Effects of aging on organic aerosol from open biomass burning smoke in aircraft and laboratory studies

If forests burn, smoke rises — that part everyone knows. If smoke carries particles, those particles scatter sunlight and seed clouds — climate modelers know that too. But if those particles keep changing chemically for days after the fire is out, transforming into something measurably different from what they started as, then every estimate of smoke's climate impact built on fresh fire measurements is built on incomplete ground. This lecture is about what Cubison and colleagues figured out when they asked what actually happens to wildfire smoke after it leaves the flame. The answer is stranger and more uncertain than expected. Biomass burning is not a minor aerosol source. It contributes substantially to the global organic aerosol burden, which recent estimates place somewhere between 150 and 300 teragrams per year. Within that, there's a crucial distinction: primary organic aerosol, or POA, is the particle mass emitted directly from the fire itself. Secondary organic aerosol, or SOA, forms later in the atmosphere when gas-phase compounds — including vapors that evaporated from the primary particles as the plume diluted — get oxidized by sunlight and chemistry, and condense back into particles. Cubison and colleagues emphasize that biomass burning POA is often semi-volatile, meaning it can partially evaporate as the plume spreads out, and some of those vapors then re-form as SOA downwind. The net result of evaporation plus new SOA formation can either increase, decrease, or leave unchanged the total aerosol mass — and that net change turns out to vary enormously from plume to plume. That variability is the central problem. You can measure what comes out of a fire reasonably well. But the aerosol keeps evolving chemically over hours to days of atmospheric transport. Existing tracers — levoglucosan in the particle phase, acetonitrile in the gas phase — are useful near the source, but levoglucosan degrades in the atmosphere, with estimated lifetimes ranging anywhere from around 15 hours to 10 days depending on conditions. So the tracer itself is moving, and the aerosol is moving, and the field needed a better way to follow both at once. The instrument at the center of Cubison's approach is the Aerodyne Aerosol Mass Spectrometer, or AMS. It flash vaporizes particles and records a detailed organic mass spectrum. From that spectrum, the authors define simple ratios — the fraction of the total organic signal sitting at particular mass-to-charge values. Two of those ratios do most of the work here. The first is f60: the fraction of the AMS signal at mass-to-charge ratio sixty, which comes from levoglucosan fragments produced when cellulose burns. Pure levoglucosan gives an f60 of about 13 percent; ambient smoke aerosol gives lower but still elevated values, because other levoglucosan-like compounds contribute. The second is f44: the fraction at mass-to-charge forty-four, which tracks oxygenated organic material and serves as a practical proxy for the oxygen-to-carbon ratio of the aerosol. Here's the key new finding: Cubison and colleagues establish that in ambient air dominated by secondary organic aerosol with no fresh fire influence, f60 clusters around a background level of about 0.3 percent, with an uncertainty of plus or minus 0.06 percent. Below that threshold, the biomass burning fingerprint is effectively gone. Above it, you can still detect the fire's chemical signature, even in highly aged, heavily oxidized air. Those two numbers — f60 and f44 — become the axes of a novel diagnostic map. Fresh smoke sits at high f60 and low f44, meaning it's full of levoglucosan-like material and not very oxidized yet. As the plume ages photochemically, it moves along a consistent trajectory: f60 drops, f44 rises. Different plumes trace slightly different slopes across that map, but they all move in the same general direction. It's a way to watch a fire's chemical legacy evolve in real time. The data to test this came from two very different settings. The first was the 2008 NASA ARCTAS aircraft campaign, where a DC-8 research aircraft intercepted biomass burning plumes at high northern latitudes, sampling across altitudes from 150 to 12,000 meters. The Arctic makes a useful natural laboratory: plumes there undergo long transport and processing times, and the clean background makes it easier to isolate the fire signal. The ARCTAS dataset included 245 classified biomass burning plumes and a 252-point Arctic background dataset for comparison. The second setting was the lab — controlled burns in the FLAME-3 study, where smoke was artificially aged in a Potential Aerosol Mass flow tube and the chemical trajectory tracked under known oxidation conditions. What Cubison and colleagues found is