Heterogeneous Chemistry of Surface Exchange
Zusammenfassung der Projektergebnisse
The new Long-Path Absorption Photometer (LOPAP) granted by the DFG for this project has opened new capabilities for the Atmospheric Chemistry Research Laboratory for the detection and reliable determination of gaseous nitric acid (HONO) in air at the ppt level in field campaigns and laboratory experiments. Two laboratory systems for the investigation of heterogeneous and photochemical heterogeneous reactions (a wettedwall and a coated-wall flow reactor) have been developed, installed and tested. The wetted-wall flow reactor has been used to determine the rate constant for the reaction of NO2 with catechol in alkaline, aqueous solution to form nitrite and the o-hydroxyphenoxy radical. No release of HONO was detected by the LOPAP at alkaline pH (as expected), and rapid uptake of interfering HONO traces from side reactions in the Teflon tubing by the liquid film within a few cm of contact was observed even at acidic pH. The photochemical coated-wall flow reactor has been tested by exposing a thin film of commercial humic substance (Fluka/Aldrich) to traces of NO2 (10 ppb) in the absence and presence of simulated sunlight. Switching on the lamps released a significant pulse of HONO to the LOPAP. A quantitative evaluation of this alternative formation route of HONO is underway. This system must be further optimized, and problems with NO2 measurements by the luminol instrument (LMA-3, Scintrex) have to be solved. After preparation, optimization and stability checks of the LOPAP instrument in the laboratory for the field campaigns IOP-1 and IOP-2, calibration was performed repeatedly in the field. Responsibility for another LOPAP instrument from the Max-Planck-Institute for Chemistry was shared with a co-worker from the Meixner group during IOP-1, and complete responsibility for both instruments was taken in IOP-2. In an intercomparison campaign these instruments were found to agree within less than 12% deviation (running side-by-side), and detection limits of 1-3 ppt were reached. In total (IOP-1 and IOP-2) about two and a half month of highly time resolved (~10 min) HONO mixing ratios are available. About one week of these deliver data at two different heights (forest floor and above canopy) for the first time. Evaluation of the data from field observation requires careful inspection of the details of various parameters synchronously and separating time periods into different meteorological and air pollution conditions. Comparing the HONO data with meteorological and chemical data obtained by the other groups leads to some preliminary conclusions: There is no significant dependence of HONO on polluting NO2 levels under the conditions we measured, except some single pollution events (advection at night). Coupling regimes which determine the exchange processes between the forest and the atmosphere by coherent structures have a clear impact on HONO distribution. Decoupling of the sub canopy leads to accumulation of HONO in the trunk space. Coupling of the whole forest to the atmosphere, whether by fully coupled canopy or coupled sub canopy by sweeps leads to equal mixing ratios (within the instruments errors) at both measurement heights. The surprising outcome that besides the 10 to 25 times higher photolysis frequency (sink) for HONO above canopy compared to the trunk space the concentrations are almost equal, points to the importance of turbulent transport. A transport time of less than 7 min is more likely to balance the concentrations than a photochemical source. To sustain the total amount of measured (50-120 ppt during daytime) HONO a photochemical source is still needed (dilution and photolysis would result in much faster loss), but cannot be quantified due to the lack of OH-Radical measurements. Therefore a detailed canopy exchange model would be needed to divide the single processes influencing mixing ratios in and above the forest. During day photolysis, dilution by up-mixing and deposition contribute to the sink terms whereas formation by the reaction of OH+NO => HONO, photochemical sources of HONO, “storage” in shaded areas and possibly reemission contribute to the source terms. From our results we conclude that at least the deposition and reemission are controlled by surface wetness which is related to relative humidity.
Projektbezogene Publikationen (Auswahl)
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2008. HONO measurements in and above a tall spruce forest canopy. EUROCHAMP Workshop: Nitrous Acid: Tropospheric Chemistry, Measurement Methods, and Future Directions, Wuppertal, March 3-6, 2008
Sörgel, M., Zetzsch, C., Wolff, V., Moravek, A., Hens, K., Meixner, F.X., Trebs, I.
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2008. The EGER 2007 Micrometeorological Experiment in the Fichtelgebirge Mountains, Germany. 28th Conference on Agricultural and Forest Meteorology, Orlando, United States of America: 28.04.2008 - 30.04.2008
Staudt, K, Siebicke, L, Serafimovich, A, Foken, T, Pyles, RD, Meixner, FX, Falge, E, Zetzsch, C
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2009. Simultaneous measurements of HONO below and above a spruce forest canopy. European Geosciences Union General Assembly 2009, Vienna: 19.04.2009 - 24.04.2009, Geophysical Research Abstracts, 11, EGU2009-10191
Sörgel, M., Trebs, I., Moravek, A., Hens, K., Zetzsch, C.
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Effects of coupling regimes and humidity on HONO concentrations in and above a spruce forest. Atmospheric Transport and Chemistry in Forest Ecosystems, Castle of Thurnau, Germany, Oct 5 - 8, 2009
Sörgel, M., Serafimovich, A., Moravek, A., Trebs, I., Meixner, F.X., Zetzsch, C.
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Statistical analysis of HONO and humidity time series measured in the EGER-Projekt. Atmospheric Transport and Chemistry; Castle of Thurnau, Germany, Oct 5 - 8, 2009
Sörgel, M; Trebs,I; Moravek, A; Meixner, F. X.; Zetzsch, C.
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Concentration differences of HONO measured below and above a tall spruce forest canopy: Photolytic lifetimes and exchange regimes. Workshop on Atmospheric Chemistry: kinetics and spectroscopy, 24-26 February 2010, Bayreuth, Germany
Sörgel, M., Serafimovich, A., Moravek, A., Trebs, I., Meixner, F. X., Zetzsch, C.
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Effects of coupling and relative humidity on HONO mixing ratios in a spruce forest. ICACGP-IGAC 2010 Conference; Halifax, Canada, July 11- 16, 2010
Sörgel, M., Trebs, I., Serafimovich, A., Moravek, A., Meixner, F.X., Zetzsch, C.
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What determines the HONO daytime source? First results from field measurements in south west Spain. European Geosciences Union General Assembly 2010, Vienna, Austria, 02 – 07 May 2010
Sörgel, M., Hosaynali Beygi, Z., Regelin, E., Bozem, H., Adame, J. A., Harder, H., Martinez-Harder, M., Held, A., Zetzsch, C.
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2011. Simultaneous HONO measurements in and above a forest canopy: influence of turbulent exchange on mixing ratio differences. Atmos. Chem. Phys., 11, 841-855
Sörgel, M., Trebs, I., Serafimovich, A., Moravek, A., Held, A., Zetzsch, C.
