Low Ozone Episodes at Amphitrite Point Marine Boundary Layer Observatory, British Columbia, Canada

At Amphitrite Point, ozone (O-3) mixing ratios are observed to drop steadily to 5-15 ppb over a period of 12 hours or less with a frequency approaching one event per week (with highest frequencies occurring in summer and fall). Analysis of 47 such O-3 depletion events reveals that low O-3 episodes are a predominantly nocturnal phenomenon associated with anticyclonic conditions characterized by light onshore or alongshore winds and an absence of fog and mist. Back-trajectories show air carried to the Amphitrite Point Observatory (APO) during depletion events remains in the marine boundary layer and is not brought to the surface from aloft. There is no strong correlation with other "criteria" pollutants (CO, NOx, SO2, PM2.5) that might be indicative of a mechanism for O-3 destruction linked to human, terrestrial, or marine pollutant sources. However, CO2 mixing ratios are observed to increase, coincident with O-3 depletion. Together, these results point to a natural marine boundary layer phenomenon in which O-3 destruction dominates O-3 production and/or replenishment by vertical mixing. While there are several candidate mechanisms, the conditions for O-3 depletion (and CO2 buildup) to occur are set by meteorology and, in particular, development of a stable marine boundary layer in which vertical mixing is suppressed. Support for this interpretation is provided by simultaneous increases in CO2 in the stable marine boundary that are indicative of an important role played by marine biogenic processes (respiration). Future research should be directed at elucidating the chemical mechanisms responsible for O-3 destruction in the coastal zone, which means that there would be a need for a much broader range of measurements at APO (including halogenated species) as well as offshore measurements of both chemical and marine boundary layer meteorological variables.