PhD Thesis Defence Presentations - Stylianos Kakavas

Secondary inorganic aerosol (nitrate, ammonia and sulfate) are major components of air pollution and contribute significantly to the acidity of atmospheric particles. Chemical transfer models often overestimate the concentrations of secondary inorganic fine particles and especially those of nitrate. This dissertation is an attempt to identify the effects of dust cations (calcium, potassium, magnesium) and atmospheric acidity on the concentrations of inorganic particles but also to better understand the particle acidity and its various dependencies.

The first part of this work examines the hypothesis that the underestimation of dust emissions in urban centers is responsible for part of the errors in the prediction of particulate concentrations in Europe from chemical transport models. Simulations with a three-dimensional chemical transport model (PMCAMx) indicate that the corresponding emissions are underestimated by an order of magnitude in the reported emissions by each European country. This hypothesis leads to improved predictions for particles with diameter less than 10 μm especially in urban areas and affects the predictions of secondary inorganic aerosol in the European atmosphere and in particular those of nitrate.

The improved emissions were used then to simulate the acidity of particles in the European atmosphere as a function of their size, location and altitude using PMCAMx CTM. The simulations indicate that there are significant differences in pH between fine and coarse particles and that pH affects the predictions of nitrates. In addition, the acidity of the particles increases significantly with height and decreases due to the non-volatile cations of dust.

In the third part of the present work, the acidity of the particles in the U.S was simulated as a function of this time and season using PMCAMx. Also, the predictions of secondary inorganic fine particles were compared with the corresponding measurements. The acidity of the particles is increased during the summer and decreased during the winter.

In the next part of the work ISORROPIA-lite a simplified and accelerated version of the thermodynamic model of inorganic particles ISORROPIA-II is presented. ISORROPIA-lite considers that inorganic particles contain water in the atmosphere even at low relative humidity and uses precalculated tables for the calculation of activity coefficients of a ionic pair. Also, ISORROPIA-lite compared to ISORROPIA-II has the ability to calculate water concentrations from organic particles and to simulate the effects it has on the thermodynamic equilibrium of inorganic particles.

Finally, the effects of water from organic particles on the concentrations and composition of fine particles were studied by applying ISORROPIA-lite to PMCAMx for a full year in the U.S. The simulations show that water from organic particles can contribute significantly to the total water concentrations of the particles and locally increase the concentrations of nitrate and ammonium salts especially in places with high levels of relative humidity and concentrations of fine particles.