Pekka Korhonen

Ternary nucleation of H2SO4, NH3, and H2O in the atmosphere

Classical theory of binary homogeneous nucleation is extended to the ternary system H2SO4-NH3-H2O. For NH3 mixing ratios exceeding about 1 ppt, the presence of NH3 enhances the binary H2SO4-H2O nucleation rate by several orders of magnitude. The Gibbs free energies of formation of the critical H2SO4-NH3-H2O cluster, as calculated by two independent approaches, are in substantial agreement. The finding that the H2SO4-NH3-H2O ternary nucleation rate is independent of relative humidity over a large range of H2SO4 concentrations has wide atmospheric consequences. The limiting component for ternary H2SO4-NH3-H2O nucleation is, as in the binary H2SO4-H2O case, H2SO4; however, the H2SO4 concentration needed to achieve significant nucleation rates is several orders of magnitude below that required in the binary case.

Modification of the Köhler Equation to Include Soluble Trace Gases and Slightly Soluble Substances

Abstract A generalized reformulation of the Kohler theory to include the effect of soluble gases and slightly soluble aerosol substances is presented. A single equation is derived that takes into account 1) the Kelvin effect; 2) the Raoult effect caused by highly soluble aerosol material (salt); 3) increase in droplet radius due to an undissolved, insoluble, or slightly soluble core; 4) contribution of solute into the droplet by a slightly soluble substance; and 5) contribution of hygroscopic material into the droplet by a soluble trace gas allowed to deplete from the gas phase because of the condensational growth of the droplets (assuming a monodisperse size distribution). Model calculations are presented for a system in which the aerosol is composed of a slightly soluble CaSO4 core coated with ammonium sulfate, and the gas phase contains HNO3. It is shown that the resulting equilibrium curves allow the occurrence of stable, unactivated droplets with radii up to about 10 μm at realistic ambient condition...

The effect of atmospheric nitric acid vapor on cloud condensation nucleus activation

Activation and growth of cloud condensation nuclei are investigated. A one-dimensional cloud model including detailed microphysics is developed. The system studied consists of NaNO3 particles and condensing water and HNO3 vapors. According to numerical simulations, enhanced concentrations of atmospheric nitric acid vapor affect cloud formation by increasing the number of cloud droplets and decreasing their mean size compared to circumstances where no condensable vapors other than water are present. It is argued that the cloud albedo increases because of these effects.

Aerosol formation during PARFORCE: Ternary nucleation of H2SO4, NH3, and H2O

A new version of a ternary nucleation (sulphuric acid-ammonia-water) model based on classical nucleation theory, but with an improved ability to predict nucleation rates over a larger temperature range (258-303 K) compared with previous work, is presented. The modeled nucleation rates are given as a function of temperature and ambient acid and ammonia concentrations. For the first time the predicted ternary nucleation rates are compared to the observed particle production rates using measured ambient sulphuric acid and ammonia concentrations as input data. The ambient gas concentrations were measured simultaneously to aerosol formation rates during the 1999 New Particle Formation and Fate in the Coastal Environment (PARFORCE) coastal field campaign at Mace Head. According to the results, daytime ambient acid and ammonia concentrations were significantly higher than required by model calculations to induce the formation of new particles by homogeneous ternary nucleation. However, binary nucleation of sulphuric acid-water molecules is not able to predict new particle formation since the binary nucleation rate is far too small. We conclude that all particle formation events observed at coastal sites can be initiated by ternary nucleation of sulphuric acid, ammonia, and water vapor. However, related studies illustrate that ambient sulphuric acid concentrations are, nevertheless, insufficient to explain observed rapid growth of particles from 1 to 3 nm sizes which can be detected by current instrumentation.

Clouds without supersaturation

Traditional Köhler theory describes the equilibrium vapour-pressure relationship between liquid solution particles and humid air. Here we present the concept of multiphase-multicomponent Köhler theory, which reveals that stable cloud droplets of size 1-10 μm could exist in air with a relative humidity of less than 100%. This may explain the occurrence of persistent large-droplet fogs or smogs such as previously existed in London and which are now found in various heavily polluted locations, near the exits of chimneys and in the plumes of volcanoes.

Changes in cloud properties due to NOx emissions

Climate forcing by anthropogenic aerosols has recently excited growing interest [Charlson et al., 1987; Charlson et al., 1990; Wigley, 1991; Charlson et al., 1992]. It has earlier been shown that increased cloud droplet concentrations can increase cloud albedos [e.g. Twomey et al., 1984]. Wigley [1991] and Charlson et al. [1990] have pointed out that the Northern Hemisphere may be warming more slowly than the Southern Hemisphere. According to them this is mainly due to the direct and indirect effects of sulphate aerosols. In this report we discuss an alternative mechanism which could enhance indirect climate forcing by aerosols. Increased concentrations of condensable vapours (such as HNO3 and HCl) in the atmosphere could activate an increased number of pre-existing aerosol particles to act as cloud condensation nuclei [Kulmala et al., 1993]. This would cause similar effects on optical properties of clouds as an increase of aerosol particles in the atmosphere.

Variations of cloud droplet concentrations and the optical properties of clouds due to changing hygroscopicity: A model study

The effects of the availability of gaseous nitric acid, hydrochloric acid, and ammonia on cloud droplet formation have been studied using an adiabatic air parcel model including detailed multicomponent condensation. The used preexisting lognormal particle distribution is bimodal in size and in hygroscopicity: four lognormal distributions were used. In each mode particles were assumed to be internally mixed; that is, they were composed partly of salt and partly of an insoluble substance. According to the simulations made, a clear positive correlation between condensable species concentrations and the number of activated cloud droplets exists. Thus trace gases can also influence the optical thickness and reflectance of clouds. The simulations show that ammonia enhances the condensation rate of acids and thus also their effect on the cloud droplet activation significantly. The change in optical thickness varies as a function of the number concentration of preexisting particles, having a maximum (Δτ/τ = 1.6) near a concentration of 1000-2000 cm -3 .

A Bargaining Model for Solving the Multiple Criteria Problem

A bargaining model for solving the multiple criteria problem is presented. The procedure assumes the framework of mathematical optimization with multiple objectives. The negotiations are started from the optimal solutions of each group member. The model identifies the most discordant group member and tries to induce him to make concessions. The process ends either in a compromise or deadlock. In addition, an experimental application to optimizing macroeconomic policy decisions in Finland is described.

Model simulation of the amount of soluble mass during cloud droplet formation

The development of the amount of hygroscopic material from nitric acid vapour in nascent cloud droplets has been studied. This phenomenon was approached first by simple calculations in order to find out maximum values of condensed hygroscopic material in droplets and in cloud water. Then, a more detailed parcel model with the binary condensation of water and nitric acid vapours was used for the simulations of the development of droplet distribution and the composition of droplets. In the parcel model vapours are condensing on the ammonium nitrate particles. According to the results of this study condensation of nitric acid could increase considerably the amount of nitrate ions in the droplets, if we compare this to the amount of nitrate ions from ammonium nitrate salt. This can lead to the enhanced amount of activated cloud droplets.

Size distributions from aerosol spark generator

Abstract An aerosol spark generator for producing ultrafine particles from metals is described. The aerosol particle size distributions from the spark generator have been measured by both Diffusion Battery-CNC and DMA-Electrometer systems. The average diameter of the generated particles can be varied to be between 2 and 15 nm. The measurements on the particle size distributions are described.