Chemosphere - Global Change Science最新文献

Based on the in situ measurement of soil moisture and nitrous oxide (N₂O) emission from a rice–wheat rotation ecosystem of southeast China and on the simulated experiments in laboratory, the impact of soil moisture on N₂O emission is investigated. By analyzing the experimental data in detail, some results could be outlined as follows: (a) It is soil moisture and temperature instead of N fertilization that determines the seasonal variation pattern of N₂O emission from the rice-based crop rotation ecosystem of southeast China. (b) Soil moisture is the most sensitive factor to regulate N₂O emission from croplands. (c) Explosive emission of N₂O from the rice-based agro-ecosystem was found to happen at the soil moisture within (110±5)% soil water holding capacity or field capacity (SWHC) or (99±9)% water-filled pore space (WFPS). When soil moisture of the rice–wheat fields is less than 105% SWHC, the N₂O emission was observed to increase exponentially vs. soil moisture. In contrast, N₂O emission was found to decrease reciprocally vs. soil moisture more than 115% SWHC. (d) The response of the N₂O emission rate from soils in fields to variations of soil moisture may be well described with a general empirical equation. For x⩽C₀% SWHC, . For x⩾C₀% SWHC, . The equation to describe the relationship between soil moisture and N₂O emission rates from incubated soil is different from that for fitting data observed in fields. Reasons for the difference still remains uncertain.

Anthropogenic emissions of chemical reactive trace gases have substantially altered the composition of the troposphere. These perturbations have caused tropospheric O₃ increases, in particular in the Northern Hemisphere. It remains, however, difficult to accurately establish O₃ trends throughout the troposphere because the number of sites where surface O₃ measurements and O₃ soundings of high quality are performed are small, especially at low latitudes and throughout the Southern Hemisphere. The longest O₃ surface measurements and sounding records are available from Europe where the upward O₃ trend seems largest, 5–20%/decade; the increase occurred, primarily before 1985. Ozone trends for other mid-latitudinal locations are generally smaller. At high latitudes in the Southern Hemisphere a negative O₃ trend is due to a reduced downward flux of ozone associated with stratospheric ozone depletion and to increased UVB levels, resulting in stronger tropospheric photochemical destruction.

After H₂O, of which tropospheric levels are not expected to change by direct emissions of H₂O from anthropogenic activities, and CO₂, tropospheric O₃ is presently the third most important greenhouse gas. Because of its significant consequences for human health and nature, the large-scale increase in tropospheric O₃ levels is to our opinion one of the most crucial environmental problems to solve during the coming decades.

Field plots of aspen and black spruce in the Alaskan boreal forest were fertilized repeatedly with nitrogen during the 1993 summer growing season, and weekly determinations of the influence of fertilization on atmospheric CH₄ oxidation were made with static chambers. Repeated fertilization with (NH₄)₂SO₄ solution or nutrient media used to culture methanotrophic or nitrifying bacteria gave a total addition of 140 or 580 kg N ha⁻¹. Time-integrated CH₄ oxidation was not significantly different in fertilized soils versus watered controls because CH₄ oxidation was localized in a subsurface soil zone that was probably not penetrated by surface-applied aqueous phase fertilizer. Insensitivity of CH₄ oxidation by these soils to a high rate of N fertilization and the low current rate of atmospheric N deposition suggest that future increases in atmospheric N deposition will not alter the sink strength of high latitude boreal forest soils in the atmospheric CH₄ budget.

An observational based analysis of ozone production for Raleigh and Charlotte, North Carolina, was performed for the years 1981–1990. A trend analysis was carried out for the 10 yr period for Raleigh. The third quartile average for Raleigh indicated a slight upward trend of about 0.5 parts per billion by volume (ppbv) per year in ozone concentration, but this may not be statistically significant. During the period studied, Raleigh was designated as out of compliance for ozone, with a classification of moderate for non-attainment areas in 1989. There were three exceedences of the National Ambient Air Quality Standard (NAAQS) of 0.12 parts per million by volume (ppmv) each in 1980, 1983, and 1987; and 13 exceedences in 1988. Based on a regression analysis, it was identified that the variability in ozone concentration in the Raleigh area is best correlated with maximum temperature and solar radiation, and also weakly correlated with daily average wind speed and wind direction. But, the local meteorological parameters could only explain 35–53% of the total variance. A delta ozone analysis was performed to obtain an estimate of the contribution to the production of ozone made by the metropolitan areas of Raleigh and Charlotte, North Carolina. During the summer of 1989, the city of Raleigh provided an average of about 25 ppbv of additional ozone to air advecting over the city. The amount of ozone produced by the metropolitan area of Charlotte for 1984–1991 averaged about 10–15 ppbv with a slight upward trend in ozone production (1.34±0.78 ppbv per year). These values are compared to a published value of 30–40 ppbv of ozone for Atlanta, Georgia, during 1979–1987.

Toxaphene is an abundant organochlorine (OC) pesticide in Great Lakes and Arctic ecosystems and the Henry's Law constant (HLC, Pa m³/mol) is a critical factor in describing its gas exchange between air and water. The HLCs for technical toxaphene and two hexachlorocyclohexane (HCHs) isomers (α- and γ-HCH) were determined by the gas stripping method over a temperature range of 10–40°C. The relationship to temperature (K) was described by $\text{log}\text{H=m/T+b}$. Parameters of this equation were: toxaphene m=−3209, b=10.42; α-HCH m=−3298, b=10.88; γ-HCH m=−3005 and b=9.51. The HLCs (Pa m³/mol) at 293.15 K were: toxaphene=0.30, α-HCH=0.43 and γ-HCH=0.18.

