Heterogeneous Reactions in Atmospheric Chemistry

Heterogeneous reactions in the atmosphere refer to chemical processes that occur at the interfaces between different phases, most commonly between gas‐phase species and solid or liquid aerosol particles. These reactions can fundamentally alter the fate of reactive radicals, trace gases and pollutants, thereby modulating key atmospheric processes such as ozone formation, radical cycling and secondary aerosol growth. In contrast to purely gas‐phase chemistry, heterogeneous pathways often involve reactive uptake, in which gas‐phase species are absorbed or adsorbed onto particle surfaces and subsequently transformed. The efficiency of these processes depends on particle composition, phase state, relative humidity and the presence of transition metals or organic coatings. Globally, heterogeneous reactions influence urban air quality, regional smog episodes and the oxidising capacity of remote tropospheric regions. For instance, uptake of hydroperoxyl radicals onto aerosol can suppress ozone formation in densely polluted regions, while metal‐catalysed conversions within aqueous aerosol phases can accelerate radical loss and shift greenhouse‐gas lifetimes. Improved understanding of these multiphase processes is essential for refining chemical transport models, guiding emission policies and predicting the interplay between air quality and climate forcing.

Research from Nature Portfolio

Recent studies have revealed a previously unrecognised photochemical regime in which aerosol‐mediated uptake of hydroperoxyl radicals dominates ozone chain termination. Modelling results indicate that, as aerosol loading has increased over recent decades, this regime has expanded from a few regions in the 1970s to widespread occurrence across Asia today. The consequence is a systematic suppression of surface ozone wherever aerosol‐inhibited chemistry prevails, highlighting a nuanced trade-off between particulate pollution control and potential ozone increases if precursor emissions are not simultaneously addressed.

Heterogeneous Reactions in Atmospheric Chemistry publication trend

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

Technical terms

Heterogeneous reactions: Chemical transformations that occur at the interface between gas-phase species and solid or liquid particles.

Hydroperoxyl radical (HO2): A key atmospheric radical involved in ozone formation and radical propagation, often subject to uptake by aerosol surfaces.

Reactive uptake coefficient (γ): A dimensionless parameter quantifying the probability that a gas-phase molecule, upon colliding with a particle surface, will be taken up and undergo reaction.

Aerosol-inhibited regime: A photochemical environment in which radical uptake by particulate matter dominates the termination of ozone-forming chain reactions.

Oxidising capacity: The ability of the atmosphere to cleanse itself through reactions driven by radicals such as OH and HO2, which determines the lifetime of pollutants and greenhouse gases.

References

1. Suppression of surface ozone by an aerosol-inhibited photochemical ozone regime. Nature Geoscience (2022).
2. Kinetics and impacting factors of HO2 uptake onto submicron atmospheric aerosols during the 2019 Air QUAlity Study (AQUAS) in Yokohama, Japan. Atmospheric Chemistry and Physics (2021).
3. The number fraction of iron-containing particles affects OH, HO2 and H2O2 budgets in the atmospheric aqueous phase. Atmospheric Chemistry and Physics (2022).