An experimental and theoretical investigation of the dry deposition of particles to snow,pine trees and artificial collectors

Submicrometer and supermicrometer ammonium sulphate particles tagged with the radioisotope ³⁵S were released upwind of natural snow, trees and artificial collectors in a large field containing micrometeorological and air monitoring equipment. At a reference height of 10 cm, the average deposition velocity for 0.7 μm diameter particles was observed to be 0.039 and 0.096 cm s⁻¹ in two experiments conducted at a wind speed of 2.7 and 2.4 m s⁻¹ under stable and unstable conditions, respectively. Supermicrometer particles having an effective diameter of 7 μm were deposited to snow at a rate of 0.16 and 0.096 cm s⁻¹ in two experiments conducted under unstable conditions at a wind speed of 2.4 m s⁻¹. Compared to snow, all artificial collectors used in this study under-collected submicrometer particles and over-collected supermicrometer particles. Individual white pine branches scavenged submicrometer particles with an average efficiency of 0.015%. Comparisons of the theoretical and observed deposition rates of submicrometer particles to snow suggest that hygroscopic growth of sulphate particles in the humid nearsurface snow layer and interception by ice needles play a role in the deposition process.     

An experimental study of the dry deposition of gaseous nitric acid to snow

A chamber placed in a constant temperature freezing room was used to study the surface resistance during deposition of HNO₃ to a snow surface. The resistance decreased with increasing temperature from larger than 5 s mm⁻¹ at − 18°C to about l s mm⁻¹ at −3°C. Measurements of gaseous and particulate nitrate concentrations during winter at a rural site in south central Sweden gave concentrations in the range of 0.4–5 μg HNO₃ m⁻¹ and 0.3–3 μg NO⁻₃ m⁻³ with a mean value of 1.3 μg HNO₃ m⁻³ and 0.7 μg NO⁻₃ m⁻³, respectively. The results indicate that for periods with temperatures below − 2°C estimated dry deposition of HNO₃ to snow is at most 4 % of measured wet deposition of nitrate in the area.     

Ecosystem effects of atmospheric deposition of nitrogen in The Netherlands

Atmospheric deposition of inorganic N, mainly ammonium volatilized from manure produced in intensive stockbreeding, on sensitive terrestrial and aquatic ecosystems in The Netherlands is in the order of 40 to 80 kg ha(-1) year(-1). Proven effects of this deposition are (i) eutrophication with N, leading to floristic changes (ii) acidification of base-poor sandy soils and of moorland pools, leading to higher concentrations of dissolved, potentially toxic metals such as Al3+, and (iii) increased levels of nitrate in groundwater below woodlands. In acid forest soils, but not in soils under heathland, nitrification and leaching of nitrate is common. However, in very poor sandy forest soils and at very high ammonium inputs, nitrification may be too slow to prevent the development of high concentrations of ammonium. Both excessive acidification and excessive levels of ammonium probably play an important role in the general forest decline, which is most severe in the southern and central parts of the country, where ammonium inputs are highest.     

Bioindicators of enhanced nitrogen deposition

Increased deposition of atmospheric N largely from intensive agriculture is affecting biodiversity and the composition of natural and semi-natural vegetation in Europe. The value of species based bioindicators such as the Ellenberg N index and measurements of total tissue N and free amino acids in key plant species, is described with reference to a mixed woodland downwind of a livestock farm in the Scottish Borders, operated for over 20 years with a measured spatial gradient of ammonia concentration (29-1.5 microg m(-3)). All the indicators examined showed a relationship with N deposition and provided some indication of vegetation change. Total tissue N and arginine concentrations were most closely linked with ammonia concentrations and N deposition, with r(2) values of >0.97 and >0.78 respectively.     

