Atmospheric Environment (v.35, #7)
List of Forthcoming Papers (I-II).
The atmospheric chemistry of sulphur and nitrogen in power station plumes by C.N. Hewitt (1155-1170).
Emissions of sulphur and nitrogen compounds from power stations represent a significant fraction of the total emissions of these elements to the atmosphere. Understanding their subsequent chemical reactions in the atmosphere is of fundamental importance as without it, a quantitative assessment of their contribution to local and regional scale air pollution is not possible. Here the atmospheric chemistry of sulphur dioxide and the oxides of nitrogen, and their resultant likely behaviour in the plumes of power stations are reviewed.
Keywords: Millenial review; Sulphur; Sulphur dioxide; Oxides of nitrogen; Nitrogen oxides; Nitric oxide; Nitrogen dioxide; Power station; Power plant; Plume; Atmospheric chemistry;
Tropospheric concentrations of the chlorinated solvents, tetrachloroethene and trichloroethene, measured in the remote northern hemisphere by C.H Dimmer; A McCulloch; P.G Simmonds; G Nickless; M.R Bassford; D Smythe-Wright (1171-1182).
A fully automated twin ECD gas chromatograph system with sample enriching adsorption–desorption primary stage was deployed on two field campaigns – Ny-Ålesund, Svalbard, Arctic Norway (July–September 1997), and the RRS Discovery CHAOS cruise of the northeast Atlantic (April–May 1998). Concentrations of an extensive set of halocarbons were detected at hourly intervals at pptv levels. We present here the results obtained for the chlorinated solvents, tetrachloroethene (PCE) and trichloroethene (TCE). Average baseline PCE and TCE concentrations of 1.77 and 0.12 pptv, respectively, were recorded in Ny-Ålesund. During pollution incidences, concentrations rose to 5.61 (PCE) and 3.18 pptv (TCE). The cruise data showed average concentrations ranging from 4.26 (PCE) and 1.66 pptv (TCE) for air masses originating over the North Atlantic and Arctic open oceans, to maxima of 15.59 (PCE) and 17.51 pptv (TCE) for polluted air masses from Northern Europe. The data sets emphasise the difficulties in defining remote sites for background tropospheric halocarbon measurements, as Ny-Ålesund research station proved to be a source of tetrachloroethene. The data also suggest possible oceanic emissions of trichloroethene in the sub-tropical ocean.
Keywords: Tetrachloroethene; Trichloroethene; Arctic troposphere; Emissions; Svalbard; Atlantic;
The water cycles of water-soluble organic salts of atmospheric importance by Changgeng Peng; Chak K Chan (1183-1192).
In this study, the water cycles of nine water-soluble organic salts of atmospheric interest were studied using an electrodynamic balance (EDB) at 25°C. Sodium formate, sodium acetate, sodium succinate, sodium pyruvate and sodium methanesulfonate (Na-MSA) particles crystallize as the relative humidity (RH) decreases and they deliquesce as the RH increases. Sodium oxalate and ammonium oxalate form supersaturated particles at low RH before crystallization but they do not deliquesce even at RH=90%. Sodium malonate and sodium maleate particles neither crystallize nor deliquesce. These two salts absorb and evaporate water reversibly without hysteresis. In most cases, the solid states of single particles resulting from the crystallization of supersaturated droplets do not form the most thermodynamically stable state found in bulk studies. Sodium formate, sodium oxalate, ammonium oxalate, sodium succinate, sodium pyruvate and Na-MSA form anhydrous particles after crystallization. Sodium acetate forms particles with a water/salt molar ratio of 0.5 after crystallization. In salts with several hydrated states including sodium formate and sodium acetate, the particles deliquesce at the lowest deliquescence relative humidity (DRH) of the hydrates. Except sodium oxalate and ammonium oxalate, all the salts studied here are as hygroscopic as typical inorganic hygroscopic aerosols. The hygroscopic organic salts have a growth factor of 1.76–2.18 from RH=10–90%, comparable to that of typical hygroscopic inorganic salts such as NaCl and (NH4)2SO4. Further study of other atmospheric water-soluble organic compounds and their mixtures with inorganic salts is needed to explain the field observations of the hygroscopic growth of ambient aerosols.
Keywords: Hygroscopic property; Crystallization; Deliquescence; Aerosol thermodynamics; Electrodynamic balance; Water activity;
Sources and concentration of nanoparticles (<10 nm diameter) in the urban atmosphere by Ji Ping Shi; Douglas E Evans; A.A Khan; Roy M Harrison (1193-1202).
