Characterizing emissions, chemistry, and health impacts of aged wildfire smoke in a western US city

Plain-language summary

What Happens to Wildfire Smoke After Days in the Air?

Hourly ground measurements reveal what multi-day-old smoke brought to Missoula.

Large collaborative field campaigns have produced detailed measurements of wildfire smoke near fires and aboard research aircraft. Far less is known about what remains in smoke after it has traveled and reacted for several days before reaching people at ground level. In September 2020, Missoula, Montana, experienced three smoke events. During smoke-affected periods, average PM₂.₅ reached 43 µg/m³—about seven times the local background—and the hourly peak reached 120 µg/m³. Much of the regional smoke had traveled and reacted for several days, although nearby fires may also have contributed. We asked what was still in the smoke when it reached the city and how its chemistry had changed along the way.

Four findings

• Ground measurements retained a chemical record of the journey. Benzene and toluene declined predictably with chemical age, while several oxygenated gases declined more slowly. The model’s OH exposure was about twice the value inferred from the ground-based chemical clock.
• More smoke particles did not always mean more ozone. Ozone rose under lighter smoke, then leveled off or fell in the heaviest smoke. Both GEOS-Chem and AIRPACT missed this change.
• Particles and gases shaped different screening-level risks. PM₂.₅ drove most of the estimated cancer risk, while measured hazardous gases drove most of the chronic non-cancer estimate.
• Getting the smoke timing right was not enough. GEOS-Chem captured the first two events but underestimated many ground-level pollutants and the resulting screening-level risk estimates.