Size-resolved carbonaceous components and water-soluble ions measurements of ambient aerosol in Beijing.

A MOUDI-120 sampler was used in Beijing to collect multi-stage samples in the summer and winter of 2013 to 2015. Thirty-three sample sets were collected during the daytime, nighttime, and different pollution levels. The actual relative humidity in the impactors was calculated for the first time. The carbonaceous components (organic and elemental carbon, OC and EC, respectively) and water-soluble inorganic ions (Na(+), NH4(+), K(+), Mg(2+), Ca(2+), Cl(-), NO3(-), and SO4(2-)) were analyzed in each sample. The characteristics of the mass concentration distribution and charge balance were discussed. On the basis of relative humidity in the impactors, aerosols less than 1.0μm were sampled under relatively dry conditions in most cases. The concentration levels for the chemical species were higher in the winter than in the summer. Three modes (condensation mode, droplet mode, and coarse mode) could be identified from the distributions of NH4(+), NO3(-), SO4(2-), Cl(-), K(+), OC and EC. The distribution characteristics for the pollution dissipation process were different from the pollution accumulation process. NO3(-) and NO2(-) contributed most of the negative electric charges in the stage below 0.1μm. In the condensation mode, the cations were dominated by NH4(+), which was sufficient to balance the anions. In the droplet mode of the heavily polluted samples, the ammonium was not sufficient to balance the anions. In the coarse mode, the positive electric charges were primarily composed of metal cations. The analyzed anions were not sufficient to neutralize the measured cations.