The first long-term concurrent measurements of trichloroacetic acid (TCA) in rainwater, in cloudwater, and in air (both gas and particle phase) are reported. Measurements were made weekly between ...June 1998 and April 2000 at a rural forested upland site in SE Scotland. Rainwater TCA concentration did not differ significantly between two elevations (602 and 275 m asl), with precipitation-weighted mean values of 0.77 and 0.70 μg L-1, respectively (n > 75). The precipitation-weighted mean concentration of TCA in cloudwater at the highest elevation was 0.92 μg L-1, yielding an average cloudwater enrichment factor of 1.2, considerably lower than for other inorganic ions measured. Rainwater and cloudwater TCA concentrations did not vary systematically with season. Since wet precipitation depth also did not vary systematically with season, the wet deposition fluxes of TCA were likewise invariant (annual fluxes at the highest elevation of 880 and 130 μg m-2, respectively, for rain and cloud interception to spruce forest). Weekly integrated concentrations of TCA in air (gas and particle) were very low (median 25 pg m-3, range <LOD−110 pg m-3). The estimated upper limit for annual dry deposition of TCA at this site was ∼20 μg m-2, assuming a deposition velocity of 2 cm s-1. Concentrations of TCA in air correlated reasonably strongly with concentrations in rainwater, with a partition ratio approximately equal to the Henry's law coefficient. On average, only about 23% of TCA measured in Edinburgh air was associated with the particle phase. These measurements are consistent with the observed high scavenging ratio of TCA (ratio of concentration in air to concentration in rainwater). Overall, these data confirm that the atmosphere is an important source of TCA to the environment and that precipitation is the dominant transfer mechanism. In line with previous work, the atmospheric deposition flux is greater than expected from the current understanding of atmospheric production of TCA from anthropogenic precursors. It is suggested that aqueous-phase processes could lead to greater atmospheric conversion of chlorinated solvent precursors to TCA than is currently accepted.
Negative-parity bands in the vicinity of 156Gd and 160Yb have been suggested as candidates for the rotation of tetrahedral nuclei. We report the observation of the odd and even-spin members of the ...lowest energy negative-parity bands in 160Yb and 154Gd. The properties of these bands are similar to the proposed tetrahedral band of 156Gd and its even-spin partner. Band-mixing calculations are performed and absolute and relative quadrupole moments deduced for 160Yb and 154Gd. The values are inconsistent with zero, as required for tetrahedral shape, and the bands are interpreted as octupole vibrational bands. The failure to observe the in-band E2 transitions of the bands at low spins can be understood using the measured B(E1) and B(E2) values.
In this study, the concentrations of phenol, four nitrated phenols, their precursors and reactants in air and cloud water, are presented. The concentrations in air and cloud water were measured ...simultaneously at the summit of Great Dun Fell (GDF). The measured concentrations were compared with emission data, leading to the conclusion, that in particular dinitrophenols are formed by atmospheric reactions, while car exhaust accounts to a significant extent for the mononitrophenols observed. The experimental results point to a formation of dinitrophenols in the liquid phase (cloud droplets). This is corroborated by flow tube experiments which show that phenol in aqueous solution reacts with N
2O
5 and ClNO
2 to form nitrophenols.
The chemical processes responsible for production of photochemical oxidants within the troposphere have been the subject of laboratory and field study throughout the last three decades. During the ...same period, models to simulate the atmospheric chemistry, transport and deposition of ozone (O
3) from individual urban sources and from regions have been developed. The models differ greatly in the complexity of chemical schemes, in the underlying meteorology and in spatial and temporal resolution. Input information from land use, spatial and temporally disaggregated emission inventories and meteorology have all improved considerably in recent years and are not fully implemented in current models. The development of control strategies in both North America and Europe to close the gaps between current exceedances of environmental limits, guide values, critical levels or loads and full compliance with these limits provides the focus for policy makers and the support agencies for the research. The models represent the only method of testing a range of control options in advance of implementation. This paper describes currently applied models of photochemical oxidant production and transport at global and regional scales and their ability to simulate individual episodes as well as photochemical oxidant climatology. The success of current models in quantifying the exposure of terrestrial surfaces and the population to potentially damaging O
3 concentrations (and dose) is examined. The analysis shows the degree to which the underlying processes and their application within the models limit the quality of the model products.
