Efficient generation of atomic chlorine by a low-temperature plasma and application to atmospheric chemistry

Short-lived reactive species, such as hydroxyl (OH) and atomic chlorine (CL) radicals, play a crucial role in atmospheric self-regulation and low temperature plasma applications. The oxidative removal of volatile organic compounds (VOC) in the atmosphere depend critically upon the local density of these reactive species. Direct measurements of radical concentration and reactivity (loss rate) are challenging in the atmosphere. Indirect techniques have been shown to be of value, notably for OH reactivity measurements. Low-temperature plasmas have potential as efficient sources of radicals at atmospheric pressure for use in these indirect techniques. In this work, atomic chlorine generated by a capacitively coupled, radio-frequency driven plasma was applied to a competitive reactivity method for measuring the reactivity of atomic chlorine in a gas sample. Argon with a small admixture (0.04–0.096%) of molecular chlorine was used as the plasma feed gas. The main production and destruction mechanisms of CL2 in the plasma were investigated by a zero-dimensional global model. Optical emission spectroscopy of the plasma identified humid air impurities through OH and N2 rotational band emission. Proton transfer reaction mass spectrometry (PTR-MS) was used to indirectly quantify the reactive species downstream of the plasma through adding VOC to the plasma effluent and monitoring the resulting mixture. Despite efforts to remove impurities in the argon gas line, ∼8×10^11 cm−3 OH was scrubbed from the plasma effluent using benzene. Additionally, (6 − 13)×10^11 cm−3 radical fragments were observed as CL2 was added to the plasma. The loss rate of atomic chlorine in a mixture of toluene/isoprene, using diethyl-ether as the reference was also attempted for the first time.