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  1. null (Ed.)
    Abstract. Short-lived highly reactive atmospheric species, such as organic peroxy radicals (RO2) and stabilized Criegee intermediates (SCIs), play an important role in controlling the oxidative removal and transformation of many natural and anthropogenic trace gases in the atmosphere. Direct speciated measurements of these components are extremely helpful for understanding their atmospheric fate and impact. We describe thedevelopment of an online method for measurements of SCIs and RO2 inlaboratory experiments using chemical derivatization and spin trappingtechniques combined with H3O+ and NH4+ chemicalionization mass spectrometry (CIMS). Using chemical derivatization agentswith low proton affinity, such as electron-poor carbonyls, we scavenge allSCIs produced from a wide range of alkenes without depleting CIMS reagentions. Comparison between our measurements and results from numericmodeling, using a modified version of the Master Chemical Mechanism, showsthat the method can be used for the quantification of SCIs in laboratoryexperiments with a detection limit of 1.4×107 molecule cm−3for an integration time of 30 s with the instrumentation used in this study. Weshow that spin traps are highly reactive towards atmospheric radicals andform stable adducts with them by studying the gas-phase kinetics of thereaction of spin traps with the hydroxyl radical (OH). We also demonstrate that spin trapadducts with SCIs and RO2 can be simultaneously probed and quantified under laboratory conditions with a detection limit of 1.6×108 molecule cm−3 for an integration time of 30 s for RO2 species with the instrumentation used in this study. Spin trapping prevents radical secondary reactions and cycling, ensuring that measurements are not biased by chemical interferences, and it can be implemented for detecting RO2 species in laboratory studies and potentially in the ambient atmosphere. 
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  2. Abstract

    Accurate estimates of aerosol refractive index (RI) are critical for modeling aerosol‐radiation interaction, yet this information is limited for ambient organic aerosols, leading to large uncertainties in estimating aerosol radiative effects. We present a semi‐empirical model that predicts the real RInof organic aerosol material from its widely measured oxygen‐to‐carbon (O:C) and hydrogen‐to‐carbon (H:C) elemental ratios. The model was based on the theoretical framework of Lorenz‐Lorentz equation and trained withn‐values at 589 nm () of 160 pure compounds. The predictions can be expanded to predictn‐values in a wide spectrum between 300 and 1,200 nm. The model was validated with newly measured and literature datasets ofn‐values for laboratory secondary organic aerosol (SOA) materials. Uncertainties ofpredictions for all SOA samples are within5%. The model suggests that‐values of organic aerosols may vary within a relatively small range for typical O:C and H:C values observed in the atmosphere.

