J.N. Cape

Organic nitrogen deposition on land and coastal environments: a review of methods and data

Despite over a century of published reports of dissolved organic nitrogen (DON) in precipitation, its implications are still being appraised. The number of studies focusing on atmospheric organic nitrogen deposition has increased steadily in recent years, but comparatively little has been done to draw together this disparate knowledge. This is partly a consequence of valid concerns about the comparability of analysis and sampling methodologies. Given the current global trends in anthropogenic nitrogen fixation, an improved qualitative and quantitative understanding of the organic nitrogen component is needed to complement the well-established knowledge base pertaining to nitrate and ammonium deposition. This global review confirms the quantitative importance of bulk DON in precipitation. This cumulative data set also helps to resolve some of the uncertainty that arises from the generally locally and temporally limited scale of the individual studies. Because of analytical and procedural changes in recent decades, assessments are made of the comparability of the data sets; caution is needed in comparisons of individual studies, but the overall trends in the compiled set are more robust. Despite the large number of reports considered, evidence for long-term temporal changes in rainwater organic nitrogen concentrations is ambiguous. With regard to sources, it is likely that some of the organic material observed is not locally generated, but undergoes extensive or long-range atmospheric transport. The compiled data set shows a land-to-sea gradient in organic nitrogen concentration. Possible precursors, reported data on the most likely component groups, and potential source mechanisms are also outlined.

The influence of altitude on rainfall composition at great dun fell

Abstract The influence of altitude on rainfall composition and wet deposition has been investigated at Great Dun Fell in northern England. Measurements of rainfall at eight altitudes between 250 and 850 m on the western slopes of the hill show marked changes in both amount and composition when orographic cloud is present and a west or southwest wind is blowing. On average (20 precipitation events from autumn 1984 and spring 1985), the rainfall at the summit (847 m) exceeded that at 250 m by a factor of 2, and concentrations of SO2−4, NO−3, Cl−, NH+4 and H+ were larger at the summit by factors of between 2.2 and 3.1. Thus, wet deposition at the summit was larger than at 250 m by about a factor of 5. The concentrations of major ions in orographie cloud at 847 m exceeded concentrations in rain by a factor of between 2.0 and 3.9. A large change occurred in the concentrations of major ions in orographie cloud with altitude, decreasing with increasing altitude from cloud base. Such changes could generally be explained by the expected dilution as cloud liquid water content increased adiabatically. When the wind was from the east or with blocked flow no increase in concentration or rainfall amount was observed.

Development and Types of Passive Samplers for Monitoring Atmospheric NO2 and NH3 Concentrations

Numerous passive samplers based on the ‘Palmes-tube’ have been developed for ambient air monitoring. In each case, the diffusion path length and/or crosssectional area are modified to achieve the desired sampling rate. ‘Tube-type’ samplers are low sensitivity samplers suitable for long-term monitoring, whereas the ‘badge-type’ samplers have faster sampling rates suited to short-term monitoring. In the U.K., diffusion tubes are widely used for monitoring nitrogen dioxide (NO2) and ammonia (NH3). The open-ended diffusion tubes are prone to positive bias caused by incursion of wind eddies, leading to a shortening of the diffusion path. By using a porous membrane at the inlet, wind incursion is prevented, but an additional diffusion resistance is imposed and it is necessary to calibrate the tubes against a reference method to obtain an effective sampling rate. For NO2 sampling, positive bias also arises from the reaction of NO with O3 within the sampler. The interference from the chemical reaction is severe close to NO sources, with errors up to 30% for curbside locations when using the ‘tubetype’ sampler. In rural areas, where NO concentrations are small relative to NO2, these errors are small. In some implementations, there is also a negative bias over long sampling periods caused by the degradation of trapped NO2. The low sampling rates of diffusion tubes make them too uncertain for use at background NH3 concentrations (<1 μg NH3 m-3) where they significantly overestimate concentrations. Badge-type samplers such as the ‘Willems badge’ samplers permit accurate sampling at low ambient NH3 concentrations, but suffer from saturation at high concentrations and sensitivity to wind speed. A passive sampler optimised for monthly measurements of NH3 is reported here, together with its application in the U.K. National Ammonia Monitoring Network.

Evidence for changing the critical level for ammonia.

The current critical level for ammonia (CLE(NH3)) in Europe is set at 8mug NH(3) m(-3) as an annual average concentration. Recent evidence has shown specific effects of ammonia (NH(3)) on plant community composition (a true ecological effect) at much smaller concentrations. The methods used in setting a CLE(NH3) are reviewed, and the available evidence collated, in proposing a new CLE(NH3) for different types of vegetation. For lichens and bryophytes, we propose a new CLE(NH3) of 1 microg NH(3) m(-3) as a long-term (several year) average concentration; for higher plants, there is less evidence, but we propose a CLE(NH3) of 3+/-1 microg NH(3) m(-3) for herbaceous species. There is insufficient evidence to provide a separate CLE(NH3) for forest trees, but the value of 3+/-1 microg NH(3) m(-3) is likely to exceed the empirical critical load for N deposition for most forest ecosystems.

