No abstract

Measurements of carbon-14 in small samples of methane from major biogenic sources, from biomass burning, and in "clean air" samples from both the Northern and Southern hemispheres reveal that methane from ruminants contains contemporary carbon, whereas that from wetlands, pat bogs, rice fields, and tundra is somewhat, depleted in carbon-14. Atmospheric (14)GH(4) seems to have increased from 1986 to 1987, and levels at the end of 1987 were 123.3 +/- 0.8 percent modern carbon (pMC) in the Northern Hemisphere and 120.0 +/- 0.7 pMC in the Southern Hemisphere. Model calculations of source partitioning based on the carbon-14 data, CH(4) concentrations, and delta(13)C in CH(4) indicate that 21 +/- 3% of atmospheric CH(4) was derived from fossil carbon at the end of 1987. The data also indicate that pressurized water reactors are an increasingly important source of (14)CH(4).

The long‐term data sets of total alkalinity (TA) (1929–2002 A.D.) and δ18O (1966–2002 A.D.) are used to investigate freshwater and brine distributions in the Arctic Ocean. Fractions of sea ice meltwater and other freshwaters (OF) (precipitation, river runoff, and freshwater carried by Pacific water implied as salinity deficit) are calculated on the basis of salinity‐TA and salinity‐δ18O relationships. Rejected brine during sea ice growth resides in surface water in the central Arctic Ocean, while net melting is found along the surface flow of water from the Pacific and Atlantic oceans. Distribution of OF at 10 m water depth suggests that Russian runoff leaves the shelf mainly west of the Mendeleyev Ridge, enters into the deep basin, and exits from the ocean through the western part of Fram Strait. The influence of Mackenzie River water is limited in the region and in depth. Accumulation of freshwater in the Canadian Basin is caused by deep penetration of OF with brine, indicating the transport of freshwater by shelf‐derived water. The major origin of shelf‐derived water entering into the upper halocline layer in the Canadian Basin should be the Chukchi and East Siberian Sea shelves, and the main freshwater sources are the salinity deficit of Pacific water and/or Russian runoff. An increase in OF inventory accompanied by an increase in brine content may suggest an increase of the shelf‐derived water supply into the western Canadian Basin in anticyclonic years.

Bimolecular rate constants for the reaction of ground-state Cl(2P3/2) atoms and for quenching of excited spin−orbit state Cl*(2P1/2) atoms in collisions with CH4, CD4, and CH2D2 have been measured at room temperature. Chlorine atoms were produced by the photolysis of CCl2F2 and HCl at 193 nm and monitored by the technique of laser-induced fluorescence at the vacuum ultraviolet region. The rate constants for the reaction of Cl atoms were found to be as follows:  CH4, (10.0 ± 1.0) × 10-14; CH2D2, (7.0 ± 0.8) × 10-14; and CD4, (8.2 ± 1.0) × 10-15 cm3 molecule-1 s-1. The absolute second-order rate constants for the removal of Cl* atoms were as follows:  CH4, (3.0 ± 0.3) × 10-11; CH2D2, (1.1 ± 0.1) × 10-10; and CO2, (1.2 ± 0.1) × 10-11 cm3 molecule-1 s-1. The removal of Cl* atoms was shown to proceed mainly by collisional quenching.

No abstract

Spatial variations in the concentration and nature of colored dissolved organic matter (CDOM) in the western Arctic Ocean were examined by three‐dimensional excitation/emission matrix (3‐D EEM) spectroscopy. CDOM profiles showed distinctive features well correlated with hydrographic characteristics. CDOM fluorescence was particularly high at depths between 40 and 200 m (up to 3 fluorescence units (Fl.U.)) in both Chukchi Sea and Beaufort Sea transects. Penetration of the high CDOM signal, formed on the shelves, into the Canada Basin was confined to the upper halocline layer (salinity of ∼33.1). This layer had distinctive 3‐D EEM fingerprints in fluorescence spectra, showing a marked terrestrial humic signature. The presence of CDOM in the halocline layer likely resulted from two main processes: the brine rejection during sea ice formation and transport across the sediment‐water interface during early diagenesis. Despite the high primary productivity in the Chukchi shelf, CDOM contribution from in situ production seemed to have little influence on the overall CDOM distributions in the study area.

Flux measurements at sites of mixed hardwood and black spruce stands from an area (C4) of the Caribou‐Poker Creek Research Watershed (CPCRW), interior Alaska, in the summer seasons of 1998, 1999, and 2000 are used to estimate the fluxes of CO2, CH4, and N2O before and after forest fires. The FROSTFIRE burning experiment was executed in typical boreal forest from 8 to 15 July 1999. The forest fire significantly decreased soil CO2 and N2O emissions, by at most 50%. On the contrary, CH4 flux from the soil increased from 7 to 142%, suggesting that the forest fire plays a role in accelerative thawing of the frozen soil, and subsequently the release of CH4 from permafrost. Most of the CH4 was oxidized in the soil after the fire; however, some was released from the soil when the permafrost maximally thawed at the end of August 1999 and September 2000. Relationships between the fluxes of trace gases and soil temperature before and after the fire showed good exponential correlations, indicating that soil temperature was one of the factors determining the fluxes of trace gases on boreal forest soils. Also, the higher soil temperature after the fire may be led to the enhanced diffusion of CO2, CH4 and N2O by microbial activity between the atmosphere and the forest soils, and to the increased fluxes of trace gases in burned black spruce stand soils. In order to understand the roles of moss and lichen mats on the black spruce stands, the net respiration by mosses and lichens was estimated with light and dark chamber measurements. Net respiration corresponds to 42 to 58% of the total soil respiration before fire. Therefore, the net respiration by moss and lichen layers was responsible for one‐half of total soil CO2 emissions. The maximum regional net respiration rate by moss and lichen mats on black spruce forest floors of central Alaska was 0.018 ± 0.009 GtC/yr, an important source of atmospheric CO2 in boreal forests. After the prescribed burn, soil respiration was attributable only to respiration by roots and microbes. The microbial respiration estimated after the fire is almost three times as high as that the calculated respiration before the fire. This finding indicates the post‐fire condition may stimulate microbial respiration because of higher nutrients and substrates in remnant soils and enhanced soil temperature. The microbial respiration can be estimated 14.7 tC/ha in burned black spruce stands over a decade after the fire, suggesting burned black spruce forests in central Alaska are a crucial source of atmospheric CO2.

Sulfur dioxide injected into the atmosphere is most likely oxidized into sulfate. Two major oxidation pathways are possible: 1) a homogeneous pathway involving gas reaction with hydroxyl radicals and 2) a heterogeneous pathway involving aqueous dissolution or aerosol reactions (Figure 1.). The relative importance of these reaction pathways conditions is controversial. Sulfur isotope ratios can be used to quantify the relative importance of these reaction pathways. However, its application was severly hampered by the fact that the isotope fractionation factor for the homogeneous pathway was not known (Caron et. al., 1986; Nriagu et. al., 1987). A significant isotope fractionation in the homogeneous SO2 oxidation is identified for the first time using an ab initio quantum mechanical calculation. By using the sulfur isotope fractionation factors we demonstrate a technique that uses measurements of the sulfur isotope ratio in gaseous SO2, aerosol SO4 and sulfate in wet precipitation to quantify the relative importance of the homogeneous and heterogeneous reaction pathways as well as the in‐cloud scavenging of sulfur dioxide for a set of isotopic observations at New Haven, CT, USA.