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  3. null (Ed.)
  4. Abstract. Oxidation of organic compounds in the atmosphere produces an immenselycomplex mixture of product species, posing a challenge for both theirmeasurement in laboratory studies and their inclusion in air quality andclimate models. Mass spectrometry techniques can measure thousands of thesespecies, giving insight into these chemical processes, but the datasetsthemselves are highly complex. Data reduction techniques that groupcompounds in a chemically and kinetically meaningful way provide a route tosimplify the chemistry of these systems but have not been systematicallyinvestigated. Here we evaluate three approaches to reducing thedimensionality of oxidation systems measured in an environmental chamber:positive matrix factorization (PMF), hierarchical clustering analysis (HCA),and a parameterization to describe kinetics in terms of multigenerationalchemistry (gamma kinetics parameterization, GKP). The evaluation isimplemented by means of two datasets: synthetic data consisting of athree-generation oxidation system with known rate constants, generationnumbers, and chemical pathways; and the measured products of OH-initiatedoxidation of a substituted aromatic compound in a chamber experiment. Wefind that PMF accounts for changes in the average composition of allproducts during specific periods of time but does not sort compounds intogenerations or by another reproducible chemical process. HCA, on the otherhand, can identify major groups of ions and patterns of behavior andmaintains bulk chemical properties like carbon oxidation state that can beuseful for modeling. The continuum of kinetic behavior observed in a typicalchamber experiment can be parameterized by fitting species' time traces tothe GKP, which approximates the chemistry as a linear, first-order kineticsystem. The fitted parameters for each species are the number of reaction stepswith OH needed to produce the species (the generation) and an effectivekinetic rate constant that describes the formation and loss rates of thespecies. The thousands of species detected in a typical laboratory chamberexperiment can be organized into a much smaller number (10–30) of groups,each of which has a characteristic chemical composition and kinetic behavior.This quantitative relationship between chemical and kinetic characteristics,and the significant reduction in the complexity of the system, provides anapproach to understanding broad patterns of behavior in oxidation systemsand could be exploited for mechanism development and atmospheric chemistrymodeling. 
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  5. Abstract. An improved understanding of the fate and properties of atmospheric aerosolparticles requires a detailed process-level understanding of fundamentalfactors influencing the aerosol, including partitioning of aerosolcomponents between the gas and particle phases. Laboratory experiments withlevitated particles provide a way to study fundamental aerosol processesover timescales relevant to the multiday lifetime of atmospheric aerosolparticles, in a controlled environment in which various characteristicsrelevant to atmospheric aerosol can be prepared (e.g., highsurface-to-volume ratio, highly concentrated or supersaturated solutions,changes to relative humidity). In this study, the four-carbon unsaturatedcompound butenedial, a dialdehyde produced by oxidation of aromaticcompounds that undergoes hydration in the presence of water, was used as amodel organic aerosol component to investigate different factors affectinggas–particle partitioning, including the role of lower-volatility“reservoir” species such as hydrates, timescales involved inequilibration between higher- and lower-volatility forms, and the effect ofinorganic salts. The experimental approach was to use a laboratory systemcoupling particle levitation in an electrodynamic balance (EDB) withparticle composition measurement via mass spectrometry (MS). In particular,by fitting measured evaporation rates to a kinetic model, the effectivevapor pressure was determined for butenedial and compared under differentexperimental conditions, including as a function of ambient relativehumidity and the presence of high concentrations of inorganic salts. Even underdry (RH<5 %) conditions, the evaporation rate of butenedial isorders of magnitude lower than what would be expected if butenedial existedpurely as a dialdehyde in the particle, implying an equilibrium stronglyfavoring hydrated forms and the strong preference of certain dialdehydecompounds to remain in a hydrated form even under lower water contentconditions. Butenedial exhibits a salting-out effect in the presence ofsodium chloride and sodium sulfate, in contrast to glyoxal. The outcomes ofthese experiments are also helpful in guiding the design of future EDB-MSexperiments. 
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  6. Abstract. Chemical ionization massspectrometry (CIMS) instruments routinely detect hundreds of oxidized organic compoundsin the atmosphere. A major limitation of these instruments is the uncertaintyin their sensitivity to many of the detected ions. We describe thedevelopment of a new high-resolution time-of-flight chemical ionization massspectrometer that operates in one of two ionization modes: using eitherammonium ion ligand-switching reactions such as for NH4+ CIMS orproton transfer reactions such as for proton-transfer-reaction massspectrometer (PTR-MS). Switching between the modes can be done within 2&thinsp;min.The NH4+ CIMS mode of the new instrument has sensitivities of upto 67&thinsp;000&thinsp;dcps&thinsp;ppbv−1 (duty-cycle-corrected ion counts per second perpart per billion by volume) and detection limits between 1 and 60&thinsp;pptv at2σ for a 1&thinsp;s integration time for numerous oxygenated volatileorganic compounds. We present a mass spectrometric voltage scanning procedurebased on collision-induced dissociation that allows us to determine thestability of ammonium-organic ions detected by the NH4+ CIMS instrument.Using this procedure, we can effectively constrain the sensitivity of theammonia chemical ionization mass spectrometer to a wide range of detectedoxidized volatile organic compounds for which no calibration standards exist.We demonstrate the application of this procedure by quantifying thecomposition of secondary organic aerosols in a series of laboratoryexperiments.

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  7. null (Ed.)
    Abstract. Formaldehyde (HCHO) has been measured from space for morethan 2 decades. Owing to its short atmospheric lifetime, satellite HCHOdata are used widely as a proxy of volatile organic compounds (VOCs; pleaserefer to Appendix A for abbreviations and acronyms), providing constraintson underlying emissions and chemistry. However, satellite HCHO products fromdifferent satellite sensors using different algorithms have received littlevalidation so far. The accuracy and consistency of HCHO retrievals remainlargely unclear. Here we develop a validation platform for satellite HCHOretrievals using in situ observations from 12 aircraft campaigns with a chemicaltransport model (GEOS-Chem) as the intercomparison method. Application tothe NASA operational OMI HCHO product indicates negative biases (−44.5 %to −21.7 %) under high-HCHO conditions, while it indicates high biases (+66.1 % to+112.1 %) under low-HCHO conditions. Under both conditions, HCHO a priorivertical profiles are likely not the main driver of the biases. By providingquick assessment of systematic biases in satellite products over largedomains, the platform facilitates, in an iterative process, optimization ofretrieval settings and the minimization of retrieval biases. It is alsocomplementary to localized validation efforts based on ground observationsand aircraft spirals. 
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