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The chemical composition of aerosol particles is crucial to understanding their formation and evolution in the atmosphere. However, very limited information is available for the chemical composition of ultrafine particles, particularly for nanoclusters and newly formed particles, due to the lack of valid analytical methods. This work reviews the online measurement techniques for characterizing the chemical composition of atmospheric clusters and sub-20 nm particles. The commonly used instrumentations are divided into two categories: direct techniques based on mass spectrometry and indirect measurement techniques mainly relying on the physical properties (e.g., hygroscopicity and volatility). The advantages of these techniques are compared and their limitations in the lab and field application are summarized. The combination of direct and indirect techniques is discussed, and this may provide more comprehensive understanding of chemical information of atmospheric clusters and particles. We propose that the newly developing instrumentations are needed to improve the collection efficiency for direct techniques, or the chemical resolution for indirect techniques. Future development should focus on obtaining simultaneous measurements of particle physical and chemical properties, which can be helpful in improving the accuracy of modeling and the understanding of particle formation and evolution.

The newly developed planar differential mobility analyzer (DMA) serving as particle sizer can achieve higher transmission and selection precision at ambient pressure compared with conventional cylindrical DMA, and show potentials on coupling with atmospheric pressure interface mass spectrometer (API-MS) for cluster detection with an additional ion mobility dimension. In this study, we assessed the performance of a commercial planar DMA (DMA P5) integrated with the home-build recirculation system. The sizing range of the system in this work is 0-3.9 nm, although larger sizes can be measured with a sheath gas flow restrictor. The resolving power under different recirculation setups (suction mode and counter flow mode) and different sheath flow rates was evaluated using electrosprayed tetra-alkyl ammonium salts (TMAI, TBAI, THAB and TDAB). The maximum resolving power of THA+ under suction and counterflow mode are 61.6 and 84.6, respectively. The sizing resolution of DMA P5 is 7-16 times higher than conventional cylindrical DMAs. The resolving power showed approximately linear correlation with v under counterflow mode, while the resolving power of THA+ under suction mode stopped linearly increase with v when the VDMA was above 3554.3V and enter a plateau due to the interference of sample flow on the laminarity of sheath flow. The transmission efficiency of DMA P5 can reach 54.3%, about one factor of magnitude higher than the commercial DMAs. The mobility spectrum of different electrosprayed tetra-alkyl ammonium salts and the mass to charge ratio-mobility 2D spectrum of sulfuric acid clusters was also characterized with the DMA P5 (-MS) system. [ABSTRACT FROM AUTHOR]