Chemosphere - Global Change Science最新文献

Gaseous methane (CH₄) emissions were determined during the winter and summer from `farrow-to-finish' (FF) swine production houses and during the summer from a `farrow-to-wean' (FW) house in 1998 in the coastal plains of North Carolina. The houses were instrumented with sensors to determine cycling of the individual forced-ventilation fans. Laser spectrometry was used to measure CH₄ concentration differences between the intake and exhaust points of the houses. Differences in CH₄ concentrations were combined with fan operation data to calculate CH₄ fluxes from the houses. During the cold winter measurement period, CH₄ fluxes averaged 6.9 g $\text{CH}{}_4\text{animal}{}^{\mathrm{-1}}\text{d}{}^{\mathrm{-1}}$ in the FF house. During summer measurement periods, CH₄ fluxes were much greater and averaged 33 and 46 g $\text{CH}{}_4\text{animal}{}^{\mathrm{-1}}\text{d}{}^{\mathrm{-1}}$ from the FF and FW houses, respectively. The much larger emissions during the summer than winter, indicate that CH₄ house emissions were primarily from fresh feces and the underground storage/wash pits containing lagoon effluent; and not directly from the animals since temperature would have little affect on direct animal emission. Emission factors based on animal units (au) of 454 kg animal⁻¹ were much greater at the FW farm with a pull-plug waste management system (7–8 day wash cycle) than at the FF farm with a periodic flush system (8 h wash cycle).

Measurements of the mixing ratios of tropospheric NO and NO_Y (defined as nitric oxide (NO) + nitrogen dioxide (NO₂) + peroxyacetyl nitrate (PAN) + nitric acid (HNO₃) + particulate nitrate (NO₃⁻) + ⋯) were made over a semi-urban area of central North Carolina at the surface (10 m) and on a tower at heights of 250 m (820 ft) and 433 m (1420 ft) above ground level (AGL) from December 1994 to February 1995. These measurements were compared with synoptic weather data and regional and local upper air soundings in an effort to characterize NO and NO_Y in the planetary boundary layer in terms of their vertical distributions, diurnal profile, and related transport mechanisms. A pronounced decreasing vertical gradient in both NO and NO_Y mixing ratios was observed, with a distinct diurnal cycle and nocturnal minimum. Furthermore, the results suggest that NO and NO_Y were mixed upward from the surface during passage of synoptic meteorological features (and their associated vertical motions). Most importantly, the data reveals that mixing ratios of NO and NO_Y at the elevated heights did not exist in sufficient concentrations above the inversion layer in the nocturnal boundary layer to be mixed downward upon breakup of the nocturnal inversion and affect surface measurements. Instead, concentrations of NO and NO_Y were apparently mixed upward during the morning and midday hours by vertical boundary layer processes. Thus, the association of observed increases in surface NO and NO_Y mixing ratios based solely on downward mixing processes is not justified in all cases, and other sources and processes for these increases must be considered, particularly over rural areas.

The flux of volatile organic chemicals from natural vegetation influences various atmospheric properties including oxidation state of the troposphere via the hydroxyl radical (OH), photochemical haze production and the concentration of greenhouse gases (CH₄, H₂O, CO). Because the Volatile Organic Compound (VOC) flux in the present-day world varies markedly with both vegetation cover and with climate, changes in the emission of VOCs may have damped or amplified past climate changes.

Here we conduct a preliminary study on possible changes in VOC emission resulting from broad scale vegetation and climate change since the Last Glacial Maximum (LGM). During the general period of the LGM (∼25–17,000 years before present {BP}), global forest cover was considerably less than in the present potential situation. The change in vegetation would have resulted in a ∼30% reduction in VOC emission at 643 Tg y⁻¹ relative to the present potential vegetation (912.9 Tg y⁻¹). Uncertainty in global vegetation cover during the LGM bounds the VOC estimate by ±15%. In contrast, during the warmer early-to-mid Holocene (8000 and 5000 BP), with greater forest extent and less desert than during the late Holocene (0 BP), emission rates of VOCs seem likely to have been higher than at present.

Further modifications in VOC emission may have been mediated by a reduction in mean tropical lowland temperatures (by around 5–6°C) decreasing the LGM VOC emission estimate by 38% relative to the warmer LGM scenario.

