Fluxes of CO 2 , water vapor, and sensible heat were measured by the eddy covariance method above a young ponderosa pine plantation in the Sierra Nevada Mountains (CA) over two growing seasons (1 June-10 September 1997 and 1 May-30 October 1998). The Mediterranean-type climate of California is characterized by a protracted summer drought, with precipitation occurring mainly from October through May. While drought stress increased continuously over both summer growing seasons, 1998 was wetter and cooler than average due to El Niño climate patterns and 1997 was hotter and drier than average. One extreme 3-day heat wave in 1997 (Days 218-221) caused a step change in the relationship between H 2 O flux and vapor pressure deficit, resulting in a change in canopy conductance, possibly due to cavitation of the tree xylem. This step change was also correlated with decreased rates of C sequestration and evapotranspiration; we estimate that this extreme climatic event decreased gross ecosystem production (GEP) by roughly 20% (4 mol C m −2 s −1) for the rest of the growing season. In contrast, a cooler, wetter spring in 1998 delayed the onset of photosynthesis by about 3 weeks, resulting in roughly 20% lower GEP relative to the spring of 1997. We conclude that the net C balance of Mediterranean-climate pine ecosystems is sensitive to extreme events under low soil moisture conditions and could be altered by slight changes in the climate or hydrologic regime.
The atmospheric degradation of HFC-134a
(CF3CFH2) proceeds via the formation of
CF3CFHO radicals.
Long path length FTIR environmental chamber techniques were used
to study the atmospheric fate of CF3CFHO radicals. Two competing reaction pathways were identified for
CF3CFHO radicals: reaction with
O2,
CF3CFHO + O2 → CF3C(O)F
+ HO2, and decomposition via C−C bond scission,
CF3CFHO + M → CF3
+ HC(O)F + M. CF3CFHO radicals were produced
by two different reactions: either via the self-reaction
of CF3CFHO2 radicals or via the
CF3CFHO2 + NO reaction. It was
found that decomposition was much
more important when CF3CFHO radicals were produced via
the CF3CFHO2 + NO reaction than when
they
were produced via the self-reaction of
CF3CFHO2 radicals. We ascribe this
observation to the formation of
vibrationally excited CF3CFHO* radicals in the
CF3CFHO2 + NO reaction. Rapid
decomposition of CF3CFHO* radicals limits the formation of CF3C(O)F and hence
CF3COOH in the atmospheric degradation of
HFC-134a. We estimate that the CF3COOH yield from
atmospheric oxidation of HFC-134a is 7−20%.
Vibrationally excited alkoxy radicals may play an important role
in the atmospheric chemistry of other organic
compounds.
We investigated key factors controlling mass and energy exchange by a young (6-year-old) ponderosa pine (Pinus ponderosa Laws.) plantation on the west side of the Sierra Nevada Mountains and an old-growth ponderosa pine forest (mix of 45- and 250-year-old trees) on the east side of the Cascade Mountains, from June through September 1997. At both sites, we operated eddy covariance systems above the canopy to measure net ecosystem exchange of carbon dioxide and water vapor, and made concurrent meteorological and ecophysiological measurements. Our objective was to understand and compare the controls on ecosystem processes in these two forests. Precipitation is much higher in the young plantation than in the old-growth forest (1660 versus 550 mm year-1), although both forests experienced decreasing soil water availability and increasing vapor pressure deficits (D) as the summer of 1997 progressed. As a result, drought stress increased at both sites during this period, and changes in D strongly influenced ecosystem conductance and net carbon uptake. Ecosystem conductance for a given D was higher in the young pine plantation than in the old-growth forest, but decreased dramatically following several days of high D in late summer, possibly because of xylem cavitation. Net CO2 exchange generally decreased with conductance at both sites, although values were roughly twice as high at the young site. Simulations with the 3-PG model, which included the effect of tree age on fluxes, suggest that, during the fall through spring period, milder temperatures and ample water availability at the young site provide better conditions for photosynthesis than at the old pine site. Thus, over the long-term, the young site can carry more leaf area, and the climatic conditions between fall and spring offset the more severe limitations imposed by summer drought.
The kinetics and pressure dependence of the reactions of NO, with CH, and CH,O have been investigated in the gas phase at 298 K, at pressures from 1 to 10 Torr. A low-pressure discharge-flow laser-induced fluorescence (LIF) technique was used. In a consecutive process, CH, reacted with NO, to form CH,O, CH, + NO, + CH,O + NO (l), which further reacted with NO, to form products, CH30 + NO, + products (2). Reaction (1) displayed Paper 3/04404A;
A discharge-flow system equipped with a laser-induced fluorescence cell to detect the ethoxyl radical and an optical absorption cell to detect the nitrate radical has been used to measure t h e rate constants for the reactions C2H5 + NO, -+ products C2H50 + NO, -+ products C,H50, + NO, -+ products(1)(2)(3) at T = 298 K and P = 2.2 Torr. The major products of these reactions are C, H, O
A discharge-flow system equipped with a laser-induced fluorescence cell to detect the methoxyl radical and a quadrupole mass spectrometer to detect both the chlorine monoxide and chlorine dioxide radicals has been used to measure the rate constants for the reactions CH, + CiO + products CH30 + CIO + products CH, + OCIO + products CH30 + OClO -+ products(1)(2)(3) (4) at f = 298 K and P = 1-3 Torr. The observed products of these reactions are CH30 for reaction (l), HOCI for reaction (2), and CH30 and CIO for reaction (3). For reaction (4), CH,OCI is a possible product. The rate constants derived for reactions (1)-( 4) are: k, = (1.3 & 0.4) x lo-'* cm3 molecule-' s-'; k, = (1.3 & 0.3) x lo-" cm3 molecule-' s-' ; k, = (1.6 & 0.3) x lo-'' cm3 molecule-'' s-' ; k, = (1.5 & 0.5) x cm3 molecule-' s-'. The likely mechanisms for reactions (1)-(4) are briefly discussed.
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