Water soluble organic nitrogen (WSON) compounds are ubiquitous in precipitation and in the planetary boundary layer, and therefore are a potential source of bioavailable reactive nitrogen. This paper ...examines weekly rain data over a period of 22 months from June 2005 to March 2007 collected in 2 types of rain collector (bulk deposition and “dry + wet” deposition) located in a semi-rural area 15 km southwest of Edinburgh, UK (N55°51′44″, W3°12′19″). Bulk deposition collectors are denoted in this paper as “standard rain gauges”, and they are the design used in the UK national network for monitoring precipitation composition. “Dry + wet” deposition collectors are flushing rain gauges and they are equipped with a rain detector (conductivity array), a spray nozzle, a 2-way valve and two independent bottles to collect funnel washings (dry deposition) and true wet deposition. On average, for the 27 weekly samples with 3 valid replicates for the 2 types of collectors, dissolved organic nitrogen (DON) represented 23% of the total dissolved nitrogen (TDN) in bulk deposition. Dry deposition of particles and gas on the funnel surface, rather than rain, contributed over half of all N-containing species (inorganic and organic). Some discrepancies were found between bulk rain gauges and flushing rain gauges, for deposition of both TDN and DON, suggesting biological conversion and loss of inorganic N in the flushing samplers.
Birdsfoot trefoil and broad-leaved dock were affected by VOCs.
A selection of herbaceous plants representing the ground flora around a typical chemical installation in the UK was exposed continuously ...for 7 weeks to a mixture of six VOCs (acetone, acetonitrile, dichloromethane, ethanol, methyl
t-butyl ether and toluene) in open-top chambers. Exposure concentrations were based on predictions of atmospheric dispersion from a single source, at a distance of approximately 2 km. The effects of continuous exposure, representing a worst-case, were measured in terms of uncontrolled water loss from leaves, leaf wettability, chlorophyll content and fluorescence, dry matter production and detailed observations of changes in plant growth and phenology. There were significant effects of VOC exposure on seed production, leaf water content and photosynthetic efficiency in some plant species. Such effects may be detectable in vegetation close to major industrial point sources of VOCs, or as a result of an accidental release of material during manufacture or transport. Some of the species tested e.g. birdsfoot trefoil (
Lotus corniculatus L.) seem to be promising as potential bioindicators for VOCs, but there may be other even more sensitive species waiting to be discovered. However, the most obvious and conveniently measured response to VOC exposure in the birdsfoot trefoil (premature senescence i.e. advanced timing of seed pod production) could easily be confused in the field with climatic influences. It is also uncertain at this stage whether any of the effects observed would lead to longer term ecological changes in natural plant communities, through biased competition between sensitive and more tolerant species.
Emissions of reactive oxidized nitrogen (NO and NO2),
collectively known as NOx, from human activities are c.
21 Tg N
annually, or 70% of global total emissions. They occur predominantly
in ...industrialized regions, largely
from fossil fuel combustion, but also from increased use of N
fertilizers. Soil emissions of NO not only make an
important contribution to global totals, but also play a part in
regulating the dry deposition of NO and NO2 (NOx)
to plant canopies. Soil microbial production of NO leads to a soil
‘compensation point’ for NO deposition or
emission, which depends on soil temperature, N and water status.
In warm conditions, the net emission of NOx
from plant canopies contributes to the photochemical formation of ozone.
