Gas chromatography-mass spectrometry (GC-MS) chromatography reveals elevated polycyclic aromatic hydrocarbons (PAH) levels in wildfire smoke over Nevada.
In 2020, several wildfires burning in the western United States caused smoke to linger over northern Nevada for weeks, affecting air quality in the region. To figure out how much of an impact this wildfire smoke had on air quality in the Reno-Sparks area, researchers from Nevada’s Desert Research Institute, in conjunction with the University of Nevada-Reno measured air pollution levels during a period without smoke (August through October 2019) and compared them to a period when smoke was present (August through October 2020). The team also collected airborne particles on filters over 24-hour periods and tested them for 104 different polycyclic aromatic hydrocarbons (PAHs)—a group of chemical compounds often linked to combustion—using gas chromatography-mass spectrometry (GC-MS). A paper based on their research was published in the journal Atmospheric Pollution Research.1
Wildfire smoke is made up of a very complicated mix of chemicals, and this mix keeps changing as the smoke travels through the atmosphere. That makes it especially hard for scientists to study how it behaves and how harmful it might be. Adding to the challenge, very few air pollutants, especially organic compounds, are monitored or regulated. For example, a standard list of 16 PAHs tracked by the EPA has been used for many years and is still common today, but research has shown that focusing on just these 16 compounds can significantly underestimate how toxic airborne particles really are—potentially missing 20% to 80% of the actual harm they cause.2,3
PM2.5 (fine particulate matter) is one pollutant that is closely regulated and commonly used to measure people's exposure to wildfire smoke. However, what that PM2.5 is made of chemically is not studied often or systematically during wildfires. This, in the opinion of the researchers, leaves a gap in the collective understanding, as it is hard to connect the chemical makeup of wildfire smoke to its broader physical properties, and even harder to link either of those to real health effects. Because of this, the researchers believe that more research is clearly needed to better understand exactly how wildfire smoke impacts human health.1,3
Using data from a U.S. Environmental Protection Agency (EPA) air-monitoring station in Reno-Sparks, the researchers found that during smoky days, levels of PM2.5, organic carbon, and elemental carbon were much higher—averaging 35.17, 16.76, and 4.91 micrograms per cubic meter, respectively, which is approximately 8 to nearly 10 times higher than levels measured on clear, smoke-free days. Ozone levels, on the other hand, only rose by about 12% during the smoky period. Out of 50 smoke-affected days studied, 18 had particulate matter levels that exceeded the EPA's official 24-hour air quality safety limit of 35 micrograms per cubic meter.1
The total amount of PAHs attached to fine particulate matter was about 6.3 times higher on smoky days (averaging 4.87 nanograms per cubic meter) compared to smoke-free days (averaging 0.77 nanograms per cubic meter). Among these, a few specific compounds—1,4-, 1,5-, and 2,3-dimethylnaphthalene—made up the largest share of PAHs found in the particulate matter during smoky periods.1
The researchers also looked at PAHs in gas form (rather than attached to particles), which they collected using special cartridges filled with an absorbent resin called XAD. They found that on smoky days, the total amount of gas-phase PAHs was about 47 times higher than the amount found attached to particulate matter, with naphthalene being the most common gas-phase compound detected.1
“Although this study had some limitations, such as data collection from only two monitoring stations and a limited number of gas-phase PAH samples to work with,” write the authors of the paper,1 “it is the first study to our knowledge to provide a detailed analysis of ambient PAHs in both the gas and PM2.5 phases, as well as other key air pollutants, by comparing periods affected by wildfire smoke to smoke-free conditions.”
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