ATMOCHEM - PHOTOchemical Formation of Nitrous Acid in the Atmosphere (PHOTONA)
Zusammenfassung der Projektergebnisse
Nitrous Acid (HONO) has attracted significant attention since recent field measurements have demonstrated that the photolysis of HONO can be the dominant source of OH radicals in the lower atmosphere. The OH radical is responsible for the degradation of most air pollutants and for the formation of harmful photo-oxidants and thus, the identification and quantification of the sources of HONO are of major importance. Different photochemical daytime sources have been proposed, however, the exact origin and the magnitude of HONO fluxes over ground surfaces are still open questions. Thus, in the PHOTONA project, HONO sources were studied in the laboratory (WP1) and over ground surfaces in the atmosphere (WP2). In the laboratory studies, heterogeneous and gas phase photochemical sources of nitrous acid (HONO) were investigated. As a potential gas phase source, HONO formation by the photolysis of different organic nitrates was investigated, which could help to explain small daytime HONO gradients. However, only very low HONO formation was observed, which cannot explain HONO formation in the atmosphere. In addition, it turned out that even the small HONO yields were caused only by the heterogeneous photosensitized reaction of the by-product NO2, making the organic nitrate photolysis even less important. As a heterogeneous HONO source, the photolysis of adsorbed nitric acid (HNO3) was investigated in the laboratory. However, in contrast to other studies, very low HNO3 photolysis frequencies for HONO formation were observed, which also cannot explain HONO levels in the daytime atmosphere. A consecutive mechanism is proposed in which NO2 is photo-chemically formed from adsorbed HNO3 at longer wavelength than in the gas and liquid phases, which is converted into HONO by the dark reaction of NO2+H2O. Based on the WP1 and other laboratory studies, only significant ground surface sources of HONO have yet been identified. In the field studies (WP2), HONO fluxes were measured above an agricultural field site near Paris during different seasons and above different types of canopies using the aerodynamic gradient (AG) method. Two LOPAPs (LOng Path Absorption Photometer) to measure HONO in two heights were intercompared on the field and showed a good ability to determine HONO fluxes. A compensation point for HONO of ~15 pptV was estimated, which indicates that HONO deposition was of minor importance during daytime. Correlation studies of the diurnal HONO flux showed strong connections with NO2 mixing ratios and long wave radiation (J(NO2)), in line with laboratory studies on the photosensitized conversion of NO2 into HONO on humic acid surfaces. This result is in excellent agreement with the laboratory results from WP1, in which the photolysis of adsorbed HNO3 – as proposed in other studies – was excluded as an important ground surface source of HONO. The comparison of all performed campaigns uncovered an additional influence of the soil temperature on the HONO flux, which can be explained by the temperature dependence of the HONO adsorption on ground surfaces. Based on the experimental data, a simple parameterization of the HONO flux at this location was derived, which can reasonably well explain all field observations and which can easily be used in models to better describe HONO ground sources in the atmosphere.
Projektbezogene Publikationen (Auswahl)
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Comparison of methods for the determination of NO-O3-NO2 fluxes and chemical interactions over a bare soil. Atmospheric Measurement and Techniques, 5, 1241-1257, 2012
Stella, P., Loubet, B., Laville, P., Lamaud, E., Cazaunau, M., Laufs, S., Bernard, F., Grosselin, B., Mascher, N., Kurtenbach, R., Mellouki, A., Kleffmann, J. and Cellier, P.
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Fluxes of Nitrous Acid (HONO) above an Agricultural Field Site near Paris. 12th IGAC Open Science Conference, "Atmospheric Chemistry in the Anthropocene", Beijing, China, 2012
Laufs S., Barnes I., Cazaunau M., Stella P., Loubet B., Bernard F., Grosselin B., Kurtenbach R., Cellier P., Mellouki W. and Kleffmann J.
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Fluxes of Nitrous Acid (HONO) above an Agricultural Field Site near Paris. Geophysical Research Abstracts., Vol. 14, EGU2012- 14471-1, 2012. EGU General Assembly, 2012
Laufs S., Cazaunau M., Stella P., Loubet B., Bernard F., Grosselin B., Kurtenbach R., Cellier P., Mellouki W. and Kleffmann J.
