Investigation of the positive artifact formation during sampling semi-volatile aerosol using wet denuders

Wet denuders are used in several steam-based semi-continuous aerosol monitors to avoid gaseous absorption artifacts and pre-humidify the air stream, while simultaneously allowing measurements of water-soluble gaseous species. Unlike dry denuders, wet denuders saturate the sample air stream with water vapor, which can lead to re-partitioning of water-soluble volatile species to the aerosol phase, thereby causing a positive artifact in aerosol measurements. This paper investigates the magnitude of the positive artifact formation occurring in wet denuders using modeling techniques. Gaseous nitric acid was used as an example of volatile water-soluble gas in both flat and annular wet denuders. We have also verified the occurrence of the positive artifact in a flat wet denuder through a laboratory experiment. The model results indicate that the magnitude of the artifact is rather limited under typical conditions being less than 2.5% of ambient nitric acid concentration for the flat denuder and less than 0.6% for the annular denuder. The magnitude of the artifact increases with condensational sink of the aerosol (i.e. with the mean aerosol size and number concentration) and aerosol water solubility. While the artifact is relatively small in the absolute sense, it could be substantial for aerosol nitrate measurements, especially in ammonia limited conditions, when the concentration of the nitric acid is high and the concentration of nitrate is low. Therefore, we recommend that the artifact is assessed regularly by replacing the wet denuder with a dry denuder.     

Monitoring Deposition of Nitrogen-Containing Compounds in a High-Elevation Forest Canopy

Measurements of airborne (gaseous and aerosol), cloud water, and precipitation concentrations of nitrogen compounds were made at Mt. Mitchell State Park (Mt. Gibbs, ~2006 m MSL), North Carolina, during May through September of 1988 and 1989, An annular denuder system was used to ascertain gaseous (nitric acid, nitrous acid, and ammonia) and particulate (nitrate and ammonium) nitrogen species, and a chemiluminescence nitrogen oxides analyzer was used to measure nitric oxide and nitrogen dioxide. Measurements of NO₃ ^? and NH₄ ⁺ ions in cloud and rain water samples were made during the same time period. Mean concentrations of gaseous nitric acid, nitrous acid, and ammonia were 1.14 μg/m³, 0.3 μg/m³, and 0.62 μg/m³ for 1988, and 1.40 μg/m³,0.3 μg/m³, and 1.47 μg/m³ for 1989, respectively. Fine particulate nitrate and ammonium ranged from 0.02 to 0.21 μg/m³ and 0.01 to 4.72 μg/m³ for 1988, and 0.1 to 0.78 μg/m³ and 0.24 to 2.32 μg/m³ for 1989, respectively. The fine aerosol fraction was dominated by ammonium sulfate particles. Mean concentrations of nitrate and ammonium ions in cloud water samples were 238 and 214 μmol/l in 1988, and 135 and 147 μmol/l in 1989, respectively. Similarly, the concentrations of NO₃ and NH₄ ⁺ in precipitation were 26.4 and 14.0 μmol/l in 1988, and 16.6 and 15.2 μmol/l in 1989, respectively. The mean total nitrogen deposition due to wet, dry, and cloud deposition processes was estimated as ~30 and ~40 kg N/ha/year (i.e., ~10 and ~13 kg N/ha/growing season) for 1988 and 1989. Based on an analytical analysis, deposition to the forest canopy due to cloud interception, precipitation, and dry deposition processes was found to contribute ~60, ~20, and ~20 percent, respectively, of the total nitrogen deposition.     

