Ignatius N. Tang

Chemical and size effects of hygroscopic aerosols on light scattering coefficients

The extensive thermodynamic and optical properties recently reported [Tang and Munkelwitz, 1994a] for sulfate and nitrate solution droplets are incorporated into a visibility model for computing light scattering by hygroscopic aerosols. The following aerosol systems are considered : NH 4 HSO 4 , (NH 4 ) 2 SO 4 , (NH 4 ) 3 H(SO 4 ), NaHSO 4 , Na 2 SO 4 , NH 4 NO 3 , and NaNO 3 . In addition, H 2 SO 4 and NaCl are included to represent freshly formed sulfate and background sea-salt aerosols, respectively. Scattering coefficients, based on 1 μg dry salt per cubic meter of air, are calculated as a function of relative humidity for aerosols of various chemical compositions and lognormal size distributions. For a given size distribution the light scattered by aerosol particles per unit dry-salt mass concentration is only weakly dependent on chemical constituents of the hygroscopic sulfate and nitrate aerosols. Sulfuric acid and sodium chloride aerosols, however, are exceptions and scatter light more efficiently than all other inorganic salt aerosols considered in this study. Both internal and external mixtures exhibit similar light-scattering properties. Thus for common sulfate and nitrate aerosols, since the chemical effect is outweighed by the size effect, it follows that observed light scattering by the ambient aerosol can be approximated, within practical measurement uncertainties, by assuming the aerosol being an external mixture. This has a definite advantage for either visibility degradation or climatic impact modeling calculations, because relevant data are now available for external mixtures but only very scarce for internal mixtures.

Composition and temperature dependence of the deliquescence properties of hygroscopic aerosols

Abstract Thermodynamic considerations are given to the hygroscopicity of inorganic salts comprising most of the ambient aerosols. Equations are derived expressing the deliquescence humidity as a function of composition and temperature for either single-salt or multicomponent aerosols. Experimental results obtained with charged salt particles individually suspended in an electrodynamic cell are compared both with theory and with available literature data on relative humidities over saturated aqueous solutions. The good agreement found between theory and experimental measurements indicates that aerosol deliquescence properties are predictable from the principle of solution thermodynamics. Furthermore, the experimental technique described here is ideally suited for providing data for complex aerosol systems about which no information is yet available.

Thermodynamic and optical properties of sea salt aerosols

Sea salt particles are constantly produced from ocean surfaces by wave-wind interactions and removed by deposition and precipitation scavenging. These particles constitute the background aerosol for light scattering in the marine boundary layer. In this work, the thermodynamic and optical properties of sea salt aerosol particles generated from seawater samples are measured at 25°C as a function of relative humidity, using a single-particle levitation technique. Water activities, densities, and refractive indices of aqueous solution droplets containing a single salt NaCl, Na2SO4, MgCl2, or MgSO4 are also reported as a function of concentration. The light-scattering properties of the sea salt aerosol are modeled by the external mixture of these four salt systems selected to approximate the sea salt composition. Good agreements are obtained. It follows that in either visibility reduction or radiative forcing calculations, both freshly produced and aged sea salt aerosols may be modeled by external mixtures of the appropriate inorganic salts, whose solution properties are now available in the literature.

Thermodynamic and optical properties of mixed-salt aerosols of atmospheric importance

Extensive water activity, density, and refractive index data at 25°C are reported for mixed-salt solutions, NaCl-KCl, NaCl-NaNO3, NaCl-Na2SO4, Na2SO4-NaNO3, and (NH4)2SO4-Na2SO4. The data are obtained from hydration experiments using the single-particle levitation technique developed recently for measuring the thermodynamic and optical properties of microdroplets. These data, covering the whole concentration range from dilute solutions to high supersaturations, provide an opportunity to explore the light-scattering properties of both internal and external mixtures of the chloride, sulfate, and nitrate aerosols of atmospheric importance. It is shown that for sulfate and nitrate aerosols as solution droplets, the light-scattering properties do not differ appreciably among all mixture types and compositions, as long as the dry-salt aerosols have the same particle-size distribution. However, for mixed-salt aerosols containing NaCl, the light-scattering properties do depend upon the composition and particle-size distribution, although not so much on the mixture type.

