A Differential Mobility Analyzer/Tandem Differential Mobility Analyzer (DMA/TDMA) system was used to measure simultaneously the size distribution and hygroscopicity of the ambient aerosol population. The system was operated aboard the National Center for Atmospheric Research/National Science Foundation (NCAR/NSF) C-130 during the 2006 Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign followed by the 2006 Intercontinental Chemical Transport Experiment – Phase B (INTEX-B) field campaign.

The research flights for the MILAGRO campaign were conducted within the Mexico City basin and the region to the northeast within the pollution plume. The aerosol within the basin is dominated by organics with an average measured κ value of 0.21 +/- 0.18, 0.13 +/- 0.09, 0.09 +/- 0.06, 0.14 +/- 0.07, and 0.17 +/- 0.04 for dry particle diameters of 0.025, 0.050, 0.100, 0.200, and 0.300 μm, respectively. As the aerosols are transported away from the Mexico City Basin, secondary organic aerosol formation through oxidation and condensation of sulfate on the aerosols surface rapidly increases the solubility of the aerosol. The most pronounced change occurs for a 0.100 μm diameter aerosol where, after 6 hours of transport, the average κ value increased by a factor of 3 to a κ of 0.29 +/- 0.13. The rapid increase in solubility increases the fraction of the aerosol size distribution that could be activated within a cloud.

The research flights for the INTEX-B field campaign investigated the evolution of the physicochemical properties of the Asian aerosol plume after 3 to 7 days of transport. The Asian aerosol within the free troposphere exhibited a bimodal growth distribution roughly 50% of the time. The more soluble mode of the growth distribution contributed between 67-80% of the overall growth distribution and had an average κ between 0.40 and 0.53 for dry particle diameters of 0.025, 0.050, 0.100, and 0.300 μm. The secondary mode was insoluble with an average κ between 0.01 and 0.05 for all dry particle diameters. Cloud condensation nuclei closure was attained at a supersaturation of 0.2% for all particles within the free troposphere by either assuming a pure ammonium bisulfate composition or a binary composition of ammonium bisulfate and an insoluble organic.