Heterogeneous Chemistry of Surface Exchange

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The heterogeneous processes, catalysed by soluble and insoluble constituents of throughfall, stemflow and soil, leading to an uptake of NO2 and release of HONO, are investigated by employing the wetted-wall flowtube and bubble column methods and a novel photochemical and heterogeneous flowtube to model compounds and to real samples. The dependence of uptake rates on pH can clarify the contributions of aromatic (i.e. phenolic), olefinic, aliphatic and carboxylic structures to the uptake process. The atmospheric relevance can then be demonstrated by selected aerosol chamber experiments at high humidities, enhancing the aerosol surface by coating hydrophilic nanoparticles, made of fused silica, with throughfall and its typical constituents. Realistic NO2 levels are employed and monitored by a Luminol instrument. Formation of nitrous acid (HONO) traces can be detected indirectly by light-induced photochemistry of OH radicals and it will be detected directly by a novel long-path absorption photometer technique (LOPAP). The formation of phytotoxic, nitrated products can be shown by analysis of aerosol samples employing ion chromatography, GC-MS, HPLC-UV and FTIR. In close cooperation with the sub-projects P2 to P4, the rate constants of the relevant heterogeneous reactions are incorporated into the atmospheric chemistry of the trace gas module of the FLAME-SVAT-CN modelling cluster.