Song-Miao Fan

Net Exchange of CO2 in a Mid-Latitude Forest.

The eddy correlation method was used to measure the net ecosystem exchange of carbon dioxide continuously from April 1990 to December 1991 in a deciduous forest in central Massachusetts. The annual net uptake was 3.7 � 0.7 metric tons of carbon per hectare per year. Ecosystem respiration, calculated from the relation between nighttime exchange and soil temperature, was 7.4 metric tons of carbon per hectare per year, implying gross ecosystem production of 11.1 metric tons of carbon per hectare per year. The observed rate of accumulation of carbon reflects recovery from agricultural development in the 1800s. Carbon uptake rates were notably larger than those assumed for temperate forests in global carbon studies. Carbon storage in temperate forests can play an important role in determining future concentrations of atmospheric carbon dioxide.

Exchange of Carbon Dioxide by a Deciduous Forest: Response to Interannual Climate Variability

The annual net uptake of CO2 by a deciduous forest in New England varied from 1.4 to 2.8 metric tons of carbon per hectare between 1991 and 1995. Carbon sequestration was higher than average in 1991 because of increased photosynthesis and in 1995 because of decreased respiration. Interannual shifts in photosynthesis were associated with the timing of leaf expansion and senescence. Shifts in annual respiration were associated with anomalies in soil temperature, deep snow in winter, and drought in summer. If this ecosystem is typical of northern biomes, interannual climate variations on seasonal time scales may modify annual CO2 exchange in the Northern Hemisphere by 1 gigaton of carbon or more each year.

Physiological responses of a black spruce forest to weather

We used eddy covariance to measure the net exchange of CO2 between the atmosphere and a black spruce (Picea mariana) forest in Manitoba for 16,500 hours from March 16, 1994 to October 31, 1996. We then partitioned net exchange into gross photosynthesis and respiration by estimating daytime respiration as a function of temperature, and used these data to define the physiological responses of the forest to weather. The annual rates of gross production and respiration by the forest were both around 8 t C ha−1 yr−1. Both photosynthetic and respiratory response were reduced in winter, recovered with warming in spring, and varied little in summer. Respiration in mid summer increased with air temperature (T air) at a Q 10 of around 2 to a rate of 2–8 μmol m−2 s−1 at 15°C. Gross photosynthesis at high light (photon flux density (PPFD) greater than 600 μmol m−2 s−1) was negligible at Tair 14°C. Gross CO2 uptake at T air > 14°C increased with increasing light at an ecosystem-level quantum yield of 0.05 mol CO2 mol−1 photons before saturating at an uptake rate of 8–18 μmol m−2 s−1 at PPFDs greater than 500–700 μmol m−2 s−1. Photosynthesis in summer did not appear limited by high evaporative demand or soil water depletion.

Evidence of inorganic chlorine gases other than hydrogen chloride in marine surface air

We report the first measurements of inorganic chlorine gases in the marine atmosphere using a new tandem mist chamber method. Surface air was sampled during four days including one diel cycle in January, 1992, at Virginia Key, Florida. Concentrations of HCl* (including HCl, ClNO3, ClNO2, and NOCl) were in the range 40 to 268 pptv and concentrations of Cl2* (including Cl2 and any HOCl not trapped in the acidic mist chamber) were in the range <26 to 254 pptv Cl. Concentrations of Cl2* increased during the night, and decreased after sunrise as HCl* concentrations increased by similar amounts. The measurements suggest an unknown source of either HOCl or Cl2 to the marine atmosphere. Photochemical model calculations indicate that photolysis of the observed Cl2* would yield a chlorine atom (Cl•) concentration of order 104–105 cm−3. Oxidation by Cl• would then represent a significant sink for alkanes and dimethylsulfide (DMS) in the marine boundary layer. The cycling of Cl• could provide either a source or a sink for O3, depending on NOX levels.

Environmental controls on the photosynthesis and respiration of a boreal lichen woodland: a growing season of whole-ecosystem exchange measurements by eddy correlation

Measurements of net ecosystem CO2 exchange by eddy correlation, incident photosynthetically active photon flux density (PPFD), soil temperature, air temperature, and air humidity were made in a black spruce (Picea mariana) boreal woodland near Schefferville, Quebec, Canada, from June through August 1990. Nighttime respiration was between 0.5 and 1.5 kg C ha−1 h−1, increasing with temperature. Net uptake of carbon during the day peaked at 3 kg C ha−1 h−1, and the daily net uptake over the experiment was 12 kg C ha−1 day−1. Photosynthesis dropped substantially at leaf-to-air vapor pressure deficit (VPD) greater than 7 mb, presumably as a result of stomatal closure. The response of ecosystem photosynthesis to incident PPFD was markedly non-linear, with an abrupt saturation at 600 μmol m−2 s−1. This sharp saturation reflected the geometry of the spruce canopy (isolated conical crowns), the frequently overcast conditions, and an increase in VPD coincident with high radiation. The ecosystem light-use efficiency increased markedly during overcast periods as a result of a more even distribution of light across the forest surface. A mechanistic model of forest photosynthesis, parameterized with observations of leaf density and nitrogen content from a nearby stand, provided accurate predictions of forest photosynthesis. The observations and model results indicated that ecosystem carbon balance at the site is highly sensitive to temperature, and relatively insensitive to cloudiness.