Whilst limited information on particle size distributions and number concentrations in cities is available, very few data on the very smallest of particles, nanoparticles, have been recorded. Measurements in this study show that road traffic and stationary combustion sources generate a significant number of nanoparticles of diameter <10 nm. Measurements at the roadside (4 m from the kerb) and downwind from the traffic (more than 25 m from the kerb) show that nanoparticles (<10 nm diameter) accounted for more than 36–44% of the total particle number concentrations. Measurements designed to sample the plume of individual vehicles showed that both a diesel- and a petrol-fuelled vehicle generated nanoparticles (<10 nm diameter). The fraction of nanoparticles was even greater in a plume 350 m downwind of a stationary combustion source. On a few occasions, a temporal association between nanoparticles in the size range 3–7 nm and solar radiation was observed in urban background air at times when no other local sources were influential, which suggests that homogeneous nucleation can also be an important source of particles in the urban atmosphere.
Keywords: Urban atmosphere; Nanoparticles; Ultrafine particles; Nucleation;
Ozone response to precursor controls: comparison of data analysis methods with the predictions of photochemical air quality simulation models by C.L Blanchard; T Stoeckenius (1203-1215).
Comparisons were made between the predictions of six photochemical air quality simulation models (PAQSMs) and three indicators of ozone response to emission reductions: the ratios of O3/NO z and O3/NO y and the extent of reaction. The values of the two indicator ratios and the extent of reaction were computed from the model-predicted mixing ratios of ozone and oxidized nitrogen species and were compared to the changes in peak 1 and 8 h ozone mixing ratios predicted by the PAQSMs. The ozone changes were determined from the ozone levels predicted for base-case emission levels and for reduced emissions of volatile organic compounds (VOCs) and oxides of nitrogen (NO x ). For all simulations, the model-predicted responses of peak 1 and 8 h ozone mixing ratios to VOC or NO x emission reductions were correlated with the base-case extent of reaction and ratios of O3/NO z and O3/NO y . Peak ozone values increased following NO x control in 95% (median over all simulations) of the high-ozone (>80 ppbv hourly mixing ratio in the base-case) grid cells having mean afternoon O3/NO z ratios less than 5 : 1, O3/NO y less than 4 : 1, or extent less than 0.6. Peak ozone levels decreased in response to NO x reductions in 95% (median over all simulations) of the grid cells having peak hourly ozone mixing ratios greater than 80 ppbv and where mean afternoon O3/NO z exceeded 10 : 1, O3/NO y was greater than 8 : 1, or extent exceeded 0.8. Ozone responses varied in grid cells where O3/NO z was between 5 : 1 and 10 : 1, O3/NO y was between 4 : 1 and 8 : 1, or extent was between 0.6 and 0.8. The responses in such grid cells were affected by ozone responses in upwind grid cells and by the changes in ozone levels along the upwind boundaries of the modeling domains.
Keywords: Peak ozone; VOC emissions; NO x emissions; Ozone response; Model corroboration;
Gas/solid partitioning of semivolatile organic compounds (SOCs) to air filters. 2. Partitioning of polychlorinated dibenzodioxins, polychlorinated dibenzofurans, and polycyclic aromatic hydrocarbons to quartz fiber filters by B.T Mader; J.F Pankow (1217-1223).
Gas/particle distributions of atmospheric semi-volatile organic compounds (SOCs) are often measured using filter/sorbent samplers. Unfortunately, the adsorption of gaseous SOCs onto a filter can cause positive biases in the measured particle-phase concentrations, and negative biases in the measured gas-phase concentrations. When quartz fiber filters (QFFs) are used, surface-area-normalized gas/quartz partition coefficient (K p,s, m3 m−2) values will be useful when estimating the magnitudes of these errors. Gas/QFF K p,s values have been reported in the literature only for polycyclic aromatic hydrocarbons (PAHs) and n-alkanes. Gas/QFF K p,s values were measured here for a series of polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), and also for a range of PAHs. Within each of the three individual compound classes, plots of log K p,s vs. log p L o (sub-cooled liquid vapor pressure) were found to be linear with slopes of approximately −1. At relative humidity (RH)=25%, the pooled log K p,s data at 25°C for the three compound classes were correlated with log p L o nearly as well (r 2=0.95) as were the data for the individual compound classes (r 2≈0.97). In general, the K p,s values for the PAHs and PCDD/PCDFs studied were found to be about a factor of 2 larger for partitioning to clean QFFs at RH=25% than for TMFs at RH=21–52%. Backup QFFs used in filter/sorbent sampling in a suburban area yielded K p,s values for PAHs at RH=37% that were significantly lower than for clean QFFs at the same RH. (This may have been the result of the adsorption of ambient organic compounds that at least partially blocked the direct adsorption of the SOCs to the QFF surface). Therefore, when QFFs are used to separate atmospheric gas- and particle-phase SOCs, corrections for compound-dependent gas adsorption artifacts for QFFs may need to be carried out using K p,s values that were obtained with ambient backup QFFs.
