The results are consistent with other inlet ionization techniques, suggesting that similar ion formation mechanisms are operative. Optimized ion yields (the combined effects of ionization probability and ion transmission) for DAI are currently in the 10 to 10 range, which is sufficient for many aerosol applications.

Atmospheric aerosol, particulate matter suspended in the air we breathe, exerts a strong impact on our health and the environment. Controlling the amount of particulate matter in air is difficult, as there are many ways particles can form by both natural and anthropogenic processes. We gain insight into the sources of particulate matter through chemical composition measurements. A substantial portion of atmospheric aerosol is organic, and this organic matter is exceedingly complex on a molecular scale, encompassing hundreds to thousands of individual compounds that distribute between the gas and particle phases. Because of this complexity, no single analytical technique is sufficient. However, mass spectrometry plays a crucial role owing to its combination of high sensitivity and molecular specificity. This review surveys the various ways mass spectrometry is used to characterize atmospheric organic aerosol at a molecular level, tracing these methods from inception to current practice, with emphasis on current and emerging areas of research. Both offline and online approaches are covered, and molecular measurements with them are discussed in the context of identifying sources and elucidating the underlying chemical mechanisms of particle formation. There is an ongoing need to improve existing techniques and develop new ones if we are to further advance our knowledge of how to mitigate the unwanted health and environmental impacts of particles.

Additional details about the experimental configurations and transmission electron microscope (TEM) imaging protocols, further methods and details on particle spreading and additional modes in scanning mobility particle size (SMPS) spectra, additional visual and numerical data pertaining to scanning electron microscopy (SEM) and atomic force microscopy (AFM) experiments, and a description of the assessment of secondary organic-and organic proxycoated samples over time. MethodsSamples of several inorganic and organic/inorganic salt systems were prepared for TEM and SEM analysis. Particles with two components comprised ammonium sulfate as well as succinic acid (>99.0%, TCI America), 2-methylglutaric acid (98%, Alfa Aesar) or PEG400 (BioUltra, Sigma-Aldrich) in a 1:1 ratio for a total of 0.05 -0.1 wt%. Single component salt systems included ammonium bisulfate (99.9%, Alfa Aesar), sodium chloride (>99.9%, VWR Chemicals), potassium chloride (>99.9%, DOT Scientific) and sodium acetate (99%, Mallinckrodt Chemicals). All such were prepared at the Pennsylvania State University using the sample generation methods described in the Methods section using carbon/copper substrates. SEM samples of both ammonium sulfate and ammonium sulfate/2-methylglutaric acid particles were prepared at the Pennsylvania State University in the same manner as TEM samples and using continuous carbon/copper TEM substrates. SEM images were obtained using an Apreo SEM (Thermo Fisher).Additionally, a series of measurements were taken over the course of several weeks to test the volatility of the shell layer of coated particles. Particles composed of 0.1 wt% succinic acid and ammonium sulfate in a 1:1 ratio were produced via the Pennsylvania State University setup described in the Methods section where the solution was aerosolized, rapidly dried using a diffusion drier, size selected, and impacted onto carbon-coated copper TEM grids. Additionally, ammonium sulfate seed particles with secondary organic coatings were generated at the Pennsylvania State University by adding dry ammonium sulfate particles into a 1 m 3 Tedlar bag (Welch Fluorocarbon) which were allowed to equilibrate followed by 150 ppb of a-pinene and 200 ppb ozone from a corona discharge tube ozone generator (Poseidon 200, Ozotech Inc.). These

Online analysis of ultrafine (<100 nm diameter) particles was performed by sending the aerosol through a condensation growth chamber (CGC) to create micrometer-size aqueous droplets that were subsequently analyzed by mass spectrometry with droplet-assisted ionization (DAI). Three experiments are reported which illustrate key performance characteristics of the method and give insight into the ion formation process: size-selected cortisone particles, size-selected secondary organic aerosol (SOA) particles, and freshly nucleated SOA under atmospherically relevant conditions. In each case, SOA was produced by α-pinene ozonolysis. For size-selected cortisone particles between 30 and 90 nm diameter and SOA particles between 30 and 70 nm, the ion signal intensity was found to be approximately independent of particle size. This observation is attributed to the formation of aqueous droplets in the CGC whose size distribution is independent of the original particle size. A consequence of this behavior is that the sensitivity of molecular detection increases as the particle size decreases, and the method is particularly well suited for new particle formation studies under atmospherically relevant conditions. This aspect of the CGC−DAI method was illustrated by the online analysis of freshly nucleated SOA samples with median diameters, number concentrations, and mass concentrations on the order of 25 nm, 10 4 cm −3 , 0.2 μg m −3 , respectively. Mass spectra of freshly nucleated SOA could be explained by condensation of highly oxidized molecules (HOMs) that subsequently reacted in the particle phase. Size-selected SOA showed increasing oligomerization with increasing particle size, which is consistent with established particle growth mechanisms.

Droplet Assisted Ionization (DAI) is a relatively new method for online analysis of aerosol droplets that enables measurement of the rate of an aerosol reaction. Here, we used DAI to study the reaction of carbonyl functionalities in secondary organic aerosol (SOA) with Girard’s T (GT) reagent, a reaction that can potentially be used to enhance the detection of SOA in online measurements. SOA was produced by α-pinene ozonolysis. Particulate matter was collected on a filter, extracted, and mixed with GT reagent in water. While the reaction hardly proceeded at all in bulk solution, products were readily observed with DAI when the solution was atomized to produce micron-size droplets. Varying the droplet transit time between the atomizer and mass spectrometer allowed the reaction rate constant to be determined, which was found to be 4 orders of magnitude faster than what would be expected from bulk solution kinetics. Decreasing the water content of the droplets, either by heating the capillary inlet to the mass spectrometer or by decreasing the relative humidity of the air surrounding the droplets in the transit line from the atomizer to the mass spectrometer, enhanced product formation. The results suggest that reaction enhancement occurs at the droplet surface, which is consistent with previous reports of reaction acceleration during mass spectrometric analysis, where a bulk solution is analyzed with an ionization method that produces aerosol droplets.