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Monoterpenes are a major component of the large quantities of biogenic volatile organic compounds that are emitted to the atmosphere each year. They have a variety of structures, which influences their subsequent reactions with OH radicals, O3, or NO3radicals and the tendency for these reactions to form secondary organic aerosol (SOA). Here we report the results of an environmental chamber study of the reaction of Δ-3-carene, an abundant unsaturated C10bicyclic monoterpene, with NO3radicals, a major nighttime oxidant. Gas- and particle-phase reaction products were analyzed in real time and offline by using mass spectrometry, gas and liquid chromatography, infrared spectroscopy, and derivatization-spectrophotometric methods. The results were used to identify and quantify functional groups and molecular products and to develop gas- and particle-phase reaction mechanisms to explain their formation. Identified gas-phase products were all first-generation ring-retaining and ring-opened compounds (ten C10and one C9monomers) with 2–4 functional groups and one C20dinitrooxydialkyl peroxide dimer. Upon partitioning to the particle phase, the monomers reacted further to form oligomers consisting almost entirely of C20acetal and hemiacetal dimers, with those formed from a hydroxynitrate and hydroxycarbonyl nitrate comprising more than 50% of the SOA mass. The SOA contained an average of 0.94, 0.71, 0.15, 0.11, 0.16, 0.13, and 7.80 nitrate, carbonyl, hydroxyl, carboxyl, ester, peroxide, and methylene groups per C10monomer and was formed with a mass yield of 56%. These results have important similarities and differences to those obtained from a previous similar study of the reaction of β-pinene and yield new insights into the effects of monoterpene structure on gas- and particle-phase reactions that can lead to the formation of a large variety of multifunctional products and significant amounts of SOA.

Partitioning of nitric acid and sulfuric acid between aqueous and organic phases may play a role in the chemistry of aqueous/organic phase-separated atmospheric aerosol particles, as well as other environmental systems. To assess the extent to which these acids partition to organic phases, organic compounds with O : C ratios ranging from 0 to 0.75 were mixed with acidified aqueous phases. The organic phase was removed and its acid content quantified with a mass-based measurement. Up to 60% of the acid was observed to partition to the organic phase, present either entirely in the dissociated form, the undissociated form, or both depending on the organic phase. Values of partitioning coefficients, Ddissand Dun, corresponding to partitioning of the dissociated and undissociated acid, were determined by fitting a simple partitioning model to the data. Values of Duncorrelated well with the O : C ratio of the organic component and to a lesser extent with the water content of the organic phase. The addition of ammonium sulfate led to an increase or decrease in partitioning of sulfuric acid to the organic phase, apparently depending on the balance between the resulting enhancement in the HSO4−concentration and the reduction in the water content of the organic phase. These results demonstrate that significant fractions of sulfuric and nitric acid can partition to the organic phase in phase-separated systems in undissociated and dissociated forms, and thus potentially participate in specific or general acid catalysis of organic reactions.

A series of C6–C132-methyl-1-alkenes were reacted with OH radicals in the presence of NOxin a Teflon environmental chamber, and molar yields of the 2-ketone products were measured using gas chromatography. Yields were corrected for secondary reactions with OH radicals and for gas-wall partitioning of the 2-methyl-1-alkene and 2-ketone, with the latter correction being determined from measurements of gas-wall partitioning of 2-ketone standards. Molar yields of 2-ketones decreased with increasing 2-methyl-1-alkene carbon number from a maximum of 0.82 for C6to a minimum of 0.34 ± 0.02 for C9–C13, which after normalization for the fraction of reaction that occurred by OH radical addition to the C═C double bond (with the rest occurring by H atom abstraction) were 0.86 and 0.39 ± 0.01. These yields were combined with branching ratios determined previously for site-specific OH radical addition to the C═C double bond and for formation of β-hydroxynitrates to determine branching ratios for decomposition and isomerization of β-hydroxyalkoxy radicals. Branching ratios for decomposition decreased with increasing 2-methyl-1-alkene carbon number from a maximum of 0.97 for C6to a minimum of 0.49 ± 0.01 for C9–C13, while the corresponding values for isomerization increased from 0.03 to 0.51 ± 0.01. The results were used to estimate absolute rate constants and activation energies for decomposition and isomerization and were also combined with previously measured yields of β-hydroxynitrates, dihydroxynitrates, trihydroxynitrates, and H atom abstraction products to obtain yields of ∼75% for the C9–C13reaction products, with the remainder likely being mostly dihydroxycarbonyls and trihydroxycarbonyls.