Deposition of nitrogen-containing compounds to an extensively managed grassland in central Switzerland

During four intensive observation periods in 1992 and 1993, dry deposition of nitrogen dioxide (NO(2)) and ammonia (NH(3)), and wet deposition of nitrogen (N) were determined. The measurements were carried out in a small, extensively managed litter meadow surrounded by intensively managed agricultural land. Dry deposition of NH(3) was estimated by the gradient method, whereas eddy correlation was used for NO(2). Rates of dry deposition of total nitrate (= nitric acid (HNO(3)) + nitrate (NO(3)(-))), total nitrite (= nitrous acid (HONO) + nitrite (NO(2)(-))) and aerosol-bound ammonium (NH(4)(+)) were estimated using deposition velocities from the literature and measured concentrations. Both wet N deposition and the vertical NH(3) gradient were measured on a weekly basis during one year. Dry deposition was between 15 and 25 kg N ha(-1) y(-1), and net wet deposition was about 9.0 kg N ha(-1) y(-1). Daily average NO(2) deposition velocity varied from 0.11 to 0.24 cm s(-1). Deposition velocity of NH(3), was between 0.13 and 1.4 cm s(-1), and a compensation point between 3 and 6 ppbV NH(3) (ppb = 10(-9)) was found. Between 60 and 70% of dry deposition originated from NH(3) emitted by farms in the neighbourhood. It is concluded that total N deposition is exceeding the critical load for litter meadows, is highly correlated to local NH(3) emissions, and that NH(3) is of utmost importance with respect to possible strategies to reduce N deposition in rural regions.     

Processes influencing secondary aerosol formation in the San Joaquin Valley during winter

Air quality data collected in the California Regional PM10/ PM(2.5) Air Quality Study (CRPAQS) are analyzed to qualitatively assess the processes affecting secondary aerosol formation in the San Joaquin Valley (SJV). This region experiences some of the highest fine particulate matter (PM(2.5)) mass concentrations in California (< or = 188 microg/m3 24-hr average), and secondary aerosol components (as a group) frequently constitute over half of the fine aerosol mass in winter. The analyses are based on 15 days of high-frequency filter and canister measurements and several months of wintertime continuous gas and aerosol measurements. The phase-partitioning of nitrogen oxide (NO(x))-related nitrogen species and carbonaceous species shows that concentrations of gaseous precursor species are far more abundant than measured secondary aerosol nitrate or estimated secondary organic aerosols. Comparisons of ammonia and nitric acid concentrations indicate that ammonium nitrate formation is limited by the availability of nitric acid rather than ammonia. Time-resolved aerosol nitrate data collected at the surface and on a 90-m tower suggest that both the daytime and nighttime nitric acid formation pathways are active, and entrainment of aerosol nitrate formed aloft at night may explain the spatial homogeneity of nitrate in the SJV. NO(x) and volatile organic compound (VOC) emissions plus background O3 levels are expected to determine NO(x) oxidation and nitric acid production rates, which currently control the ammonium nitrate levels in the SJV. Secondary organic aerosol formation is significant in winter, especially in the Fresno urban area. Formation of secondary organic aerosol is more likely limited by the rate of VOC oxidation than the availability of VOC precursors in winter.     

Altitude-dependent wet,dry and occult nitrogen deposition in an Alpine Region

From November 1995 to October 1996 and from October 1997 to September 1998, samples of wet precipitation, cloud water, as well as of reactive gases and particulate matter, were collected at three elevational levels (920 m, 1280 m and 1758 m a.s.l.) in Achenkirch, Austria. The samples were analysed for ammonium and nitrate in wet precipitation and in cloud water, for ammonia, nitric acid and nitrogen dioxide in the gas phase and for particulate ammonium and particulate nitrate in aerosol. Total nitrogen deposition was calculated combining measured concentrations in wet, dry and occult depositions with the corresponding deposition fluxes. Two multilayer deposition models were used for the calculation of dry and occult deposition. The total nitrogen input in 1995/96 was estimated to be 29 kg N ha⁻¹a⁻¹ at the Christlumkopf station (1758 m), 20 kg N ha⁻¹a⁻¹ at the Christlumalm station (1280 m) and 28 kg N ha⁻¹a⁻¹ at the Talboden station (930 m). Respective data for the 1997/98 observation period were 31 kg N ha⁻¹a⁻¹ at the Christlumkopf station (1758 m) and 18 kg N ha⁻¹a⁻¹ at the Mühleggerköpfl station (920 m). Critical Loads of nitrogen for coniferous forests were exceeded significantly near-source regions represented by areas of intense agricultural use and at high elevation sites.