Agricultural ammonia emissions inventory and spatial distribution in the North China Plain.

An agricultural ammonia (NH(3)) emission inventory in the North China Plain (NCP) on a prefecture level for the year 2004, and a 5 x 5 km(2) resolution spatial distribution map, has been calculated for the first time. The census database from Chinas statistics datasets, and emission factors re-calculated by the RAINS model supported total emissions of 3071 kt NH(3)-N yr(-1) for the NCP, accounting for 27% of the total emissions in China. NH(3) emission from mineral fertilizer application contributed 1620 kt NH(3)-N yr(-1), 54% of the total emission, while livestock emissions accounted for the remaining 46% of the total emissions, including 7%, 27%, 7% and 5% from cattle, pigs, sheep and goats, and poultry, respectively. A high-resolution spatial NH(3) emissions map was developed based on 1 x 1 km land use database and aggregated to a 5 x 5 km grid resolution. The highest emission density value was 198 kg N ha(-1) yr(-1).

Modelling photochemical oxidant formation, transport, deposition and exposure of terrestrial ecosystems.

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.

The great dun fell cloud experiment 1993: An overview

The 1993 Ground-based Cloud Experiment on Great Dun Fell used a wide range of measurements of trace gases, aerosol particles and cloud droplets at five sites to study their sources and sinks especially those in cloud. These measurements have been interpreted using a variety of models. The conclusions add to our knowledge of air pollution, acidification of the atmosphere and the ground, eutrophication and climate change. The experiment is designed to use the hill cap cloud as a flow-through reactor, and was conducted in varying levels of pollution typical of much of the rural temperate continental northern hemisphere in spring-time.

The cycling of organic nitrogen through the atmosphere

Atmospheric organic nitrogen (ON) appears to be a ubiquitous but poorly understood component of the atmospheric nitrogen deposition flux. Here, we focus on the ON components that dominate deposition and do not consider reactive atmospheric gases containing ON such as peroxyacyl nitrates that are important in atmospheric nitrogen transport, but are probably not particularly important in deposition. We first review the approaches to the analysis and characterization of atmospheric ON. We then briefly summarize the available data on the concentrations of ON in both aerosols and rainwater from around the world, and the limited information available on its chemical characterization. This evidence clearly shows that atmospheric aerosol and rainwater ON is a complex mixture of material from multiple sources. This synthesis of available information is then used to try and identify some of the important sources of this material, in particular, if it is of predominantly natural or anthropogenic origin. Finally, we suggest that the flux of ON is about 25 per cent of the total nitrogen deposition flux.

The influence of altitude on wet deposition. Comparison between field measurements at Great Dun Fell and the predictions of a seeder-feeder model.

The influence of topography on rainfall rate and chemical composition has been investigated at Great Dun Fell in northern England. The measurements at eight different altitudes between 250 and 850 m above sea-level on the western slopes of Great Dun Fell (GDF) show, in the presence of a cap cloud on the hill and a west or southwest wind flow, a marked increase in both rainfall amount and concentrations of major ions with altitude. Three case studies of the variation in rainfall rate and chemical composition with altitude when the seeder-feeder mechanism was operating are described and compared with model predictions. Associated measurements of the cap cloud microphysics and chemistry were also made. The case studies show close agreement between model predictions and measured values for changes in chemical composition and wet deposition with altitude. This pattern of wet deposition is a frequent occurrence at GDF, but exceptions do occur, for example when the upstream flow is blocked or the wind speed is small, no increase in the concentration or rainfall amount was observed.

The contamination of rain samples by dry deposition on rain collectors

A series of short-term experiments at a rural site in eastern Scotland showed that dry deposition of SO2 gas onto pyrex glass rain collectors in dry weather approached saturation after 24 h and contributed c. 80 % of dry-deposited S, the remaining 20 % being attributable to Particulate sulphate. A comparison over 3 years of soluble sulphate from funnel washings after dry days with daily concentrations of SO2 and particulate sulphate was well fitted by a linear dependence on Particulate sulphate and a logarithmic dependence on SO2 concentrations. Particulate sulphate contributed 34 (+- 7) % of the dry deposition on dry days, although there was a marked seasonal variation in the total dry deposit of S on the collector which was independent of SO2 and particulate sulphate concentrations. A comparison of monthly and daily collectors at the same site from April to September was consistent with these estimates of dry deposition on dry days. An extrapolation to sites with similar collectors in northern Britain showed that between 1S and 35 % of the monthly collected non-marine sulphate could have been dry-deposited on the collector.