Increased VOC emissions due to forest expansion and increased tropical temperatures since the LGM may have served as a significant driver of climate change over the last 15 ka y through the influence of VOC oxidation; this can impact tropospheric radiative balance through reductions in the concentration of OH, increasing the concentration of CH₄.

The error limits on past VOC emission estimates are large, given the uncertainties of present-day VOC emission rates, paleoecosystem distribution, tropical paleoclimatic conditions, and physiological assumptions regarding controls over VOC emission. Nevertheless, the potential significance of changes in natural VOC emission over the last 20 ka and their influence on climate are an important unknown that should at least be borne in mind as a limit on the understanding of past atmospheric conditions. Elucidation of the role of VOCs in climate change through paleoclimatic general circulation model simulations may improve understanding of past and future changes in climate.

The concentration of atmospheric methane in Beijing is still increasing, although its annual trend has significantly decreased from 2.0% yr⁻¹ in 1985–1989 to 0.5% yr⁻¹ in 1990–1997. The seasonal variability of methane concentration apparently appeared in a double-peak pattern with one peak in winter and the other in summer. It is known that the seasonal inter-annual variations of atmospheric methane in Beijing are different from year to year. From 1986 to 1997, the concentrations of atmospheric methane increased by 184 ppbv, in which about 37% was due to its increase in winter and 21% in summer. After 1993, the trends of methane concentration in summer, which are mainly due to emission from biogenic sources, are negative, while their trends in winter, which are mainly due to emission from non-biogenic sources, are positive with a value of about 25 ppbv yr⁻¹. As a result, the seasonal inter-annual trends from 1993 to 1997 were mainly due to the increase of methane emission from non-biogenic sources in winter. It implies, therefore, that in Beijing the biogenic sources have been decreasing but the non-biogenic ones, such as fossil fuel combustion, have increased.

Major ion and trace metal concentrations were determined in aerosols and cloud water at a site in the Himalayan Mountains of Northern Pakistan. In spite of the fact that the site is well removed from significant urban/industrial pollution sources the SO²⁻₄ concentrations in some of the samples were as high as those observed in North America. Concentrations of Se, Tl, Pb, Cl, Cd, Sb, Zn, and As in aerosols were highly enriched relative to average crustal abundances indicating significant anthropogenic contributions. Cloud water concentrations of major ions and trace elements are reported for 18 samples from six different clouds. The pH varied between 5.3 and 6.8 in spite of the fact that the SO²⁻₄ concentration approached 300 μmol in some samples, values often observed in the northeastern US. Selenium was used as a tracer to determine in-cloud production of SO²⁻₄ in these clouds and in three of the six clouds 40–60% of the observed SO²⁻₄ came from in-cloud production.

The transport of gases forming in the sedimentary (active) layer of water basins and coming from the atmosphere is discussed. Diffusion equations are used to derive differential equations describing a change in the concentrations in the sedimentary layer with depth. The asymptotic equations describing a change in gas concentration with active layer depth have been derived. It is shown that the rate of gas generation (W) and the position of the upper boundary of bubbling (h) are related via the relationship W∼h⁻². A comparison has been made with experimental data.

The effect of green manure amendment, flooding treatment and crop season on methane emission from paddy fields in Taiwan was investigated from August 1994 to July 1996. Sesbania amendment stimulated methane emission and the effect was more significant at the early growth stage of rice. Methane emission was higher in continuous flooding treatment than that in intermittent irrigation. Both redox potential and methane emission showed significant differences between these two irrigation systems. Methane concentration increased sharply with the depth of soil in the intermittent irrigation system due to oxidation; whereas it increased moderately in the continuous flooding treatment. The seasonal methane flux in the first crop season with chemical fertilizer was between 2.73 and 5.23 g m⁻²; while the value was between 10.54 and 10.56 g m⁻² in the second crop season. In the case of Sesbania amendment in the second crop season, the seasonal methane flux in the first crop season was 6.35 g m⁻²; while the value was between 14.43 and 30.12 g m⁻² in the second crop season. Total methane emission in the second crop season was about two to five-fold higher than that in the first crop season.