Twin-cuvette measurement technique for investigation of dry deposition of O₃ and PAN to plant leaves under controlled humidity conditions

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.