Exchange of airborne organophosphorus pesticides with pine needles

Abstract

Organophosphate (OP) pesticide residues have been found in rain, snow, fog and air samples collected in the Sierra Nevada mountains. The fate of these residues in this ecosystem is poorly understood. A major component of the Sierra Nevada ecosystem is the varied plant biomass. Interaction between chemicals vapors and plant material has been demonstrated for various semivolatile organic compounds. This research addresses the hypothesis that the extensive forests of the westem slopes of the Sierra Nevada mountains, which lie in the predominant downwind direction from the agricultural Central Valley of California, might serve as sinks for airborne OPs. The main paths of vapor-leaf interaction are cuticular and stomatal. More specifically, interactions may include sorption of residues to surface dust particles, or to the needle surface, solubilization in the cuticular wax of the needle, and penetration to the interior of the needle either by absorption through the stomata or migration through the cuticle. Analytical methods, which can separately assess surface, cuticle adsorbed, cuticle dissolved, and internal needle tissue residues, have been applied in two experimental situations. One involved measuring the vapor-needle distribution coefficient for several OPs exposed as vapors to pine branches in a laboratory chamber. A second, involving air and needle analyses for trees placed in, and downwind from a diazinon-treated orchard, provided outdoor distribution coefficients for comparison with chamber data. This paper addresses these two experimental situations, along with preliminary calculations of possible loading values for forests in the Sierra Nevadas. The results support the hypothesis that forests act as sinks for airborne pesticide vapors.