Aerosol Ammonium Research Articles

Global modeling of nitrate and ammonium: Interaction of aerosols and tropospheric chemistry

Global radiative forcing of nitrate and ammonium aerosols has mostly been estimated from aerosol concentrations calculated at thermodynamic equilibrium or using approximate treatments for their uptake by aerosols. In this study, a more accurate hybrid dynamical approach (HDYN) was used to simulate the uptake of nitrate and ammonium by aerosols and the interaction with tropospheric reactive nitrogen chemistry in a three‐dimensional global aerosol and chemistry model, Umich/IMPACT, which also treats sulfate, sea salt and mineral dust aerosol. The calculated sulfate, ammonium and nitrate aerosol concentrations show good agreement with the available ground‐based measurements over both ocean and land areas. The global annual average nitrate aerosol burden is 0.16 Tg N, with 43% (i.e., 0.079 Tg N) in the fine mode (D < 1.25 μm) that scatters most efficiently. The global annual average ammonium burden is 0.29 Tg N with 92% in the fine mode. A sensitivity study with a thermodynamic equilibrium model underestimates the fine‐mode nitrate aerosol burden by 25%, because of the excessive nitrate formation on coarse aerosols. These underpredictions are especially important in the remote continents or over the oceans, where the availability of the total nitrate is limited. We also examined two common approaches used to treat nitrate and ammonium aerosols in global models, including the first‐order gas‐to‐particle approximation based on uptake coefficients (UPTAKE) and a simple hybrid method that combines the former with an equilibrium model (HYB). The two methods calculate higher nitrate aerosol burdens than HDYN by +106% and +47%, respectively. Both fine‐ and coarse‐model nitrate aerosols are overestimated by UPTAKE, but the overestimation by HYB is mainly due to uptake of nitrate by the coarse aerosols. As a result, HYB calculates lower surface concentrations of the fine‐mode nitrate aerosol by up to 50% over most continental areas, compared to HDYN. Surface HNO3 and NOx concentrations are underpredicted by HYB by up to 90% and 5%, respectively. Since the reaction of N2O5 on sulfate aerosols is not included in the UPTAKE method, the NOx burden and surface concentrations are overestimated by 56% and a factor of 2–5, respectively. These results suggest the importance of using the more accurate hybrid dynamical method in the estimates of both aerosol forcing and tropospheric ozone chemistry.

Simulation of nitrate aerosol concentrations over East Asia with the model system RAMS-CMAQ

The Models-3 Community Multi-scale Air Quality modeling system (CMAQ) coupled with the Regional Atmospheric Modeling System (RAMS) is used to analyse the geographical and the seasonal characteristics of nitrate aerosol (ANO3) concentration distributions over East Asia. Three-dimensional concentrations in January,March, April, July and October of 2001 are simulated, and for the evaluation of model performances, the simulated values of wind direction, wind speed, temperature, specific humidity, nitric oxide (NO), nitrogen oxide (NO2), ANO3 and ammonium aerosol (ANH4) are compared with the observational data obtained onboard the P-3B aircraft during its four consecutive flights over the South China Sea, the East China Sea, the Yellow Sea and the water areas to the east of Japan conducted on 13, 17, 18 and 21 March 2001. The observed data are 5-min averaged and the model results with a 1-hour temporal resolution are interpolated to the aircraft location and time using trilinear interpolation. Comparison shows that the observed values exhibit strong temporo-spatial variations, and the model reproduces these variations reasonably well. The simulated values of wind direction, wind speed, temperature and specific humidity are generally in good agreement with the observed ones, their correlation coefficients reach 0.66, 0.94, 0.99 and 0.96, respectively. Comparison also shows that the average mixing ratios of modelled and observed NO, NO2, ANO3 and ANH4 agree reasonably well with each other, the correlation coefficient for ANO3 and ANH4 are larger than 0.8, but the simulated standard deviations are smaller than the observed one, and the correlation coefficient for NO and NO2 are 0.48 and 0.44.Analysis of horizontal distributions of monthly averaged ANO3 concentrations in the boundary layer indicates that the ANO3 mixing ratios have noticeable differences among the four seasons. Generally the concentrations are high in the winter, spring and fall and low in the summer, and seasonal variations are typically strongest over China and Japan. Highest concentrations are found over eastern China where emissions of nitrogen oxides and ammonia are high and long-range transport may cause elevated concentrations in remote areas under favourable meteorological conditions, but the seasonal variation in the ANO3 concentrations is heavily influenced by the changes in precipitation and temperature.

