Particle Falls: How It Works

 Behind the visual spectacle of Particle Falls lies complicated science. As you view Particle Falls—whether in person or in video clips—read below to learn more about how the work processes particulate data. 


Particle Falls displays concentrations of particulates in bursts of bright color over a constant background of falling blue light. Increasing frequency of the crackling dots of color indicates a greater concentration of particles. The image updates with new particulate data every 15 seconds. 


An instrument called a nephelometer monitors and samples the air to generate the data for Particle Falls. Pictured below, the nephelometer used in this art installation is an E-Sampler developed and manufactured by Met One Instruments. The E-Sampler combines two different technologies: light scattering and the gravimetric filter method. Light scattering operates by pulsing a beam of light through a sample of air. The particulates in the air scatter the beam of light, which is collected and concentrated onto a photo diode in the E-Sampler. That light is then converted into an electric signal, which is proportional to the concentration of particulates in the air. That electric signal is converted to usable data and sent to the projector via a computer software program.   Also used in scientific analysis is the E-Sampler’s gravimetric filter system. After measurement via light scatter the sample is drawn onto a filter. The filter can be taken for lab analysis as a second method of measurement of air particles.


The Particle Falls nephelometer measures particles in a gas sample that are smaller than 2.5 micrometers (or PM, particulate matter) in diameter. Particulate matter comprises water droplets and a variety of small particles made up of organic chemicals, soil, dust, metals, and acids like sulfates or nitrates, among other materials. Particles smaller than 10 micrometers can pass into the nose and throat, thus posing potential health risks to humans. 

Just how small is 2.5 PM? Check out the graphic below. Particles fitting this description are called “fine” particles and can come from a variety of combustion sources, like diesel exhaust. Not visible to the naked eye, these particles are key components of air pollution in Philadelphia, leading to Philadelphia’s ranking as one of the cities in the United States with the most polluted air. 


For more information on the science of air-quality monitoring and the health effects of air pollution and particulate matter, check out the Resources page.