Impact of ambient humidity on the generation of OH in the effluent of an atmospheric pressure plasma jet

Atmospheric pressure plasma jets are commonly used in plasma medicine for their ability to deliver reactive species precisely to target areas. During transport through ambient air, these species can undergo secondary reactions, forming new reactive compounds. This study focuses on the post-plasma generation of hydroxyl radicals (OH) in the effluent of the COST reference microplasma jet, operated with pure helium. Employing laser-induced fluorescence spectroscopy, the spatial distribution of the OH density is resolved in all three dimensions as the helium effluent interacts with a controlled, humidified air atmosphere. OH is predominantly observed at the interface between the helium core and the ambient atmosphere with a density in the range of 10¹³ cm⁻³. Increasing the relative humidity in the ambient air shifts the OH density peak closer to the jet’s nozzle in all directions, indicating that moisture penetrates deeper into the helium flow. The radical’s absolute density distribution is further compared to those obtained with humidified feed gas and potential generation mechanisms are discussed.