Multi-year composite view of ozone enhancements and stratosphere-to-troposphere transport in dry intrusions of northern hemisphere extratropical cyclones

We examine the role of extratropical cyclones in stratosphere-to-troposphere (STT) exchange with cyclone-centric composites of O3 retrievals from the Microwave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer (TES), contrasting them to composites obtained with the Modern-Era Retrospective-analysis for Research and Applications (MERRA and MERRA-2) reanalyses and the GEOS-Chem chemical transport model. We identify 15,978 extratropical cyclones in the northern hemisphere (NH) for 2005-2012. The lowermost stratosphere (261 hPa) and middle troposphere (424 hPa) composites feature a 1,000 km-wide O3 enhancement in the dry intrusion (DI) airstream to the southwest of the cyclone center, coinciding with a lowered tropopause, enhanced potential vorticity, and decreased H2O. MLS composites at 261 hPa show that the DI O3 enhancements reach a 210 ppbv maximum in April. At 424 hPa, TES composites display maximum O3 enhancements of 27 ppbv in May. The magnitude and seasonality of these enhancements are captured by MERRA and MERRA-2, but GEOS-Chem is a factor of two too low. The MERRA-2 composites show that the O3-rich DI forms a vertically aligned structure between 300 and 800 hPa, wrapping cyclonically with the warm conveyor belt. In winter and spring DIs, O3 is enhanced by 100 ppbv or 100-130% at 300 hPa, with significant enhancements below 500 hPa (6-20 ppbv or 15-30%). We estimate that extratropical cyclones result in a STT flux of 119±56 Tg O3 yr-1, accounting for 42±20 % of the NH extratropical O3 STT flux. The STT flux in cyclones displays a strong dependence on westerly 300 hPa wind speeds.

Learning to see the invisible: discovery and measurement of ozone

Ozone is a key trace constituent of the atmosphere that is interesting for multiple reasons, including its ability to serve both as a screen against harmful solar radiation and as an aggressor against human health. However, methods for accurately detecting and measuring ozone were required before the behavior of ozone in the atmosphere and the effect of human activity on that behavior could be understood. This paper traces out the history of technologies and practices in ozone monitoring that have made this understanding possible, from nineteenth century chemical indicators to modern, laser-based detection technologies. Key insights include the importance of interactions between theorizing and observation in the process of scientific discovery, the importance of intercomparisons between different types of instruments, the way in which public policy concerns changed the pace and direction of ozone monitoring in the 1970s, and the importance of long-term environmental monitoring data to both improving our understanding of earth systems and protecting human health and the environment.