Keywords: Air; Sampling; Adsorption; Artifacts; SOCs;
An electronic nose for odour annoyance assessment by Fabio Di Francesco; Beatrice Lazzerini; Francesco Marcelloni; Giovanni Pioggia (1225-1234).
Although in most cases annoying atmospheric emissions do not menace public health, they are less and less tolerated because of the effects on quality of life. Several approaches have been proposed to face this problem but none of them offers a completely satisfying solution. The development of electronic noses, which promise to mimic human sense of smell by means of a sensor array and a pattern recognition model, offers new interesting perspectives. In this paper, an electronic nose based on conducting polymer sensors and a fuzzy logic-based pattern recognition system is tested with waste water samples, obtaining 87% recognition rate on the test set. Current limits of this new technology are discussed and a strategy for their overcoming is proposed.
Keywords: Odour; Pollution; Waste waters; Conducting polymer sensors; Fuzzy classification;
Aromatic hydrocarbons in the atmospheric environment: Part I. Indoor versus outdoor sources, the influence of traffic by Elke Ilgen; Natascha Karfich; Karsten Levsen; Jürgen Angerer; Peter Schneider; Joachim Heinrich; H.-Erich Wichmann; Lothar Dunemann; Jutta Begerow (1235-1252).
Six aromatic hydrocarbons (benzene, toluene, ethylbenzene and the three isomeric xylenes) were monitored in the indoor and outdoor air of 115 private non-smoker homes (∼380 rooms), about half of which were located in two city streets in Hannover (Northern Germany) with high traffic density, the other half in rural areas with hardly any traffic at all. This environmental monitoring was complemented by human biomonitoring (i.e. the determination of aromatic hydrocarbons in blood and exhaled air). Particular attention was paid to benzene as a result of its carcinogenicity. In the city streets with high traffic density, an average benzene concentration of 3.1 μg m−3 and in the rural areas of 1.8 μg m−3 was found in these non-smoker homes (all data=geometric means), which reflects the influence of the traffic (automobile exhaust) on the benzene level found indoors. Source identification is also possible by determining the indoor/outdoor (I/O) concentration ratio. For the rooms facing the city street, this I/O ratio is close to 1 for all aromatic hydrocarbons studied with the exception of toluene (I/O=3.5), while in the rural areas I/O ratios for the individual compounds ranging in 6–9 were determined, with the exception of benzene where the I/O ratio is only 1.5. These I/O ratios in the city street with high traffic density indicate that an equilibrium between indoor and outdoor air is almost reached. Indoor sources prevail only in the case of toluene. In contrast, in the rural area, indoor sources dominate for all aromatic hydrocarbons except benzene, the indoor level of which is mainly influenced by the outdoor air even in areas of very low traffic density. However, weak indoor sources must exist also for this compound even in non-smoker homes. The internal exposure of the non-smoking inhabitants of these homes to benzene is very low. Depending on the living area, mean values of 61–67 ng l−1 benzene in blood and 0.9–1.2 μg m−3 in the exhaled air were found.
Keywords: Aromatic hydrocarbons; Benzene; Indoor air; Indoor/outdoor ratio; Biomonitoring; Blood; Exhalation air;
Aromatic hydrocarbons in the atmospheric environment – Part II: univariate and multivariate analysis and case studies of indoor concentrations by Elke Ilgen; Karsten Levsen; Jürgen Angerer; Peter Schneider; Joachim Heinrich; H.-Erich Wichmann (1253-1264).