On the equilibrium partial pressures of nitric acid and ammonia in the atmosphere

Abstract Water-soluble salts of sulfate and nitrate have been shown to constitute a substantial portion of the ambient aerosol. Unlike sulfuric acid which has a high affinity for water, NH 3 and HNO 3 are relatively volatile and may transfer between the gaseous phase and the suspended solution droplets. The chemical equilibrium for the NH 3 -HNO 3 -H 2 SO 4 -H 2 O system at 25°C is considered. Equilibrium partial pressures of NH 3 and HNO 3 ranging from a few hundredths to a few hundred ppbv are calculated for droplets of various sulfate and nitrate compositions. The effects of relative humidity and solution pH value on the atmospheric NH 3 and HNO 3 concentrations are studied analytically. The results indicate that, while the HNO 3 partial pressure depends strongly on relative humidity and the nitrate-to-sulfate ratio in solution, the NH 3 partial pressure varies only slightly with humidity but inversely with the hydrogen-ion concentration.

The relative importance of atmospheric sulfates and nitrates in visibility reduction

Atmospheric visibility is largely determined by the concentration, size distribution and chemical composition of the ambient aerosol. In the present paper, total extinction coefficients were calculated for mixed NH4NO3-(NH4)2SO4 aerosols of two different compositions under various humidity conditions. Pertinent thermodynamic and optical properties were measured at 25° C and incorporated into a computer model for droplet growth and Mie calculations. Log-normal particle size distributions with mass median diameters characteristic of the Los Angeles aerosol were assumed throughout the study. The results show that the extent of light extinction by aerosol particles depends on whether the aerosol is present as a multicomponent solution droplets of identical composition (internal mixture) or as a mixture of single-salt aerosols (external mixture|. The extinction coefficients calculated for externally mixed sulfate — nitrate aerosols compare remarkably well with published statistical correlations derived from the Los Angeles visibility data. In addition, it is shown that, for externally mixed sulfate-nitrate aerosols, the sulfate indeed contributes overwhelmingly to visibility reduction, a finding in agreement with general observations for atmospheric aerosols.

An Improved Nonenzymic Method for the Determination of Gas-phase Peroxides.

We report an improved method for sampling and realtime determination of gaseous hydrogen peroxide, hydroxymethyl hydroperoxide (HMHP), and methyl hydroperoxide (MHP) in the atmosphere. The analytical method is based on the hydroxylation of benzoic acid by Fenton reagent [Fe(II) and H[sub 2]O[sub 2]] to form the fluorescent product, hydroxybenzoic acid. Fluorescence intensity is enhanced by complexation with aluminum ion. A novel sampling device with a surfaceless intake permitting collection of gas-phase peroxides without inlet line losses is described. SO[sub 2] interference in the measurements has been fully characterized, and experimental conditions have been specified for minimizing such interference. Significant concentrations of organic peroxides were observed when the improved method was fielded during the Southern Oxidant Study/Southern Oxidant Research Program on Ozone Nonattainment (SOS/SORP-ONA) in Atlanta, GA. in August 1992 and during the North Atlantic Regional Experiment (NARE) in Nova Scotia in August 1993. 44 refs., 5 figs., 3 tabs.

Accommodation coefficient of gaseous NO2 on water surfaces

Abstract Mass accommodation (or sticking) coefficients for trace gases on water surfaces play an important role in the atmospheric heterogeneous processes. In particular, the rate of trace gas incorporation into cloud or fog droplets has recently received much attention because of its relevance to acid precipitation. In the present study, experimental results are reported on the interfacial mass transfer of gaseous NO2 into aqueous phase. The accommodation coefficient for NO2, α = (6.3 ± 0.7) × 10−4 at 0°C, has been measured for the first time using a stop-flow multiple-pass laser absorption technique. This result indicates that, in clouds or fogs, NO2 mass transport into liquid droplets may be limited by the gas-droplet interfacial resistance under certain atmospheric conditions.

Laboratory study of sulfuric acid spill characteristics pertaining to maritime accidents

Concentrated sulfuric acid and oleums are routinely transported in bulk quantities on U.S. and international waterways. It is conceivable that, in the event of an accident, an acid spill could occur with consequences detrimental to both man and the environment. In the present paper, several acid-spill scenarios are briefly described, and the results from laboratory experiments designed to simulate the two most important types of acid-spill accidents are reported. In the first case, the convective mixing of concentrated sulfuric acid with water is shown to be governed by the buoyancy force arising from changes in acid concentration and released heat of dilution. In the second case, experiments with oleums have resulted in the formation of dense clouds of acid aerosols well within the respirable particle size range.