Measuring Ammonia Concentrations and Emissions from Agricultural Land and Liquid Surfaces: A Review

Aerial ammonia concentrations (C g) are measured using acid scrubbers, filter packs, denuders, or optical methods. Using C g and wind speed or airflow rate, ammonia emission rate or flux can be directly estimated using enclosures or micrometeorological methods. Using nitrogen (N) recovery is not recommended, mainly because the different gaseous N components cannot be separated. Although low cost and replicable, chambers modify environmental conditions and are suitable only for comparing treatments. Wind tunnels do not modify environmental conditions as much as chambers, but they may not be appropriate for determining ammonia fluxes; however, they can be used to compare emissions and test models. Larger wind tunnels that also simulate natural wind profiles may be more useful for comparing treatments than micrometeorological methods because the latter require larger plots and are, thus, difficult to replicate. For determining absolute ammonia flux, the micrometeorological methods are the most suitable because they are nonintrusive. For use with micrometeorological methods, both the passive denuders and optical methods give comparable accuracies, although the latter give real-time C g but at a higher cost. The passive denuder is wind weighted and also costs less than forced-air C g measurement methods, but it requires calibration. When ammonia contamination during sample preparation and handling is a concern and separating the gas-phase ammonia and aerosol ammonium is not required, the scrubber is preferred over the passive denuder. The photothermal interferometer, because of its low detection limit and robustness, may hold potential for use in agriculture, but it requires evaluation. With its simpler theoretical basis and fewer restrictions, the integrated horizontal flux (IHF) method is preferable over other micrometeorological methods, particularly for lagoons, where berms and land-lagoon boundaries modify wind flow and flux gradients. With uniform wind flow, the ZINST method requiring measurement at one predetermined height may perform comparably to the IHF method but at a lower cost.

Dry and wet deposition of inorganic nitrogen compounds to a tropical pasture site (Rondônia, Brazil)

Abstract. The input of nitrogen (N) to ecosystems has increased dramatically over the past decades. While total (wet + dry) N deposition has been extensively determined in temperate regions, only very few data sets of N wet deposition exist for tropical ecosystems, and moreover, reliable experimental information about N dry deposition in tropical environments is lacking. In this study we estimate dry and wet deposition of inorganic N for a remote pasture site in the Amazon Basin based on in-situ measurements. The measurements covered the late dry (biomass burning) season, a transition period and the onset of the wet season (clean conditions) (12 September to 14 November 2002) and were a part of the LBA-SMOCC (Large-Scale Biosphere-Atmosphere Experiment in Amazonia – Smoke, Aerosols, Clouds, Rainfall, and Climate) 2002 campaign. Ammonia (NH3), nitric acid (HNO3), nitrous acid (HONO), nitrogen dioxide (NO2), nitric oxide (NO), ozone (O3), aerosol ammonium (NH4+) and aerosol nitrate (NO3-) were measured in real-time, accompanied by simultaneous meteorological measurements. Dry deposition fluxes of NO2 and HNO3 are inferred using the ''big leaf multiple resistance approach'' and particle deposition fluxes are derived using an established empirical parameterization. Bi-directional surface-atmosphere exchange fluxes of NH3 and HONO are estimated by applying a ''canopy compensation point model''. N dry and wet deposition is dominated by NH3 and NH4+, which is largely the consequence of biomass burning during the dry season. The grass surface appeared to have a strong potential for daytime NH3 emission, owing to high canopy compensation points, which are related to high surface temperatures and to direct NH3 emissions from cattle excreta. NO2 also significantly accounted for N dry deposition, whereas HNO3, HONO and N-containing aerosol species were only minor contributors. Ignoring NH3 emission from the vegetation surface, the annual net N deposition rate is estimated to be about −11 kgN ha-1 yr-1. If on the other hand, surface-atmosphere exchange of NH3 is considered to be bi-directional, the annual net N budget at the pasture site is estimated to range from −2.15 to −4.25 kgN ha-1 yr-1.