     

Photochemical oxidant pollution and vegetation: effects of mixtures of gases, fog and particles

Photochemical 'smog' contains mixtures of gases (e.g. ozone, nitrogen dioxide), and dry particles (e.g. nitrates). Intermittent fog in the same geographical area can be acidic with high concentrations of nitric acid. Results from recent field studies in the Los Angeles Basin have emphasized the relative toxicity of these components of photochemical air pollution. Studies have focused on gaseous+fog or gaseous+dry particulate effects on conifers, gaseous+fog effects on crops, and the effects of trace pollutants produced during generation of ozone on crops. Data from these studies indicate that direct alterations in growth and physiological responses were observed only with gaseous pollutants (primarily ozone), or repeated applications of highly acidic fogs (pH < 2.7). Direct particle dry deposition effects are unclear. Few interactions have been found between gaseous pollutants and acidic fog. Charcoal-filtered open-top chambers are highly effective in removing pollutants in the following order: fog (100%) > peroxyacetyl nitrate > ozone > nitrogen dioxide > sulfur dioxide > nitrate ion > ammonium ion > sulfate ion. However, nitric oxide concentrations are higher in charcoal-filtered chambers than in ambient air. The studies point out the importance of considering other components of photochemical pollution in addition to ozone, especially when investigating subtle, long-term effects on vegetation.     

Processes Influencing Secondary Aerosol Formation in the San Joaquin Valley during Winter

Abstract

Air quality data collected in the California Regional PM₁₀/PM_2.5 Air Quality Study (CRPAQS) are analyzed to qualitatively assess the processes affecting secondary aerosol formation in the San Joaquin Valley (SJV). This region experiences some of the highest fine particulate matter (PM_2.5) mass concentrations in California (≤188 μg/m³ 24-hr average), and secondary aerosol components (as a group) frequently constitute over half of the fine aerosol mass in winter. The analyses are based on 15 days of high-frequency filter and canister measurements and several months of wintertime continuous gas and aerosol measurements. The phase-partitioning of nitrogen oxide (NO_x)-related nitrogen species and carbonaceous species shows that concentrations of gaseous precursor species are far more abundant than measured secondary aerosol nitrate or estimated secondary organic aerosols. Comparisons of ammonia and nitric acid concentrations indicate that ammonium nitrate formation is limited by the availability of nitric acid rather than ammonia. Time-resolved aerosol nitrate data collected at the surface and on a 90-m tower suggest that both the daytime and nighttime nitric acid formation pathways are active, and entrainment of aerosol nitrate formed aloft at night may explain the spatial homogeneity of nitrate in the SJV. NO_x and volatile organic compound (VOC) emissions plus background O₃ levels are expected to determine NO_x oxidation and nitric acid production rates, which currently control the ammonium nitrate levels in the SJV. Secondary organic aerosol formation is significant in winter, especially in the Fresno urban area. Formation of secondary organic aerosol is more likely limited by the rate of VOC oxidation than the availability of VOC precursors in winter.     

Determination of atmospheric nitrogen input to Lake Greenwood, South Carolina: part 2--Gaseous measurements and modeling

The Reedy River branch of Lake Greenwood, SC, has repeatedly experienced summertime algal blooms, upsetting the natural system. A series of experiments were carried out to investigate atmospheric nitrogen (N) input into the lake. N was examined because of the insignificant phosphorus dry atmospheric flux and the unique nutrient demands of the dominant algae (Pithophora oedogonia) contributing to the blooms. Episodic atmospheric measurements during January and March 2001 have shown that the dry N flux onto the lake ranged from 0.9 to 17.4 kg N/ha-yr, and on average is caused by nitric acid (HNO3; 31%), followed by nitrogen dioxide (NO2; 23%), fine ammonium (NH4+; 20%), coarse nitrate (NO3-; 16%), fine NO3 (5%), and coarse NH4+ (5%). Similar measurements in Greenville, SC (the upper watershed of the Reedy River), showed that the dry N deposition flux there ranged from 1.4 to 9.7 kg N/ha-yr and was mostly caused by gaseous deposition (40% NO2 and 40% HNO3). The magnitude of this dry N deposition flux is comparable to wet N flux as well as other point sources in the area. Thermodynamic modeling showed low concentrations of ammonia, relative to the particulate NH4+ concentrations.     

