Synergistic Uptake by Acidic Sulfate Particles of Gaseous Mixtures of Glyoxal and Pinanediol

The uptake of gaseous organic species by atmospheric particles can be affected by the reactive interactions among multiple co-condensing species, yet the underlying mechanisms remain poorly understand. Here, the uptake of unary and binary mixtures of glyoxal and pinanediol by neutral and acidic sulfate particles is investigated. These species are important products from the oxidation of volatile organic compounds (VOCs) under atmospheric conditions. The uptake to acidic aerosol particles greatly increased for a binary mixture of glyoxal and pinanediol compared to the unary counterparts. The strength of the synergism depended on the particle acidity and water content (i.e., relative humidity). The greater uptake was up to 2.5× to 8× at 10% relative humidity (RH) for glyoxal and pinanediol, respectively. At 50% RH, it was 2× and 1.2× for the two species. Possible mechanisms of acid-catalyzed cross reactions between the species are proposed to explain the synergistic uptake. The proposed mechanisms are applicable to a broader extent across atmospheric species having carbonyl and hydroxyl functionalities. The results thus suggest that synergistic uptake reactions can be expected to significantly influence the gas-particle partitioning of VOC oxidation products under atmospheric conditions and thus greatly affect their atmospheric transport and lifetime.

Quantifying the Role of the Relative Humidity-Dependent Physical State of Organic Particulate Matter in the Uptake of Semivolatile Organic Molecules

The uptake of gas-phase dicarboxylic acids to organic particulate matter (PM) was investigated to probe the role of the PM physical state in exchange processes between gas-phase semivolatile organic molecules and organic PM. A homologous series of probe molecules, specifically isotopically labeled ¹³C-dicarboxylic acids, was used in conjunction with aerosol mass spectrometry to obtain a quantitative characterization of the uptake to organic PM for different relative humidities (RHs). The PM was produced by the dark ozonolysis of unlabeled α-pinene. The uptake of ¹³C-labeled oxalic, malonic, and α-ketoglutaric acids increased stepwise by 5 to 15 times with increases in RH from 15 to 80%. The enhanced uptake with increasing RH was explained primarily by the higher molecular diffusivity in the particle phase, as associated with changes in the physical state of the organic PM from a nonliquid state to a progressively less-viscous liquid state. At high RH, the partitioning of the probe molecules to the particle phase was more associated with physicochemical interactions with the organic PM than that with the co-absorbed liquid water. Uptake of the probe molecules also increased with a decrease in volatility along the homologous series. This study quantitatively shows the key roles of the particle physical state in governing the interactions of organic PM with semivolatile organic molecules.

Experimental and model estimates of the contributions from biogenic monoterpenes and sesquiterpenes to secondary organic aerosol in the southeastern United States

Atmospheric organic aerosol (OA) has important impacts on climate and human health but its sources remain poorly understood. Biogenic monoterpenes and sesquiterpenes are important precursors of secondary organic aerosol (SOA), but the amounts and pathways of SOA generation from these precursors are not well constrained by observations. We propose that the less-oxidized oxygenated organic aerosol (LO-OOA) factor resolved from positive matrix factorization (PMF) analysis on aerosol mass spectrometry (AMS) data can be used as a surrogate for fresh SOA from monoterpenes and sesquiterpenes in the southeastern US. This hypothesis is supported by multiple lines of evidence, including lab-in-the-field perturbation experiments, extensive ambient ground-level measurements, and state-of-the-art modeling. We performed lab-in-the-field experiments in which the ambient air is perturbed by the injection of selected monoterpenes and sesquiterpenes, and the subsequent SOA formation is investigated. PMF analysis on the perturbation experiments provides an objective link between LO-OOA and fresh SOA from monoterpenes and sesquiterpenes as well as insights into the sources of other OA factors. Further, we use an upgraded atmospheric model and show that modeled SOA concentrations from monoterpenes and sesquiterpenes could reproduce both the magnitude and diurnal variation of LO-OOA at multiple sites in the southeastern US, building confidence in our hypothesis. We estimate the annual average concentration of SOA from monoterpenes and sesquiterpenes in the southeastern US to be roughly 2 μg m-3.

Highly Viscous States Affect the Browning of Atmospheric Organic Particulate Matter

Initially transparent organic particulate matter (PM) can become shades of light-absorbing brown via atmospheric particle-phase chemical reactions. The production of nitrogen-containing compounds is one important pathway for browning. Semisolid or solid physical states of organic PM might, however, have sufficiently slow diffusion of reactant molecules to inhibit browning reactions. Herein, organic PM of secondary organic material (SOM) derived from toluene, a common SOM precursor in anthropogenically affected environments, was exposed to ammonia at different values of relative humidity (RH). The production of light-absorbing organonitrogen imines from ammonia exposure, detected by mass spectrometry and ultraviolet-visible spectrophotometry, was kinetically inhibited for RH < 20% for exposure times of 6 min to 24 h. By comparison, from 20% to 60% RH organonitrogen production took place, implying ammonia uptake and reaction. Correspondingly, the absorption index k across 280 to 320 nm increased from 0.012 to 0.02, indicative of PM browning. The k value across 380 to 420 nm increased from 0.001 to 0.004. The observed RH-dependent behavior of ammonia uptake and browning was well captured by a model that considered the diffusivities of both the large organic molecules that made up the PM and the small reactant molecules taken up from the gas phase into the PM. Within the model, large-molecule diffusivity was calculated based on observed SOM viscosity and evaporation. Small-molecule diffusivity was represented by the water diffusivity measured by a quartz-crystal microbalance. The model showed that the browning reaction rates at RH < 60% could be controlled by the low diffusivity of the large organic molecules from the interior region of the particle to the reactive surface region. The results of this study have implications for accurate modeling of atmospheric brown carbon production and associated influences on energy balance.