Numerical simulation with a comprehensive chemical transport model of nitrate, sulfate, and ammonium aerosol distributions over east asia

The transport and chemical production processes of nitrate, sulfate, and ammonium aerosols over East Asia were investigated by use of the Models-3 Community Multi-scale Air Quality (CMAQ) modeling system coupled with the Regional Atmospheric Modeling System (RAMS). For the evaluation of the model's ability in depicting their 3-dimensional concentration distributions and temporal variations, modeled concentrations of nitrate, sulfate, and ammonium aerosols are compared with the observations obtained at a ground station in Japan in March 2001 and onboard of an aircraft DC-8 on 18 and 21 March 2001 during the Transport and Chemical Evolution over the Pacific (TRACE-P) field campaign. Comparison shows that simulated values of nitrate, sulfate, and ammonium aerosols are generally in good agreement with their observed data, and the model captures most important observed features, and reproduces temporal and spatial variations of nitrate, sulfate, and ammonium aerosol concentrations reasonably well, e.g., the timing and locations of the concentration spikes of nitrate, sulfate, and ammonium aerosols are well reproduced, but large discrepancies between observed and simulated values are also clearly seen at some points and some times due to the coarse grid resolution and uncertainties of the emissions used in this study. This comparison results indicate that CMAQ is able to simulate the distributions of nitrate, sulfate, and ammonium aerosols and their related species in the troposphere over East Asia reasonably well.

Effect of ambient NH3 levels on PM2.5 composition in the Great Smoky Mountains National Park

Abstract Continuous measurements were made of gaseous NH 3 , gaseous HNO 3 , PM 2.5 sulfate, aerosol mass, air temperature, and relative humidity (RH), and 24-h filter samples of PM 2.5 aerosols were analyzed to determine the effects of ambient NH 3 levels on PM 2.5 aerosol composition. These measurements were conducted from 2 to 15 August 2002, at Look Rock, TN, at an air monitoring station located on a mountain ridge (elevation ∼800 m MSL) on the southwestern edge of the Great Smoky Mountains National Park. A strong diurnal pattern was observed for gaseous NH 3 levels at this site with higher daytime and much lower NH 3 during the night hours. The diurnal variability of both gaseous NH 3 concentrations and RH suggests that conditions controlling aerosol nitrate formation and acidic aerosol neutralization are complex. The observations during this 2-week long sampling period showed that measurable NH 4 NO 3 aerosol was not present under the ambient conditions encountered. However, modest perturbations from observed ambient conditions could have enabled NH 4 NO 3 aerosol production. The extent of neutralization of sulfate aerosol collected on filters indicates that there was insufficient regional NH 3 to neutralize the acidic sulfate aerosols to the degree predicted from local NH 3 and RH conditions. Incomplete neutralization of acidic sulfate could also result if neutralization proceeds more slowly under field conditions compared to laboratory conditions or model predictions. Continuous measurements of aerosol ammonium and/or acidity are needed to determine if ammonia availability or kinetic limitations (or both) restrict neutralization of acidic sulfate aerosols, a key factor for modeling and exposure studies.

Jupiter's ammonia clouds—localized or ubiquitous?