Phosphorus supply influences heathland responses to atmospheric nitrogen deposition

On an upland moor dominated by pioneer Calluna vulgaris and with an understorey of mosses and lichens, experimental plots were treated with factorial combinations of nitrogen (N) at +0 and +20kg Nha(-1)yr(-1), and phosphorus (P) at +0 and +5kg Pha(-1)yr(-1). Over the 4-year duration of the experiment, the cover of the Calluna canopy increased in density over time as part of normal phenological development. Moss cover increased initially in response to N addition but then remained static; increases in cover in response to P addition became stronger over time, eventually causing reductions in the cover of the dominant Calluna canopy. Lichen cover virtually disappeared within 4 years in plots receiving +20kg Nha(-1)yr(-1) and also in separate plots receiving +10kg Nha(-1)yr(-1), but this effect was reversed by the addition of P.     

An experimental investigation of incremental reactivities of volatile organic compounds

California has adopted a set of VOC reactivity factors for regulatory purposes that is based on a model of the ozone formation process. These incremental reactivity factors (derived by Carter) describe the amount of ozone each exhaust VOC will form under a certain set of conditions in an urban atmosphere. The main objective of this study is to measure reactivity factors using smog chamber experiments, and to compare the measurements to the Carter factors. A new facility was constructed explicitly for this study. The facility has four identical smog chambers and a temperature-controlled enclosure for the chambers. The chambers are irradiated using a set of filtered xenon arc lamps to approximate “sunlight”. The reactivities of 14 individual VOCs representative of those found in automobile exhaust and several mixtures of VOCs have been measured. The measured and Carter-reactivity factors were highly correlated, suggesting that the chemical model used by Carter accounts for the reactivities of a wide range of compounds with dramatically different uncertainties in their mechanisms. The measured results, in general, are consistent with the Carter-reactivity factors for comparing the relative reactivities of VOCs in the atmosphere. However, additional kinetic and mechanistic studies of many VOC species including aromatic isomers are needed to improve reactivity scales.     

The effects of atmospheric nitrogen deposition on the soil chemistry of coniferous forests in The Netherlands

Nitrogen fluxes, particularly those of ammonium, are extremely high in Dutch forests. In soils exposed to high ammonium deposition, acidification, eutrophication or a combination of both processes may occur. In addition to the amounts of ammonium deposited, the rate of soil nitrification determines which process takes place. A nation-wide investigation, in which three coniferous tree species were involved, was carried out to study the relation between deposition fluxes, measured by means of throughfall and bulk samplers, and the chemical composition of the soil. The ammonium deposition accounted directly for the high ammonium content and the high ammonium/cation ratios in the soil. In the top layer of most of the forest soils which were investigated nitrification rates were low. In these stands ammonium/cation ratios in the soil often reflected ammonium/cation ratios in throughfall water. Even in soils with relatively high nitrification rates, ammonium concentrations exceeded those of nitrate in the top layer of the mineral soil, indicating that ammonium deposition was more important than nitrification rate in determining the predominant form of nitrogen.     

Diagnostic indicators of elevated nitrogen deposition

Tissue N content of mosses, which has been shown to be an indicator of enhanced N, was studied at a range of locations dominated either by wet or dry deposited and oxidised and reduced forms of N. Tissue N responded differently to wet and dry deposited N. For a 1 kg ha(-1) y(-1) increase in N deposition, tissue N increased by 0.01% at wet deposition sites but by 0.03% at sites dominated by dry deposited NH3. Tissue N at wet deposition sites responded more to concentrations of NO3- and NH4+ in precipitation (r(2) 0.63) than to total N deposition (r(2) 0.27), concentration explaining 66% of the variation in tissue N, wet deposition 33%. The study clearly concludes that tissue N concentration in mosses provides a good indication of N deposition at sites where deposition is dominated by NH3, and is also valuable in identifying vegetation exposed to large concentrations of NH4+ or NO3-, in wet deposition dominated areas, such as hilltops and wind exposed woodland edges.     

