Tropospheric vertical column densities of NO.sub.2 over managed dryland ecosystems : MAX-DOAS measurements vs. 3-D dispersion model simulations based on laboratory-derived NO emission from soil samples

We report on MAX-DOAS observations of NO.sub.2 over an oasis-ecotone-desert ecosystem in NW China. There, local ambient NO.sub.2 concentrations originate from enhanced biogenic NO emission of intensively managed soils. Our target oasis "Milan" is located at the southern edge of the Taklimakan desert, very remote and well isolated from other potential anthropogenic and biogenic NO.sub.x sources. Four observation sites for MAX-DOAS measurements were selected, at the oasis centre, downwind and upwind of the oasis, and in the desert. Biogenic NO emissions in terms of (i) soil moisture and (ii) soil temperature of Milan oasis (iii) different land-cover type sub-units (cotton, Jujube trees, cotton/Jujube mixture, desert) were quantified by laboratory incubation of corresponding soil samples. Net potential NO fluxes were up-scaled to oasis scale by areal distribution and classification of land-cover types derived from satellite images using GIS techniques. A Lagrangian dispersion model (LASAT, Lagrangian Simulation of Aerosol Transport) was used to calculate the dispersion of soil emitted NO into the atmospheric boundary layer over Milan oasis. Three-dimensional (3-D) NO concentrations (30 m horizontal resolution) have been converted to 3-D NO.sub.2 concentrations, assuming photostationary state conditions. NO.sub.2 column densities were simulated by suitable vertical integration of modelled 3-D NO.sub.2 concentrations at those downwind and upwind locations, where the MAX-DOAS measurements were performed. Downwind-upwind differences (a direct measure of Milan oasis' contribution to the areal increase of ambient NO.sub.2 concentration) of measured and simulated slant (as well as vertical) NO.sub.2 column densities show excellent agreement. This agreement is considered as the first successful attempt to prove the validity of the chosen approach to up-scale laboratory-derived biogenic NO fluxes to ecosystem field conditions, i.e. from the spatial scale of a soil sample (cm.sup.2) to the size of an entire agricultural ecosystem (km.sup.2).