From an analysis of the Galileo Near Infrared Imaging Spectrometer (NIMS) data, Baines et al. (Icarus 159 (2002) 74) have reported that spectrally identifiable ammonia clouds (SIACs) cover less than 1% of Jupiter. Localized ammonia clouds have been identified also in the Cassini Composite Infrared Spectrometer (CIRS) observations (Planet. Space Sci. 52 (2004a) 385). Yet, ground-based, satellite and spacecraft observations show that clouds exist everywhere on Jupiter. Thermochemical models also predict that Jupiter must be covered with clouds, with the top layer made up of ammonia ice. For a solar composition atmosphere, models predict the base of the ammonia clouds to be at 720 mb, at 1000 mb if N/H were 4×solar, and at 0.5 bar for depleted ammonia of 10 −2×solar (Planet. Space Sci. 47 (1999) 1243). Thus, the above NIMS and CIRS findings are seemingly at odds with other observations and cloud physics models. We suggest that the clouds of ammonia ice are ubiquitous on Jupiter, but that spectral identification of all but the freshest of the ammonia clouds and high altitude ammonia haze is inhibited by a combination of (i) dusting, starting with hydrocarbon haze particles falling from Jupiter's stratosphere and combining with an even much larger source—the hydrazine haze; (ii) cloud properties, including ammonia aerosol particle size effects. In this paper, we investigate the role of photochemical haze and find that a substantial amount of haze material can deposit on the upper cloud layer of Jupiter, possibly enough to mask its spectral signature. The stratospheric haze particles result from condensation of polycyclic aromatic hydrocarbons (PAHs), whereas hydrazine ice is formed from ammonia photochemistry. We anticipate similar conditions to prevail on Saturn.

Size-segregated characterization of atmospheric aerosols in Taipei during Asian outflow episodes

To investigate the size resolved physico-chemical properties of aerosols transported from the eastern Asian Continent following the winter monsoons, an intensive field experiment was conducted in Taipei, Taiwan in January 2003. Mass concentration and the abundance of major functional groups in size-segregated aerosol samples were measured. The size distribution parameters of the aerosol mass and the associated functional groups were analyzed. The results of the infrared spectroscopic analysis showed that the contents of sulfate and ammonium in Taipei's sub-micron aerosols were significantly increased due to continental pollution. In addition, the Asian outflow aerosols contained higher levels of silicate and nitrate in the super-micron size range. The size-resolved functionalities of the aerosol samples collected during Asian outflow episodes were compared with those of local pollution samples. This comparison revealed that ammonium and sulfate generally coexisted in sub-micron aerosols, suggesting that there are substantial emissions of ammonia to neutralize the acidity of sulfate in the region from eastern China to northern Taiwan. The nitrate had a bimodal distribution over the size range between 0.1 and 10 μm. A significant enhancement was found in coarse nitrate during the Asian outflow episodes, implying heterogeneous reactions of nitric acid on the surface of dust and sea salt particles during the transport processes. High concentrations of sub-micron ammonium nitrate aerosols were observed prior to the frontal passage and served as an important indicator of local photochemical pollution. The aliphatic and carbonyl carbon in the aerosols were suggested as being mostly due to local pollution in Taipei. Nevertheless, our analysis found that the aerosols of Asian outflows had a higher infrared absorbance ratio of carbonyl to aliphatic carbon than those of the local pollution. The enrichment of carbonyl carbon in the Asian outflow aerosols is most likely due to the emissions of biomass burning aerosols in the eastern China.

Three‐dimensional simulations of inorganic aerosol distributions in east Asia during spring 2001

