A TECHNIQUE FOR IN SITU MEASUREMENT OF OZONE IN TROPOSPHERIC VOLCANIC PLUMES USING TWO PORTABLE, LOW-COST, SENSORS

Chemical models suggest atmospheric ozone (O₃) is very important in altering volcanic plume halogen-halide chemistry. It has been proposed that chemical reactions involving catalytic destruction of atmospheric O₃ are responsible for converting volcanic bromine, typically emitted as hydrogen bromide (HBr), to bromine monoxide (BrO) in plumes downwind of volcanoes. BrO has been detected in several tropospheric volcanic plumes worldwide, implying that the destruction of tropospheric O₃ by volcanic emissions may be common and widespread. Thus, measurements of O₃ in tropospheric volcanic plumes are desirable in order to assess the impact of volcanic hydrogen-halide emissions on tropospheric O₃ budgets, and as a potential volcanic hazard monitoring tool because hydrogen halides are generally emitted from magmas at low pressures (i.e. close to the surface). However, very few measurements of O₃ in volcanic plumes have been reported. One reason for the paucity of O₃ measurements in volcanic plumes is because commonly used UV-absorption O₃ sensors have cross-sensitivities to sulfur dioxide (SO₂), a major constituent in volcanic gases. In order to address this problem, we first determined the response of a mini closed-path UV-absorption (253.7 nm) O₃ sensor to SO₂ and then used simultaneous, independent, measurements from a mini voltammetric SO₂ sensor to calculate and subtract the SO₂ interference from the O₃ measurements. Laboratory experiments show that ambient levels of O₃ can be measured in mixtures of ambient air with several ppm SO₂ using this approach. Initial in situ airborne measurements using this method, conducted in the concentrated tropospheric plume from Redoubt Volcano, Alaska, (~5,000 t/d SO₂) in June 2009, indicate O₃ depletions of several tens of ppb from ambient mixing ratios in the plume several miles downwind of the volcano. We speculate that this depletion is due to reactions involving volcanic HBr and possibly other hydrogen halides. The equipment used in the study is readily available, portable, and battery-powered, and it costs less than most passive spectroscopic equipment currently used in volcanic gas studies.