Christodoulos Pilinis

ISORROPIA: A New Thermodynamic Equilibrium Model for Multiphase Multicomponent Inorganic Aerosols

A computationally efficient and rigorous thermodynamic model that predicts the physical state and composition of inorganic atmospheric aerosol is presented. One of the main features of the model is the implementation of mutual deliquescence of multicomponent salt particles, which lowers the deliquescence point of the aerosol phase.The model is used to examine the behavior of four types of tropospheric aerosol (marine, urban, remote continental and non-urban continental), and the results are compared with the predictions of two other models currently in use. The results of all three models were generally in good agreement. Differences were found primarily in the mutual deliquescence humidity regions, where the new model predicted the existence of water, and the other two did not. Differences in the behavior (speciation and water absorbing properties) between the aerosol types are pointed out. The new model also needed considerably less CPU time, and always shows stability and robust convergence.

Sensitivity of direct climate forcing by atmospheric aerosols to aerosol size and composition

We evaluate, using a box model, the sensitivity of direct climate forcing by atmospheric aerosols for a “global mean” aerosol that consists of fine and coarse modes to aerosol composition, aerosol size distribution, relative humidity (RH), aerosol mixing state (internal versus external mixture), deliquescence/crystallization hysteresis, and solar zenith angle. We also examine the dependence of aerosol upscatter fraction on aerosol size, solar zenith angle, and wavelength and the dependence of single scatter albedo on wavelength and aerosol composition. The single most important parameter in determining direct aerosol forcing is relative humidity, and the most important process is the increase of the aerosol mass as a result of water uptake. An increase of the relative humidity from 40 to 80% is estimated for the global mean aerosol considered to result in an increase of the radiative forcing by a factor of 2.1. Forcing is relatively insensitive to the fine mode diameter increase due to hygroscopic growth, as long as this mode remains inside the efficient scattering size region. The hysteresis/deliquescence region introduces additional uncertainty but, in general, errors less than 20% result by the use of the average of the two curves to predict forcing. For fine aerosol mode mean diameters in the 0.2–0.5 μm range direct aerosol forcing is relatively insensitive (errors less than 20%) to variations of the mean diameter. Estimation of the coarse mode diameter within a factor of 2 is generally sufficient for the estimation of the total aerosol radiative forcing within 20%. Moreover, the coarse mode, which represents the nonanthropogenic fraction of the aerosol, is estimated to contribute less than 10% of the total radiative forcing for all RHs of interest. Aerosol chemical composition is important to direct radiative forcing as it determines (1) water uptake with RH, and (2) optical properties. The effect of absorption by aerosol components on forcing is found to be significant even for single scatter albedo values of ω=0.93–0.97. The absorbing aerosol component reduces the aerosol forcing from that in its absence by roughly 30% at 60% RH and 20% at 90% RH. The mixing state of the aerosol (internal versus external) for the particular aerosol considered here is found to be of secondary importance. While sulfate mass scattering efficiency (m2 (g SO42−)−1) and the normalized sulfate forcing (W (g SO42−)−1) increase strongly with RH, total mass scattering efficiency (m2 g−1) and normalized forcing (W g−1) are relatively insensitive to RH, wherein the mass of all species, including water, are accounted for. Following S. Nemesure et al. (Direct shortwave forcing of climate by anthropogenic sulfate aerosol: sensitivity to particle size, composition, and relative humidity, submitted to Journal of Geophysical Research, 1995), we find that aerosol feeing achieves a maximum at a particular solar zenith angle, reflecting a balance between increasing upscatter fraction with increasing solar zenith angle and decreasing solar flux (from Rayleigh scattering) with increasing solar zenith angle.

Continued development of a general equilibrium model for inorganic multicomponent atmospheric aerosols

Abstract A model is presented that predicts the total quantities of ammonium, chloride, nitrate and water contained in atmospheric aerosols, their physical state and their distribution among aerosol particles of different sizes. The model is based on the thermodynamic equilibrium calculation of the ammonium/chloride/nitrate/sodium/sulfate/water system. The existence of water in the aerosol phase at low relative humidities is shown to be explained. Observed aerosol concentrations at Long Beach, California during 30–31 August 1982 are successfully predicted.

Continued development and testing of a new thermodynamic aerosol module for urban and regional air quality models

A computationally efficient and rigorous thermodynamic model (ISORROPIA) that predicts the physical state and composition of inorganic atmospheric aerosol is presented. The advantages of this particular model render it suitable for incorporation into urban and regional air quality models. The model is embodied into the UAM-AERO air quality model, and the performance is compared with two other thermodynamic modules currently in use, SEQUILIB 1.5 and SEQUILIB 2.1. The new model yields predictions that agree with experimental measurements and the results of the other models, but at the same time proves to be much faster and computationally efficient. Using ISORROPIA accelerates the thermodynamic calculations by more than a factor of six, while the overall speed-up of UAM-AERO is at least twofold. This speedup is possible by the optimal solution of the thermodynamic equations, and the usage of precalculated tables, whenever possible.

