Stratosphere-troposphere Exchange of Ozone and its Changes in the Future

臭氧会引起呼吸问题和破坏植被，因此，人们对了解对流层臭氧的来源及其随时间的变化非常感兴趣。由于平流层的臭氧浓度高于对流层，两者之间的空气混合，称为平流层-对流层交换(STE)，是对流层臭氧的重要来源。但平流层的贡献很难量化，再分析产品对臭氧STE的直接估计与基于对流层臭氧预算残差的估计不同。另一个令人担忧的问题是，两个重要的再分析产品(ERA5和MERRA2)对40年STE趋势的估计显示出巨大的差异。最近，首席调查员(PI)发现了臭氧STE计算的一个潜在的重大缺陷，即他们只考虑了中高纬度地区的STE，那里的臭氧输送方向是向下进入对流层，并且大部分臭氧STE发生在那里。与预期相反，他发现热带臭氧STE是上升的，因此是对流层的损失，不能忽视，因为它使全球臭氧STE减少了约35%。这一发现，以及相关高质量卫星观测的提供，需要对臭氧STE进行重新审查，以解决三个问题：1)基于观测数据的臭氧STE及其相关不确定性是什么？2)不同再分析产品对臭氧STE的巨大差异的主要原因是什么？3)臭氧STE在气候变化中将如何变化？STE计算使用的方法是假设热带对流层顶位于等温面(当今气候中的380K等熵)，而温带对流层顶位于380K等温线以下的位涡表面(3.5PVU)。该项目使用来自CloudSat、Calypso、MODIS和COSMIC等卫星任务的数据，结合辐射传输代码，计算产生观测STE估计所需的加热率和大气密度。关于未来STE变化的工作审查了化学-气候模型倡议的气候模拟。一个令人担忧的问题是，由于温室气体增加，平流层翻转环流(布鲁尔-多布森环流)预计将加强，这可能导致中高纬度臭氧STE下降的增加。另一个原因是热带对流层顶预计将上升，这可能会通过热带STE减少臭氧对平流层的损失。鉴于地面臭氧的有害影响以及更好地估计其来源和可能的未来演变的实用价值，这项工作具有社会利益。此外，PI还开展公众宣传活动，包括以K-12科学参与为重点的多项活动。国际地球物理协会还积极开发数据可视化软件和技术，其中一些软件和技术曾在美国地球物理联盟数据可视化大赛中亮相。此外，该项目还为两名研究生提供支持和培训。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.