Building on Ray Leuning's legacy: extending the WPL approach to open path laser-based gas analyzers

Ray Leuning's contributions have touched almost every facet of quantitative ecophysiology in which LI-COR instruments have been used, including leaf gas exchange, measuring and modeling stomatal conductance, measuring and modeling canopy and ecosystem fluxes, and providing ground truth for satellite data. Our association and friendship with Ray go back to the mid-1980's and his work on mass balance analysis of closed path photosynthesis systems and properly partitioning leaf resistance into stomatal and boundary layer components. Beyond this, the landmark Web-Pearman-Leuning corrections (WPL, 1980, Qtr J Royal Meteorol Soc 106, 85) have guided all of our efforts in analyzing open path eddy covariance measurements for the last 30 years, and his tube attenuation analysis is an important part of closed path measurement analysis. It is thus an honor to participate in this session honoring Ray, and I want to focus on one element of Ray's legacy as a foundation for moving into the future. Many of the new generation of gas analyzers use laser-based methods. These new instruments come in closed path and open path configurations, and the considerations Ray put forward concerning tube attenuation and water sorption for closed path instruments still apply. But here we shall discuss some new requirements imposed by open path laser-based instruments that continue to rely on foundations Ray helped to build. WPL corrections for effects of thermal expansion and water vapor dilution are required for measurements made with laser-based instruments just as they are for non-dispersive infrared (NDIR) instruments. One important difference, however, is that NDIR-based instruments do not require a knowledge of temperature for their calibration, while laser-based instruments do require such knowledge. Thus, with laser-based instruments, temperature has two effects: the thermal expansion effect and a calibration effect, both of which require knowledge of instantaneous temperature, which is usually not known. Similarly, water vapor has a dilution effect and a pressure broadening effect, both of which must be accounted. In this paper we will discuss the origins and treatment of these effects and show how they are incorporated into the WPL formulation that Ray helped to create. Ray's legacy has touched all of us at many levels and it will continue to live on, long into the future. Thank you, Dr. Ray Leuning.