Characterizing airborne nanoparticles in six Chinese cities based on their interactions with natural air ions†

Airborne nanoparticles (NPs) are particles with a diameter smaller than 100 nm, which can significantly influence global climate, regional air quality, and human health. The interactions between airborne nanoparticles and atmospheric ions are ubiquitous, which also condition the charge state of nanoparticles. To deepen our understanding of nanoparticles in different regions of China and explore their interactions with air ions, we conducted a one-year measurement of airborne nanoparticle number size distributions in six Chinese cities. Six homemade bipolar scanning mobility particle sizers were applied to scan both positively and negatively charged nanoparticles. The annual average number concentrations of nanoparticles (N_NPs) are 5880 ± 3140 # cm⁻³ (Beijing), 6280 ± 2910 # cm⁻³ (Shanghai), and 5440 ± 3370 # cm⁻³ (Wuhan) in the three urban sites, and 5320 ± 3440 # cm⁻³ (Shenzhen), 3440 ± 2370 # cm⁻³ (Zhuhai) and 2440 ± 1870 # cm⁻³ (Kunming) in the three suburban sites. N_NPs account for 65.6%–80.4% of the total particle number concentration in the six cities. Besides, N_NPs contributed by new particle formation in suburban areas are comparable to or even higher than those in urban areas. In Beijing and Shanghai, N_NPs decreased by 55.2% and 66.4% from 2013 to 2023, respectively. Ion mobility, composition, and concentration are the parameters governing the charge state of nanoparticles. In Beijing, we found that the ion mobility distribution and nanoparticle charge state vary at the same time, and the composition of negative cluster ions are mainly composed of inorganic nitrogen-containing ions, inorganic sulfur-containing ions, and organic ions.