Abstract
Abstract. We present a dynamic twin-cuvette system for quantifying the trace-gas exchange fluxes between plants and the atmosphere under controlled temperature, light, and humidity conditions. Compared with a single-cuvette system, the twin-cuvette system is insensitive to disturbing background effects such as wall deposition. In combination with a climate chamber, we can perform flux measurements under constant and controllable environmental conditions. With an Automatic Temperature Regulated Air Humidification System (ATRAHS), we are able to regulate the relative humidity inside both cuvettes between 40 and 90 % with a high precision of 0.3 %. Thus, we could demonstrate that for a cuvette system operated with a high flow rate (> 20 L min−1), a temperature-regulated humidification system such as ATRAHS is an accurate method for air humidification of the flushing air. Furthermore, the fully automatic progressive fill-up of ATRAHS based on a floating valve improved the performance of the entire measurement system and prevented data gaps. Two reactive gas species, ozone (O3) and peroxyacetyl nitrate (PAN), were used to demonstrate the quality and performance of the twin-cuvette system. O3 and PAN exchange with Quercus ilex was investigated over a 14 day measurement period under controlled climate chamber conditions. By using O3 mixing ratios between 32 and 105 ppb and PAN mixing ratios between 100 and 350 ppt, a linear dependency of the O3 flux as well as the PAN flux in relation to its ambient mixing ratio could be observed. At relative humidity (RH) of 40 %, the deposition velocity ratio of O3 and PAN was determined to be 0.45. At that humidity, the deposition of O3 to the plant leaves was found to be only controlled by the leaf stomata. For PAN, an additional resistance inhibited the uptake of PAN by the leaves. Furthermore, the formation of water films on the leaf surface of plants inside the chamber could be continuously tracked with our custom built leaf wetness sensors. Using this modified leaf wetness sensor measuring the electrical surface conductance on the leaves, an exponential relationship between the ambient humidity and the electrical surface conductance could be determined.
Highlights
The atmosphere–biosphere exchange of various trace gas species plays an important role for the climate and ecosystem interaction
The comparison of fluxes between a single and the twin – cuvette system revealed an overestimation of fluxes by the single-cuvette system for both O3 (8.3 %) and peroxyacetyl nitrate (PAN) (21.4 %), which is due to unconsidered effects of wall deposition
With Automatic Temperature Regulated Air Humidification System (ATRAHS) the relative humidity inside the cuvette could be controlled with a high precision of 0.3 %
Summary
The atmosphere–biosphere exchange of various trace gas species plays an important role for the climate and ecosystem interaction. The removal and emission of trace gases by the biosphere represents a significant factor, and its understanding is essential for atmospheric chemistry and the calculation of global trace gas budgets. While there is an increasing interest in the underlying mechanism of trace gas exchange of plants, various methods to determine the exchange flux of trace gases exist – in the field and under controlled laboratory conditions. For flux measurements on ecosystem level micrometeorological methods such as the eddy covariance or gradient method are used, causing only minimal disturbance (Horst and Weil, 1995). S. Sun et al.: Twin-cuvette measurement technique
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