Ion chemistry for atmospheric size-segregated aerosol and depositions at an offshore site of Yangtze River Delta region, China

- Size distribution of ions is studied at an offshore site of a heavy polluted region.
- Characteristic of ions during a haze-precipitation-fog transition course is analyzed.
- The anion to cation ratios, Cl− depletion and nss-SO42 −/NO3− ratios are size-dependant.
- Sea salt can be ignored at this site with its mass percentage in PM10 of 0.12%-2.1%.
- Dry deposition fluxes and ion concentrations exhibit multi-linear form correlation.

An intensive sampling campaign was conducted in the autumn of 2012 to study the size distribution of inorganic ions and ion deposition at an offshore site of Yangtze River Delta region (YRD), China. Particles in < 0.43, 0.43-0.65, 0.65-1.1, 1.1-2.1, 2.1-3.3, 3.3-4.7, 4.7-5.8, 5.8-9.0 and 9.0-10.0 μm were collected and analyzed by ion chromatography for NH4+, Na+, Mg2 +, K+, Ca2 +, F−, Cl−, NO3− and SO42 −. The average mass concentrations in the nine particle sizes ranged from 13.1 to 38.7 μg m− 3, accounting for 5.5%-16.8% of the total mass. Concentrations exhibit bi-modal distribution, peaking at 0.65-1.1 μm and 3.3-4.7 μm. PM1.1, PM2.1 and PM2.1-10 account for 41%, 56% and 44% of PM10 implying the dominance of finer particles. Different ions hold different size distribution patterns. The concentrations of NO3−, SO42 −, NH4+, K+ and Ca2 + exhibit the highest values when compared to literature values indicating the serious air pollution situation in YRD. A haze-precipitation-fog transition course is captured. The precipitation can decrease all the particles in the nine sizes by 20%-62% and the fog formation process can promote the particles' accumulation, with their mass concentrations increasing by 26%-232% except for those in 5.8-9.0 μm and 9.0-10 μm. Mass percentages of SO42 −, NO3− and NH4+ exhibit decreases during precipitation and increases during fog formation course. The anion to cation ratios are lower than 1.0 indicating the anion deficiency and they exhibit significant negative relationship with the particle diameter. The Cl-dep (%) is higher for particles larger than 0.65 μm, increasing from 1.3% to 61% with size added. Sea salt only accounts for 0.12%-2.1% of PM10. The ratios of nss-SO42 −/NO3− decrease with increasing particle size, from 1.68 to 0.69, and are well fit as linear form (r = − 0.74, p = 0.02). It indicates that stationary sources make more contributions to smaller particles and vehicle emission is more important for larger particles. The dry deposition fluxes for F−, Cl−, NO3− and SO42 − are 3.10, 0.59, 22.99 and 17.04 mg/(m2 d). They exhibit multi-linear correlation with corresponding ion concentrations and the coefficients are higher for coarse particles. More researches are needed to obtain the distribution, transportation and deposition characteristic of ions at Chinese coastal area.