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Nouvelles sources hétérogènes de radicaux atmosphériques – Neo-Rad

Submission summary

In recent decades, air pollution has become one of the most important problems of megacities. Therefore tackling the issue of the air pollution is not new but it is also not solved! Open issues are linked to the transformation, induced by radicals, of a large panel of volatile organic compounds (so called VOCs) turning them into harmful/harmless products (such as ozone) or secondary organic aerosols (so called SOA). There are on-going efforts to characterise these issues and associated impacts (both in terms of health and air quality). Trying to understand the mechanisms of these transformations and to simulate the levels of radicals requires an in-depth understanding of the radicals interconversion mechanisms and their sources and sinks. However, significant discrepancy has been observed for the variability and absolute concentrations of the hydroxyl radical (OH), arguably the most important oxidizer in the atmosphere. More precisely, while the measurement/model agreement concerning OH radicals is roughly acceptable for sub-urban exercises, it is not satisfactory for city center situations where a significant underestimation of the calculated radical concentrations at high NOx was found. The missing source of the OH radicals was tentatively assumed to be the photolysis of nitrous acid, HONO, formed by heterogeneous or liquid phase transformation of NO2 and emitted to the atmosphere. Recently, the importance of the heterogeneously formed HONO emissions as a source of OH radicals has been confirmed by several studies aimed at a better characterization of the mechanism of HONO formation. This new HONO and hence OH radical source involve the photoconversion of NO2 on solid organic coated surfaces… that appear to be ubiquitous! Indeed urban surfaces are complex with a high organic content from various sources (combustion producing complex aromatics such as PAHs, car exhaust, solvents, degradation of waste, deposition of VOCs from biogenic origin, …). Accordingly, the presence of these urban surfaces with an organic coverage may represent an important source of nitrous acid and other species in the urban environment and may have a strong impact on the radicals concentrations and consequently on the building up ozone pollution and other air quality issues. Indeed urban surfaces are still considered mainly as sink for species from the gas phase (deposition). But, the recent recognition of such a new heterogeneous source for HONO therefore also shines a new light on the role of surfaces in affecting the urban air chemistry. As mentioned above, recent advances in our knowledge are finally hints that these surfaces are quite reactive with potential feedbacks on the urban level of radicals (and associated impacts on the air we breathe). Obviously there is a gap to fill for a better understanding of the radical formation in the urban atmosphere. Radicals, because of their short lifetime, are probably the most important species that need to be monitored in the urban environment in order to understand the impact of heterogeneous surface reactions converting gas phase species. Since radicals are the key species of the photo-oxidation cycles, which form ozone, oxidize nitrogen oxide, produce strong acids and turn primary volatile organic compounds into secondary species and aerosols (which are toxic and often also carcinogens), this subject is of utmost importance for air quality in the urban environment. The goal of this project is thus to identify, evaluate and monitor new heterogeneous sources of radicals in the urban atmosphere. The overall outcome of this project is expected to be of crucial importance for assessing the radicals budget in urban environments, which is a prerequisite for any further air quality issue. Accordingly, this project will bring to the community a better understanding of the radical sources in the planetary boundary layer in order to better assess the oxidizing capacity of the troposphere in the level where people live and will therefore fill a gap in our current knowledge. A scientific consortium composed of 6 research groups from 4 different laboratories was constituted to achieve this goal through a multidisciplinary approach based on laboratory activities (WP1), simulation chambers experiments (WP2) and modelling activities (WP3). Indeed, the whole project was designed around the strong interactions between the combined skills and knowledge and the consequent added value therefore should comprise not only (i) a detailed understanding of the studied processes, but also (ii) a validated operational parameterization of these processes, and finally (iii) the updated chemical scheme to assess the radical budget and hence estimate the impact of the new heterogeous sources of radicals.

Project coordination


The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.



Help of the ANR 400,000 euros
Beginning and duration of the scientific project: - 48 Months

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