Nitrogen deposition in California forests: a review

Atmospheric concentrations and deposition of the major nitrogenous (N) compounds and their biological effects in California forests are reviewed. Climatic characteristics of California are summarized in light of their effects on pollutant accumulation and transport. Over large areas of the state dry deposition is of greater magnitude than wet deposition due to the arid climate. However, fog deposition can also be significant in areas where seasonal fogs and N pollution sources coincide. The dominance of dry deposition is magnified in airsheds with frequent temperature inversions such as occur in the Los Angeles Air Basin. Most of the deposition in such areas occurs in summer as a result of surface deposition of nitric acid vapor (HNO3) as well as particulate nitrate (NO3-) and ammonium (NH4+). Internal uptake of gaseous N pollutants such as nitrogen dioxide (NO2), nitric oxide (NO), HNO3, peroxyacetyl nitrate (PAN), ammonia (NH3), and others provides additional N to forests. However, summer drought and subsequent lower stomatal conductance of plants tend to limit plant utilization of gaseous N. Nitrogen deposition is much greater than S deposition in California. In locations close to photochemical smog source areas, concentrations of oxidized forms of N (NO2, HNO3, PAN) dominate, while in areas near agricultural activities the importance of reduced N forms (NH3, NH4+) significantly increases. Little data from California forests are available for most of the gaseous N pollutants. Total inorganic N deposition in the most highly-exposed forests in the Los Angeles Air Basin may be as high as 25-45 kg ha(-1) year(-1). Nitrogen deposition in these highly-exposed areas has led to N saturation of chaparral and mixed conifer stands. In N saturated forests high concentrations of NO3- are found in streamwater, soil solution, and in foliage. Nitric oxide emissions from soil and foliar N:P ratios are also high in N saturated sites. Further research is needed to determine the ecological effects of chronic N deposition, and to develop appropriate management options for protecting water quality and managing plant nutrient resources in ecosystems which no longer retain excess N.     

Atmospheric deposition of nitrogen emitted in the Metropolitan Area of Buenos Aires to coastal waters of de la Plata River

The Metropolitan Area of Buenos Aires (MABA) is the third mega-city in Latin America. Atmospheric N emitted in the area deposits to coastal waters of de la Plata River. This study describes the parameterizations included in DAUMOD-RD (v.3) model to evaluate concentrations of nitrogen compounds (nitrogen dioxide, gaseous nitric acid and nitrate aerosol) and their total (dry and wet) deposition to a water surface. This model is applied to area sources and CALPUFF model to point sources of NO_x in the MABA. The models are run for 3 years of hourly meteorological data, with a spatial resolution of 1 km². Mean annual deposition is 69, 728 kg-N year^?1 over 2 339 km² of river. Dry deposition contributions of N-NO₂, N-HNO₃ and N-NO₃^? to this value are 44%, 22% and 20%, respectively. Wet deposition of N-HNO₃ and N-NO₃^? represents 3% and 11% of total annual value, respectively. This very low contribution results from the rare occurrence of rainy hours with wind blowing from the city to the river. Monthly dry deposition flux estimated for coastal waters of MABA varies between 7 and 13 kg-N km^?2 month^?1. These results are comparable to values reported for other coastal zones in the world.     

