Introduction to Aerosol Sampling

Introduction to Aerosol Sampling, narrated by Tom Peters of the University of Iowa. The source of an aerosol defines its size and concentration. Concentration can be described by different metrics, including number, surface area, or mass concentration. Hot sources, such as welding and combustion engines, create vapors that nucleate or condense to form particles typically smaller than 1 micrometer in diameter. These particles are known as nano, ultrafine, and fine particles, or more commonly as fume, smoke, and smog. They dominate particle number concentration and can be an important component of mass distributions. In contrast, mechanical sources, such as grinding and sanding, break apart a bulk material up into coarse particles that are primarily larger than 1 micrometer. These aerosols we know as dust, mist, and spray. They often dominate particle mass distributions but contribute little to number concentration. Why are we interested in sampling aerosols anyway? One reason is to show compliance with an exposure limit, such as comparing titanium dioxide concentrations measured in the breathing zone to the NIOSH recommended exposure limit. Another reason is to identify and characterize hazardous sources, such as determining where particle concentrations are highest in a facility that uses carbon nanotubes to strengthen sporting equipment. Sampling can also be used to demonstrate the effectiveness of controls, such as measuring particle concentrations with and without an exhaust hood in operation, or to evaluate a research hypothesis to relate silica dust concentrations to miner lung health in an epidemiological study. Lastly, sampling is an important step in evaluating risk. Sampling can be used to determine exposure and/or dose of a group of workers, or to evaluate whether concentrations exceed levels defined to be immediately dangerous to life and health in emergency situations. Ideally, we need a tricorder as originally seen carried by Spock in a 1960s Star Trek episode. The tricorder is a multifunction, handheld device used to sense environmental conditions, analyze and record data, and estimate risk. The use I have in mind for such a device is to report particle number, surface area, and mass concentration by size and composition. Although we don’t have such a device, we do have other instruments that we can use together to assess aerosol exposures. The method of aerosol sampling may be manual or direct-reading. In manual methods, particles are physically collected, usually onto a filter. These filters are then weighed to determine the mass of the collected particles or analyzed chemically to determine the mass of a certain composition of particles. This weight is then divided by the air volume sampled to obtain mass concentration. In contrast, direct-reading methods use some property of an aerosol to obtain a direct readout of concentration. For example, aerosol photometers use the fact that the amount of light scattered by an assembly of particles is related to their mass concentration. There are different types of instruments with some called size integrated, while others are called size resolved. So, here I show a continuous distribution of the actual number concentration by size. An instrument that integrates over many sizes will provide a single value representing the average particle number concentration across many diameters. In contrast, a sizing instrument provides number concentration in many different size bins. Because these data are size resolved, they can be used to estimate particle surface area and mass concentration by size. We also need to think about the characteristics of the contaminant that we are trying to measure. The source of the particles is important. Often times, particles are emitted as a byproduct of an industrial process. Welding fumes are an example of fine and nanoparticles emitted by a common industrial process. In contrast, the product may be the contaminant of interest, such as engineered nanoparticles. In this case, we need to be able to differentiate the product from the byproduct. Also, we need to know the particle size range we are interested in. For coarse and fine particles, mass-based sampling often has enough sensitivity for detection because these particles have sufficient mass to weigh. Gravimetric sampling is favorable because of historical continuity to past measurements. In contrast, other types of instruments are often needed because the mass of nanoparticles is insignificant compared to larger particles. In this case, we either need new samplers to collect nanoparticles alone or instruments that measure number or surface area concentration. A good manual sampling method will have the following characteristics. The collection medium is safe with a high capture efficiency for the contaminant of interest. The quantity of material collected is less than the capacity of the collection system. The collected material is chemically stable until analyzed. The signal is well above background noise. We need to collect sufficient mass to be above limits of detection of the analysis method. The location of the sampling is also important. Some sampling is done in a specific “area” of interest, such as that shown in the picture at the left near a kitchen grill. This type of sampling allows a lot of flexibility in the types of instruments that can be used. For example, we can move a cart with fairly large instruments into an area to get detailed measurements of particles within that area. Other times, we sample within the breathing zone of a worker, which we call “personal” sampling. This sampling requires small, unobtrusive
equipment that can be worn by the worker. In the picture at right, a worker is wearing a lapel-type sampler that collects particles with an efficiency mimicking aspiration and/or deposition of particles within the respiratory tract. An airflow pump, typically worn on the belt of the worker, is used to pull air through the sampler to simulate breathing. Personal sampling is usually required to demonstrate compliance with occupational exposure limits. Obtaining a sample of particles that are representative of the environment is a major concern. First, particles must be aspirated into an inlet. Then, they must be transported to a filter for collection or a sensor for detection. Aspiration and transport are dependent on the characteristics of the particle and of the airflow. Aspiration also depends on whether particles are being sampled from moving air, such as in a duct, or still air. Transport through something like a tube or a duct is characterized by penetration, which is 1 minus the collection efficiency. So, in this plot, a penetration of 100% means that all particles pass through the tube or are
transported to the other side, and 0% means that all of the particles hit the walls of the tube and do not transport to the other side. Very small particles (like the red one in the image) tend to diffuse to the walls, resulting in low penetration. So shorter tubes are better for nanoparticles. Medium-sized particles (like the orange particle), ‘go with the flow’ and have high penetration efficiency. Big particles (like the green particle) settle due to gravity and tend wind up on the bottom walls of the tube and have low penetration, which again favors short tubes. Bends can cause particles to deposit on walls due to inertia. So minimizing the number of bends in a tube is favorable. There are many different types of sampling instruments, and we can group these sampling instruments into categories including manual instruments, those that are size integrated such as filter samplers or size-selective samplers; those that are size-resolved manual samplers, such as impactors or devices that collect samples for microscopy; or direct-reading instruments that provide indication of particle concentration integrated over many particle sizes, such as photometers and condensation particle counters; or that provide size-resolved data in real-time — the optical particle counter, aerodynamic particle sizer, and scanning mobility particle sizer.