The concentrations of the aromatic hydrocarbons benzene, toluene, ethylbenzene and the isomeric xylenes (BTEX) have been determined in the indoor air of 115 private non-smoker homes (∼380 individual rooms) situated in areas with an extreme traffic situation, i.e. in city streets (street canyons) with high traffic density and in rural areas with hardly any traffic at all. The influence of the traffic on the indoor concentration was apparent in the high traffic area. In order to identify other factors influencing the BTEX concentrations, the data and additional questionnaires were analyzed by univariate and multivariate analysis. The analysis was supplemented by some case studies. It is shown that meteorology (the seasons), the type of room (e.g. living room versus bedroom), the ventilation and, in particular, garages in the house strongly influence the indoor concentration of BTEX. Thus, the indoor BTEX level is significantly higher in winter than in summer. Moreover, garages with a connecting door to the living quarters lead to high indoor concentrations of aromatic hydrocarbons in these rooms. In addition, the storage of solvents and hobby materials, and also the presence of smoking guests increase the BTEX level. If rooms are directly heated by coal or wood, the BTEX level is higher compared to the use of gas heating. Surprisingly, no correlation was found between the building materials used and the BTEX level. Case studies were carried out for two homes with an integrated garage (and a connecting door to the living rooms) and for seven homes where redecoration work was carried out during sampling. In both instances, a pronounced increase was observed in the BTEX concentration.
Keywords: Aromatic hydrocarbons; Benzene; Indoor; Sources; Multivariate analysis;
Aromatic hydrocarbons in the atmospheric environment. Part III: personal monitoring by E. Ilgen; K. Levsen; J. Angerer; P. Schneider; J. Heinrich; H.-E. Wichmann (1265-1279).
As part of a larger study, personal sampling of the aromatic hydrocarbons benzene, toluene, ethylbenzene and the isomeric xylenes (BTEX) was carried out by 55 nonsmoking volunteers for a period of 14 days. Thirty-nine persons lived in a rural area near Hannover (Germany) with hardly any traffic at all, while 16 persons lived in a high-traffic city street in Hannover. The personal exposure level of the persons in the rural area (some commuting to Hannover) was: 2.9, 24.8, 2.4 and 7.7 μg m−3 for benzene, toluene, ethylbenzene and the sum of xylenes, respectively, while the corresponding data for the high traffic city streets were 4.0, 22.2, 2.8 and 9.7 μg m−3 (geometric means). Four microenvironments have been monitored which contribute to the total exposure to BTEX, i.e. the home, the outdoor air, the workplace and the car cabin. The most important microenvironment for non-working persons is the private home. The concentration of most BTEX in the private home is almost equal to the personal exposure level, demonstrating that the indoor pollution in the home makes by far the highest contribution to the total exposure. For working people (mostly office workers), the workplace is the second most important microenvironment contributing to the total BTEX exposure. Taking all working persons into consideration (independent of the location of their private home) the personal exposure level is higher by a factor of 1.2–1.4 than that of the workplace (for toluene this factor is 2.2). As already found by others, very high BTEX concentrations may be found in car cabins, in particular, if the engine is gasoline-driven. In the cabin of 44 cars in the rural/urban area average benzene concentrations (geometric mean) of 12/14 μg m−3 and a maximum value of ∼550 μg m−3 were found. On average, the participating volunteers drove their car for 45 min day−1 (i.e. 3% of the day). Nevertheless, the car cabin constitutes about 10% of the total benzene exposure. Refueling of the car during the 14-day sampling period has only a small effect on the personal exposure level.
Keywords: Benzene; Aromatic hydrocarbons; Personal sampling; Workplaces; Indoor air; Car cabins;
Characterization of emissions from portable household combustion devices: particle size distributions, emission rates and factors, and potential exposures by Cheng-Wei Fan; Junfeng (Jim) Zhang (1281-1290).
A series of source tests were conducted to characterize emissions of particulate matter (PM), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and total hydrocarbon (THC ) from five types of portable combustion devices. Tested combustion devices included a kerosene lamp, an oil lamp, a kerosene space heater, a portable gas range, and four unscented candles. All tests were conducted either in a well-mixed chamber or a well-mixed room, which enables us to determine emission rates and emission factors using a single-compartment mass balance model. Particle mass concentrations and number concentrations were measured using a nephelometric particle monitor and an eight-channel optical particle counter, respectively. Real-time CO concentrations were measured with an electrochemical sensor CO monitor. CO2, CH4, and THC were measured using a GC-FID technique. The results indicate that all particles emitted during steady burning in each of the tested devices were smaller than 1.0 μm in diameter with the vast majority in the range between 0.1 and 0.3 μm. The PM mass emission rates and emission factors for the tested devices ranged from 5.6±0.1 to 142.3±40.8 mg h−1 and from 0.35±0.06 to 9.04±4.0 mg g−1, respectively. The CO emission rates and emission factors ranged from 4.7±3.0 to 226.7±100 mg h−1 and from 0.25±0.12 to 1.56±0.7 mg g−1, respectively. The CO2 emission rates and emission factors ranged from 5500±700 to 210,000±90,000 mg h−1 and from 387±45 to 1689±640 mg g−1, respectively. The contributions of CH4 and THC to emission inventories are expected to be insignificant due both to the small emission factors and to the relatively small quantity of fuel consumed by these portable devices. An exposure scenario analysis indicates that every-day use of the kerosene lamp in a village house can generate fine PM exposures easily exceeding the US promulgated NAAQS for PM2.5.