Effects of geographical location and land use on atmospheric deposition of nitrogen in the State of Connecticut

A network of eight monitoring stations was established to study the atmospheric nitrogen concentration and deposition in the State of Connecticut. The stations were classified into urban, rural, coastal and inland categories to represent the geographical location and land use characteristics surrounding the monitoring sites. Nitrogen species including nitrate, ammonium, nitric acid vapor and organic nitrogen in the air and precipitation were collected, analyzed and used to infer nitrogen concentrations and dry and wet deposition flux densities for the sampling period from 1997 through 1999, with independently collected meteorological data. Statistical analyses were conducted to evaluate the spatial variations of atmospheric concentration and deposition fluxes of total nitrogen in Connecticut. A slightly higher atmospheric concentration of total nitrogen was observed along the Connecticut coastline of Long Island Sound compared to inland areas, while the differences of nitrogen deposition fluxes were insignificant between coastal and inland sites. The land use characteristics surrounding the monitoring sites had profound effects on the atmospheric nitrogen concentration and dry deposition flux. The ambient nitrogen concentration over the four urban sites was averaged 38.9% higher than that over the rural sites, resulting a 58.0% higher dry deposition flux in these sites compared to their rural counterparts. The local industrial activities and traffic emissions of nitrogen at urban areas had significant effects on the spatial distribution of atmospheric nitrogen concentration and dry deposition flux in the State. Wet and total deposition fluxes appeared to be invariant between the monitoring sites, except for high flux densities measured at Old Greenwich, a monitoring station near to and downwind of the New York and New Jersey industrial complexes.     

Preliminary measurements of summer nitric acid and ammonia concentrations in the Lake Tahoe Basin air-shed: implications for dry deposition of atmospheric nitrogen

Over the past 50 years, Lake Tahoe, an alpine lake located in the Sierra Nevada mountains on the border between California and Nevada, has seen a decline in water clarity. With significant urbanization within its borders and major urban areas 130 km upwind of the prevailing synoptic airflow, it is believed the Lake Tahoe Basin is receiving substantial nitrogen (N) input via atmospheric deposition during summer and fall. We present preliminary inferential flux estimates to both lake surface and forest canopy based on empirical measurements of ambient nitric acid (HNO3), ammonia (NH3), and ammonium nitrate (NH4NO3) concentrations, in an effort to identify the major contributors to and ranges of atmospheric dry N deposition to the Lake Tahoe Basin. Total flux from dry deposition ranges from 1.2 to 8.6 kg N ha-1 for the summer and fall dry season and is significantly higher than wet deposition, which ranges from 1.7 to 2.9 kg N ha-1 year-1. These preliminary results suggest that dry deposition of HNO3 is the major source of atmospheric N deposition for the Lake Tahoe Basin, and that overall N deposition is similar in magnitude to deposition reported for sites exposed to moderate N pollution in the southern California mountains.     

Sensitivity analyses of factors influencing CMAQ performance for fine particulate nitrate

Improvement of air quality models is required so that they can be utilized to design effective control strategies for fine particulate matter (PM_2.5). The Community Multiscale Air Quality modeling system was applied to the Greater Tokyo Area of Japan in winter 2010 and summer 2011. The model results were compared with observed concentrations of PM_2.5 sulfate (SO₄^2-), nitrate (NO₃^?) and ammonium, and gaseous nitric acid (HNO₃) and ammonia (NH₃). The model approximately reproduced PM_2.5 SO₄^2? concentration, but clearly overestimated PM_2.5 NO₃^? concentration, which was attributed to overestimation of production of ammonium nitrate (NH₄NO₃). This study conducted sensitivity analyses of factors associated with the model performance for PM_2.5 NO₃^? concentration, including temperature and relative humidity, emission of nitrogen oxides, seasonal variation of NH₃ emission, HNO₃ and NH₃ dry deposition velocities, and heterogeneous reaction probability of dinitrogen pentoxide. Change in NH₃ emission directly affected NH₃ concentration, and substantially affected NH₄NO₃ concentration. Higher dry deposition velocities of HNO₃ and NH₃ led to substantial reductions of concentrations of the gaseous species and NH₄NO₃. Because uncertainties in NH₃ emission and dry deposition processes are probably large, these processes may be key factors for improvement of the model performance for PM_2.5 NO₃^?.

