Particle formation in the atmosphere from gas-phase precursors has been observed around the world; however, our fundamental understanding of the key species responsible and mechanisms involved remains uncertain. Recent laboratory studies demonstrated that acid-base reactions involving methanesulfonic acid (CH3SO3H, MSA) and small alkylamines may contribute significantly to new particle formation. To date, most of the investigations have been focused on particle number concentration and size distribution measurements in combination with quantum chemistry predictions of the most stable clusters. Here, we present the first measurements of the size-resolved chemical composition of sub-20 nm particles generated in a custom flow reactor from the reaction of MSA with methylamine (MA) in the presence or absence of NH3 using thermal desorption chemical ionization mass spectrometry (TDCIMS). A novel design of the TDCIMS inlet was evaluated, and the measurement of the chemical composition of particles was extended down to 5 nm in diameter, the smallest size yet reported for this method. MSA-MA particles with diameters smaller than 9 nm were found to be more acidic, with an acid/base molar ratio of 1.8 ± 0.4 (1σ) for 5 nm particles compared to the larger particles which were neutral. A similar acid/base molar ratio trend was observed when NH3 was added to the MSA-MA combination. In the MSA-MA-NH3 system, the MA/NH3 molar ratio was higher than 1 (up to 2.6) for all particle sizes despite the much larger concentration of NH3 in the gas phase (the gas-phase MA/NH3 ratio was ∼0.23), indicating that MA is a key component in particle formation from this system. The potential reasons for this based on previous calculations of small clusters in this system are discussed.

Ambient ionization processes are becoming more widely used for the measurement of atmospherically relevant particles and gases. We report here ambient ionization mass spectra utilizing a commercial tincture of iodine and a piezoelectric discharge gun (PDG) to generate the ionizing reagents. Analytes include Cl2, Br2, HNO3, the C1-C9 series of saturated monocarboxylic acids, benzoic acid, 2,2-dimethylpropanoic acid, 9-decenoic acid, and trichloroacetic acid. While Cl2 and Br2 form the [M + I]- iodide adducts, HNO3 and the organic acids show unexpected peaks corresponding to [2M - 2H + I]-. For HNO3, the new ion formed is interpreted as the [NO3-···IONO2] complex, where IONO2 is likely formed upon reaction of HOI with gaseous NO3-. Similarly, for the organic acids, the [2M - 2H + I]- peaks are interpreted as [RC(O)O-···IOC(O)R] complexes formed by association of RC(O)O- with acyl hypoiodites [RC(O)OI]. It is proposed that the association of (1) Cl2 and Br2 with I-, (2) IONO2 with NO3- ions, and (3) RC(O)OI with carboxylate ions occurs via non-covalent halogen bonding. The results suggest the possibility that halogen bonding may play a role in chemical transformations in the atmosphere, particularly in particles where concentrations of iodinated species may be significant.

Organic peroxides are key intermediates in the atmosphere but are challenging to detect, especially in the particle phase, due to their instability, which has led to substantial gaps in the understanding of their environmental effects. We demonstrate that matrix-assisted ionization in vacuum (MAIV) mass spectrometry (MS), which does not require an ionization source, enables in situ characterization of peroxides and other products in the surface layers of organic particles. Hydroxyl radical oxidation of glutaric acid particles yields hydroperoxides and organic peroxides, which were detected with signals of the same order of magnitude as the major, more stable products. Product identification is supported by MS/MS analysis, peroxide standards, and offline high-resolution MS. The peroxide signals relative to the stable carbonyl and alcohol products are significantly larger using MAIV compared to those in the offline bulk analysis. This is also the case for analysis using fast, online easy ambient sonic-spray ionization mass spectrometry. These studies demonstrate the advantage of MAIV for the real-time characterization of labile peroxides in the surface layers of solid particles. The presence of peroxides on the surface may be important for surface oxidation processes as well as for the toxicity of inhaled particles.

The environmental fate of neonicotinoids (NNs) is a key regulatory issue due to their widespread distribution, mobility, and role in honeybee colony collapse disorder. Here, we explore the potential matrix effects of the commercial formulations on the photochemistry of two NNs, dinotefuran (DNF) and nitenpyram (NPM). The commercial formulations and the pure reagents, both embedded in KBr pellets, were irradiated at wavelengths from 254 to 350 nm. Loss of the NN and product formation were followed using Fourier transform infrared spectroscopy and direct analysis in real time mass spectrometry. Quantum yields for loss of NN in commercial formulations were within experimental error of the pure NN. However, quantum yields for loss in KBr were consistently smaller than those previously measured for thin films of pure compounds, suggesting increased quenching of excited intermediates in the dense solid KBr matrix. At 350 nm where NPM absorbs, the quantum yield for loss in the formulation was about half that of the pure compound, which was attributed to a reduction in the NPM absorption cross section due to the presence of water taken up by additional formulation ingredients. Major products of DNF and NPM photolysis were the same in the formulation and in the pure compound. However, in contrast to previous thin film measurements, unexpected infrared peaks at 2222 and 2136 cm-1 during photolysis of DNF in KBr were identified as N2O trapped in the KBr matrix and NO+, respectively. Overall, the presence of other compounds in the formulations did not significantly alter the photochemistry compared to the pure compounds.

Understanding how gases interact with and are incorporated into atmospheric secondary organic aerosol particles is crucial for predicting particle effects on climate and human health. This work examined how three gaseous organic nitrates (ON) are taken up into viscous particles formed from the ozonolysis of α-pinene (AP). Experiments were performed in a flow reactor at room temperature under dry conditions, either with or without an OH scavenger present, with constant ozone and variable AP concentrations. Each ON was introduced independently into the flow reactor and was present during particle formation/growth. ON gas-phase concentrations were determined by gas chromatography−mass spectrometry, and particle phase concentrations were measured by high-resolution time-of-flight aerosol mass spectrometry. Partition coefficients (KSOA) for each ON were independent of the initial AP concentration, except for 2-ethylhexyl nitrate which was undetectable in the particles at the lowest AP concentration. Measured KSOA values were larger than those previously determined for equilibrium partitioning, which points to a potential burying mechanism for incorporation of ON during particle growth. Estimated effective net uptake coefficients (γΟΝ) were found to increase with initial AP concentration. Concentrations of gas-phase oxidation products (including dimers and autoxidation products) predicted using an updated master chemical mechanism increased with AP concentration, with little change in the overall species distribution, consistent with increased trapping/burying of ON during particle growth and thus increased values of γΟΝ. These results provide further evidence that kinetically controlled burying can contribute significantly to particle growth, provided that the incoming gas-phase molecules have sufficient residence time on the particle surface to become buried via subsequent gas−surface collisions.