GFDL - Geophysical Fluid Dynamics Laboratory

Larry W. Horowitz – Research

I am a Physical Scientist at the NOAA Geophysical Fluid Dynamics Laboratory (GFDL). My research focuses on tropospheric and stratospheric chemistry and aerosols. On this page, I describe the goals of my research, and some of the techniques I use to address these goals.

View information about the AM3 chemistry-climate model, one of the principal tools I use in my research.

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Ozone (O3) is of central importance to the chemistry of the troposphere. Ozone is:

  • a major air pollutant, detrimental to public health and vegetations
  • the precursor of the hydroxyl radical, which is the primary oxidant in the atmosphere
  • a climatically-important greenhouse gas

Ozone is produced within the troposphere as the result of reactions involving nitrogen oxides (NOx), carbon monoxide, and hydrocarbons. Increases in the emissions of these ozone precursors over the past century (from fossil fuel and biomass burning) are believed to have resulted in increases in tropospheric ozone concentrations over industrial regions, and probably on a global scale.

Major questions my research addresses include:

  • What controls the global distribution of ozone in the troposphere?
  • How have anthropogenic emissions altered the distribution of ozone?
  • What role does deep convection in the tropics play in controlling the production of ozone in the upper troposphere?
  • What impact do changes in tropospheric ozone have on climate?
  • How will projected future emission scenarios alter the chemistry of the atmosphere?
  • How does the oxidizing capacity of the atmosphere change in response to varying anthropogenic emissions?

In order to answer these questions, I am using the global three-dimensional chemistry-climate model AM3 to simulate the distributions of ozone and its chemical precursors throughout the troposphere. Using AM3, and comparing model results with observations, I test our understanding of chemical and dynamical processes in the atmosphere. A goal of my work is to improve our understanding of the complex nonlinear dependence of ozone concentrations on emissions from sources such as fossil fuel combustion and biomass burning.

The oxidizing capacity of the atmosphere, which is the ability of the atmosphere to remove pollutants emitted, depends heavily on the abundance of ozone in the tropics. However, there have been relatively few measurements of ozone and its precursors in the tropical troposphere. As a result, the mechanisms controlling ozone in this region are poorly understood. Global three-dimensional models can be of great use in helping to explain these mechanisms in a manner consistent with the few available observations. Future land use changes and industrialization in the tropics are expected to lead to an increased anthropogenic perturbation to ozone in this region. There is a complicated interaction in the tropics between surface emissions, convection, lightning, and large-scale circulation. Using MOZART, which contains representations of all of these processes, together with available observations of ozone and its precursors in the tropics, I plan to study the mechanisms controlling the distribution of tropical tropospheric ozone. I will also evaluate how well these various processes are currently represented in global chemistry models.

I got my Ph.D. from the Atmospheric Sciences Group in the Division of Engineering and Applied Sciences at Harvard University. My advisor was Professor Daniel Jacob. In my dissertation research, I utilized a variety of models to study tropospheric chemistry. In my early work, I used a photochemical model to perform 0-dimensional and 1-dimensional simulations. I updated our photo-oxidation mechanism for isoprene and studied the effects of these modifications on the production of ozone and organic nitrates, using comparisons with observations as a guide. I also developed a simplified chemical mechanism for incorporation into a 3-dimensional chemical tracer model, and evaluated the accuracy of this mechanism against our complete mechanism. I developed and used both continental-scale and global versions of this model to study the export of reactive nitrogen from the continental boundary layer and its effect on the abundance of nitrogen oxides in the remote troposphere.

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