YorkSpace has migrated to a new version of its software. Access our Help Resources to learn how to use the refreshed site. Contact diginit@yorku.ca if you have any questions about the migration.
 

Research publications

Permanent URI for this collection

Browse

Recent Submissions

Now showing 1 - 20 of 244
  • ItemOpen Access
    Seasonal variability and trends of volatile organic compounds in the lower polar troposphere
    (American Geophysical Union, 2003-01-01) Rudolph, J.; Stein, O.; Rohrer, F.; Koppmann, R.; Brauers, T.; Gautrois, M.
  • ItemOpen Access
    Determination of Peroxyacetylnitrate (PAN) in Unpolluted Areas
    (Taylor & Francis, 1985) Diederich, S.; Rudolph, J.; Vierkorn-Rudolph, B.
  • ItemOpen Access
    Problems Conncted with the Analysis of Halocarbons and Hydrocarbons in the Non-Urban Armosphere
    (Taylor & Francis, 1989) Khedim, A.; Johnen, J.; Rudolph, J.
  • ItemOpen Access
  • ItemOpen Access
    The Use of Automated "On Line" Gaschromatrography for the Monitoring of Organic Trace Gases in the Atmosphere at Low Levels
    (Taylor & Francis, 1990) Pilwat, G.; Khedim, A.; Johnen, F.J.; Rudolph, J.
  • ItemOpen Access
    Application of absolute principal component analysis to size distribution data: identification of particle origins
    (Copernicus Publications, 2007) Chan, T.W.; Mozurkewich, M.
    Absolute principal component analysis can be applied, with suitable modifications, to atmospheric aerosol size distribution measurements. This method quickly and conveniently reduces the dimensionality of a data set. The resulting representation of the data is much simpler, but preserves virtually all the information present in the original measurements. Here we demonstrate how to combine the simplified size distribution data with trace gas measurements and meteorological data to determine the origins of the measured particulate matter using absolute principal component analysis. We have applied the analysis to four different sets of field measurements that were conducted at three sites in southern Ontario. Several common factors were observed at all the sites; these were identified as photochemically produced secondary aerosol particles, regional pollutants (including accumulation mode aerosol particles), and trace gas variations associated with boundary layer dynamics. Each site also exhibited a factor associated specifically with that site: local industrial emissions in Hamilton (urban site), processed nucleation mode particles at Simcoe (polluted rural site), and transported fine particles at Egbert (downwind from Toronto).
  • ItemOpen Access
    Simplified representation of atmospheric aerosol size distributions using absolute principal component analysis
    (Copernicus Publications, 2007) Chan, T.W.; Mozurkewich, M.
    Principal component analysis provides a fast and robust method to reduce the data dimensionality of an aerosol size distribution data set. Here we describe a methodology for applying principal component analysis to aerosol size distribution measurements. We illustrate the method by applying it to data obtained during five field studies. Most variations in the sub-micrometer aerosol size distribution over periods of weeks can be described using 5 components. Using 6 to 8 components preserves virtually all the information in the original data. A key aspect of our approach is the introduction of a new method to weight the data; this preserves the orthogonality of the components while taking the measurement uncertainties into account. We also describe a new method for identifying the approximate number of aerosol components needed to represent the measurement quantitatively. Applying Varimax rotation to the resultant components decomposes a distribution into independent monomodal distributions. Normalizing the components provides physical meaning to the component scores. The method is relatively simple, computationally fast, and numerically robust. The resulting data simplification provides an efficient method of representing complex data sets and should greatly assist in the analysis of size distribution data.
  • ItemOpen Access
    An inverse modeling procedure to determine particle growth and nucleation rates from measured aerosol size distributions
    (Copernicus Publications, 2006) Verheggen, B.; Mozurkewich, M.
    Classical nucleation theory is unable to explain the ubiquity of nucleation events observed in the atmosphere. This shows a need for an empirical determination of the nucleation rate. Here we present a novel inverse modeling procedure to determine particle nucleation and growth rates based on consecutive measurements of the aerosol size distribution. The particle growth rate is determined by regression analysis of the measured change in the aerosol size distribution over time, taking into account the effects of processes such as coagulation, deposition and/or dilution. This allows the growth rate to be determined with a higher time-resolution than can be deduced from inspecting contour plots ("banana-plots''). Knowing the growth rate as a function of time enables the evaluation of the time of nucleation of measured particles of a certain size. The nucleation rate is then obtained by integrating the particle losses from time of measurement to time of nucleation. The regression analysis can also be used to determine or verify the optimum value of other parameters of interest, such as the wall loss or coagulation rate constants. As an example, the method is applied to smog chamber measurements. This program offers a powerful interpretive tool to study empirical aerosol population dynamics in general, and nucleation and growth in particular.
