Dust composition and mixing state inferred from airborne composition measurements during ACE-Asia C130 Flight #6

In this study 3.5 min average bulk aerosol composition data recorded from aircraft during ACE-ASIA C130 Flight #6 are used to investigate the extent of fine particle (Dp 50 < 1.3 μm) mixing among various inorganic aerosol particle constituents. Dust-laden air masses that had mixed with urban air pollutants over the Beijing and Tianjin region were subsequently recorded in the marine boundary layer over the Yellow Sea during the ACE-ASIA mission. An analysis of correlations and molar ratios between cations and anions suggest that dust particles are largely composed of CaCO3 (i.e., the dust particles are fresh), and that NH4+ and K+ are likely associated with SO42− and NO3−. The results point to an external mixture of mineral dust and urban pollutant particles. Aerosol simulations carried out assuming various degrees of internal mixing (by Ca2+ mass) between dust and urban particles indicate the observed 2NH4+–SO42− association exists only when most of the fine-mode dust particles exist externally to SO42−. In this case, the degree of external mixing (by Ca2+ mass) is estimated to be larger than 70%.