The Campaign on Atmospheric Aerosol Research network of China (CARE-China) is a long-term project for the study of the spatio-temporal distributions of physical aerosol characteristics as well as the chemical components and optical properties of aerosols over China. This study presents the first long-term data sets from this project, including 3 years of observations of online PM_2.5 mass concentrations (2012–2014) and 1 year of observations of PM_2.5 compositions (2012–2013) from the CARE-China network. The average PM_2.5 concentration at 20 urban sites is 73.2 µg m⁻³ (16.8–126.9 µg m⁻³), which was 3 times higher than the average value from the 12 background sites (11.2–46.5 µg m⁻³). The PM_2.5 concentrations are generally higher in east-central China than in the other parts of the country due to their relatively large particulate matter (PM) emissions and the unfavourable meteorological conditions for pollution dispersion. A distinct seasonal variability in PM_2.5 is observed, with highs in the winter and lows during the summer at urban sites. Inconsistent seasonal trends were observed at the background sites. Bimodal and unimodal diurnal variation patterns were identified at both urban and background sites. The chemical compositions of PM_2.5 were analysed at six paired urban and background sites located within the most polluted urban agglomerations – North China Plain (NCP), Yangtze River delta (YRD), Pearl River delta (PRD), North-east China region (NECR), South-west China region (SWCR) – and the cleanest region of China – the Tibetan Autonomous Region (TAR). The major PM_2.5 constituents across all the urban sites are organic matter (OM, 26.0 %), SO₄²⁻ (17.7 %), mineral dust (11.8 %), NO₃⁻ (9.8 %), NH₄⁺ (6.6 %), elemental carbon (EC) (6.0 %), Cl⁻ (1.2 %) at 45 % RH and unaccounted matter (20.7 %). Similar chemical compositions of PM_2.5 were observed at background sites but were associated with higher fractions of OM (33.2 %) and lower fractions of NO₃⁻ (8.6 %) and EC (4.1 %). Significant variations of the chemical species were observed among the sites. At the urban sites, the OM ranged from 12.6 µg m⁻³ (Lhasa) to 23.3 µg m⁻³ (Shenyang), the SO₄²⁻ ranged from 0.8 µg m⁻³ (Lhasa) to 19.7 µg m⁻³ (Chongqing), the NO₃⁻ ranged from 0.5 µg m⁻³ (Lhasa) to 11.9 µg m⁻³ (Shanghai) and the EC ranged from 1.4 µg m⁻³ (Lhasa) to 7.1 µg m⁻³ (Guangzhou). The PM_2.5 chemical species at the background sites exhibited larger spatial heterogeneities than those at urban sites, suggesting different contributions from regional anthropogenic or natural emissions and from long-range transport to background areas. Notable seasonal variations of PM_2.5-polluted days were observed, especially for the megacities in east-central China, resulting in frequent heavy pollution episodes occurring during the winter. The evolution of the PM_2.5 chemical compositions on polluted days was consistent for the urban and nearby background sites, where the sum of sulfate, nitrate and ammonia typically constituted much higher fractions (31–57 %) of PM_2.5 mass, suggesting fine-particle pollution in the most polluted areas of China assumes a regional tendency, and the importance of addressing the emission reduction of secondary aerosol precursors including SO₂ and NO_x. Furthermore, distinct differences in the evolution of [NO₃⁻]∕[SO₄²⁻] ratio and OC∕EC ratio on polluted days imply that mobile sources and stationary (coal combustion) sources are likely more important in Guangzhou and Shenyang, respectively, whereas in Beijing it is mobile emission and residential sources. As for Chongqing, the higher oxidation capacity than the other three cities suggested it should pay more attention to the emission reduction of secondary aerosol precursors. This analysis reveals the spatial and seasonal variabilities of the urban and background aerosol concentrations on a national scale and provides insights into their sources, processes and lifetimes.

