Abstract
Abstract. We report on MAX-DOAS observations of NO2 over an oasis–ecotone–desert ecosystem in NW China. There, local ambient NO2 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 NOx 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 NO2 concentrations, assuming photostationary state conditions. NO2 column densities were simulated by suitable vertical integration of modelled 3-D NO2 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 NO2 concentration) of measured and simulated slant (as well as vertical) NO2 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 (cm2) to the size of an entire agricultural ecosystem (km2).
Highlights
Emissions of nitric oxide (NO) are important in regulating chemical processes of the atmosphere (Crutzen, 1987)
Ambient NOx is a key catalyst in atmospheric chemistry: during the atmospheric oxidation of hydrocarbons its ambient concentration determines whether ozone (O3) is photochemically generated or destroyed in the troposphere (Chameides et al, 1992)
Milan oasis can be geomorphologically classified as a “mountain–oasis–ecotone–desert system (MOED system)” consisting of Gobi desert, a salty transition zone surrounding the oasis, and dryland farming with irrigation
Summary
Emissions of nitric oxide (NO) are important in regulating chemical processes of the atmosphere (Crutzen, 1987). 25 Tg a−1 in terms of mass of N), biogenic NO emissions from soils have been estimated to range between 6.6 and 9.6 Tg a−1 (Denman et al, 2007). The considerable uncertainty about the range of soil biogenic NO emissions stems from widely differing estimates of the NO emission. The uncertainties in the NO emission data from semi-arid, arid and hyper-arid regions are very large (mainly due to a very small number of measurements being available). These ecosystems, are considered to contribute more than half to the global soil NO source (Davidson and Kingerlee, 1997), and make up approx. These ecosystems, are considered to contribute more than half to the global soil NO source (Davidson and Kingerlee, 1997), and make up approx. 40 % of planet Earth’s total land surface (Harrison and Pearce, 2000)
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