Jeremy M. Hales

Fundamentals of the theory of gas scavenging by rain

Abstract The calculation of precipitation scavenging rates for trace gases in the atmosphere is complicated by a number of factors, including reversible sorption behavior, liquid phase mixing and chemical reaction. Despite the large quantity of work that has been devoted to other areas in the field, there exists at present no generalized basis for analyzing the variety of gas scavenging problems that are of potential importance to the atmospheric sciences. The analysis presented in this paper provides such a basis. This analysis examines relationships between the fundamental equations of continuity for a trace contaminent in the air-precipitation system, and suggests classifications for various limiting situations of interest. Criteria for approach of these limiting cases are derived. The general problem of modelling macrophysical systems in the atmosphere is discussed as it relates to the pertinent microphysical phenomena of drop-gas interactions.

Solubility of ammonia in water at low concentrations

Abstract Comparisons of ammonia concentrations in air and in rainwater indicate that a substantial discrepancy exists between observed values and those calculated on the basis of conventional solubility theory, which is based on extrapolation of solubility measurements obtained at higher concentrations. This paper presents the results of ammonia solubility measurements at low concentrations, which have been conducted in an attempt to resolve this anomaly. These results have demonstrated that existing solubility theory, which presumes dissolution to occur via an interphase transport step plus a dissociation of aqueous-phase ammonia to form ammonium ion, provides an adequate description of true behavior at environmental concentrations whenever pure water or pure water plus strong acid is utilized as a solvent. The results are presented in terms of a Henrys Law constant for the interphase transport step NH 3 | g ⇌ H 1 NH 3 | aq which is given as log 10 H 1 = −1.69 + 1477.7 T ( dimensionless ) , where T is the absolute temperature (K), based on molar units for both gas- and liquid-phase concentrations. Previously-measured values of the ammonia dissociation constant were utilized with the experimental data to derive the above expression for H1. Limited tests in the presence of carbon dioxide indicate that previous calculations of ammonia solubility in combination with carbon dioxide at atmospheric concentrations are high by about an order of magnitude. These results are not conclusive and should be verified by further experiments. At the present time, however, they suggest that a substantial restructuring of many concepts regarding the aqueous-phase behavior of environmental ammonia may be necessary.

Statistical aspects of the washout of polydisperse aerosols

When considering the washout properties of polydisperse aerosols, one must specify whether the washout is in terms of number of aerosol particles, or in terms of length, area, or mass of aerosol. This follows from the fact that the effective washout coefficient is a function of the particle size distribution of the aerosol being removed. Sample calculations, using collection efficiencies covering particle radii in the range 0.001 < a < 10 μm. and lognormal raindrop and particle size spectra show that the washout coefficient for a polydisperse aerosol is generally larger than it would be for a monodisperse aerosol of radius equal to some typically used distribution parameter. Thus, it is difficult, if not practically impossible, to deduce generalized washout properties of aerosols from experimental measurements involving polydisperse aerosols.

Wet removal of sulfur compounds from the atmosphere

Abstract This paper presents a brief overview of our current capability to calculate sulfur scavenging rates. The general wet removal process can be decomposed into several individual pathways. These include direct sulfur dioxide scavenging, direct sulfate scavenging and combined scavenging and chemical reaction. Modeling approaches for these pathways are discussed and pertinent research areas for improvement of our present modeling capability are recommended. At the present time the calculation of direct sulfur dioxide scavenging appears to be well in hand, although more careful network measurements of dissolved sulfur dioxide concentrations in rainwater are needed to establish the relative importance of this phenomenon on a regional scale. Direct sulfate scavenging presents a more difficult calculational problem and much more data regarding particle-size relationships of sulfate-containing aerosols is required before an adequate understanding of this pathway can be achieved. Sulfur scavenging via the pathway of sulfur dioxide absorption followed by aqueous-phase conversion is expected to be rate limited by the chemical conversion step under most circumstances. Much additional information regarding aqueous-phase transformation chemistry is necessary before a reliable calculational basis for describing this pathway can be obtained.