In this paper, aerosol composition and size distributions in east Asia are simulated using a comprehensive chemical transport model. Three‐dimensional aerosol simulations for the TRACE‐P and ACE‐Asia periods are performed and used to help interpret actual observations. The regional chemical transport model, STEM‐2K3, which includes the on‐line gas‐aerosol thermodynamic module SCAPE II, and explicitly considers chemical aging of dust, is used in the analysis. The model is found to represent many of the important observed features. The Asian outflow during March and April of 2001 is heavily polluted with high aerosol loadings. Under conditions of low dust loading, SO2 condensation and gas phase ammonia distribution determine the nitrate size and gas‐aerosol distributions along air mass trajectories, a situation that is analyzed in detail for two TRACE‐P flights. Dust is predicted to alter the partitioning of the semivolatile components between the gas and aerosol phases as well as the size distributions of the secondary aerosol constituents. Calcium in the dust affects the gas‐aerosol equilibrium by shifting the equilibrium balance to an anion‐limited status, which benefits the uptake of sulfate and nitrate, but reduces the amount of aerosol ammonium. Surface reactions on dust provide an additional mechanism to produce aerosol nitrate and sulfate. The size distribution of dust is shown to be a critical factor in determining the size distribution of secondary aerosols. As much of the dust mass is found in the supermicron mode (70–90%), appreciable amounts of sulfate and nitrate are found in the supermicron particles. For sulfate the observations and the analysis indicate that 10–30% of sulfate is in the supermicron fraction during dust events; in the case of nitrate, more than 80% is found in the supermicron fraction.

Gas-particle interactions above a Dutch heathland: I. Surface exchange fluxes of NH<sub>3</sub>, SO<sub>2</sub>, HNO<sub>3</sub> and HCl

Abstract. A field measurement campaign was carried out over a Dutch heathland to investigate the effect of gas-to-particle conversion and ammonium aerosol evaporation on surface/atmosphere fluxes of ammonia and related species. Continuous micrometeorological measurements of the surface exchange of NH3, SO2, HNO3 and HCl were made and are analyzed here with regard to average fluxes, deposition velocities (Vd), canopy resistances (Rc) and canopy compensation point for NH3. Gradients of SO2, HNO3 and HCl were measured with a novel wet-denuder system with online anion chromatography. Measurements of HNO3 and HCl indicate an Rc of 100 to 200 s m-1 during warm daytime periods, probably at least partly due to non-zero acid partial pressures above NH4NO3 and NH4Cl on the leaf surfaces. Although it is likely that this observation is exacerbated by the effect of the evaporation of airborne NH4+ on the gradient measurements, the findings nevertheless add to the growing evidence that HNO3 and HCl are not always deposited at the maximum rate. Ammonia (NH3) fluxes show mainly deposition, with some periods of significant daytime emission. The net exchange could be reproduced both with an Rc model (deposition fluxes only) using resistance parameterizations from former measurements, as well as with the canopy compensation point model, using parameterizations derived from the measurements. The apoplastic ratio of ammonium and hydrogen concentration (Γs=[NH4+]/[H+]) of 1200 estimated from the measurements is large for semi-natural vegetation, but smaller than indicated by previous measurements at this site.

Effects of the physical state of tropospheric ammonium-sulfate-nitrate particles on global aerosol direct radiative forcing