Moreover, the effect of NOx emissions
from soil is to reduce net rates of NO2 deposition to
terrestrial surfaces over large areas. Increasing anthropogenic emissions of NOx have led to an
approximate doubling in surface O3 concentrations
since the last century. NOx acts as a catalyst for the
production of O3 from volatile organic compounds (VOCs).
Paradoxically, emission controls on motor vehicles might lead to increases
in O3 concentrations in urban areas. Removal of NO and NO2 by dry deposition is regulated to some
extent by soil production of NO; the major
sink for NO2 is stomatal uptake. Long-term flux measurements
over
moorland in Scotland show very small
deposition rates for NO2 at night and before mid-day of
1–4 ng NO2-N m−2 s−1,
and similar emission rates during
afternoon. The bi-directional flux gives 24-h average deposition velocities
of only
1–2 mm s−1, and implies a long
life-time for NOx due to removal by dry deposition. Rates of removal of O3 at the ground are also influenced
by stomatal uptake, but significant non-stomatal uptake
occurs at night and in winter. Measurements above moorland showed 40%
of total annual flux was stomatal, with
60% non-stomatal, giving nocturnal and winter deposition velocities of
2–3 mm s−1 and daytime summer values
of 10 mm s−1. The stomatal uptake is responsible for adverse
effects on vegetation. The critical level for O3
exposure (AOT40) is used to derive a threshold O3
stomatal
flux for wheat of 0·5 μg m−2 s−1.
Use of modelled
stomatal fluxes rather than exposure might give more reliable estimates
of yield
loss; preliminary calculations
suggest that the relative grain yield reduction (%) can be estimated as
38 times the stomatal ozone flux (g m−2)
above the threshold, summed over the growing season.
The reaction of ozone (O
3) with α-pinene has been studied as a function of temperature and relative humidity and in the presence of wax surfaces that simulate a leaf surface. The objective was to ...determine whether the presence of a wax surface, in which α-pinene could dissolve and form a high surface concentration, would lead to enhanced reaction with O
3. The reaction of O
3 itself with the empty stainless steel reactor and with aluminium and wax surfaces demonstrated an apparent activation energy of around 30
kJ
mol
−1 for all the surfaces, similar to that observed in long-term field measurements of O
3 fluxes to vegetation. However, the absolute reaction rate was 14 times greater for aluminium foil and saturated hydrocarbon wax surfaces than for stainless steel, and a further 5 times greater for beeswax than hydrocarbon wax. There was no systematic dependence on either relative or absolute humidity for these surface reactions over the range studied (20–100% RH). Reaction of O
3 with α-pinene occurred at rates close to those predicted for the homogeneous gas-phase reaction, and was similar for both the empty reactor and in the presence of wax surfaces. The hypothesis of enhanced reaction at leaf surfaces caused by enhanced surface concentrations of α-pinene was therefore rejected. Comparison of surface decomposition reactions on different surfaces as reported in the literature with the results obtained here demonstrates that the loss of ozone at the earth's surface by decomposition to molecular oxygen (i.e. without oxidative reaction with a substrate) can account for measured ‘non-stomatal’ deposition velocities of a few mm
s
−1. In order to quantify such removal, the effective molecular surface area of the vegetation/soil canopy must be known. Such knowledge, combined with the observed temperature-dependence, provides necessary input to global-scale models of O
3 removal from the troposphere at the earth's surface.
Workable pollution abatement policies and effective legislation must be based on sound science. However, despite many years of research, there are still uncertainties about the effects of atmospheric ...nitrogen compounds on crops and other vegetation. This paper reviews the current state of knowledge of the main compounds, focussing on the concentrations and combinations of pollutants that occur in rural areas. The sources, concentrations and effects of oxidised, reduced and organic nitrogen compounds are considered in turn, then the effects of deposited nitrogen on ecosystems are discussed. Research priorities on the effects of deposited nitrogen in Europe and the USA are compared. Finally, the review leads to a list of issues for discussion and recommendations for research.
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