Division of Environmental and Occupational Health
School of Public Health, University of Minnesota
Box 807 Mayo, 420 Delaware Street, S.E.
Minneapolis, MN 55455
Tel: 612-626-5428
Fax: 612-626-0650
887 19th Avenue S.E.#1
Minneapolis, MN 55414
Tel: 612-617-8044
"Faith" is a fine invention
-- Emily Dickinson
Click here for my curriculum vitae.
My primary research interests are in developing new technologies for measurement of airborne contaminants, and in developing mathematical methods for modeling and analyzing environmental measurements. The focus of both these interests is the development of more effective and accurate methods to assess health-related human exposure. I also have parallel, complementary interests in aerosol sampling, particle morphology characterization using fractals, and control of particulate emissions.
Estimating a continuous physical parameter from a set of discrete measurements of related quantities is a problem that crops up repeatedly in many areas of science and environmental science in particular. All such problems can be classified under the rubric of data inversion, otherwise known as parameter estimation or model calibration. My studies in this area comprise developing and adapting mathematical techniques to obtain aerosol size distributions by deconvolving measurements. These measurements could be anything from optical light extinctions to masses collected by the stages of a cascade impactor. Such problems are usually 'ill-posed' in that there is not sufficient information available from the measurements to obtain a unique solution for the desired property. Thus many solutions could satisfy the same set of measurements. The most interesting aspect of this research lies in handling data uncertainties and quantifying the sensitivity to errors in measurements.
Such dosimetry requires information about appropriate health-related size fractions, and thus the particle aerodynamic size distributions of inhaled aerosols. Frequently, in industrial hygiene, this involves using cascade impactors. Recovering particle size distributions from cascade impactors is again an ill-posed problem. Two mathematical techniques were developed and applied to handle such data. A comparison of the efficacy of the two methods in determining various aerosol fractions such as the inhalable, thoracic, and respirable was presented in another paper.
A consequence of this work was the articulation of a health-based rationale for the measurement of particle size distributions for workplace aerosols. In most practical situations, aerosol exposures involve the inhalation of many different types of particles, distinguished not only by their size but also by their chemical, physical or biological characteristics. Aerosol exposures are characterized in terms of their particle size (which governs their ability to reach given parts of the respiratory tract), the relative rate at which that species is biologically available in a region of the respiratory tract, and the toxic effect of the species. This work provides a framework for assessing the dose of inhaled material based on the use of an aerosol spectrometer.
A consistent theme in all of the above work has been the evaluation of the amount of information needed to solve a particular problem, and conversely, how to obtain a solution in the absence of complete information. These issues were formalized in a probabilistic Bayesian framework to analyze inversion problems. The amount of knowledge regarding a physical quantity is described by a probability distribution. In the Bayesian view, a measurement process serves to refine previous knowledge of physical parameters by narrowing their probability distributions. This has proved to be a valuable insight and has generalized the issues from the narrow scope of determining particle size distributions to a broader conception of evaluating exposures in the face of uncertain information.
This is a new and very interdisciplinary area in industrial hygiene research involving not only aerosol sampling and measurement, and data handling methodologies, but also expert judgments in industrial hygiene, decision analysis, and synthesizing consensus among experts. In many areas of industrial hygiene, exposure estimates and decisions based on them are made with incomplete knowledge of working conditions, and a few actual measurements that are assumed to be representative. While such approaches are often necessary due to various constraints, it is important to conduct these assessments within a rational framework that allows the quantification of the uncertainties and biases, while incorporating both subjective opinions and objective evidences.
This is an area of research that is complementary to my work on aerosol dosimetry. The motivation for this work is based on the needs expressed by national and international standards-setting bodies to measure aerosol exposures in a manner that reflects the health risk and studies the physical factors governing the sampling characteristics of bluff-body aerosol samplers. The studies include experiments to characterize the flow patterns around such samplers and measure the efficiency with which particles enter the sampling instrument from ambient air. Another component is modeling based on concepts of inertial transport of particles in the distorted airflow around the samplers.
In addition to developing methodologies for handling measurements, I have conducted more fundamental research on aerosol formation processes. Combustion aerosols are formed by coagulation and sintering of submicron primary particles. When water vapor condenses on irregularly shaped, chain-like agglomerates, subsequent evaporation tends to collapse the agglomerate structure producing porous, near-isometric structures (Ramachandran and Reist, Aerosol Science and Technology, 1995). This change in morphology will affect the deposition characteristics of aerosol in the respiratory system, thereby changing exposure.
The other main thread of my research interests is technology for particulate control. This involves both the development of models to predict the performance of particle collectors as well as their empirical design. I have developed an optimization procedure for design of reverse-flow cyclones that yields the highest collection efficiency for a given pressure drop. The motivation for this research was to design high efficiency cyclones to prevent damage to gas turbines from fine fly ash in pressurized fluidized bed combustion. A paper based on this research was published in Aerosol Science and Technology (Ramachandran et al., 1991). A similar procedure was developed for rotary-flow cyclones, and the results were published in Filtration and Separation (Ramachandran et al., 1994). I have also written a chapter on Particulate Control in the Environmental Engineer's Handbook (Ed. David H. Liu).
My doctoral dissertation work was in the area of optical remote sensing: determination of aerosol concentration and size distribution from light extinction measurements at multiple wavelengths and the use of computed tomography in mapping spatial distributions of aerosols. Most particle and gas measuring instruments are point samplers. Since computed tomography can non-invasively determine spatial distributions of particles and gases, it is the seed for an extremely fertile area of research with wide applications in monitoring industrial workplace environments, hazardous waste sites, and stack emissions.
Non-health-related effects of aerosols such as visibility impairment have been another research interest. A study the effect of chemical composition of aerosols on light scattering and atmospheric visibility is being published (Ramachandran and Reist, Journal of Aerosol Science, 1996, in press).
I teach PubH 5171 every fall semester. Click here for course details.
I am married to Candace Lee Pilon. She is a professional bee-keeper. We have three bee-hives in our backyard and have spent many a morning watching the bees go about their business.
We have a daughter, Sandhya, age 2 years. She is a budding pianist and gymnast.
Check out the Division of Environmental and Occupational Health Homepage at the University of Minnesota.
My wife also imports hand-knotted Indian wool carpets made by women in Southern India (no child-labor involved). The women have been taught to hand knot carpets and this is a business that promotes self-sufficiency in poor communities. If you are interested in looking at pictures of handknotten Indian wool carpets, here are some red carpets, blue carpets, brown carpets, ivory designs, and a Chinese design.