Abstract. The effect of aqueous versus crystalline sulfate-nitrate-ammonium tropospheric particles on global aerosol direct radiative forcing is assessed. A global three-dimensional chemical transport model predicts sulfate, nitrate, and ammonium aerosol mass. An aerosol thermodynamics model is called twice, once for the upper side (US) and once for lower side (LS) of the hysteresis loop of particle phase. On the LS, the sulfate mass budget is 40% solid ammonium sulfate, 12% letovicite, 11% ammonium bisulfate, and 37% aqueous. The LS nitrate mass budget is 26% solid ammonium nitrate, 7% aqueous, and 67% gas-phase nitric acid release due to increased volatility upon crystallization. The LS ammonium budget is 45% solid ammonium sulfate, 10% letovicite, 6% ammonium bisulfate, 4% ammonium nitrate, 7% ammonia release due to increased volatility, and 28% aqueous. LS aerosol water mass partitions as 22% effloresced to the gas-phase and 78% remaining as aerosol mass. The predicted US/LS global fields of aerosol mass are employed in a Mie scattering model to generate global US/LS aerosol optical properties, including scattering efficiency, single scattering albedo, and asymmetry parameter. Global annual average LS optical depth and mass scattering efficiency are, respectively, 0.023 and 10.7 m2 (g SO4-2)-1, which compare to US values of 0.030 and 13.9 m2 (g SO4-2)-1. Radiative transport is computed, first for a base case having no aerosol and then for the two global fields corresponding to the US and LS of the hysteresis loop. Regional, global, seasonal, and annual averages of top-of-the-atmosphere aerosol radiative forcing on the LS and US (FL and FU, respectively, in W m-2) are calculated. Including both anthropogenic and natural emissions, we obtain global annual averages of FL=-0.750, FU=-0.930, and DFU,L=24% for full sky calculations without clouds and FL=-0.485, FU=-0.605, and DFU,L=25% when clouds are included. Regionally, DFU,L=48% over the USA, 55% over Europe, and 34% over East Asia. Seasonally, DFU,L varies from 18% in DJF to 75% in SON over the USA. The global annual average contribution from anthropogenic aerosol is FL=-0.314 and FU=-0.404, which yield normalized direct radiative forcings (G) of GL=-205 W (g SO4-2)-1 and GU=-264 W (g SO4-2)-1.

Ambient ammonia and ammonium aerosol across a region of variable ammonia emission density

Abstract We present 1 year of ambient ammonia (NH3), ammonium (NH4+), hydrochloric acid (HCl), chloride (Cl−), nitric acid (HNO3), nitrate (NO3−), nitrous acid (HONO), sulfur dioxide (SO2), and sulfate (SO42−) concentrations at three sites in the Coastal Plain region of North Carolina. The three sites, Clinton, Kinston, and Morehead City, are located in counties with total NH3 emission densities of 4800, 2280, and 320 kg NH3-N km−2 yr−1, respectively. Average NH3 concentrations were 5.32, 2.46, and 0.58 μg m−3 at Clinton, Kinston, and Morehead City, respectively. Average NH4+ concentrations were 1.84, 1.25, and 0.91 μg m−3, and total concentrations of inorganic (NH4++NO3−+ SO42−+Cl−) particulate matter with aerosol diameters

IMAGES‐SCAPE2: A modeling study of size‐ and chemically resolved aerosol thermodynamics in a global chemical transport model

Chemically and size‐resolved aerosol mass size distributions are simulated online using the Intermediate Model of Global Evolution of Species (IMAGES). Aerosol‐phase thermodynamics is coupled to this chemical transport model using the Simulating Composition of Atmospheric Particles at Equilibrium (SCAPE2) module. Emphasis is placed on the equilibrium concentrations of aerosol ammonium, nitrate, sulfate, and water content subject to the interaction with ions from sea salt and dust. Transport for both aerosol and gas‐phase species is solved using a semi‐Lagrangian scheme driven by monthly mean climatological variables provided by European Centre for Medium‐Range Weather Forecasts (ECMWF) analysis. This study presents estimates of annual average dry mass size distributions at selected stations in North America and Europe. Simulations are performed to observe the major differences between the coupled IMAGES‐SCAPE2 model and IMAGES alone for key aerosol species. Nitrate concentrations predicted with IMAGES‐SCAPE2 compare better with observations than those calculated with IMAGES alone. Model results show that aerosol size distributions are generally bimodal. This study finds that over polluted continental regions, the presence of sea‐salt and dust aerosol potentially increases the formation of sulfate by 20–80%. Nitrate formation is enhanced by 14–60%, whereas ammonium formation is decreased by 20–60%. Nitrate plays an important role to determine the size distributions in many European stations. The presence of water changes the general shape of the size distribution, placing maximum values on larger size ranges.

