Browsing by Author "Khlystov, Andrey"

We have developed and applied novel experimental techniques for determination of thermodynamic and kinetic properties of semi-volatile organic aerosols. The thermodynamic properties investigated were the saturation pressure, enthalpy of vaporization and activity coefficient, and the kinetic property was the evaporation coefficient.

The thermodynamic properties were determined using the integrated volume method (IVM), which relies on measurements of aerosol particle concentrations at different thermodynamic equilibrium states. The measured decrease in particle concentration upon heating in a flow tube, a thermodenuder, can be correlated with saturation pressure and enthalpy of vaporization via the IVM equation, which was derived from fundamental principles, namely the Clausius-Clapeyron relation, mass conservation, and ideal gas law. The main advantage of the IVM over other methods reported in the literature is that the other methods use kinetic-based techniques to measure thermodynamic properties which requires assumptions on the usually unknown evaporation coefficient; the IVM, on the other hand, is equilibrium-based and thus requires no assumption on the evaporation coefficient. We have applied the IVM to C-4, -6, -7, and -9 dicarboxylic acid aerosols, which are pertinent to atmospheric secondary organic aerosols. Saturation pressure and enthalpy of vaporization values obtained for these compounds were respectively 3.7E-4 Pa and 88 kJ/mol, 3.4E-5 Pa and 135 kJ/mol, 7.2E-5 Pa and 149 kJ/mol, and 1.4E-5 Pa and 145 kJ/mol.

The IVM was also used to determine activity coefficients of individual compounds in binary mixtures as a function of their mole fractions. We demonstrated this method using the following four model systems. System 1: adipic acid - pimelic acid, which illustrated polar organic - polar organic interactions. Non-ideal behavior was observed with activity coefficients around three at infinite dilution. System 2: adipic acid - dioctyl sebacate, which illustrated polar organic - non-polar organic interactions. The compounds in this experiment did not form a solution. System 3: adipic acid - ammonium sulfate, which illustrates polar organic - inorganic interactions. The compounds in this experiment did not form a solution. System 4: adipic acid - ambient extracts, which illustrated the potential use of the method to study partitioning behavior of individual components in a complex matrix approximating that of real ambient aerosol. The measured activity coefficient of adipic acid was less than unity for the tested range of mixing ratios, indicating suppression of volatility of this compound in ambient organic matrix.

We have investigated three controversial issues associated with the IVM as well as other methods which utilize thermodenuders and/or aerosol generation by spray atomization and drying: 1) equilibration time scales in thermodenuders, 2) the need of an activated carbon (AC) denuder in the cooling section, and 3) the effect of residual solvent on measured thermodynamic properties of aerosols generated by spray atomization and drying. Both numerical simulations and experiments showed that the aerosols approached equilibrium within reasonable residence times (15 s - 30 s) for aerosol size distributions typical for laboratory measurements. We have also performed dimensional analysis on the problem of equilibration in TDs, and derived a dimensionless equilibration parameter which can be used to determine the residence time needed for an aerosol of given sized distribution and kinetic properties to approach equilibrium. Using both model simulations and experiments, we have shown that with aerosol size distributions relevant to both ambient and laboratory measurements re-condensation in the cooling section, with and without an AC denuder, was negligible. Thus, there is no significant benefit in using an AC denuder in the cooling section. To investigate the effect of residual solvent on measured thermodynamic properties, we compared measurements of saturation pressure and enthalpy of vaporization of C-6 (adipic) and C-9 (azelaic) dicarboxylic acid aerosols generated by atomization of aqueous solutions to those generated by homogeneous condensation using a modified Sinclair - La Mer generator. We found no statistically significant difference between the tested aerosol generation methods, indicating that residual solvent carried by the particles had no impact on the measurements.

To determine the evaporation coefficient, we developed the integrated volume - tandem differential mobility analysis (IV-TDMA) method. This thermodenuder-based method allows separate determination of the three parameters governing aerosol evaporation, namely, saturation pressure, surface free energy, and evaporation coefficient. Saturation pressure was determined using the IVM, while evaporation coefficient and surface free energy were determined by fitting particle evaporation rates measured under non-equilibrium conditions to a numerical model of the evaporation process. Evaporation coefficient was determined in a size range where surface free energy effects were negligible, allowing for single parameter optimization. We obtained evaporation coefficient and surface free energy values of 0.07 and 0.15 J/m2, 0.08 and 0.17 J/m2 and 0.24 and 0.23 J/m2 for C-4, -6, and -7 dicarboxylic acids, respectively.

Ultrafine particles (UFP) emitted by traffic have been associated with health risks for people living and working near major roadways. Studies have shown that people living in near-roadway communities experience higher risk of aggravated asthma, respiratory diseases and even childhood leukemia. Sharp concentration gradients of UFP have been reported near major highways with the concentration decreasing rapidly away from the road. Dispersion of UFP downwind of a road depends on many parameters, such as the atmospheric stability and wind speed. Presence of different structures such as noise barriers and vegetation can greatly influence the dispersion and downwind concentrations of UFP. These structures can block the traffic emissions and increase vertical mixing. In addition, vegetation can reduce UFP by deposition processes. Two sets of experiments were conducted in this thesis to investigate the effect of barriers on UFP deposition and dispersion.

The first set of experiments was performed in a wind tunnel facility to address UFP deposition to vegetation barriers solely. Two analytical models were proposed to characterize UFP dry deposition to vegetation measured during the wind tunnel experiment. The first model was derived from the filtration theory to explain UFP dry deposition to pine and juniper branches. The model agrees well with the experimental data indicating that pine and juniper branches can be treated as fibrous filters. The fiber diameters of pine derived from the experimental data were also similar to the physical diameters of pine needles; thus, providing further evidence that vegetation can be regarded as fibers. The second model was derived from the continuity equation and can predict the branch-scale dry deposition of UFP using conventional canopy properties such as the drag coefficient and leaf area density. Both models agree with the measurement results to within 20%.

The second set of experiments was done in three near-roadway environments to investigate the effects of barriers on the dispersion and dry deposition of UFP. We used mobile and stationary measurements to obtain the spatial and temporal variability of UFP. Both mobile and stationary measurements indicated that vegetation and noise barriers can reduce downwind UFP concentrations through dispersion and dry deposition by 20-60 %.

In conclusion, the effect of barriers on UFP dispersion and deposition has been characterized in this thesis. Two analytical models were also proposed from the wind tunnel experiments to characterize dry deposition and agreed well with the measurement results. The analytical model could benefit future climate and air quality models.