Nitrous acid (HONO) source and sink processes in a disturbed forest ecosystem - interactions of heterogeneous chemistry and turbulent transport
Final Report Abstract
In this project, HONO source and sink processes were quantified based on comprehensive experimental data sets obtained in various field experiments and additional laboratory studies. In the field, vertical profile measurements of HONO, NO, NO2, and O3 were made in the lowest 2 m of the atmosphere above a forest clear-cut and in the trunk space of a Norway spruce forest. These data contributed to an improved interpretation of horizontal and vertical transport of reactive nitrogen species and ozone in the framework of the EGER field measurements 2011. Net HONO fluxes at forest floors were measured for the first time in this project. In general, the forest floor is mainly a net sink for HONO, while the clearcut acts as a net sink for HONO at night, and as a net source during the day. When investigating HONO soil emissions, the measured potential soil emission fluxes were several orders of magnitude smaller than fluxes calculated from a model based on soil nitrite concentrations and soil pH. The parameterization of the vertical transport, i.e. the transfer velocity, was identified as a key factor for this large discrepancy. With respect to chemical formation of HONO from NO2, the light-induced conversion of NO2 on organic surfaces was found to be independent of light for most of the day, contrary to our expectation. The calculations suggest that this may be a general feature when NO2 mixing ratios are in the order of 1 ppb or below. HONO formation from photolysis of adsorbed nitrate was estimated to be of the same order of magnitude as estimated HONO fluxes when using empirical parameterizations. Overall, HONO photolysis was found to be the main sink during daytime. In addition to these comprehensive measurements in a disturbed forest ecosystem, a unique data set for high-alpine reactive nitrogen studies was obtained from HONO measurements at Mount Zugspitze. In this environment, instead of plant surface-related HONO sources, which are probably important in forest ecosystems, atmospheric volume sources are expected to be important. Also, in a laboratory study the light-dependent heterogeneous conversion of NO2 was found to be a potentially important indoor HONO formation process with a strong humidity dependence. For our atmospheric measurements, the dependence of HONO formation and removal processes on humidity conditions was investigated under dry conditions, wet conditions, and in a transition regime. As expected, we have observed a slightly positive correlation of HONO mixing ratios and relative humidity under dry conditions after removing long-term trends and the diurnal cycle by Singular Spectrum Analysis. While there were no obvious correlations in the transition regime, the observed drop of HONO mixing rations at relative humidities above 90 % is consistent with uptake of HONO in liquid water films under wet conditions. Direct measurements of the leaf surface wetness with an updated sensor system provided valuable data sets, which were used to successfully parameterize the formation of liquid films on vegetation surfaces in an atmosphere-vegetation model. Overall, the project successfully quantified several key HONO source and sink processes in a disturbed forest ecosystem, which helps to constrain the atmospheric reactive nitrogen budget. Also, some important new findings, e.g. on the light-dependent formation of HONO, will help to better understand changes in atmospheric HONO mixing ratios, the role of HONO in the reactive nitrogen cycle, and its potential contribution to the atmospheric oxidative capacity.
Publications
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(2011) Interactions of heterogeneous chemistry and turbulent transport of nitrous acid in a disturbed forest ecosystem. iLEAPS Science Conference 2011, Garmisch-Partenkirchen, Germany
Held, A., Sörgel, M., Trebs, I., Serafimovich, A., Zetzsch, C.
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(2011) Near surface profiles of HONO, NOx, O3 and their relation to surface wetness. iLEAPS Science Conference 2011, Garmisch-Partenkirchen, Germany
Sörgel, M., Held, A.
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(2012) Daytime formation of HONO measured close to the surface: Indication for a light driven process? AGU Fall Meeting 2012, San Francisco, USA
Sörgel, M., Held, A.
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(2014) Formation of indoor nitrous acid (HONO) by light-induced NO2 heterogeneous reactions with white wall paint. Environ. Sci. Pollut. Res. 21, 9259-9269
Bartolomei, V., Sörgel, M., Gligorovski, S., Gomez Alvarez, E., Gandolfo, A., Strekowski, R., Quivet, E., Held, A., Zetzsch, C., Wortham, H.
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A comparison of measured HONO uptake and release with calculated source strengths in a heterogeneous forest environment. Atmospheric Chemistry and Physics, 15,16, 9237-9251, 2015
Sörgel, M., Trebs, I., Wu, D., Held, A.