Quantification on the source/receptor relationship of primary pollutants and secondary aerosols by a Gaussian plume trajectory model: Part III—Asian dust-storm periods

Characteristics of pollutants at heights in the top of the Planetary Boundary Layer (PBL) are collected and used in a local-scale model. A subsidence mechanism is developed to quantify the concentration fraction from the top PBL to simulate PM concentration during Asian dust-storm (ADS) periods. The results show that using the data measured at a mountain station, which is very vulnerable to ADS, as the top boundary conditions for the air quality model can capture all the PM2.5 episodes due to local sources and ADS events, at a low-altitude urban station. The correlation coefficient (r2) of daily PM2.5−10 concentration has increased from 0.17 to 0.62 by incorporating the subsidence mechanism, and that of PM2.5 increases as well. The model results of nitrate, sulfate and ammonium aerosol in fine radii can be compared with observations. According to our analysis, five out of eight PM2.5 or PM10 episode days occurred on ADS days in the past 4 years (1999–2002). During ADS episodes in 2000, 12% of PM2.5 and 53% of PM2.5−10 were from ADS dust. In addition, two dry deposition algorithms are evaluated; the algorithm of Seinfeld and Pandis (Atmospheric Chemistry and Physics from Air Pollution to Climate Change, Wiley, New York, 1998, 1057pp.) is suggested in this case study.

Isotopic evidence for a marine ammonia source

Ammonium salts represent a major component of fine mode marine aerosol that play an important role in climate regulation, atmospheric pH control and nutrient supply to the oceans. We report here the results of analyses of aerosol samples collected in remote areas of the Atlantic Ocean. We show the isotopic abundance of aerosol ammonium varies with concentration, with isotopically light ammonium (−5 to −8‰) associated with the lowest ammonium concentrations compared to isotopically heavy ammonium (+10‰) associated with higher ammonium concentrations. We interpret the light ammonium signature as arising from marine emissions of ammonia. These results provide further evidence for the existence of a marine ammonia source and offers a method to define the relative contribution of marine and terrestrial ammonium to aerosols in remote areas and thereby investigate the role of these emissions in climate regulation.

Results from an informal intercomparison of ammonia measurement techniques

NH3 measurements made using a chemical ionization mass spectrometry (CIMS) method and a molybdenum‐oxide, converter‐difference (MoOx‐CD) approach were compared with those made using the citric acid denuder method. The citric acid denuder, which has fared well in a variety of other evaluations, was used as the benchmark method in the current intercomparison. A very high degree of correlation, r2 > 0.9, between the three ammonia measurement techniques was found. For ammonia mixing ratios greater than approximately 100 pptv, the denuder method and the CIMS technique were found to agree to within about 20%. However, the MoOx‐CD measurement results were larger by approximately 70% than the denuder measurements. For our measurements, we conclude that the MoOx‐CD method measured the sum of gas‐phase NH3 collected by the denuder tube and aerosol ammonium (NH4+) that was simultaneously collected on Teflon filters. The methods and the comparison along with the results of the ancillary measurements are described.

Atmospheric nitrogen deposition to Barnegat Bay

To characterize atmospheric nitrogen deposition to Barnegat Bay, measurements of nitrate and ammonium were made, both in precipitation and in aerosol particulate matter, at coastal Tuckerton in southern New Jersey from February 1999 to February 2001. Results show that the concentrations of nitrate and ammonium in the ambient air exhibit strong temporal variation. The concentrations in precipitation ranged from 2.26 to 84.6 μmol l −1 (mean: 29.1 μmol l −1) for nitrate and from 0.625 to 107 μmol l −1 (mean: 18.3 μmol l −1) for ammonium. Most of the pH values in precipitation fall below 4.5, suggesting an acidic nature for precipitation. The low spring-time mean δ 15N values in rain for both ammonium (−4.6‰) and nitrate (−6.03‰) indicate that increased agricultural and soil emissions as well as anthropogenic emissions contribute to atmospheric nitrate and ammonium. The average concentrations in the aerosol phase were 2.04 μg m −3 for nitrate and 0.497 μg m −3 for ammonium. High concentrations of aerosol nitrate were associated with both submicrometer and supermicrometer particles, while aerosol ammonium was primarily concentrated on particles of ∼0.5 μm in diameter. High wet deposition rates of nitrate and ammonium were observed during the summer in both 1999 and 2000, with average summer-time rates of ∼4.12 mmol m −2 month −1 for nitrate and 3.18 mmol m −2 month −1 for ammonium. On an annual basis, the total atmospheric deposition rates were 32.9 mmol m −2 yr −1 for nitrate and 20.3 mmol m −2 yr −1 for ammonium, and wet deposition accounted for >88% of the total deposition. The annual atmospheric input to the Barnegat Bay ecosystem is estimated to be 9.2×10 6 mol yr −1 for nitrate and 5.7×10 6 mol yr −1 for ammonium.

