The role of functional groups in the understanding of secondary organic aerosol formation mechanism from α-pinene

The infrared spectra (IR) analysis in combination with electrospray ionization high-resolution orbitrap mass spectra (ESI-HRMS) can provide new insight into the overall structural feature and specific molecules of secondary organic aerosol (SOA). In this study, the functional group signature of SOA produced from OH and O₃ channel oxidation of α-pinene is characterized based on the IR and ESI-HRMS. The IR spectra of SOA from the OH channel show strong absorptions of hydrogen bonded OH groups and weak absorptions of CO groups, while the absorptions of CO are more abundant than OH in the O₃ channel. A linear relationship between the ratio of functional group absorption area (S_O-H/S_C=O) and the group number ratio of n_O-H/n_C=O is obtained. The ratios of n_O-H/n_C=O in the O₃ and H₂O₂ systems of SOA are estimated to be 0.60 and 3.91, respectively. The ESI-HRMS results show that organic acids are the major products in both the O₃ and NO₂ systems. In contrast to the O₃ channel, alcohols are more abundant from the OH channel. The major compounds of SOA from the H₂O₂ system are confirmed to be formed by autoxidation of first generation RO₂ radicals. The n_O-H/n_C=O ratio obtained by IR is in good agreement with that by MS. Thus, the ratio of n_O-H/n_C=O can be used to characterize SOA formation from different oxidation channels. In α-pinene-NO₂ irradiations, the ratio of n_O-H/n_C=O is 0.83, which is quite close to that from the O₃ system, but totally different from that in the H₂O₂ system. This strongly supports that the O₃ channel plays a key role in the formation of SOA from the α-pinene-NO₂ system. The similarity of both products and the n_O-H/n_C=O ratios between the α-pinene-O₃ and α-pinene-NO₂ systems strongly states that a stabilized Criegee intermediate (SCI) is a key factor controlling SOA formation.

Evidence for a kinetically controlled burying mechanism for growth of high viscosity secondary organic aerosol

Secondary organic aerosol (SOA) particles are ubiquitous in air and understanding the mechanism by which they grow is critical for predicting their effects on visibility and climate. The uptake of three organic nitrates into semi-solid SOA particles formed by α-pinene ozonolysis either with or without an OH scavenger was investigated. Four types of experiments are presented here. In Series A, uptake of the selected organic nitrates (2-ethylhexyl nitrate (2EHN); β-hydroxypropyl nitrate (HPN); β-hydroxyhexyl nitrate (HHN)) into impacted SOA particles was interrogated by attenuated total reflectance (ATR)-FTIR. In this case, equilibrium was reached and partition coefficients (K_SOA = [-ONO₂]_SOA/[-ONO₂]_air) were measured to be K^2EHN = (3.2-11) × 10⁴, K^HPN = (4.4-5.4) × 10⁵, and K^HHN = (4.9-9.0) × 10⁶. In Series B, SOA particles were exposed on-the-fly to gas phase organic nitrates for comparison to Series A, and uptake of organic nitrates was quantified by HR-ToF-AMS analysis, which yielded similar results. In Series C (AMS) and D (ATR-FTIR), each organic nitrate was incorporated into the SOA as the particles formed and grew. The incorporation of the RONO₂ was much larger in Series C and D (during growth), exceeding equilibrium values determined in Series A and B (after growth). This suggests that enhanced uptake of organic nitrates during SOA formation and growth is due to a kinetically controlled "burying" mechanism, rather than equilibrium partitioning. This has important implications for understanding SOA formation and growth under conditions where the particles are semi-solid, which is central to accurately predicting properties for such SOA.

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.

Probing surfaces of atmospherically relevant organic particles by easy ambient sonic-spray ionization mass spectrometry (EASI-MS)

EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Both ambient and laboratory-generated particles can have a surface composition different from the bulk, but there are currently few analytical techniques available to probe these differences. Easy ambient sonic-spray ionization mass spectrometry (EASI-MS) was applied to solid, laboratory-generated particles with core–shell morphologies formed from a variety of dicarboxylic acids. The soft ionization facilitated parent peak detection for the two compounds, from which the depth probed could be determined from the relative signal intensities. Two different configurations of a custom-made nebulizer are reported that yield different probe depths. In the “orthogonal mode,” with the nebulizer ∼10 centimeters away from the particle stream and at a 90° angle to the MS inlet, evaporation of the nebulizer droplets forms ions before interaction with the particles. The probe depth for orthogonal mode EASI-MS is shown to be 2–4 nm in these particle systems. In the “droplet mode”, the nebulizer and particle streams are in close proximity to each other and the MS inlet so that the particles interact with charged liquid droplets. This configuration resulted in full dissolution of the particles and gives particle composition similar to that from collection on filters and extraction of the particles (bulk). These studies establish that EASI-MS is a promising technique for probing the chemical structures of inhomogeneous airborne organic particles.

