Heterogeneous Chemistry of Atmospheric Nitrous Acid

Heterogeneous chemistry of atmospheric nitrous acid (HONO) encompasses the set of processes occurring at the interface of gas phase and condensed matter—surfaces, aerosols and ground substrates—that govern the formation, redistribution and photolysis of this trace gas. HONO acts as a primary source of the hydroxyl radical (OH) upon photolysis, driving the oxidative capacity of the troposphere and influencing secondary pollutant formation such as ozone and particulate nitrate. Generating significant HONO levels at night via NO₂ conversion on surfaces, and releasing HONO during the day through photolytic or renoxification pathways, alters diurnal patterns of oxidants. Recent work has elucidated factors that modulate these processes, including relative humidity, surface composition and solar irradiance. The interplay of surface-mediated production, aerosol photochemistry and gas–surface exchange underpins gaps in models that formerly underestimated early morning OH and NOₓ cycling, with ramifications for urban air quality and regional climate forcing.

Research from Nature Portfolio

Recent studies have confirmed that nitrous acid functions as an intermediate in an external NOₓ cycling pathway that regenerates active nitrogen oxides from their oxidised reservoir. Field measurements onboard airborne platforms have demonstrated daytime HONO production in low-NOₓ regions, reconciling persistent discrepancies between observed and modelled HONO/NO₂ ratios and atypical diurnal profiles lacking a midday trough. This mechanism of external cycling amplifies OH radical production by up to forty per cent in dilute NOₓ conditions, highlighting a missing contribution in chemical transport models and emphasising the need to include surface-mediated and photolytic feedbacks in global NOₓ budgets.

Heterogeneous Chemistry of Atmospheric Nitrous Acid publication trend

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

Technical terms

Heterogeneous chemistry: Chemical processes occurring at interfaces between gas and condensed phases, such as on aerosol particles or surfaces.

Renoxification: Photochemical recycling of oxidised nitrogen species back to reactive NOₓ via intermediate compounds like HONO.

Photolysis: Breakdown of a chemical compound by solar radiation, leading to formation of radicals such as OH.

Hydroxyl radical (OH): A highly reactive oxygen radical generated predominantly by water-vapour and HONO photolysis, crucial for atmospheric oxidation.

NOₓ: Collective term for nitrogen oxides, NO and NO₂, which participate in ozone formation and radical chemistry.

Photostationary state: A dynamic equilibrium where concentrations of photochemically active species are balanced by their formation and loss rates under steady illumination.

References

1. Synthesizing evidence for the external cycling of NOx in high- to low-NOx atmospheres. Nature Communications (2023).
2. Extensive field evidence for the release of HONO from the photolysis of nitrate aerosols. Science Advances (2023).
3. Atmospheric NOx oxidation as major sources for nitrous acid (HONO). npj Climate and Atmospheric Science (2023).