that both settings produce the same story in f44 to f60 space. Field plumes and lab-aged smoke follow remarkably similar trajectories: f44 rises, f60 falls, with aging. One spring plume had traveled roughly 6,000 kilometers from southern Russia to Alaska, and another summer plume had crossed roughly 8,000 kilometers from near Beijing to western Canada. Both still showed f60 values clearly above the 0.3 percent background. The levoglucosan fingerprint is more persistent than its known chemistry would suggest. The lab results from the FLAME-3 study, where the fastest f60 decay corresponded to an estimated levoglucosan lifetime of about five days under standard hydroxyl radical exposure conditions, align with that picture. The convergence of field and chamber data validates the approach. There's also a statistically significant distinction in f60 between highly oxygenated aerosol of biomass burning origin and highly oxygenated aerosol from other sources. In the springtime Arctic of 2008, the background f60 was elevated above levels seen in ground-based campaigns without fire influence, consistent with a substantial biomass burning organic aerosol contribution to the Arctic aerosol burden that year. Now the harder question: does aging actually create more aerosol mass, or does it just rearrange the chemistry of what's already there? The metric the field uses for this is the change in organic aerosol mass normalized to excess carbon monoxide, which acts as a dilution-corrected fire tracer. And here the data get genuinely messy. Across six field datasets, measured values of the change in organic aerosol mass normalized to excess carbon monoxide range from negative 0.01 to positive 0.05 grams of organic aerosol per gram of excess carbon monoxide. Some plumes lose net aerosol mass with aging. Some gain it. The mean across those datasets is 0.013 plus or minus 0.011 grams per gram. Expressed differently, the average net enhancement above emitted primary organic aerosol — what the authors call the change in organic aerosol mass over primary organic aerosol — is about 19 percent, but with a standard deviation of 18 percent. That's not a tight result. The authors are clear that this scatter is real, not an artifact. Different fire types, fuels, photochemical ages, and atmospheric conditions all drive genuinely different outcomes. Some individual studies, like Yokelson and colleagues' work on Yucatan agricultural fires and DeCarlo and colleagues' measurements near Mexico City, found clear enhancement. Others, like Akagi and colleagues on California brush fires, found a small decrease. Chamber experiments during FLAME-3 showed the same fuel-dependent variability. The chemical trajectory in f44 to f60 space is consistent; the mass budget is not. Those are two different things, and keeping them distinct is important. The global implications follow directly from that uncertainty. Cubison and colleagues scale their measurements using two approaches. In the first, they multiply the mean change in organic aerosol mass normalized to excess carbon monoxide ratio by the Intergovernmental Panel on Climate Change estimate of global biomass burning carbon monoxide emissions — 508 teragrams of carbon monoxide per year — to get a global net organic aerosol source from biomass burning aging of seven plus or minus six teragrams per year. In the second, they multiply the change in organic aerosol mass over primary organic aerosol ratio by a biomass burning primary organic aerosol emission inventory of 41 teragrams per year, arriving at an alternative estimate of eight plus or minus seven teragrams per year. Both approaches converge on roughly five percent of total global organic aerosol source estimates. But in both cases, the error bar is nearly as large as the central estimate. That's not a failure — it's an honest accounting of what the available field data can and cannot support. The lasting contribution of this work is the f44 to f60 framework itself: a way to simultaneously track the decay of biomass burning's chemical fingerprint and the rise of oxidation state across aging plumes, validated against both aircraft measurements and controlled laboratory experiments. The eight teragrams per year figure is imprecise, but not negligible — and it sits against a backdrop of increasingly frequent and intense fire seasons globally. The open question the authors flag is still open: because some plumes gain net aerosol mass and some lose it, we still cannot reliably predict whether a major fire event will make the atmosphere's aerosol burden heavier or lighter downwind. More aircraft campaigns that track individual plumes through their full atmospheric lifetime are what would settle it. Until then, the smoke keeps changing, and the ledger stays uncertain. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.