Implications: The Community Multiscale Air Quality modeling system clearly overestimated the concentration of fine particulate nitrate in the Greater Tokyo Area of Japan, which was attributed to overestimation of production of ammonium nitrate. Sensitivity analyses were conducted for factors associated with the model performance for nitrate. Ammonia emission and dry deposition of nitric acid and ammonia may be key factors for improvement of the model performance.     

Comparisons of coarse-mode aerosol nitrate and ammonium at two polluted coastal sites

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.     

Atmospheric conversion of sulfur dioxide to particulate sulfate and nitrogen dioxide to particulate nitrate and gaseous nitric acid in an urban area

Sulfur dioxide, nitrogen dioxide, particulate sulfate and nitrate, gaseous nitric acid, ozone and meteorological parameters (temperature and relative humidity) were measured during the winter season (1999-2000) and summer season (2000) in an urban area (Dokki, Giza, Egypt). The average particulate nitrate concentrations were 6.20 and 9.80 microg m(-3), while the average gaseous nitric acid concentrations were 1.14 and 6.70 microg m(-3) in the winter and summer seasons, respectively. The average sulfate concentrations were 15.32 microg m(-3) during the winter and 25.10 microg m(-3) during the summer season. The highest average concentration ratio of gaseous nitric acid to total nitrate was found during the summer season. Particulate sulfate and nitrate and gaseous nitric acid concentrations were relatively higher in the daytime than those in the nighttime. Sulfur conversion ratio (Fs) and nitrogen conversion ratio (Fn) defined in the text were calculated from the field measurement data. Sulfur conversion ratio (Fs) and nitrogen conversion ratio (Fn) in the summer were about 2.22 and 2.97 times higher than those in the winter season, respectively. Moreover, sulfur conversion ratio (Fs) and nitrogen conversion ratio (Fn) were higher in the daytime than those in the nighttime during the both seasons. The sulfur conversion ratio (Fs) increases with increasing ozone concentration and relative humidity. This indicates that the droplet phase reactions and gas phase reactions are important for the oxidation of SO2 to sulfate. Moreover, the nitrogen conversion ratio (Fn) increases with increasing ozone concentration, and the gas phase reactions are important and predominant for the oxidation of NO2 to nitrate.     

Exploration of heterogeneous chemistry in model atmospheric particles using extended X-ray absorption fine structure analysis

As models of the composition and heterogeneous chemical reactions of the troposphere undergo refinement, novel application of state-of-the-art analytical techniques will be necessary to propound realistic characterizations of mineral dust chemistry. In this study, strontium carbonate particles treated with gaseous nitric acid and nitrogen dioxide were examined with X-ray absorption fine structure analysis (EXAFS). The X-ray spectra of carbonate and nitrate standards were fitted to ab initio calculations, which were used to determine the structure and consistency of strontium nitrate formed on strontium carbonate. By examining differences in mean square radial displacement and lattice spacing values obtained for bulk Sr(NO₃)₂ as compared to Sr(NO₃)₂ formed on SrCO₃, EXAFS proves effective as a tool for investigating the local structure and composition of heterogeneous aerosol particles. The implications of findings on reacted strontium carbonate for atmospheric models of calcium carbonate aerosol are discussed.     