Air-snow exchange of HNO3 and NOy at Summit, Greenland

A novel crystalline monohydrate of 7-[D- alpha -amino- alpha -(p-hydroxyphenyl)acetamido]-3-methyl-3-caphem-4-carboxylic acid is prepared and found to be a stable useful form of the cephalosporin antibiotic especially advantageous for pharmaceutical formulations.

Regional budgets for nitrogen oxides from continental sources: Variations of rates for oxidation and deposition with season and distance from source regions

Measurements of nitrogen deposition and concentrations of NO, NO2, NOy (total oxidized N), and O3 have been made at Harvard Forest in central Massachusetts since 1990 to define the atmospheric budget for reactive N near a major source region. Total (wet plus dry) reactive N deposition for the period 1990–1996 averaged 47 mmol m−2 yr−1 (126 μmol m−2 d−1, 6.4 kg N ha−1 yr−1), with 34% contributed by dry deposition. Atmospheric input adds about 12% to the N made available annually by mineralization in the forest soil. The corresponding deposition rate at a distant site, Schefferville, Quebec, was 20 mmol m−2 d−1 during summer 1990. Both heterogeneous and homogeneous reactions efficiently convert NOx to HNO3 in the boundary layer. HNO3 is subsequently removed rapidly by either dry deposition or precipitation. The characteristic (e-folding) time for NOx oxidation ranges from 0.30 days in summer, when OH radical is abundant, to ∼1.5 days in the winter, when heterogeneous reactions are dominant and O3 concentrations are lowest. The characteristic time for removal of NOx oxidation products (defined as NOy minus NOx) from the boundary layer by wet and dry deposition is ∼1 day, except in winter when it decreases to 0.6 day. Biogenic hydrocarbons contribute to N deposition through formation of organic nitrates but are also precursors of reservoir species, such as peroxyacetylnitrate, that may be exported from the region. A simple model assuming pseudo first-order rates for oxidation of NOx, followed by deposition, predicts that 45% of NOx in the northeastern U.S. boundary layer is removed in 1 day during summer and 27% is removed in winter. It takes 3.5 and 5 days for 95% removal in summer and winter, respectively.

Biosphere/atmosphere CO2 exchange in tundra ecosystems: Community characteristics and relationships with multispectral surface reflectance

The spatial and temporal patterns of many of the factors controlling CO2 exchange are related to characteristics of the vegetated surface which can potentially be monitored using multispectral remote sensors. Realization of this potential depends, in part, on an improved understanding of ecosystem processes and their relationship to variables which are accessible to remote sensing techniques. We examined these relationships using portable, climate-controlled, instrumented enclosures to measure CO2 exchange rates in selected tundra sites near Bethel, Alaska. Rates were related to vegetation community type and climatic variables. Exchange rates in enclosures were compared to exchange measurements obtained by eddy correlation on a 12-m micrometeorological tower. For an average light input of 37 einsteins/day during 20 midsummer days, the empirically modeled exchange rate for a representative area of vegetated tundra was 1.2 ±1.1 (95% confidence interval) g CO2 m−2 d−1. This was comparable to a tower measured exchange over the same time period of 1.1 ±1.1 (95% confidence interval) g CO2 m−2 d−1. Net exchange in response to varying light levels was compared for two major community types, wet meadow and dry upland tundra, and to the net exchange measured by the micrometeorological tower technique. Portable radiometers were used to measure the multispectral reflectance properties of the sites. These properties were then related to exchange rates with the goal of providing a quantitative foundation for the use of satellite remote sensing to monitor biosphere/atmosphere CO2 exchange in the tundra biome. The Normalized Difference Vegetation Index (NDVI) and the near-infrared/red reflectance ratio (SR) computed from surface reflectance were strongly correlated with net CO2 exchange for both upland and wet meadow vegetation. However, the form of the relationship was distinct from measured correlations in other ecosystems, suggesting that global surveys may require adjustment for geographical differences in exchange processes.

Emissions of ethene, propene, and 1‐butene by a midlatitude forest

Measurements of nonmethane hydrocarbon concentrations and gradients above Harvard Forest (42°32′ N, 72°11′ W) are reported for January through December 1993, along with inferred whole-ecosystem emission rates for ethene, propene, and 1-butene. Emissions were calculated using a micrometeorological technique where the ratio of observed CO2 fluxes and gradients were multiplied by the observed hydrocarbon gradients. Average emissions of ethene, propene, and 1-butene during summer were 2.63, 1.13, and 0.41 × 1010 molecules cm−2 s−1, respectively. Emission of these olefins was correlated with incident solar radiation, implying a source associated with photosynthesis. In the northeastern United States, summertime biogenic emissions of propene and 1-butene exceed anthropogenic emissions, and biogenic emissions of ethene contribute approximately 50% of anthropogenic sources. Our measurements suggest that terrestrial biogenic emissions of C2-C4 olefins may be significant for atmospheric photochemistry.

Air-snow exchange of HNO3 and NO y at Summit, Greenland

A novel crystalline monohydrate of 7-[D- alpha -amino- alpha -(p-hydroxyphenyl)acetamido]-3-methyl-3-caphem-4-carboxylic acid is prepared and found to be a stable useful form of the cephalosporin antibiotic especially advantageous for pharmaceutical formulations.