Polar organic tracers in PM2.5 aerosols from an inland background area in Southwest China: Correlations between secondary organic aerosol tracers and source apportionment

PM_2.5 aerosol samples were collected over 12 hr and 24 hr intervals in an inland background area, Gongga Mountain National Nature Reserve (hereafter shortened to Gongga), during the summer of 2011. Polar organic tracers, inorganic ions and meteorological data were measured. The purpose of this work was to investigate the variation patterns, formation and sources of the secondary organic aerosol tracers in the studied atmosphere. The average concentrations of isoprene oxidation products, α-pinene oxidation products, β-caryophyllinic acid, sugars, sugar alcohols and anhydrosugars were 88.6 ± 106.1, 3.6 ± 5.7, 0.13 ± 0.30, 13.6 ± 13.1, 31.9 ± 31.4 and 14.8 ± 10.7 ng/m³ respectively in all aerosol samples. The aged α-pinene second organic aerosol (SOA) tracers (i.e., 3-hydroxyglutraric acid (3HGA), 3-hydroxy-2,2-dimethylglutaric acid (HDMGA), 3-acetylpentandioic acid (APDA) and 3-methyl-1,2,3-butanetricarboxylic acid (MBTCA)) correlated significantly with each other in the 24 hr PM_2.5 aerosol samples, indicating that OH· is the major factor controlling the formation of these α-pinene SOA tracers. Using the positive matrix factorization (PMF) model and the tracer-based source apportionment method, we calculated that isoprene oxidation products, α-pinene oxidation products, sesquiterpene oxidation products, biomass burning, fungi spores and anthropogenic SOA accounted for 21.9% ± 5.5%, 8.4% ± 2.1%, 3.0% ± 0.7%, 5.2% ± 5.3%, 5.0% ± 6.2% and 31.4% ± 7.8% of organic carbon respectively during the sampling period.

Kinetics, mechanisms and ionic liquids in the uptake of n-butylamine onto low molecular weight dicarboxylic acids

Atmospheric particles adversely affect visibility, health, and climate, yet the kinetics and mechanisms of particle formation and growth are poorly understood. Multiphase reactions between amines and dicarboxylic acids (diacids) have been suggested to contribute. In this study, the reactions of n-butylamine (BA) with solid C3-C8 diacids were studied at 296 ± 1 K using a Knudsen cell interfaced to a quadrupole mass spectrometer. Uptake coefficients for amines on the diacids with known geometric surface areas were measured at initial amine concentrations from (3-50) × 10¹¹ cm^-3. Uptake coefficients ranged from 0.7 ± 0.1 (2σ) for malonic acid (C3) to <10^-6 for suberic acid (C8), show an odd-even carbon number effect, and decrease with increasing chain length within each series. Butylaminium salts formed from evaporation of aqueous solutions of BA with C3, C5 and C7 diacids (as well as C8) were viscous liquids, suggesting that ionic liquids (ILs) form on the surface during the reactions of gas phase amine with the odd carbon diacids. Predictions from the kinetic multi-layer model of aerosol surface and bulk chemistry (KM-SUB) were quantitatively consistent with uptake occurring via dissolution of the underlying diacid into the IL layer and reaction with amine taken up from the gas phase. The butylaminium salts formed from the C4 and C6 diacids were solids, and their uptake coefficients were smaller. These experiments and kinetic modeling demonstrate the unexpected formation of ILs in a gas-solid reaction, and suggest that ILs should be considered under some circumstances in atmospheric processes.

Knudsen cell studies of the uptake of gaseous ammonia and amines onto C3–C7 solid dicarboxylic acids

While atmospheric particles affect health, visibility and climate, the details governing their formation and growth are poorly understood on a molecular level.

Introductory lecture: atmospheric chemistry in the Anthropocene

The term “Anthropocene” was coined by Professor Paul Crutzen in 2000 to describe an unprecedented era in which anthropogenic activities are impacting planet Earth on a global scale. Greatly increased emissions into the atmosphere, reflecting the advent of the Industrial Revolution, have caused significant changes in both the lower and upper atmosphere. Atmospheric reactions of the anthropogenic emissions and of those with biogenic compounds have significant impacts on human health, visibility, climate and weather. Two activities that have had particularly large impacts on the troposphere are fossil fuel combustion and agriculture, both associated with a burgeoning population. Emissions are also changing due to alterations in land use. This paper describes some of the tropospheric chemistry associated with the Anthropocene, with emphasis on areas having large uncertainties. These include heterogeneous chemistry such as those of oxides of nitrogen and the neonicotinoid pesticides, reactions at liquid interfaces, organic oxidations and particle formation, the role of sulfur compounds in the Anthropocene and biogenic–anthropogenic interactions. A clear and quantitative understanding of the connections between emissions, reactions, deposition and atmospheric composition is central to developing appropriate cost-effective strategies for minimizing the impacts of anthropogenic activities. The evolving nature of emissions in the Anthropocene places atmospheric chemistry at the fulcrum of determining human health and welfare in the future.

Role of the reaction of stabilized Criegee intermediates with peroxy radicals in particle formation and growth in air

We investigate the particle formation mechanism from ozonolysis, and find that it is highly dependent on the structure of the alkene.