactions, biomass burning, fossil fuel combustion and secondary formation via heterogeneous hydrolysis of dinitrogen pentoxide on particle surfaces. The balance between gas‐phase photochemistry and multiphase processes within cloud droplets and aerosol influences regional air quality, the lifetime of greenhouse gases and particulate formation. Recent advances in measurement techniques and modelling have illuminated the complexity of interfacial mechanisms and the global distribution of halogen‐mediated oxidation pathways, underscoring their significance for climate feedbacks and pollution‐control strategies.

Research from Nature Portfolio

Recent studies have employed advanced quantum‐chemical methods to unravel the molecular details of dinitrogen pentoxide reactions in microdroplet environments. High‐level calculations have quantified both bulk and interfacial hydrolysis rates and shown that ammonolysis at aqueous surfaces can dominate under elevated ammonia concentrations. These findings have led to a new parameterisation of heterogeneous N₂O₅ uptake, offering improved mechanistic representation in regional and global chemistry models. The enhanced description of interfacial processes refines predictions of nocturnal nitrogen‐oxide removal, particulate nitrate formation and chlorine activation in aerosol‐rich settings.

Atmospheric Chemistry of Reactive Halogen Compounds publication trend

The graph below shows the total number of articles in atmospheric chemistry of reactive halogen compounds across all publications each year (not limited to Nature Index journals).

Technical terms

Halogen radical: A highly reactive atom or molecule of chlorine, bromine or iodine with an unpaired electron that initiates oxidative cycles in the atmosphere.

Dinitrogen pentoxide (N₂O₅): A nocturnal reservoir species formed from nitrogen dioxide and ozone, which undergoes heterogeneous hydrolysis or ammonolysis on aerosol surfaces.

Hydrolysis: A multiphase reaction in which N₂O₅ reacts with water to form nitric acid on or within aqueous particles.

Ammonolysis: A surface reaction in which N₂O₅ reacts with ammonia to produce chloride‐containing or nitrogen‐containing products, competing with hydrolysis.

Heterogeneous uptake: The process by which gas‐phase species are transferred to and react on solid or liquid aerosol surfaces.

Nitryl chloride (ClNO₂): A chlorine‐containing reservoir formed from N₂O₅ and particulate chloride that photolyzes to release chlorine radicals at sunrise.

Aerosol interface: The boundary region of a particle where gas‐phase species interact with the condensed phase, often exhibiting distinct kinetics from bulk solutions.

Reactive uptake coefficient (γ): A dimensionless parameter quantifying the probability that a gas‐phase molecule will react upon colliding with an aerosol surface.

References

1. Mechanistic insight into the competition between interfacial and bulk reactions in microdroplets through N2O5 ammonolysis and hydrolysis. Nature Communications (2024).
2. Significant chlorine emissions from biomass burning affect the long-term atmospheric chemistry in Asia. National Science Review (2024).
3. The role of chlorine in global tropospheric chemistry. Atmospheric Chemistry and Physics (2019).
4. Significant concentrations of nitryl chloride sustained in the morning: investigations of the causes and impacts on ozone production in a polluted region of northern China. Atmospheric Chemistry and Physics (2016).