Moving Platforms for Micrometeorological and Trace Gas Profiling Very Close to the Soil-Air Interface

While transport of trace gases in the soil is due to molecular diffusion (10-6 < Dsoil < 10-5 m2s-1), turbulent diffusion (0.1 < KZ < 1 m2s-1) is overwhelming in the adjacent atmospheric sur-face layer. Therefore, the soil-air interface is characterized by a dramatic change in gaseous diffusion and consequently of residence times: near-surface fluxes of (particularly) reactive trace gases become prone of (strong) vertical gradients, particularly if characteristic times of transport are (much) longer than those of (photo-)chemical reactions. The most prominent candidate for such a chemistry-turbulence interaction is the NO–NO2–O3 triad.

The challenge for any kind of profile and/or flux measurements within the first tens of centi-metres above the soil surface is, first of all, the sheer size of sensors and platform structures: a vertically dense array of sensors is out of question at least due to unavoidable flow distortion. Furthermore, application of a series of individual (reactive) trace gas analyzers along a verti-cal profile is much too costly and the quality of current analyzers will not provide the neces-sary accuracy to identify significant vertical concentration gradients. Therefore, application of a single meteorological/trace gas sensor (or one single combination of sensors) on a moving platform becomes a convincing alternative (however, to the cost of non-simultaneous profile measurements).

We designed a custom-built moving platform with a combination of meteorological sensors and a trace gas intake tube in two different variants: (a) the carrier is moved by a toothed drive belt along a vertical mast through a programmable step motor (max. vertical resolution: 103 steps/mm), and (b) sensors and intake tube are mounted at both ends of a rod which ro-tates from its initially vertical position by 180° through a programmable step motor (max. angular resolution: 3x103 steps/deg). In both cases, the allocation of observed signals of sen-sors and analyzers to a defined height (above ground) is complex due to the temporal response of sensors (analyzers) and the linear or circular movement of the carrier. In an earlier study (Mayer et al., 2009) we have developed a simple algorithm to unravel these signals for their unique allocation to height – provided that response times of sensors and analyzers are known and carefully characterized. In case of very long response times (few minutes; NO, NO2 ana-lyzers) we had to deviate from the preferred continuous to stepwise moving of the carrier.

We will report on results of (a) laboratory experiments on sensor and analyzer response times, (b) simulations concerning the effect of different speeds of movement (continuous and step-wise) on potential vertical resolution of meteorological (air temperature, relative humidity, wind speed) and trace gas (NO, NO2, O3, CO2, H2O) quantities, and (c) first field studies in arid (Milan Oasis, Xinjiang, P.R. China) and temperate climates (Mainz-Finthen, Germany).

Mayer, J. C., Hens, K, Rummel, U., Meixner, F.X. and Foken, T.(2009): Moving measurement platforms – spe-cific challenges and corrections, Meteorologische Zeitschrift, Vol. 18, No. 5, p. 477-488.