Science1 publisher3 min readPublished
Seventeen of the 20 most abundant PAHs in Reno's smoke sit outside the EPA's priority list
A DRI team compared Reno's smoke-affected autumn of 2020 with the smoke-free months of 2019 and found every pollutant class elevated, including more than 100 toxic compounds that fall outside routine air monitoring.
The Scientist · Science desk
What happened
- Researchers at DRI compared smoke-free days in Reno from August to October 2019 with smoke-affected days over the same months in 2020, publishing the results on Aug. 8 in Atmospheric Pollution Research.
- Fine particulate matter exceeded EPA standards on 18 of the 50 smoke-affected days the team sampled.
- Ground-level ozone rose about 12 percent on smoke-affected days, which the study describes as a limited increase.
- The authors conclude that occasional monitoring of the EPA's 16 priority PAHs is likely to give an insufficient assessment of overall airborne toxicity.
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Why it matters
- constraint PM2.5 stayed inside the federal standard on 32 of the 50 smoke days, so an advisory system keyed to particles describes only part of the exposure this study measured.
- decision An agency that wants PAH numbers has to decide to buy gas-phase sampling, because the particle instruments already sitting in the network do not produce them.
- exposure Samburova's list of sensitive groups includes athletes and outdoor workers, whose dose is set by the outdoor concentrations sampled here rather than by indoor air.
- precedent If the 16-compound list underrepresents the mixture over Reno, other cities downwind of California face the same gap in fire years like 2020 and 2021.
The comparison runs across two different autumns: smoke-free days from August to October 2019 set against smoke-affected days from August to October 2020 [1]. That isolates smoke, and it also leaves a year between the two samples, so anything else that changed in Reno's air over that year travels inside the same contrast.
The ozone result shows the limit of that. Smoke has a chemical route to ozone: its volatile organic compounds react with free radicals in the presence of nitrogen oxides and sunlight [20]. The increase the team measured was small, and some smoky days came in below the local average, so the authors concluded that vehicle exhaust and other pollutants remain the main sources of ground-level ozone in the region [7].
The study covered PM2.5, organic carbon, elemental carbon, ozone and more than 100 PAHs [2]. Those compounds behave differently from particles in a way that matters for measurement: they form in incomplete combustion, exist in both gas and particle form, and are not tracked routinely the way PM2.5 is [22]. In this sample the particle phase held about 2 percent of the PAHs identified [4]. Particulate PAHs alone were around six times higher on smoke days, and methyl- and dimethylnaphthalenes were the most abundant compounds [9]. Naphthalene is on the EPA's hazardous air pollutant list [11]. The team also notes that higher molecular weight PAHs are the more toxic and more bioaccumulative class [19].
Three of the EPA's 16 priority PAHs appeared among the 20 most abundant compounds measured [12]. Seventeen of that top 20 are compounds the priority list does not include [3]. One figure in the account does double duty: 47 is both the top of the reported 6.3-to-47-fold range for PAH enhancement on smoke days [8] and the ratio of mean gas-phase to particle-phase concentration [10]. Those are two different comparisons.
On the particle side, PM2.5 breached EPA standards on 36 percent of the smoke-affected days [1], and on the worst days concentrations ran 1.1 to 2.8 times the National Air Quality Standard [5]. The heaviest smoke arrived Aug. 19-22 and Sept. 11-17, 2020, while hundreds of damaging wildfires burned in California [18].
"With limited monitoring on the contents of smoke, we don't yet have a great understanding of the full range of public health impacts, and this study can help us identify which air pollutants are most important to monitor closely in the future," said Vera Samburova, an atmospheric scientist at DRI and one of the study's lead authors [14].
This study measured ambient air across 50 smoke days, not health outcomes. The asthma link in the region comes from earlier work that found a strong association between elevated wildfire PM2.5 and emergency room visits at Reno and Sparks hospitals [17].
What to watch
- Whether Nevada air regulators add gas-phase PAH sampling to routine monitoring or keep the network keyed to PM2.5.
- Whether the enhancement factors hold in a second smoke season, or whether California's 2020-2021 fire count made this one an outlier.
- Health work that tests emergency room visits against measured PAH concentrations rather than PM2.5 alone.