DETERMINATION OF THE BIOGENIC EMISSION RATES OF SPECIES CONTRIBUTING TO VOC IN THE SAN-JOAQUIN VALLEY OF CALIFORNIA

As part of an extensive effort to characterize biogenic hydrocarbon em ission rates in the San Joaquin Valley and surrounding areas during th e SJVAQS/AUSPEX field experimental period, July-August 1990, measureme nts were made for the first time of isoprene, terpene, and other VOC e mission rates from blue oak (Quercus douglasii), foothill pine (Pinus sabiniana), and a ground cover plant called tarweed (Holocarpha sp.) a t a rural site near Mariposa, CA. A flow-through plant enclosure metho d was used to measure the emission flux rates from these species; the plant limb or whole plant was flushed with clean air just prior to hyd rocarbon sampling. Samples of the plant emissions were collected on Te nax GC or Tenax GC-Carbosieve S-II cartridges and analysed by gas chro matography-Fourier transform infrared-mass spectrometry (GC-FTIR-MS). Quantifiable biogenic emissions from two blue oak specimens consisted only of isoprene, with an average emission rate of 8.4 mug g-1 dry bio mass h-1. Emission rates (above the detection of about 0.05 mug g-1 h- 1) from two foothill pine specimens consisted mostly of alpha-pinene; an average emission rate of 0.64 mug g-1 h-1 of alpha-pinene was obser ved. The tarweed species emitted both alpha- and beta-pinenes, along w ith other terpene and oxygenated species, some of which have been tent atively identified. The emission rates of biogenic hydrocarbons from f oothill pine and blue oak species as determined in this study make the se species potentially significant contributors to summertime VOC leve ls in the San Joaquin Valley of California, based on vegetation classi fication data and the predominant summer meteorology.