Water content of atmospheric aerosols

Abstract The water content of atmospheric aerosol particles is calculated, both above and below the classic deliquescent point. The existence of water in the paniculate phase is predicted to be strongly dependent on the aerosol composition. Liquid water may constitute a significant mass fraction of the aerosol concentration at r.h.

Chemical composition differences in fog and cloud droplets of different sizes

Abstract The distribution of acidity and solute concentration among the various droplet sizes in a fog or cloud and the effect of the evaporation-condensation cycle on the composition and size distribution of atmospheric aerosol is studied. Significant total solute concentration differences can occur in aqueous droplets inside a fog or cloud. For the fog simulated here, during the period of dense fog, the solute concentration in droplets larger than 10 μm diameter increased with size, in such a way that droplets of diameter 20 μm attain a solute concentration that is a factor of 3.6 larger than that in the 10 μm droplets. Droplets on which most of the liquid water condenses have access to most of the reacting medium for in situ S(IV) oxidation and are therefore preferentially enriched in sulfate. The gas and aqueous-phase chemical processes result in an increase of the total solute mass concentration nonuniform over the droplet spectrum for a mature fog. These chemical processes tend to decrease the total solute mass concentration differences among the various droplet sizes. Low cooling rates of the system also tend to decrease these concentration differences while high cooling rates have exactly the opposite effect. The mass/size distribution of the condensation nuclie influences quantitatively, but not qualitatively, the above concentration differences.

Development and evaluation of an Eulerian photochemical gas-aerosol model

Abstract A three-dimensional Eulerian model that simulates the concentrations of gaseous pollutants and the size-composition distribution of multicomponent atmospheric aerosols has been developed and used to study the evolution of the aerosol-size distribution and composition in the South Coast Air Basin of California. The predictions of the model are compared with ambient measurements of sulfate, nitrate, sodium, chloride and ammonium aerosol.

MADM : A new Multicomponent Aerosol Dynamics Model

A Multicomponent Aerosol Dynamics Model (MADM) capable of solving the condensation/evaporation equation of atmospheric aerosols is presented. Condensable species may be organic and/or inorganic. For the inorganic constituents the equilibrium model ISORROPIA is used to predict the physical state of the particle, i.e., whether the aerosol is liquid or solid. The mass transfer equations for the fluxes for solid atmospheric particles are developed. MADM is able to simulate aerosol deliquescence, crystallization, solid to solid phase transitions, and acidity transitions. Aerosols of different sizes can be in different physical states (solid, liquid, or partially solid and partially liquid). Novel constraints on the electroneutrality of the species flux between the gas and aerosol phases are presented for both liquid and solid aerosols. These constraints aid in the stability of the algorithm, yet still allow changes in aerosol acidity. As an example, MADM is used to predict the dynamic response of marine aerosol entering an urban area.

Mathematical modeling of the dynamics of multicomponent atmospheric aerosols

Abstract A model is developed to simulate the dynamics of multicomponent atmospheric aerosols, including new particle formation by homogeneous heteromolecular nucleation, gas-to-particle conversion, coagulation and dry deposition. Both equilibrium and non-equilibrium aspects involving sulfate, nitrate and ammonium compounds are considered. The model is used to predict the dynamics of the composition of the aerosol observed on an air trajectory in the Los Angeles basin on 31 August 1982.

Particle shape and internal inhomogeneity effects on the optical properties of tropospheric aerosols of relevance to climate forcing

Direct climate forcing by atmospheric aerosols is a strong function of the aerosol scattering coefficient, single-scatter albedo, and upscatter fraction. Recent estimates of forcing assume that atmospheric particles are homogeneous spheres. In this paper we attempt to quantify the uncertainty associated with this assumption. We have developed a model, NEPH3, which calculates all the optical properties relevant to direct forcing. The model is tested against measured scattering coefficients from Barbados, assuming that the aerosol particles are homogeneous spheres, stratified spheres, and spheroids. The correlation coefficient of model predictions with measurements is 0.98, for the April 5 to May 3 period, while it is 0.86 for the low dust period of April 14 to May 3. The model indicates that the scattering and extinction coefficients and as a result, the single-scatter albedo are insensitive to the shape and the structure of the particles throughout the measurement period. On the other hand, our calculations show that volume equivalent spherical-nonspherical differences in the backscatter fraction can be large, especially in cases of nonhygroscopic (dust) aerosol, or when the relative humidity is low enough so that the particles are dry. The shape of the particles seems to be important in upscatter fraction calculations too, especially for small solar zenith angles and super-micron-sized particles. Therefore for the size distributions measured in this study, the assumption that atmospheric aerosols are homogeneous spheres may introduce errors as high as 300% in direct forcing estimations, especially when the Sun is increasingly close to the zenith.