As an important part of atmospheric particulate matter, nanoparticles have a significant impact on human health, atmospheric environment, as well as climate. The physical and chemical properties of nanoparticles, especially the chemical compositions, play an important role in these effects. This review surveys the various analytical techniques at home and abroad used to measure airborne nanoparticles. For each technique, its operation principle and typical instrument configuration are summarized, and representative examples reported in previous work are also introduced to illustrate its advantages and disadvantages. In addition, the future directions of instrumental development and application are outlined and discussed.

Ultra-fine particles emitted by motor vehicles which have high number concentration and surface area, are one of the main sources of air pollution,. Accurate on-line monitoring is of significance for vehicle emission assessment and control measures. With the addition of particle number (PN) control index in vehicle emission standards in EU and China, the requirements of on-line monitoring technology are also increasing. At present, diffusion charging method and condensation nuclear particle count method are mainly used to measure the number of ultra-fine particles emitted by motor vehicles, and aerodynamics and electro-migration classification methods are mainly used to measure the particle size distribution. The research status of on-line monitoring technology of ultra-fine particles emitted by motor vehicles at home and abroad is summarized, the principle, characteristics and application status of several main developed measurement technologies and instruments are mainly introduced, and the future development of the technologies is prospected.

Currently most of the aerosol chargers have poor penetration and charging efficiencies for 1 ～ 3 nm aerosols. A new soft X-ray aerosol charger is developed and evaluated. While maintaining similar intrinsic charging efficiency to the other chargers, its penetration efficiency is significantly increased through the improvement of the structure design. Laboratory evaluations show that, compared with commercialized TSI 3088 soft X-ray charger, the penetration efficiencies of the new developed charger for sub-3 nm aerosols have been increased by 175% ～ 300% and 115% ～ 173% under 1.0 and 2.5 L · min －1 flow rates, respectively. Meanwhile, the intrinsic charging efficiencies of the new charger for 10 ～ 40 nm and sub-3 nm aerosols is basically consistent with the intrinsic charging efficiencies estimated by a well-accepted Fuchs steady state approximation formula and those of the other similar aerosol chargers. Therefore, compared with the existing commercial chargers, the new developed charger is beneficial for its higher extrinsic charging efficiency for sub-3 nm aerosols, which indicates its potential application value.

Performance comparison between a custom-made soft X-ray neutralizer (CM-SXR) and a commercial counterpart (TSI-SXR, advanced aerosol neutralizer, model 3088, TSI, USA) is made by using a scanning mobility particle sizer (SMPS). The results show that for all the tested particles (i.e., Polystyrene latex particles, ammonium sulfate particles, sodium chloride particles, and room air particles) with a size greater than 20 nm, the particle number concentration measured by CM-SXR is higher than that measured by TSI-SXR when the differential mobility analyzer (DMA) is operated under the negatively-charged particle mode, and the differences can be up to 40%. However, opposite results have been found when the DMA is operated at the positively-charged mode, that is the particle number concentration measured by TSI-SXR is higher than that measured by CM-SXR and the differences are up to 77%. Possible reasons accounting for the above differences are discussed, and it is deduced that the mounting position of the soft X-ray in the two neutralizers, which results in different exposures of the X-ray and the differences of the residence time when the sample passes the two different neutralizers, is likely the main reason leading to different positively-and negatively-charged particle distribution inside the neutralizers.

Accurately measuring the size distribution of ultrafine particles in the exhaust gas of motor vehicles is of great significance for air pollution and human health risk assessment. Based on previously developed unipolar charger, plate differential mobility analyzer (PDMA) and Faraday cup electrometer (FCE) module, a portable ultrafine particle sizer was developed, which can monitor size distribution of ultrafine particles from 20 to 500 nm in the exhaust gas of motor vehicles in real time. The calibration experiment of the self-developed particle size spectrometer was carried out. It is shown that the particle size peak measured by the portable motor vehicle ultrafine particle sizer is within ± 5% of the 40 nm and 100 nm single particle size selected by TSI 3081A/3082 and the difference between the number concentration measured by the sizer and that measured by Pegasor particle sensor (PPS-M) is ± 10%. Furthermore, field experiments were carriedout on different types of vehicles. The results show that the concentration of particulate matter in the exhaust gas of gasoline and diesel vehicles is mainly concentrated in the range of particle size less than 40 nm. In addition, in the contrast experiment, the median diameter of the peak measured by the self-developed particle sizer is 20 nm, and the median diameter measured by the commercial electrical low pressure impactor (ELPI) is 17 nm. The difference between the two particle sizers is 15%, indicating that the two particles sizers have good consistency.

