Continuous Monitoring Methods for Metal Air Pollution

Metal air pollution is a side effect of many industrial, technological, agricultural, and domestic processes. Mines, smelters, foundries, pesticide producers, oil refineries, air traffic that uses leaded fuel, coal-burning power plants, and other sources all produce heavy metal pollution, and this list of sources is ever-expanding.

The first heavy metal pollution regulations in the U.S. emerged in the late 1960s, and recent regulations have expanded to affect specific industries more directly. For example, a final rule was released by the U.S. Environmental Protection Agency (EPA) on September 10, 2020, that set National Emissions Standards for Hazardous Air Pollutants (NESHAP) for steel and iron foundries. Further regulations are expected that will impact businesses across a wide range of industries.

Heavy Metals are Ubiquitous

The term ‘heavy metals’ describes a group of metal and metalloid elements that have relatively high density and toxicity, including arsenic, boron, copper, lead, mercury, platinum, zinc, and others. Most pose no health concern in their original state as naturally occurring oxide or elemental depositions in the Earth’s crust, and some are even essential to life. When they are used and processed for industrial purposes, heavy metals can be released into water, soil, and the air, and can accumulate in living beings over time to become a health concern. Since these elements are not organic, they do not biodegrade and, in many cases, can be carried long distances in the air and water.

Heavy Metals Monitoring Considerations

As the use of heavy metals across multiple industries has increased since the start of the industrial age, the need to measure and monitor them has also grown. Analytical options for measurement can roughly be split into (1) long-established, wet-chemical procedures, and (2) modern spectroscopic methods.

The most common spectroscopic methods use offsite laboratory analysis, including inductively coupled plasma-mass spectrometry (ICP-MS), which isolates the metallic target analyte and uses an energy source to excite its atomic electronic structure.

Although fixed-laboratory analysis dominates the field, there are a few instruments that have been able to replicate these off-site procedures to provide data in continuous real-time or near-real time. Below is a breakdown of a few continuous metals air pollution monitoring instruments that are currently commercially available or in development. As with all monitoring projects, the choice of approach is dependent on the variety of goal-specific requirements.

1. Xact 625 (SailBri Cooper Environmental): The Xact 625 is a well-established, adapted laboratory instrument that uses X-ray Fluorescence (XRF) spectroscopy, a common method of elemental analysis that relies on x-rays to provide excitation of the element’s electronic state. The target aerosol is collected onto a tape that advances for each sampling period and is then analyzed in near-real time. Because this instrument is based on laboratory methodology that has been adapted for near-real time use in the field, it still requires significant infrastructure, including a climate-controlled shelter and access to continuous power. The detection limits for the Xact 625 are on the order of single-digit ng/m3 for short-term sampling times of 15 minutes, and the instrument can detect a wide range of elements. It is EPA-certified through the Environmental Technology Verification program and is the most prolific instrument for this application in the U.S. The instrument literature references an EPA Compendium Method, which enhances regulatory acceptance. The instrument also has an Other Test Method designation.
2. PX-375 (Horiba): The Horiba instrument also uses XRF to analyze a collected aerosol on an advancing tape. Fewer elements are detectable with the PX-375 than the Xact 625, but it does have similar single-digit detection limits. Like the Xact 625, the PX-375 requires a climate-controlled shelter and access to continuous power. While it is not as widely used in the U.S., the instrument literature does cite Compendium Method IO-3.3, a document released by the EPA in June 1999 that provides guidance on how to detect inorganic compounds in ambient air. This means data collected on this instrument should be accepted by regulatory agencies.
3. Toxic Metal Aerosol Real-Time Analysis/TARTA (UC Davis): The TARTA is a prototype instrument that uses spark-induced breakdown spectroscopy to separate the physical elements of an aerosol with a high-voltage spark. The same spark also provides the excitation for the electronic spectroscopy that is necessary for analysis, making this process more efficient than some other methods.This instrument was developed by the University of California, Davis with funding from the California Air Resources Board (CARB). CARB stipulated that the resulting instrument needed to support low-cost, community use, which has largely been achieved as the instrument costs at least ten times less than other commercial instruments. Unfortunately, it also has a smaller range of detectable elements.

Key Takeaways

While the ability to conduct continuous real-time or near-real time heavy metal air pollution monitoring is still developing, the technology is rapidly improving. This is an area of regulation and technology to keep an eye on over the coming years.