Keywords: Portable combustion devices; Air pollution; Particulate matter; Carbon monoxide; Pollution exposure;
A mass transfer model for simulating VOC sorption on building materials by X. Yang; Q. Chen; J.S. Zhang; Y. An; J. Zeng; C.Y. Shaw (1291-1299).
The sorption of volatile organic compounds (VOCs) by different building materials can significantly affect VOC concentrations in indoor environments. In this paper, a new model has been developed for simulating VOC sorption and desorption rates of homogeneous building materials with constant diffusion coefficients and material–air partition coefficients. The model analytically solves the VOC sorption rate at the material–air interface. It can be used as a “wall function” in combination with more complex gas-phase models that account for non-uniform mixing to predict sorption process. It can also be used in conjunction with broader indoor air quality studies to simulate VOC exposure in buildings.
Keywords: Numerical model; Indoor air quality; Diffusion coefficient; Partition coefficient; Environmental chamber; Gypsum board;
Emission of specific pollutants from a compression ignition engine. Influence of fuel hydrotreatment and fuel/air equivalence ratio by E Zervas; X Montagne; J Lahaye (1301-1306).
A compression ignition engine is used for the study of the fuel (one reference and one hydrotreated) and the fuel/air equivalence ratio influence on the exhaust emissions of specific pollutants. Under the experimental conditions used, seven hydrocarbons, nine aldehydes and three organic acids are detected in the exhaust gas. No alcohols are detected under these conditions, indicating that these compounds are emitted only if they (or probably other oxygenated compounds) are introduced in the fuel. Fuel hydrotreatment decreases most of the exhaust pollutants, the four toxics and also the quantity of the ozone that could be formed from the exhaust gas. It also changes the composition of exhaust gas: it increases the proportion of methane, benzene, formaldehyde, acetaldehyde, acroleine, and propionic acid, while it decreases the proportion of all other pollutants detected. Fuel/air equivalence ratio also decreases most of the exhaust emissions, the emission of the total toxics and the quantity of the ozone that could be formed. It also changes the proportion of each pollutant in exhaust gas: the percentages of methane, benzene, acetone and acetic acid increase, while those of the other pollutants detected decrease. The majority of the specific pollutants detected corresponds to organic acids, followed by hydrocarbons and aldehydes.
Keywords: Aldehydes; Compression ignition engines; Hydrocarbons; Organic acids; Ozone;
Comparisons of aerosol nitrogen isotopic composition at two polluted coastal sites by S.G. Yeatman; L.J. Spokes; P.F. Dennis; T.D. Jickells (1307-1320).
Atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters. Stable nitrogen isotope data can provide important information regarding the sources and processing of atmospheric fixed-nitrogen species and is thus important in controlling eutrophication. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland and also aboard the RRS Challenger in the Eastern Atlantic Ocean. Aerosol concentrations of ammonium and nitrate were determined prior to δ 15N isotopic analysis. For both species a significant difference in mean isotopic composition was seen between samples from Weybourne (+6±6‰ for ammonium and +7±6‰ for nitrate) and Mace Head and RRS Challenger campaigns (−9±8‰ for ammonium and −1±3‰ for nitrate). At each site a strong dependence of isotopic composition on the geographical origin of the sampled air mass was also observed. For aerosol ammonium, marine and terrestrially dominated samples were found to be isotopically distinct, perhaps reflecting the presence of oceanic sources of ammonia in addition to anthropogenic or natural terrestrial sources. Further distinctions were made within terrestrially dominated samples, possibly indicative of different types of animal husbandry regimes or other forms of anthropogenic activity. For aerosol nitrate, there was found to be generally less variation between samples at each site, although at Weybourne a significant difference was observed between the mean isotopic composition of samples originating from the northern UK (+15±3‰) and that of those originating from the southern UK (+10±3‰), suggesting that aerosol δ 15N data might possibly facilitate source apportionment between NO x emissions from power stations and those from vehicle exhausts. The nitrate data also appeared to show seasonality with higher concentrations and lower δ 15N values seen in the summer.
