Modeling and Data Analysis of PEM-Tropics B Observations

 Principal Investigators:

Shaw C. Liu and Yuhang Wang, Georgia Institute of Technology

  Summary

A three-year funding is requested to make a modeling and data analysis and interpretation study of the observations that will be made by DC-8 and P3-B during PEM-Tropics B. The study would focus on two subjects. The primary subject would be the photochemistry and budgets of O3 and HOx. A pivotal part of this subject is to evaluate and understand the sources, photochemistry and transport of NOx as well as other O3 precursors such as primary and secondary NMHCs and CO. The secondary subject would be the budget and photochemistry of SO2. A combination of modeling and data analysis/interpretation approaches would be used. We plan to use, depending on the situation, a three-dimensional global model, a three-dimensional mesoscale/regional model, a one-dimensional model, a box model, and/or a combination of them to simulate, analyze, and interpret the observations. All models may be needed because they are complementary and each has its advantages and limitations. Proven analytical techniques, including correlation between key tracers and with various air masses, would be applied to both observations and model results to deduce useful information of the two subjects. In addition, the PEM-Tropics B observations would be compared and analyzed in a systematic way against other airborne experiments over remote atmosphere conducted by GTE as well as other programs.

Major Objective

• To advance the understanding of a selected set of the questions raised above.

• To quantify the budgets of O3, NOx, and SO2 over the region of PEM-Tropics B Experiment.

Key Scientific Questions

1. Do we have a closure on the HOx photochemistry?i.e. is the observed OH concentration consistent with the result of a steady state model that is well constrained by observations of key species? Are NMHCs important as the source of HOx in the upper troposphere as indicated in some of the previous GTE experiments?

2. What are the major sources of NOx that contribute to the NOx distribution over the tropical Pacific? How much does each source contribute? Is the biomass burning source during PEM-Tropics B as important as that during PEM-Tropics A? How important is the source from lightning? Is the NOx production from NH3 important? How much NOx is transported from the boundary layer to the free troposphere or vise versa?

3. To what extent does the long-range transport of organic nitrates contribute to the concentrations of NOx and production of O3? Is recycling from other odd nitrogen species such as HNO3 significant compared to the primary sources (or emissions)? If so, what is the mechanism(s)? Is the recycling rate altitude dependent? What does this imply in regard to the HOx photochemistry and photochemical O3 production?

4. Is there clear evidence showing that the significant O3 photochemical production rate in the free marine troposphere calculated in several modeling studies is quantitatively correct? This question is closely coupled to the questions about HOx.

5. Is long-range transport from continents the dominant source of all NMHCs over the tropical Pacific? Which NMHCs if any and how much emissions are from the oceans? What does the difference in the concentrations of NMHCs between biomass and non-biomass seasons tell us about the emissions from biomass burning and those from terrestrial biosphere?

6. What are the relative contributions to the budgets of SO2 and sulfate by major known sources: DMS, volcanic emissions, fossil fuel combustion, and biomass burning? How do the contributions depend on the altitude? What is relative importance of gas phase oxidation compared to liquid phase oxidation of SO2 to sulfate?

7. For most of the above questions, what does the contrast between PEM-Tropics B and PEM-Tropics A observations imply? In this regard, one of the pivotal questions is how does seasonal change in biomass burning impact on the budgets of key trace gases and aerosols. Another general question is how convection affects the distributions and budgets of trace species, including NOx, HOx, O3, and sulfur species.

Proposed Approach

Models are essential for placing in the broader context for an airborne experiment like PEM-Tropics B that will provide only a snap shot of the tropical Pacific during the northern hemispheric spring of 1999. We plan to use, depending on the situation, a three-dimensional model, a three-dimensional mesoscale/regional model, a one-dimensional model, a box model, and/or a combination of them to simulate, analyze, and interpret the observations. These models are needed because they are complementary and each has its advantages and limitations. For example, a box model calculation is usually carried out by fixing the key species and ambient conditions to observed values and calculate other species of interest by neglecting the transport effect. This has a clear advantage over 3-D models in terms of studying the fast photochemistry because, in order to preserve its internal consistency, a 3-D model needs to be run in prognostic rather than diagnostic mode. Thus, the 3-D model is usually not able to reproduce closely the observed values, especially for those species with short or intermediate lifetimes such as NOx and H2O2. As a result, the box model tends to give a more accurate evaluation of the fast photochemical processes than the 3-D model, e.g. concentrations of OH and HO2. On the other hand, a box model obviously can not compete with a 3-D model in investigating the history and transport processes of an air mass or any other problems that are significantly affected by atmospheric transport processes.