A field-based approach for deterimining ATOFMS instrument sensitities to ammonium and nitrate.

Aerosol time-of-flight mass spectrometry (ATOFMS) instruments measure the size and chemical composition of individual particles in real-time. ATOFMS chemical composition measurements are difficult to quantify, largely because the instrument sensitivities to different chemical species in mixed ambient aerosols are unknown. In this paper, we develop a field-based approach for determining ATOFMS instrument sensitivities to ammonium and nitrate in size-segregated atmospheric aerosols, using tandem ATOFMS-impactor sampling. ATOFMS measurements are compared with collocated impactor measurements taken at Riverside, CA, in September 1996, August 1997, and October 1997. This is the first comparison of ion signal intensities from a single-particle instrument with quantitative measurements of atmospheric aerosol chemical composition. The comparison reveals that ATOFMS instrument sensitvities to both NH4+ and NO3- decline with increasing particle aerodynamic diameter over a 0.32-1.8 microm calibration range. The stability of this particle size dependence is tested overthe broad range of fine particle concentrations (PM1.8) = 17.6 +/- 2.0-127.8 +/- 1.8 microg m(-3)), ambient temperatures (23-35 degrees C), and relative humidity conditions (21-69%), encountered during the field experiments. This paper describes a potentially generalizable methodology for increasing the temporal and size resolution of atmospheric aerosol chemical composition measurements, using tandem ATOFMS-impactor sampling.

Controlling Factors of Weak Acid and Base Concentrations in Urban Dewwater—Comparison of Dew Chemistry with Rain and Fog Chemistry—

Abstract Dewwater was collected from January 1993 to December 2000 in Yokohama, Japan, and analyzed for weak acid anions with other major chemical components. The volume-weighted average and medium pH of the dewwater were 4.88 and 6.08, respectively, at our sampling site. Weak acids (carbonate, nitrite, sulfite, formate, and acetate ions) were the major species in the dewwater and were several times higher than those in the rain- and fogwater. The concentrations of these species and ammonium ion can be estimated by the resistance model. The estimated values of N(III) correlate relatively well with the measured values, which shows the origin of N(III) in dewwater is nitrous acid in the ambient air. On the other hand, ammonium ion in the dewwater originated from not only gaseous ammonia but also aerosol ammonium ions. Hydrogensulfite ions in the dewwater which originated from sulfur dioxide are an important species as the precursor of the sulfate and they were stabilized by forming hydroxyalkanesulfonate (HASA). Formic acid and acetic acid in the dewwater originated mainly from gas, while the oxalic acid is from aerosol.

Altitude-dependent wet, dry and occult nitrogen deposition in an Alpine region (Achenldrch, Austria, 920 m - 1758 m a.s.l.).

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, 1,280 m and 1,758 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(-1)a(-1) at the Christlumkopf station (1,758 m), 20 kg N ha(-1)a(-1) at the Christlumalm station (1,280 m) and 28 kg N ha(-1)a(-1) at the Talboden station (930 m). Respective data for the 1997/98 observation period were 31 kg N ha(-1)a(-1) at the Christlumkopf station (1,758 m) and 18 kg N ha(-1)a(-1) at the Mülhleggerköpfl 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.