ABSTRACT

Ozone causes breathing problems and damages vegetation, thus there is considerable interest in understanding the sources of ozone in the troposphere and how they change over time. Because ozone concentration is higher in the stratosphere than the troposophere the mixing of air between the two, referred to as stratosphere-troposphere exchange (STE), is an important source of tropospheric ozone. But the stratospheric contribution is hard to quantify and direct estimates of ozone STE from reanalysis products differ from estimates based on tropospheric ozone budget residuals. A further concern is that estimates of the 40-year STE trend from two prominent reanalysis products (ERA5 and MERRA2) show substantial differences.

Recently the Principal Investigator (PI) found a potentially important shortcoming of ozone STE calculations, which is that they only consider STE in the middle and high latitudes, where the direction of ozone transport is downward into the troposphere and where the bulk of the ozone STE occurs. Contrary to expectations he found that the tropical ozone STE, which is upward and thus a loss for the troposphere, cannot be neglected as it reduces the global ozone STE by about 35%. This finding, along with the availability of relevant high-quality satellite observations, warrants a reexamination of ozone STE addressing three questions: 1) what is the ozone STE and associated uncertainty based on observational data? 2) what are the main factors responsible for the large discrepancies in ozone STE from different reanalysis products? 3) How will ozone STE change in a changing climate?

The STE calculations use a methodology in which the tropical tropopause is assumed to lie on an isentropic surface (the 380K isentrope in present-day climate) while the extratropical tropopause lies on a potential vorticity surface (3.5PVU) below the 380K isentrope. The project uses data from satellite missions including CloudSat, CALYPSO, MODIS, and COSMIC, in combination with radiative transfer codes, to calculate the heating rates and atmospheric density needed to create observational STE estimates. The work on future STE change examines climate simulations from the Chemistry-Climate Model Initiative. One concern is that the stratospheric overturning circulation (the Brewer-Dobson circulation) is expected to strengthen due to greenhouse gas increases, which could lead to an increase in downward ozone STE in the middle and high latitudes. Another is that the tropical tropopause is expected to rise, which could reduce the loss of ozone to the stratosphere through tropical STE.

The work is of societal interest given the harmful effects of ground-level ozone and the practical value of better estimates of its sources and likely future evolution. In addition, the PI conducts public outreach including multiple events focusing on K-12 engagement in science. The PI has also been active in developing data visualization software and techniques, some of which were featured in the American Geophysical Union Data Visualization Competition. In addition, the project provides support and training to two graduate students.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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