CALIFORNIA OZONE DEPOSITION EXPERIMENT - METHODS, RESULTS, AND OPPORTUNITIES

The California Ozone Deposition Experiment (CODE) is a program of obse rvations and modeling to improve estimates of the rate of removal of t ropospheric ozone at the earth's surface used in grid-based photochemi cal models of ozone production, transport, and removal. The purpose of CODE is to test, diagnose and improve treatment of dry deposition of ozone and other gaseous species. CODE supports a larger air quality me asurement and modeling effort comprised of the San Joaquin Valley Air Quality Study (SJVAQS) and Atmospheric-Utilities Signatures: Predictio ns and Experiments (AUSPEX) joined as SJVAQS/AUSPEX Regional Model Ada ptation Project (SARMAP). However, the CODE data are also applicable t o a variety of boundary layer and turbulence problems. This paper desc ribes the field methods and data collected during summer (10 July thro ugh 6 August) of 1991 in the San Joaquin Valley (SJV) of California an d introduces several related papers. General comparisons and conclusio ns from all the participants are summarized. The core elements of the CODE field effort consisted of a research aircraft for spatial coverag e and three ground sites located in a cotton field, grape vineyard, an d very dry (senescent) annual grassland. A major portion of the SJV is represented by these three vegetation types. The eddy covariance meth od is used to compute the vertical fluxes of ozone, carbon dioxide, wa ter vapor, sensible heat and momentum. For the first half of the study period, flights were made mainly for comparison with tower-based flux es. Subsequent flights were over other vegetation types and to conduct special studies. In addition to the vertical fluxes, the ground-site data include individual leaf measurements of stomatal conductance, rad iative leaf temperature, wetness of surrogate leaves, soil temperature profiles and heat flux, soil composition and water content, mean nitr ogen oxide and ozone concentrations, solar and net radiation, photosyn thetically active radiation, and vertical profiles of wind, temperatur e, ozone and water vapor. Aircraft data also include reflected short-w ave radiation, surface greenness index and radiative surface temperatu re. Several factors simplify analyses: a nearly constant synoptic situ ation, lack of cloud cover, low-level (30 m) flights and land use char acterized by extensive homogeneous areas with well defined interfaces. Repeated five-km aircraft runs, necessary for a representative flux c alculation, were commonly made over a single crop type. In addition, a partial (60%) solar eclipse on 11 July provides an opportunity to exa mine the influence of light intensity upon the plant-atmosphere exchan ge of carbon dioxide and ozone via stomatal activity.