Secondary organic aerosols (SOA) are a major contributor to fine particulate mass and wield substantial influences on the Earth’s climate and human health. Despite extensive research in recent years, many of the fundamental processes of SOA formation and evolution remain poorly understood. Most atmospheric aerosol models use gas/particle equilibrium partitioning theory as a default treatment of gas-aerosol transfer, despite questions about potentially large kinetic effects. We have conducted fundamental SOA formation experiments in a Teflon environmental chamber using a novel method. A simple chemical system produces a very fast burst of low-volatility gas-phase products, which are competitively taken up by liquid organic seed particles and Teflon chamber walls. Clear changes in the species time evolution with differing amounts of seed allow us to quantify the particle uptake processes. We reproduce gas- and aerosol-phase observations using a kinetic box model, from which we quantify the aerosol mass accommodation coefficient (α) as 0.7 on average, with values near unity especially for low volatility species. α appears to decrease as volatility increases. α has historically been a very difficult parameter to measure with reported values varying over 3 orders of magnitude. We use the experimentally constrained model to evaluate the correction factor (Φ) needed for chamber SOA mass yields due to losses of vapors to walls as a function of species volatility and particle condensational sink. Φ ranges from 1–4.

Viscosity of monoterpene‐derived secondary organic aerosols (SOAs) as a function of temperature and relative humidity (RH), and dry SOA glass transition temperatures are reported. Viscosity was measured using coalescence time scales of synthesized 100 nm dimers. Dry temperature‐dependent SOA viscosity was similar to that of citric acid, coal tar pitch, and sorbitol. The temperature where dry viscosity was 106Pa·s varied between 14 and 36 °C and extrapolated glass transition varied between −10 and 20 °C (±10 °C). Mass fragment f44obtained with an Aerosol Chemical Speciation Monitor was anticorrelated with viscosity. Viscosity of humidified Δ3‐carene and α‐pinene SOAs exceeded 106Pa·s for all subsaturated RHs at temperatures <0 and –5 °C, respectively. Steep viscosity isopleths at 106Pa·s were traced for these across (temperature, RH) conditions ranging from (approximately −5 °C, 100%) and (approximately 36 °C, 0%). Differences in composition and thus hygroscopicity can shift humidified viscosity isopleths for SOAs at cold tropospheric temperatures. Airborne particles in the environment can be harmful to human health and are part of the climate system. These particles and any harmful substances they may carry can be broken down by oxidants or removed by water. This is easier if the particles are liquid and becomes more difficult if particles are semisolid (like peanut butter) or solid (like glass). However, viscosity is hard to measure for nanoscale airborne particles. Recent advances have made this possible. In this study we measured the viscosity of several types of oxidized organic aerosols at different temperatures and humidities. We collided and melted together 100 nm particles in a continuous flow system. Without moisture, the particles were as hard as pitch and melted between 14 and 36 °C. At temperatures 20° colder they could be considered as hard as glass. The chemical marker for more oxidized material was correlated with softer particles. Below −5° we were unable to liquefy the particles even with high relative humidity. The particles melted at about −5° at 100% relative humidity and at 36° dry, with intermediate points connecting these extremes. We found that the composition and water solubility of the particles affects their viscosity at cold temperatures. Differences in composition and hygroscopicity shift humidified viscosity isopleths for secondary organic aerosols at cold temperaturesTemperature dependence of viscosity for dry secondary organic aerosol was similar to that of citric acid, coal tar pitch, and sorbitolFor monoterpene‐derived secondary organic aerosols, the temperature where viscosity was 106Pa·s was anticorrelated with oxidation state