AIR-SURFACE EXCHANGE OF NONMETHANE ORGANIC-COMPOUNDS AT A GRASSLAND SITE - SEASONAL-VARIATIONS AND STRESSED EMISSIONS

Emissions of nonmethane organic compounds (NMOCs) were measured by a s tatic enclosure technique at a grassland site in the midwestern United States during the growing seasons over a 2-year period. A mixture of nonmethane hydrocarbons (NMHCs) and oxygenated hydrocarbons (OxHCs) wa s emitted from the surface at rates exhibiting large seasonal and year -to-year variations. The average emission rate (and standard error) of the total NMOCs around noontime on sunny days during the growing seas ons for the 2-year period was 1,300 +/- 170 mu g m(-2) h(-1) (mass of the total NMOCs pet area of enclosed soil surface per hour) or 5.5 +/- 0.9 mu g g(-1) h(-1) (mass of the total NMOCs per mass of dry plant b iomass in an enclosure per hour), with about 10% and 70% of the emissi ons being composed of tentatively identified NMHCs and OxHCs, respecti vely. Methanol was apparently derived from both the soil and vegetatio n and exhibited an average emission rate of 460 +/- 73 mu g m(-2) h(-1 ) (1.4 +/- 0.2 mu g g(-1) h(-1)), which was the largest emission among the NMOCs. The year-to-year variation in the precipitation pattern gr eatly affected the NMOC emission rates. Emission rates normalized to b iomass density exhibited a linear decrease as the growing season progr essed. The emission rates of some NMOCs, particularly the OxHCs, from vegetation subjected to hypoxia, frost, and physical stresses were sig nificantly greater than the average values observed at the site. Emiss ions of monoterpenes (alpha- and beta-pinene, limonene, and myrcene) a nd cis-3-hexen-1-ol were accelerated during the flowering of the plant s and were much greater than those predicted by algorithms that correl ated emission rates with temperature. Herbaceous vegetation is estimat ed to contribute about 40% and 50% of the total NMOC and monoterpene e missions, respectively, in grasslands; the remaining contributions are from woody species within grasslands. Contributions of isoprene emiss ions from herbaceous vegetation in grasslands are negligible. Grasslan ds are estimated to contribute about 10% of the total biogenic NMOC em issions in the United States.