The effects of excess nitrogen deposition on young Norway spruce trees. Part II the vegetation

The effects of wet-deposited nitrogen on soil acidification and the health of Norway spruce were investigated in a pot experiment using an open-air spray/drip system. Nitrogen was applied as ammonium ((NH(4))(2)SO(4)) or nitrate (HNO(3)/NaNO(3)) in simulated rain to either the soil or the foliage. Symptoms of forest decline as observed in the field were not reproduced, and there was no evidence of direct toxicity. Treatments did, however, have significant effects on tree nutrition. Both NH(+)(4) and NO(-)(3) treatment applied to the foliage lowered foliar K concentrations. NH(+)(4) to a greater extent. Soil-applied NH(+)(4) reduced foliar Mg concentrations and increased foliar Al and Fe. Soil-applied NO(-)(3) significantly reduced foliar P concentrations, and at high doses prevented the alleviation of P deficiency by fertiliser. These effects could be important in some field situations. Ammonium deposition is predicted to be more damaging than nitrate deposition, although the latter may be critical for forests where P status is marginal, such as in parts of the British uplands.     

The effects of excess nitrogen deposition on young Norway spruce trees. Part I the soil

The effects of wet-deposited nitrogen on soil acidification and the health of Norway spruce were investigated in a pot experiment using an open-air spray/drip system. Nitrogen was applied as ammonium ((NH(4))(2)SO(4)) or nitrate (HNO(3)/NaNO(3)) in simulated rain to either the soil or the foliage for a period of two years five months. Symptoms of forest decline were not reproduced. Adverse effects relating to soil acidification and N saturation were observed and depended on the chemical form of N. The plant-soil system absorbed most of the soil-applied NH(+)(4) at doses of up to 65 kgN ha(-1) year(-1) but only 54% at a dose of 125 kgN ha(-1) year(-1). About 60% of soil-applied NO(-)(3) was absorbed in all treatments. Ammonium treatments acidified the soil, increased base cation leaching, and mobilised acidic cations. Nitrification was not the major source of acidity, however. Nitrate inputs increased soil pH. Critical loads calculated using current criteria were 60-120 and 30-60 kgN ha(-1) year(-1) for ammonium and nitrate, respectively. Ammonium is more likely to damage forest ecosystems, however, illustrating the need for care in the definition of critical loads.     

Vegetation composition of roadside verges in Scotland: the effects of nitrogen deposition, disturbance and management

Vehicular emissions of NO(x) and NH(3) result in elevated concentrations of nitrogen at roadside verges. To determine the extent that vehicular nitrogen emissions, disturbance and management affect the vegetation composition of road verges, a survey of 92 verges in Scotland was carried out with sites stratified by background nitrogen deposition and road type. NO(x) and NH(3) concentrations were monitored at 15 key sites for a year, and showed a decreasing gradient with increasing distance from the road. Ellenberg fertility indices of the vegetation communities also showed a general decrease with increasing distance from the road, but there was no straightforward correlation with NO(x) and NH(3) air concentrations between sites. Cover of bare ground, ruderal species and salt-tolerant species were highest at the verge edge. The proximity of the verge to traffic is important both in terms of NO(x) and NH(3) gradients, but also for deposited salt, grit and physical disturbance.     

Concentrations of ammonia and nitrogen dioxide at roadside verges, and their contribution to nitrogen deposition

Bimonthly integrated measurements of NO2 and NH3 have been made over one year at distances up to 10 m away from the edges of roads across Scotland, using a stratified sampling scheme in terms of road traffic density and background N deposition. The rate of decrease in gas concentrations away from the edge of the roads was rapid, with concentrations falling by 90% within the first 10 m for NH3 and the first 15 m for NO2. The longer transport distance for NO2 reflects the production of secondary NO2 from reaction of emitted NO and O3. Concentrations above the background, estimated at the edge of the traffic lane, were linearly proportional to traffic density for NH3 (microg NH3 m(-3) = 1 x 10(-4) x numbers of cars per day), reflecting emissions from three-way catalysts. For NO2, where emissions depend strongly on vehicle type and fuel, traffic density was calculated in terms of 'car equivalents'; NO2 concentrations at the edge of the traffic lane were proportional to the number of car equivalents (microg NO2 m(-3) = 1 x 10(-4) x numbers of car equivalents per day). Although absolute concentrations (microg m(-3)) of NH3 were five times smaller than for NO2, the greater deposition velocity for NH3 to vegetation means that approximately equivalent amounts of dry N deposition to road side vegetation from vehicle emissions comes from NH3 and NO2. Depending on traffic density, the additional N deposition attributable to vehicle exhaust gases is between 1 and 15 kg N ha(-1) y(-1) at the edge of the vehicle lane, falling to 0.2-10 kg N ha(-1) y(-1) at 10 m from the edge of the road.     

