Chemical characterization of water-soluble species in PM2.5 aerosols in Beijing, China

In recent years, Beijing has been long suffered from heavy air pollution with markedly high concentrations of fine particles (the aerodynamic diameter of particles ≤ 2.5 <; ×m, PM
_2.5
) in the atmosphere. The chemical species of particles are key indicators for sources apportionment, which are complicated and still not understood in details. To characterize the PM
_2.5
and chemical compositions in Beijing, daily samples of PM
_2.5
were collected in January (winter), April (spring), July (summer) and October (autumn) in 2014. The gravimetric mass and water soluble ions including five anions (SO
₄
²
, NO
₃
^-
, Cl
^-
) and three cations (K
⁺
, Ca
²⁺
, Mg
²⁺
, Na
⁺
, NH
₄
⁺
) were analyzed. The annual mean PM
_2.5
mass concentration was 105.9 ± 42.9 μg m
^-3
in 2014. The concentration was the highest (164.5 ± 115.5 ±g m
^-3
) in autumn, which was 1.5, 2.3 and 2.2 times higher than those in spring, summer and winter, respectively. And the water-soluble ions were the major components of fine particles, which accounted for 48.1 ± 12.5% of annual mean total particulate mass. The estimated result of ion balance showed the aerosol was acidic in haze days during sampling period. Secondary inorganic aerosol were the dominant parts, contributing 85.0%, 87.5%, 95.3% and 93.2% to the total measured ions in winter, spring, summer and autumn, respectively. Sulfate had the highest concentration in autumn, while the lowest concentration was in spring. The concentrations of nitrate and ammonium also were highest in autumn, which had the lowest concentration in summer. Additionally, the highest concentration of K
⁺
was the result of the large-scale straw burning events in autumn in current study.