A comprehensive general circulation model has been coupled with a tropospheric chemistry model (TCM) using “on-line” actinic flux calculation. The three-dimensional global distribution of OH was calculated and is presented in some detail. A 2-year integration generated a volume- and pressure-weighted global, tropospheric, annual mean OH concentration of 8.4×10⁵ molecules/cm³. Over 70 gas phase reactions involving 28 chemical species were solved, using a two-step backward differentiation formula (BDF) combined with Gauss–Seidel iteration. The set of chemical equations was solved every model hour. “On-line” actinic flux calculation allows for photo-radiation feedback between the two model components. Local changes in clouds and radiatively active gas concentrations directly affect the availability of actinic flux which has a direct impact on photochemistry through the photolysis rate constant. The actinic flux was efficiently calculated in each grid cell every model hour by the delta-Eddington radiation scheme of the general circulation model. The spectral resolution of the radiation scheme was 5 nm between 200 and 400 nm, 2 nm between 245 and 350 nm, and 25 nm between 350 and 700 nm. This provided for accurate calculations in the photolytically active spectral regions of O₃ and NO₂.

One of the most important aspects of global change is that of stratospheric ozone depletion and the resulting increase in UV radiation reaching the surface of the Earth. Some 70% of the Earth surface is covered by water containing an extremely complicated milieu of organic and inorganic chemical species. The photochemical production and transformation of various greenhouse and chemically reactive gases in the ocean has been a focus of much study over the last century. We assess the implications of increased UV radiation on aquatic and marine boundary layer biogeochemistry with a focus on trace gases that exchange between the ocean and the atmosphere. CO₂, DMS, CO, OCS, CH₄, N₂O, non-methane hydrocarbons (NMHCs) and organohalogens are considered within the context of changing surface ocean UV fluxes and various feedbacks upon the integrated climate system. Links between the upper ocean photochemical environment and the lower atmosphere are stressed. Once in the atmosphere, these gases each play a different role in modulating several aspects of atmospheric chemistry and by implication atmospheric circulation and climate dynamics such as precipitation patterns, surface temperatures and surface–atmosphere substance exchange. We augment the conceptual models proposed with new observational data on surface ocean concentrations from the southern hemisphere obtained under a range of UV exposures.

The ambient levels of methane in the urban environment of Delhi were measured during November 1994–June 1995, at 13 sites varying in anthropogenic activities and traffic density. The methane levels in the ambient urban environment of Delhi varied from 1703 to 9492 ppbv, with an average concentration of 4121 ± 354 ppbv, exhibiting diurnal and seasonal variation. The likely reasons for the elevated methane level (above the global average of 1737 ppbv) in the urban environment of Delhi and its implications for air qualities have been discussed.

Vertical fluxes of a series of monoterpenes emitted by orange trees along the Spanish Mediterranean coast have been measured in the atmospheric surface layer in June 1997 by the relaxed eddy accumulation (REA) and vertical gradient (VG) methods. In both approaches, the products to be analysed were trapped on Tenax-TA coated steel tubes and subsequently analysed by GC–MS. The concentrations obtained were treated in combination with micrometeorological observations made by sonic anemometers. The fluxes obtained by the two methods are weak but show similar behaviour. The influence of chemical destruction on the observed monoterpene fluxes can be investigated by calculating the Damköhler number, i.e. the ratio of the dynamic and chemical characteristic timescale by considering that the chemical destruction pathways for monoterpenes occurred through reactions with ozone and OH, the values obtained for the Damköhler number were all lower than 0.05, confirming the low influence of chemical destruction on the flux measurements.

To understand the long-term and short-term variations in monoterpene emissions from Australian native trees and the factors which influence these variations, the seasonal variations in monoterpene emissions for 15 Eucalyptus species as well as the monthly and diurnal variations in monoterpene emissions for E. globulus were investigated using a dynamic flow chamber technique. Distinct seasonal variations both in chemical characteristics of monoterpene emissions and the monoterpene emission rates (normalised to 30°C) were characterised. The normalised monoterpene emission rates showed high emission rates during summer and low rates during other seasons for many Eucalyptus species. For example, the average normalised total monoterpene emission rate on a leaf mass basis (μg g⁻¹ h⁻¹) for E. globulus in summer was 5.4, 2.9 in autumn, 2.3 in spring and 1.5 in winter. The seasonal variation patterns appeared to be species-specific. The maximum values of monthly average total emission rates for E. globulus occurred in January (9.4 μg g⁻¹ h⁻¹ or 1.6 mg m⁻² h⁻¹), and the minimum values were in July (0.74 μg g⁻¹ h⁻¹ or 0.16 mg cm⁻² h⁻¹). The measured diurnal emission rates showed that there was a maximum emission rate at noon and two emission peaks at midnight and pre-dawn. Leaf temperature could be responsible for the diurnal variation in monoterpene emission rate under nonwetting leaf conditions. Temperature, light and relative humidity showed correlations with the variation in the average normalised monoterpene emission rate in E. globulus, but not with the variation in monoterpene emission composition. The variation in emission composition may be mainly controlled by the factors associated with leaf age. An algorithm for estimating monthly temperature-independent mean monoterpene emission rate from E. globulus was developed.