Detection of stratospheric ozone intrusions by windprofiler radars

Ozone is critical to the chemical and thermal balance of Earth's troposphere, controlling the capacity of this layer of the atmosphere to oxidize and remove other pollutants via the formation of hydroxyl radicals. Tropospheric ozone is also an important greenhouse gas. Photochemistry in the lower troposphere is the major source of tropospheric ozone, but the intrusion of stratospheric ozone into the troposphere is also a factor in the overall climatology budget. Using the latest windprofiler radars, together with ozonesonde launches and computer modelling, Hocking et al. have identified rapid changes in radar tropopause height as a diagnostic for the intrusion of stratospheric ozone to lower altitudes, a technique that may help to establish the mechanism involved in the phenomenon. Stratospheric ozone attenuates harmful ultraviolet radiation and protects the Earth’s biosphere1. Ozone is also of fundamental importance for the chemistry of the lowermost part of the atmosphere, the troposphere1,2,3,4,5,6,7,8. At ground level, ozone is an important by-product of anthropogenic pollution7, damaging forests and crops5,6, and negatively affecting human health9. Ozone is critical to the chemical and thermal balance of the troposphere10 because, via the formation of hydroxyl radicals, it controls the capacity of tropospheric air to oxidize and remove other pollutants1. Moreover, ozone is an important greenhouse gas, particularly in the upper troposphere1. Although photochemistry in the lower troposphere is the major source of tropospheric ozone2,7,11, the stratosphere–troposphere transport of ozone12,13,14,15,16,17,18,19 is important to the overall climatology, budget and long-term trends of tropospheric ozone3,4,8,12. Stratospheric intrusion events, however, are still poorly understood. Here we introduce the use of modern windprofiler radars20,21,22 to assist in such transport investigations. By hourly monitoring the radar-derived tropopause height23,24,25 in combination with a series of frequent ozonesonde balloon launches, we find numerous intrusions of ozone from the stratosphere into the troposphere in southeastern Canada. On some occasions, ozone is dispersed at altitudes of two to four kilometres, but on other occasions it reaches the ground, where it can dominate the ozone density variability. We observe rapid changes in radar tropopause height immediately preceding these intrusion events. Such changes therefore serve as a valuable diagnostic for the occurrence of ozone intrusion events. Our studies emphasize the impact that stratospheric ozone can have on tropospheric ozone, and show that windprofiler data can be used to infer the possibility of ozone intrusions, as well as better represent tropopause motions in association with stratosphere–troposphere transport.