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  1. Wildfire smoke contains numerous different reactive organic gases, many of which have only recently been identified and quantified. Consequently, their relative importance as an oxidant sink is poorly constrained, resulting in incomplete representation in both global chemical transport models (CTMs) and explicit chemical mechanisms. Leveraging 160 gas-phase measurements made during the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE-CAN) aircraft campaign, we calculate OH reactivities (OHRs) for western U.S. wildfire emissions, smoke aged >3 days, smoke-impacted and low/no smoke-impacted urban atmospheres, and the clean free troposphere. VOCs were found to account for ∼80% of the total calculated OHR in wildfire emissions, with at least half of the field VOC OHR not currently implemented for biomass burning (BB) emissions in the commonly used GEOS-Chem CTM. To improve the representation of OHR, we recommend CTMs implement furan-containing species, butadienes, and monoterpenes for BB. The Master Chemical Mechanism (MCM) was found to account for 88% of VOC OHR in wildfire emissions and captures its observed decay in the first few hours of aging, indicating that most known VOC OH sinks are included in the explicit mechanisms. We find BB smoke enhanced the average total OHR by 53% relative to the low/no smoke urban background, mainly due to the increase in VOCs and CO thus promoting urban ozone production. This work highlights the most important VOC species for daytime BB plume oxidation and provides a roadmap for which species should be prioritized in next-generation CTMs to better predict the downwind air quality and health impacts of BB smoke. 
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    Free, publicly-accessible full text available January 19, 2024
  2. Wildfires are an important atmospheric source of primary organic aerosol (POA) and precursors for secondary organic aerosol (SOA) at regional and global scales. However, there are large uncertainties surrounding the emissions and physicochemical processes that control the transformation, evolution, and properties of POA and SOA in large wildfire plumes. We develop a plume version of a kinetic model to simulate the dilution, oxidation chemistry, thermodynamic properties, and microphysics of organic aerosol (OA) in wildfire smoke. The model is applied to study the in-plume OA in four large wildfire smoke plumes intercepted during an aircraft-based field campaign in summer 2018 in the western United States. Based on estimates of dilution and oxidant concentrations before the aircraft first intercepted the plumes, we simulate the OA evolution from very close to the fire to several hours downwind. Our model results and sensitivity simulations suggest that dilution-driven evaporation of POA and simultaneous photochemical production of SOA are likely to explain the observed evolution in OA mass with physical age. The model, however, substantially underestimates the change in the oxygen-to-carbon ratio of the OA compared to measurements. In addition, we show that the rapid chemical transformation within the first hour after emission is driven by higher-than-ambient OH concentrations (3×10 6 -10 7 molecules cm -3 ) and the slower evolution over the next several hours is a result of lower-than-ambient OH concentrations (<10 6 molecules cm -3 ) and depleted SOA precursors. Model predictions indicate that the OA measured several hours downwind of the fire is still dominated by POA but with an SOA fraction that varies between 30% and 56% of the total OA. Semivolatile, heterocyclic, and oxygenated aromatic compounds, in that order, were found to contribute substantially (>90%) to SOA formation. Future work needs to focus on better understanding the dynamic evolution closer to the fire and resolving the rapid change in the oxidation state of OA with physical age. 
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  3. The evolution of organic aerosol (OA) and brown carbon (BrC) in wildfire plumes, including the relative contributions of primary versus secondary sources, has been uncertain in part because of limited knowledge of the precursor emissions and the chemical environment of smoke plumes. We made airborne measurements of a suite of reactive trace gases, particle composition, and optical properties in fresh western US wildfire smoke in July through August 2018. We use these observations to quantify primary versus secondary sources of biomass-burning OA (BBPOA versus BBSOA) and BrC in wildfire plumes. When a daytime wildfire plume dilutes by a factor of 5 to 10, we estimate that up to one-third of the primary OA has evaporated and subsequently reacted to form BBSOA with near unit yield. The reactions of measured BBSOA precursors contribute only 13 ± 3% of the total BBSOA source, with evaporated BBPOA comprising the rest. We find that oxidation of phenolic compounds contributes the majority of BBSOA from emitted vapors. The corresponding particulate nitrophenolic compounds are estimated to explain 29 ± 15% of average BrC light absorption at 405 nm (BrC Abs405) measured in the first few hours of plume evolution, despite accounting for just 4 ± 2% of average OA mass. These measurements provide quantitative constraints on the role of dilution-driven evaporation of OA and subsequent radical-driven oxidation on the fate of biomass-burning OA and BrC in daytime wildfire plumes and point to the need to understand how processing of nighttime emissions differs.

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  4. Abstract

    Wildfire emissions affect downwind air quality and human health. Predictions of these impacts using models are limited by uncertainties in emissions and chemical evolution of smoke plumes. Using high‐time‐resolution aircraft measurements, we illustrate spatial variations that can exist within a plume due to differences in the photochemical environment. Horizontal and vertical crosswind gradients of dilution‐corrected mixing ratios were observed in midday plumes for reactive compounds and their oxidation products, such as nitrous acid, catechol, and ozone, likely due to faster photochemistry in optically thinner plume edges relative to darker plume cores. Gradients in plumes emitted close to sunset are characterized by titration of O3in the plume and reduced or no gradient formation. We show how crosswind gradients can lead to underestimated emission ratios for reactive compounds and overestimated emission ratios for oxidation products. These observations will lead to improved predictions of wildfire emissions, evolution, and impacts across daytime and nighttime.

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