Search for: All records

Abstract Aqueous‐phase uptake and processing of water‐soluble organic compounds can promote secondary organic aerosol (SOA) production. We evaluated the contributions of aqueous‐phase chemistry to summertime urban SOA at two sites in New York City. The relative role of aqueous‐phase processing varied with chemical and environmental conditions, with evident daytime SOA enhancements (e.g., >1 μg/m³) during periods with relative humidities (RH) exceeding 65% and often higher temperatures. Oxygenated organic aerosol (OOA) production was also sensitive to secondary inorganic aerosols, in part through their influence on aerosol liquid water. On average, high‐RH periods exhibited a 69% increase in less‐oxidized OOA production in Queens, NY. These enhancements coincided with southerly backward trajectories and greater inorganic aerosol concentrations, yet showed substantial intra‐city variability between Queens and Manhattan. The observed aqueous‐phase SOA production, even with historically low sulfate and nitrate aerosol loadings, highlights both opportunities and challenges for continued reductions in summertime PM_2.5in urban communities. 

Abstract Isoprene (C₅H₈) is the non-methane hydrocarbon with the highest emissions to the atmosphere. It is mainly produced by vegetation, especially broad-leaved trees, and efficiently transported to the upper troposphere in deep convective clouds, where it is mixed with lightning NO_x. Isoprene oxidation products drive rapid formation and growth of new particles in the tropical upper troposphere. However, isoprene oxidation pathways at low temperatures are not well understood. Here, in experiments at the CERN CLOUD chamber at 223 K and 243 K, we find that isoprene oxygenated organic molecules (IP-OOM) all involve two successive$${{{\rm{OH}}}}^{\bullet}$$ ${\mathrm{OH}}^{\bullet }$oxidations. However, depending on the ambient concentrations of the termination radicals ($${{{{\rm{HO}}}}_{2}}^{\bullet},\,{{{\rm{NO}}}}^{\bullet}$$ ${{\mathrm{HO}}_2}^{\bullet },{\mathrm{NO}}^{\bullet }$, and$${{{\rm{NO}}}}_{2}^{\bullet}$$ ${\mathrm{NO}}_2^{\bullet }$), vastly-different IP-OOM emerge, comprising compounds with zero, one or two nitrogen atoms. Our findings indicate high IP-OOM production rates for the tropical upper troposphere, mainly resulting in nitrate IP-OOM but with an increasing non-nitrate fraction around midday, in close agreement with aircraft observations. 

Abstract Aircraft observations have revealed ubiquitous new particle formation in the tropical upper troposphere over the Amazon^1,2and the Atlantic and Pacific oceans^3,4. Although the vapours involved remain unknown, recent satellite observations have revealed surprisingly high night-time isoprene mixing ratios of up to 1 part per billion by volume (ppbv) in the tropical upper troposphere⁵. Here, in experiments performed with the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber, we report new particle formation initiated by the reaction of hydroxyl radicals with isoprene at upper-tropospheric temperatures of −30 °C and −50 °C. We find that isoprene-oxygenated organic molecules (IP-OOM) nucleate at concentrations found in the upper troposphere, without requiring any more vapours. Moreover, the nucleation rates are enhanced 100-fold by extremely low concentrations of sulfuric acid or iodine oxoacids above 10⁵ cm⁻³, reaching rates around 30 cm⁻³ s⁻¹at acid concentrations of 10⁶ cm⁻³. Our measurements show that nucleation involves sequential addition of IP-OOM, together with zero or one acid molecule in the embryonic molecular clusters. IP-OOM also drive rapid particle growth at 3–60 nm h⁻¹. We find that rapid nucleation and growth rates persist in the presence of NO_xat upper-tropospheric concentrations from lightning. Our laboratory measurements show that isoprene emitted by rainforests may drive rapid new particle formation in extensive regions of the tropical upper troposphere^1,2, resulting in tens of thousands of particles per cubic centimetre. 

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. 

Abstract. Aromatic hydrocarbons make up a large fraction of anthropogenic volatile organic compounds and contribute significantly to the production of tropospheric ozone and secondary organic aerosol (SOA). Four toluene and four 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation experiments were performed in an environmental chamber under relevantpolluted conditions (NOx∼10 ppb). An extensive suite of instrumentation including two proton-transfer-reaction mass spectrometers (PTR-MS) and two chemical ionisation mass spectrometers (NH4+ CIMS and I− CIMS) allowed for quantification of reactive carbon in multiple generations of hydroxyl radical (OH)-initiated oxidation. Oxidation of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate compounds, as well as ring-scission products such as epoxides and dicarbonyls. We show that the oxidation of bicyclic intermediate products leads to the formation of compounds with high oxygen content (an O:C ratio of up to 1.1). These compounds, previously identified as highly oxygenated molecules (HOMs), are produced by more than one pathway with differing numbers of reaction steps with OH, including both auto-oxidation and phenolic pathways. We report the elemental composition of these compounds formed under relevant urban high-NO conditions. We show that ring-retaining products for these two precursors are more diverse and abundant than predicted by current mechanisms. We present the speciated elemental composition of SOA for both precursors and confirm that highly oxygenated products make up a significant fraction of SOA. Ring-scission products are also detected in both the gas and particle phases, and their yields and speciation generally agree with the kinetic model prediction. 

Glass transitions from liquid to semi-solid and solid phase states have important implications for reactivity, growth, and cloud-forming (cloud condensation nuclei and ice nucleation) capabilities of secondary organic aerosols (SOAs). The small size and relatively low mass concentration of SOAs in the atmosphere make it difficult to measure atmospheric SOA glass transitions using conventional methods. To circumvent these difficulties, we have adapted a new technique for measuring glass-forming properties of atmospherically relevant organic aerosols. Aerosol particles to be studied are deposited in the form of a thin film onto an interdigitated electrode (IDE) using electrostatic precipitation. Dielectric spectroscopy provides dipole relaxation rates for organic aerosols as a function of temperature (373 to 233 K) that are used to calculate the glass transition temperatures for several cooling or heating rates. IDE-enabled broadband dielectric spectroscopy (BDS) was successfully used to measure the kinetically controlled glass transition temperatures of aerosols consisting of glycerol and four other compounds with selected cooling and heating rates. The glass transition results agree well with available literature data for these five compounds. The results indicate that the IDE-BDS method can provide accurate glass transition data for organic aerosols under atmospheric conditions. The BDS data obtained with the IDE-BDS technique can be used to characterize glass transitions for both simulated and ambient organic aerosols and to model their climate effects.