Y. Fukui et Pv. Doskey, AIR-SURFACE EXCHANGE OF NONMETHANE ORGANIC-COMPOUNDS AT A GRASSLAND SITE - SEASONAL-VARIATIONS AND STRESSED EMISSIONS, J GEO RES-A, 103(D11), 1998, pp. 13153-13168
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.