  • ItemOpen Access
    Aerosol particle size distributions in the lower Fraser Valley: evidence for particle nucleation and growth.
    (Copernicus Publications, 2004) Mozurkewich, M.; Chan, T.W.; Aklilu, Y.; Verheggen, B.
    Particle size distributions from 9 to 640nm diameter were measured at Eagle Ridge in the lower Fraser Valley from 13 August to 1 September 2001 as part of the Pacific 2001 Air Quality Study. The site was on top of a ridge, about 300m above the valley floor, in a predominantly agricultural area about 70km ESE of Vancouver. To further characterize the particles, their hygroscopic properties (affinity for water) were measured. The maximum of the number distributions was generally between 40 and 100nm diameter, but the number distribution was sometimes dominated by ultrafine particles with diameters below 40nm. These ultrafine particles, which appeared to some extent on all days, were frequently associated with elevated levels of CO and NOx, as expected for fresh vehicular emissions. The appearance of these fresh emissions was most pronounced when the growing mixed layer reached the altitude of the site. In contrast, pronounced nucleation events occurred on the five cleanest days; these resulted in particle number concentrations as high as 5x104 particles cm-3 and growth rates of 5 to 10nmhr-1. Nucleation appears to have been triggered when the UV flux reached about 25Wm-2. The growth of these newly formed particles was probably driven by the photochemical oxidation of biogenic organic compounds. Dramatic growth events were also observed on the afternoons of the more polluted days; these produced an extremely narrow mode σ<0.3) at a diameter of about 40nm. Rainy days showed low number concentrations with the size distributions shifted to small sizes. On one of these days there was evidence of nucleation not far from the site; this may have been occurring in the vicinity of the clouds.
  • ItemOpen Access
    Determination of external and internal mixing of organic and inorganic aerosol components from hygroscopic properties of sub micrometer particles during a field study in the lower Fraser Valley
    (Taylor & Francis, 2004) Aklilu, Y.; Mozurkewich, M.
    A tandem differential mobility analyzer (TDMA) system was developed to measure particle growth factors over a range of relative humidities from 50-85%. This system was deployed in the Lower Fraser Valley, and data was collected at Golden Ears Provincial Park and Eagle Ridge Mountain during the Pacific 2001 field study. Following humidification, particles either had a single hygroscopic mode or a bimodal distribution with more hygroscopic and less hygroscopic modes. Growth factors for the less hygroscopic mode were found to be consistent throughout the study. Monomodal distributions were the more common result. Their growth factor ranged between that of the less and the more hygroscopic modes. Using observed and calculated growth factors, relative organic and inorganic fractions were estimated. Particles at these sites were largely organic. Organic fractions near unity were found at Golden Ears forest and during rainy days at Eagle Ridge.
  • ItemOpen Access
    Laboratory studies of bromide oxidation in the presence of ozone: Evidence for glass-surface mediated reaction
    (Springer Verlag, 2002) Anastasio, C.; Mozurkewich, M.
    The reaction of sodium bromide particles in the presence of ozone was studied in a flow system both under dark conditions and with 254 nm radiation. We found that there was significant formation of gaseous bromine (probably Br2) in the presence of ozone in the dark, and that bromide deposited to the walls of the Pyrex reaction flask was its source. The observed rate of gaseous bromine formation in these experiments was approximately 100–1000 times faster than expected based on the knownrate constant for aqueous reaction of bromide with ozone. While the mechanism responsible for this enhanced reactivity was not identified, based on previous reports we suggest that the glass surface converted ozone to more reactive species, such as hydroxyl radical, which in turn oxidized bromide. In the presence of 254 nm radiation, rates of gaseous bromine collection were further enhanced, likely as a result of increased radical production in the system, and wall-deposited bromide was also the source of the gaseous bromine. In these `light' experiments, there was a significant decline in ozone mixing ratios, consistent with bromine radical chemistry. These results suggest the possibility that ozone reacting with internally mixed silicate/sea-salt particles might be a significant mechanism for the oxidation of particulate halides, and subsequent release of photoactive halogen species, in the marine boundary layer.
  • ItemOpen Access
    Measurement of the coagulation rate constant for sulphuric acid particles as a function of particle size
    (Elsevier, 2001) Chan, T.W.; Mozurkewich, M.