Hygroscopicity is one of the most important physicochemical properties of aerosol particles and also plays indispensable roles in many other scientific and technical fields. A myriad of experimental techniques, which differ in principles, configurations and cost, are available for investigating aerosol hygroscopicity under subsaturated conditions (i.e., relative humidity below 100 %). A comprehensive review of these techniques is provided in this paper, in which experimental techniques are broadly classified into four categories, according to the way samples under investigation are prepared. For each technique, we describe its operation principle and typical configuration, use representative examples reported in previous work to illustrate how this technique can help better understand aerosol hygroscopicity, and discuss its advantages and disadvantages. In addition, future directions are outlined and discussed for further technical improvement and instrumental development.

Calcium- and magnesium-containing salts are important components for mineral dust and sea salt aerosols, but their physicochemical properties are not well understood yet. In this study, hygroscopic properties of eight Ca- and Mg-containing salts, including Ca(NO₃)₂⚫4H₂O, Mg(NO₃)₂⚫6H₂O, MgCl₂⚫6H₂O, CaCl₂⚫6H₂O, Ca(HCOO)₂, Mg(HCOO)₂⚫2H₂O, Ca(CH₃COO)₂⚫H₂O and Mg(CH₃COO)₂⚫4H₂O, were investigated using two complementary techniques. A vapor sorption analyzer was used to measure the change of sample mass with relative humidity (RH) under isotherm conditions, and the deliquescence relative humidities (DRHs) for temperature in the range of 5–30 ^∘C as well as water-to-solute ratios as a function of RH at 5 and 25 ^∘C were reported for these eight compounds. DRH values showed large variation for these compounds; for example, at 25 ^∘C DRHs were measured to be ∼ 28.5 % for CaCl₂⚫6H₂O and >95 % for Ca(HCOO)₂ and Mg(HCOO)₂⚫2H₂O. We further found that the dependence of DRH on temperature can be approximated by the Clausius–Clapeyron equation. In addition, a humidity tandem differential mobility analyzer was used to measure the change in mobility diameter with RH (up to 90 %) at room temperature, in order to determine hygroscopic growth factors of aerosol particles generated by atomizing water solutions of these eight compounds. All the aerosol particles studied in this work, very likely to be amorphous under dry conditions, started to grow at very low RH (as low as 10 %) and showed continuous growth with RH. Hygroscopic growth factors at 90 % RH were found to range from 1.26 ± 0.04 for Ca(HCOO)₂ to 1.79 ± 0.03 for Ca(NO₃)₂, and the single hygroscopicity parameter ranged from 0.09–0.13 for Ca(CH₃COO)₂ to 0.49–0.56 for Ca(NO₃)₂. Overall, our work provides a comprehensive investigation of hygroscopic properties of these Ca- and Mg-containing salts, largely improving our knowledge of the physicochemical properties of mineral dust and sea salt aerosols.

Aerosol deposition is a major source of soluble Fe in open oceans, affecting marine biogeochemistry and primary production. However, Fe fractional solubility, a key parameter in estimating deposition fluxes of soluble aerosol Fe, is still highly uncertain. Abundance and fractional solubility of aerosol Fe in fine (1 μm) particles was measured at Qingdao (a coastal city in northern China) in November‐December 2019. Average concentrations of total and soluble Fe were found to be 798 ± 466 and 7.7 ± 14.5 ng/m³ in coarse particles, and 801 ± 534 and 7.3 ± 7.6 ng/m³ in fine particles. Fe solubility was significantly lower in coarse particles (average: 0.80 ± 1.03%) than fine particles (average 1.29 ± 1.41%). Compared to clean days, total Fe concentration was substantially increased during dust and haze days; however, Fe solubility was significantly reduced in dust days and elevated in haze days. Acid processing significantly enhanced Fe solubility in both fine and coarse particles, and the contribution of primary emission to Fe solubility enhancement was important for fine particles but minor for coarse particles. Higher Fe solubility (>1%) in fine and coarse particles was usually observed at high aerosol acidity (pH < 4) and high RH (>60%), suggesting critical roles of aerosol acidity and water content in regulating aerosol Fe solubility.