半干半湿法脱硫灰循环利用中试试验研究

半干半湿法脱硫产生的脱硫灰中混有大量有效钙和粉煤灰 ,本文对其进行了循环利用机理分析及试验研究 ,证实脱硫灰循环大大提高了钙的利用率 ,降低了钙硫比 ,但循环倍率增加到 2倍以上时 ,脱硫效率增速变缓。脱硫灰的循环利用对降低脱硫运行费用是非常有效的。     

Atmospheric nitrogen deposition in south-east Scotland: Quantification of the organic nitrogen fraction in wet,dry and bulk deposition

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.     

Regional and historical variation in the nitrogen content of Racomitrium lanuginosum in Britain in relation to atmospheric nitrogen deposition

The moss Racomitrium lanuginosum (Hedw.) Brid. is an important component of the drier parts of ombrotrophic mires and montane heaths in north-western Britain. The extent and quality of the montane heaths dominated by R. lanuginosum has declined in recent decades, perhaps in part due to the effects of acidic deposition at high elevations. This paper examines the effect of atmospheric nitrogen deposition, which has increased during this century, on the nitrogen content of R. lanuginosum in Britain. The nitrogen content of the moss reflects the magnitude of the atmospheric supply being least in north-western Scotland and greatest (as much as six-fold greater) near to urban centres in northern England. This regional difference was less marked (only approx. two-fold) during the 19th century (as revealed from the analysis of herbarium specimens) when nitrogen concentrations were appreciably lower. Transplant studies both between regions and between sites within a mountain system demonstrated the importance of atmospheric deposition in determining the tissue nitrogen concentration of the moss. The results are discussed in relation to the potential importance of the enhanced atmospheric nitrogen supply to the normally nitrogen-impoverished montane heaths, and to the growth and persistence of the moss.     

Atmospheric dynamics and ozone cycles related to nitrogen deposition in the western Mediterranean

Recent results from EU-sponsored projects have shed new light on the workings of the atmosphere in the Mediterranean Basin, where a large airmass becomes trapped and nearly stagnant over the sea during the summer. This work reviews the developments which have led to our present understanding of air pollution dynamics and associated meteorological processes in this region. In summer, the sea breezes combine with upslope winds to create recirculations along the coasts and within the western Mediterranean basin, with residence times in the order of days. Under strong insolation these recirculations become "large natural photo-chemical reactors" where most of the NOx emissions and other precursors are transformed into oxidants, acidic compounds, aerosols and ozone, which can exceed European Union directives for several months of the year. The objective of this paper is to evaluate available experimental evidence and complementary modelling results to help in the interpretation of observed ozone cycles and to anticipate possible scenarios for anthropogenic nitrogen deposition in the region.     

Response of stable carbon isotope in epilithic mosses to atmospheric nitrogen deposition

Epilithic mosses are characterized by insulation from substratum N and hence meet their N demand only by deposited N. This study investigated tissue C, total Chl and δ¹³C of epilithic mosses along 2 transects across Guiyang urban (SW China), aiming at testing their responses to N deposition. Tissue C and total Chl decreased from the urban to rural, but δ¹³C_moss became less negative. With measurements of atmospheric CO₂ and δ¹³CO₂, elevated N deposition was inferred as a primary factor for changes in moss C and isotopic signatures. Correlations between total Chl, tissue C and N signals indicated a nutritional effect on C fixation of epilithic mosses, but the response of δ¹³C_moss to N deposition could not be clearly differentiated from effects of other factors. Collective evidences suggest that C signals of epilithic mosses are useful proxies for N deposition but further works on physiological mechanisms are still needed.