Highly oxygenated organic molecules (HOMs) are important precursors for secondary organic aerosols (SOAs). Gaseous sulfuric acid was used as an analogue of HOMs to estimate the ionization efficiency ofHOMs in a chemical ionization atmospheric-pressure-interface time-of-flight mass spectrometer (CI-APi-ToF) equipped with a nitrate chemical ionization source, then a mass-dependent transmission correction for HOMs was performed using a high-resolution half-mini differential mobility analyzer (HR-DMA) in tandem of the ToF-MS, and hence a semi-quantitative method of atmospheric HOMs was established. Based on the method established, a field campaign was then carried out in Beijing from February to March 2018, during which about 110 HOMs species were identified together with their temporal profiles and elemental composition. The observation indicates that hydroxyl radicals play an important role in the formation of HOMs, whereas nitrogen oxides are also involved. Most of the observed HOMs can be classified as extremely low volatility organic compounds (ELVOCs), and followed by low volatility organic compounds (LVOCs), which implies that condensation of HOMs contributes to the formation of secondary organic aerosol in this particular region.

Diethylene glycol-based aerosol size spectrometers have been extensively employed to measure sub-3 nm size-resolved particle concentrations in recent years. Quantification of the performance of these instruments for accurate measurement of particle size distribution is essential and significant. Comparison between diethylene glycol-based scanning mobility size spectrometer (DEG-SMPS) and scanning mode particle size magnifier (PSM) was performed. Duringthe laboratory calibrations, to obtain more reliable data by the PSM, the particle size bin larger than 4 nm is recommended to be set during calibration to reduce the influence of large particles during the inversion and increase the inverted size range of the PSM results as well. It is shown that the results of PSM and DEG-SMPS are almost consistent when measuring the number concentration of particles below 3 nm. The correlation of the number concentration between the two instruments in this study is high, with r2> 0.75 for each day. However, the slope of the total number concentration of PSM versus that of DEG-SMPS varies from 1.4 to 4.5 in this study, which most likely due to the variations in the chemical composition of newly formed particles. In addition, some non-negligible uncertainties, such as the uncertainty caused by the detection efficiency calibration and the uncertainty in the aerosol charge fraction, may also contribute to the difference in the intercomparison. DEG-SMPS classifies particles with DMA and thus can achieve a superior sizing resolution over PSM. Compared with DEG-SMPS, PSM performs better in environment with low particle concentrations, including the days with weak nucleation, due to the fact that PSM completely avoids the low charging efficiency issue of sub-3 nm particles and is less likely affected by the counting uncertainty of a typical CPC in the SMPS system in“magnification stage”. In general, PSM and DEG-SMPS are both adequate for measuring sub-3 nm particle size distributions to better understand new particle formation processes, even some non-negligible uncertainties in the data need to be solved in future research.

The thermodynamic process of atmospheric aerosol particles is mainly derived from the non-ideal mixing of multiple substances, which includes liquid-liquid phase separation, hygroscopic-volatilization,and non-equilibrium mass transfer. Relevant physical and chemical parameters are the basis for understanding aerosol evolution process, analyzing evolution motivation and predicting evolution path. Thus accurate single particle measurement is the key to obtain these important parameters. In this study, single aerosol droplet capture without contact for a long time is realized by using the self-developed aerosol Raman optical tweezers system, and the hygroscopic-volatilization thermodynamic evolution process of aerosol droplets in the actual atmosphere is simulated by changing the ambient relative humidity around aerosol particles. Through measuring the cavity resonance Raman signal of the single droplet particle and combining with the appropriate physical models, we accurately measured the particle size, refractive index, diffusion coefficient, volatile flux and other important physicochemical parameters of the sodium chloride, sucrose and citric acid in the hygroscopic-volatile process. Moreover, the effects of relative humidity on hygroscopic-volatilization process for organic and inorganic aerosols, as well as the possible phase transitions such as glassy and gel transition, are investigated, which provide an important reference for understanding the hygroscopic-volatilization process of actual atmospheric aerosol.