    A new method for the determination of coagulation rate constants for monodisperse, neutral particles is described. In this method, a differential mobility analyzer (DMA) is used to prepare a monodisperse aerosol and a second DMA is used to separate the coagulation products from the original monodisperse particles. The experiments are carried out under initial rate conditions so that typically 5–9% of the monomer particles undergo coagulation. Experimental results at 298±1 K for H2SO4/H2O particles with diameters of 49–127 nm and a composition of 72–73% H2SO4 by mass gave enhancement factors, relative to rate constants calculated for hard spheres, that vary from about 1.2 for the largest particles to 2.8 for the smallest particles. Fitting these results to a theoretical expression accounting for van der Waals forces gives a Hamaker constant of (6.4±2.6)×10−13 erg. We also give convenient formulas for computing coagulation enhancement factors from the Hamaker constant.
  • ItemOpen Access
    Observation of a SO2 induced nucleation event in the atmosphere
    (Elsevier, 1998) Verheggen, B.; Mozurkewich, M.
  • ItemOpen Access
    A novel injector for low temperature flow kinetic studies
    (American Institute of Physics, 1994) Henry, B.E.; Fried, A.; Fox, J.; Mozurkewich, M.; Calvert, J.G.
    A new enclosed injector system for low temperature kinetic studies is described. This system offers a number of advantages over more conventional designs of moveable inlet injectors. The enclosed injector prevents the introduction of contaminants into the flow reactor. In conventional designs, ambient contaminants adsorbed onto the outside of the injector could be transported into the flow reactor. The enclosed injector exposes the reactant gas to the same length of inlet tubing at a uniform temperature regardless of the injector position. In addition, the enclosed injector minimizes temperature gradients in the flow reactor caused by heat transfer along the injector. This is a particular problem when the flow reactor is operated near atmospheric pressure at reduced temperatures. Finally, this design is significantly more compact than conventional injector designs. This system has been successfully employed in studies of the heterogeneous reaction of N2O5 with sulfuric acid aerosols over the temperature range 225–293 K.
  • ItemOpen Access
    Aerosol growth and the condensation coefficient for water: a review
    (Taylor & Francis, 1986) Mozurkewich, M.
    The transfer of gas phase species to aerosols depends critically on the condensation (or sticking) coefficient. Reported values for water on water vary from 0.03 to 1. Theoretical arguments indicate that the condensation coefficient should be near unity for polar species on an aqueous surface. As long as heat transfer is properly accounted for, measurements on bulk water support this conclusion. The theory of aerosol growth is reviewed and a somewhat modified form is presented. Experimental measurements of aerosol growth are consistent with a condensation coefficient of unity but indicate that the thermal accommodation coefficient may be somewhat smaller. Aerosols grown on natural condensation nuclei may have smaller condensation coefficients owing to the presence of organic films.
  • ItemOpen Access
    Self-reaction of HO2 and DO2: negative temperature dependence and pressure effects
    (Wiley-Blackwell, 1985) Mozurkewich, M.; Benson, S.W.
    The negative temperature dependence, pressure dependence, and isotope effects of the self-reaction of HO2 are modeled, using RRKM theory, by assuming that the reaction proceeds via a cyclic, hydrogen-bonded intermediate. The negative temperature dependence is due to a tight transition state, with a negative threshold energy relative to reactants, for decomposition of the intermediate to products. A symmetric structure for this transition state reproduces the observed isotope effect. The weak pressure dependence for DO2 self-reaction is due to the approach to the high-pressure limit. Addition of a polar collision partner, such as ammonia or water vapor, enhances the rate by forming an adduct that reacts to produce deexcited intermediate. A detailed model is presented to fit the data for these effects. Large ammonia concentrations should make it possible to reach the high-pressure limit of the self-reaction of HO2.
  • ItemOpen Access
    Optimization of a heat engine based on dissipative system
    (American Institute of Physics, 1983) Mozurkewich, M.; Berry, R.S.
    A new class of heat engine is analyzed in which the working fluid operates in a dissipative process, never in equilibrium. The conditions are found for stability and for the generation of work. Then the optimal path is found for operating the general dissipative engine by means of optimal control theory. The optimal cycle consists of arcs of constant power and of approximately instantaneous adiabats. If the heat flow is a function of temperature only, then the constant power arcs become isotherms. An upper bound is found to the power output.Two examples are worked out in detail: a light‐driven dissipative engine whose absorption is a step function of temperature, and a light‐driven dissipative engine whose working fluid undergoes a chemical reaction (isomerization), absorbing light in the isomeric form favored at high temperatures.
  • ItemOpen Access
    Optimal paths for thermodynamic system: the ideal Otto cycle
    (American Institute of Physics, 1982) Mozurkewich, M.; Berry, R.S.
    We apply the method of optimal control theory to determine the optimal piston trajectory for successively less idealized models of the Otto cycle. The optimal path has significantly smaller losses from friction and heat leaks than the path with conventional piston motion and the same loss parameters. The resulting increases in efficiency are of the order of 10%.