Determination of Atmospheric Nitrogen Input to Lake Greenwood,South Carolina: Part 2—Gaseous Measurements and Modeling

Abstract

The Reedy River branch of Lake Greenwood, SC, has repeatedly experienced summertime algal blooms, upsetting the natural system. A series of experiments were carried out to investigate atmospheric nitrogen (N) input into the lake. N was examined because of the insignificant phosphorus dry atmospheric flux and the unique nutrient demands of the dominant algae (Pithophora oedogonia) contributing to the blooms. Episodic atmospheric measurements during January and March 2001 have shown that the dry N flux onto the lake ranged from 0.9 to 17.4 kg N/ha-yr, and on average is caused by nitric acid (HNO₃; 31%), followed by nitrogen dioxide (NO₂; 23%), fine ammonium (NH₄ ⁺; 20%), coarse nitrate (NO₃ ^?; 16%), fine NO₃ ⁺ (5%), and coarse NH₄ ⁺ (5%). Similar measurements in Greenville, SC (the upper watershed of the Reedy River), showed that the dry N deposition flux there ranged from 1.4 to 9.7 kg N/ha-yr and was mostly caused by gaseous deposition (40% NO₂ and 40% HNO₃). The magnitude of this dry N deposition flux is comparable to wet N flux as well as other point sources in the area. Thermodynamic modeling showed low concentrations of ammonia, relative to the particulate NH₄ ⁺ concentrations.     

A laboratory investigation of the routes of HNO3 dry deposition to coniferous seedlings

White pine, Norway spruce and red spruce seedlings were exposed to nitric acid vapor concentrations of 10 to 120 ppb in constant stirred tank reactors. Nitric acid dry deposition rates were determined from both the change in nitric acid concentration in the reactor flow stream and from the amount of nitrogen recovered from the seedlings. Nitric acid labeled with 15N was used to distinguish dry-deposited nitrogen in the plant from the nitrogen that was already present. It was found that dry deposition occurs via three routes: surface deposition, trans-cuticular deposition, and stomatal deposition. Resistance to surface deposition is very low (< 4.8 m2-s mol(-1)) for a freshly washed surface, but increases as the surface adsorption sites are occupied. Resistance to trans-cuticular uptake averaged 206 m2-s mol(-1). Stomatal resistance can be calculated from the rate of water diffusion out of the plant. Eighty per cent of the nitric acid deposited via the trans-cuticular and stomatal routes was assimilated by the plant. However, none of the nitric acid deposited on the surface was assimilated. In rural areas with coniferous forests, the combination of low ambient nitric acid concentrations and low initial surface resistance means that most nitric acid will be dry deposited on the tree surface, and thus will not be directly assimilated.     

Estimation of nitrogen balance between the atmosphere and Lake Balaton and a semi natural grassland in Hungary

The paper summarises the results to determine the fluxes of different N-compounds within the atmosphere and an aquatic and a terrestrial ecosystems, in Hungary. In the exchange processes of N-compounds between atmosphere and various ecosystems the deposition dominates. The net deposition fluxes are -730, -1270 and -1530 mg Nm(-2)yr(-1) for water, grassland, and forest ecosystems, respectively. For water, the main source of nitrogen compounds is the wet deposition. Ammonia gas is close to the equilibrium between the water and the air. For grassland the dry flux of nitric acid and ammonia is also an important term beside the wet deposition. Dry deposition to terrestrial ecosystems is roughly two times higher than wet deposition. A total of 8-10% of the nitrates and NH(x) deposited to terrestrial ecosystems are re-emitted into the air in the form of nitrous oxide (N2O) greenhouse gas.     

Understanding particulate matter formation in the California San Joaquin Valley: conceptual model and data needs

Quantitative information from the 1995 Integrated Monitoring Study (IMS95) is used to develop a conceptual model, which describes the chemical characteristics and the physical processes responsible for the accumulation of PM in the San Joaquin Valley of California. One significant finding of the conceptual model is the sensitivity of ammonium nitrate (46% of winter PM_2.5) and nitric acid to oxidants, which may be VOC-sensitive rather than NO_x-sensitive. Key gaps in current knowledge are identified using the conceptual model, e.g., the relative sensitivity of winter oxidants to VOC and NO_x, mechanistic details of secondary organic aerosol formation, mechanisms of dispersion under calm conditions, and the importance of dry deposition. Some recommendations are also provided for the formulation of air quality models suitable to address the accumulation of PM in the San Joaquin Valley.