Keywords: Aerosol; Nitrate; Ammonium; Isotopes; Fractionation;
Comparisons of coarse-mode aerosol nitrate and ammonium at two polluted coastal sites by S.G Yeatman; L.J Spokes; T.D Jickells (1321-1335).
Direct atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters and can be enhanced by heterogeneous reactions between gaseous atmospheric nitrogen species and aerosol sea salt, which increase deposition rates. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland. Major-ion aerosol concentrations were determined and temporal patterns were interpreted with the use of air-mass back trajectories. Low levels of terrestrially derived material were seen during periods of clean, onshore flow, with respective concentration ranges for nitrate and ammonium of 0.47–220 and below detection limit to 340 nmol m−3. Corresponding levels of marine derived material during these periods were high, with sodium concentrations ranging from 39 to 1400 nmol m−3. Highest levels of terrestrially derived material were seen during polluted, offshore flow, where the air had passed recently over strong source regions of the UK and northern Europe, with concentration ranges of nitrate and ammonium of 5.6–790 and 9.7–1000 nmol m−3, respectively. During polluted flow ∼40–60% of the nitrate was found in the coarse mode (>1 μm diameter) and under clean marine conditions almost 100% conversion was seen. In addition, our data suggests strong evidence for dissolution/coagulation processes that also shift nitrate to the coarse mode. Furthermore, such processes are thought also to give rise to the size-shifting of aerosol ammonium, since significant coarse-mode fractions of this species (∼19–45%) were seen at both sites. A comparison of the relative importance of nitrate and ammonium in the overall dry deposition of inorganic fixed-nitrogen at each site indicates that at Weybourne the mass-weighted dry deposition velocity of the latter is around double that seen at Mace Head with its resultant contribution to the overall inorganic nitrogen dry flux exceeding that of nitrate.
Keywords: Aerosol; Nitrate; Ammonium; Size-shift; Coarse-mode;
Can the study of nitrogen isotopic composition in size-segregated aerosol nitrate and ammonium be used to investigate atmospheric processing mechanisms? by S.G. Yeatman; L.J. Spokes; P.F. Dennis; T.D. Jickells (1337-1345).
Atmospheric fixed-nitrogen deposition can contribute to eutrophication in coastal and estuarine waters. Stable nitrogen isotope data can provide important information regarding the sources and processing of atmospheric fixed-nitrogen species and is thus important in controlling eutrophication. Size-segregated aerosol samples were collected from two coastal sites: Weybourne, England and Mace Head, Ireland and also aboard the RRS Challenger in the Eastern Atlantic Ocean. Aerosol concentrations of ammonium and nitrate were determined prior to δ 15N isotopic analysis. The isotopic enrichment factor, ε, was calculated for both the species by subtracting the respective δ 15N values of the coarse-mode fraction (>1 μm diameter) from those of the fine-mode fraction (<1 μm diameter). Variations in this parameter were observed as weak functions of the percentage of each species in the coarse mode and of meteorological condition. As a result, the presence of two different size-shift processes (dissociation/gas scavenging and dissolution/coagulation) is proposed, consistent with similar arguments based upon major-ion data obtained from the same suite of samples. Dissolution/coagulation processes appear to exhibit negative isotopic enrichment factors whereas dissociation/gas scavenging processes appear to exhibit positive enrichment factors, suggesting that they may be reversible and thermodynamically driven. In offshore-flowing air masses just entering the marine environment, transferral of nitrate to the coarse mode by initial dissociation of NH4NO3 followed by scavenging of the HNO3 produced appears to be significantly more important than in samples of onshore, marine dominated air. In contrast, ammonium appears to be transferred to the coarse mode during offshore flow largely via the dissolution and coagulation of aerosol ammonium, nitrate and sulphate-containing species. During onshore flow, the uptake of gaseous NH3, arising from the continued dissociation of NH4NO3, seems to become increasingly important.
Keywords: Aerosol; Nitrate; Ammonium; Nitrogen isotopes; Enrichment factor;
New Directions: Fugitive emissions identified by chemical fingerprinting by Jon Peters; Ag Stephens (1347-1348).