Peroxyacetyl nitrate solubility and decomposition rate in acidic water

Abstract Recent investigations have shown that peroxyacetyl nitrate (PAN) is soluble in acidic water samples. A Henrys law constant of 5 ± 1 M atm −1 has been determined. PAN decays in a first order manner in these solutions, with k = 7.0 ± 0.3 × 10 −3 min −1 at 5°C. The principal anionic product was nitrate ion.

Ozone Formation Related to Power Plant Emissions

A curious and unambiguous event of excess ozone formation downwind of a power plant has been analyzed. A kinetic model used to simulate the event indicates that the excess ozone can be reconciled entirely on the basis of excess nitrogen oxides in the polluted air.

Solubility of sulfur dioxide in water at low concentrations

Abstract Solubility of low-concentration SO 2 in water was measured using a flow technique. Concentrations of SO 2 in the gas phase ranged between 0.00178 and 0.1075 μmole 1 −1 ; the temperature of the experiment was held constant at 25.1 °C. The influence of acidity on solubility was examined by adding HCl to the water solutions thereby modifying pH over the range between approximately 3 and 7. Experimental results agree well with values predicted on the basis of solubility theory used in conjunction with solubility and dissociation constants published previously.

Precipitation Scavenging of Urban Pollutants by Convective Storm Systems

Abstract During the summer periods of 1972 and 1973 a precipitation chemistry network for the analysis of rainborne urban pollutants was operated in a region surrounding the St. Louis metropolitan area. The purposes of this network were to assess the effectiveness of convective storms in removing urban pollutants and to provide a data base for scavenging model development. Designed on the basis of a material balance of pollutant over the city, this network concentrated on elucidation of the scavenging behavior of the inorganic, nonmetallic species NH4+, NO3−, NO2−, SO2, SO4= and H+. Quantities of rainborne material deposited on the network downwind of the city, comparable to the urban pollution burden, indicated precipitation scavenging to be a highly efficient removal mechanism. Much of the observed rainborne sulfate and nitrate appears to have been incorporated into the rain by scavenging of gaseous precursors. This finding implies strongly that a rapid oxidation of SO2 to sulfate occurs in cloud system...

Mechanistic analysis of precipitation scavenging using a one-dimensional, timevariant model

Abstract A one-dimensional, time-variant model of a storm system is presented. The model is coded in modular form, and as a consequence it contains a great deal of flexibility for general interpretive use. Besides describing cloud and precipitation processes, the model possesses the capability to predict the precipitationscavenging of both reactive and nonreactive pollutants. Example computations are presented to illustrate the codes application for scavenging analysis. This example is based upon an orographie rain storm, and utilizes highly parameterized descriptions of cloud and precipitation phenomena. The primary pollutants of interest are SO 2 and sulfate. Sulfate aerosol is assumed to be scavenged by a nucleation mechanism, while SO 2 scavenging proceeds via the aqueous-phase reaction SO 2 + O 3 → SO 4 2− . Sulfate concentrations and deposition patterns obtained via these example calculations are not set forth as definitive characterizations of the scavenging process. They do, however, reflect some of the behavior commonly exhibited by real storm systems. More detailed application of this basic model is suggested for use in the future analysis of wet-deposition phenomena.

Smick—A scavenging model incorporating chemical kinetics

Abstract A scavenging model has been developed as an advanced tool for assessing the wet deposition of reactive gases and aerosols from point-source plumes. The model describes both the microscopic and macroscopic aspects of the plume-rain interaction, and predicts the concentrations of pollutant in rain at ground-level points below the plume. Microscopic interactions treated by the model include simultaneous mass transfer and chemical reaction, both outside and within the falling raindrops. Reversibility is incorporated within the mass-transfer description, thus allowing a realistic treatment of gas scavenging. Application of the model to the washout of SO2 from power plant plumes, both with and without the assumption of chemical reaction within droplets, is described. These results indicate that sulfate levels found in rainwater below power-plant plumes are not adequately explained by traditional aqueous-phase conversion mechanisms involving oxidation of SO2 by oxygen, at least when such conversion is limited to the below-cloud region. The model has been documented in users-manual form, and is suggested for use in the general assessment of deposition impacts of pollution sources, such as fossil power plants.