Biogenic emission of nitric oxide (NO) from soils as a strong chemical sink for near-surface ozone: some comments on history, background and recent findings

In the dry season of 1992, measurements in the Kalahari region (southern Africa, SAFARI'92), occasionally revealed extremely low surface concentrations of ozone (O₃) in a fully developed very strong and shallow nocturnal boundary layer. That observation has raised reasonable suspicion that these extremely low concentrations are due to more than dry deposition of ozone to the bare soil surface. A long series (1993-2011) of similar field measurements in Africa, Brazil, Germany, and China followed with quite similar results, leading to the hypothesis, that the necessary sink of O₃ is just the near-surface titration of ozone by nitric oxide (NO), which is biogenic emitted from microbes in the top soil layer.

The first evidence for this O₃ sink on a spatially large scale was published by Gao, Desjardins, McPherson, and Schuepp (1995a,b) for the California Ozone Deposition Experiment (CODE) in the San Joaquin Valley of California during July and August 1991. Aircraft eddy covariance O₃ fluxes (5 km averages), obtained by one of Desjardins' famous Twin Otter flight missions, were shown to correlate very well with the surface greenness index as long as only surfaces with active vegetation have been considered (dominating O₃ stomatal uptake). However, over-proportional O₃ fluxes have been found for recently cultivated bare soil, newly cut hay, cities and near highways (strong biogenic and anthropogenic surface emissions of NO).

Meanwhile, there is much more knowledge about the nature and amount of biogenic NO soil emissions -  from mechanistic, laboratory, and field experiments, as well as from mechanistic, local, regional and global modelling. Furthermore, about a decade, concurrent measurements of NO-NO₂-O₃ concentrations and fluxes in the field (Africa (savannah), Brazil (primary rainforest), Germany (agricultural plots, forests), and China (oasis agriculture)) are available to study the strong interaction between turbulent transport, biological uptake, and biogenic soil emissions within the surface–atmosphere exchange of the NO-NO₂-O₃ triad.

We will report on experimental findings of field measurements with respect to the suspected near-surface chemical O₃ sink, as well as on the mechanistic background of the ubiquitous NO emission from soils. First attempts to model the turbulence-chemistry driven surface exchange of the NO-NO₂-O₃ triad will also be discussed.