During the summer of 2002, a modified tandem differential mobility analyzer (TDMA) was used to examine the size-resolved hydration state of the ambient aerosol in Southeast Texas. Although there were slight variations in the measured properties over the course of the study, the deliquescent particles observed were almost always present as metastable aqueous solutions. A relative humidity (RH) scanning TDMA system was used to measure the deliquescence/crystallization properties of ambient aerosol populations in the same region. During August, sampling was conducted at a rural site in College Station, and in September at an urban site near the Houston ship channel. Measurements from both sites indicate cyclical changes in the composition of the soluble fraction of the aerosol, which are not strongly linked to the local aerosol source. The observations show that as temperature increases and RH decreases, the hysteresis loop describing the RH-dependence of aerosol hygroscopic growth collapses. It is proposed that this collapse is due to a decrease in the ammonium to sulfate ratio in the aerosol particles, which coincides with increasing temperature and decreasing RH. This cyclical change in aerosol acidity may influence secondary organic aerosol (SOA) production and may exaggerate the impact of the aerosol on human health. The compositional changes also result in a daily cycle in crystallization RH that is in phase with that of the ambient RH, which reduces the probability that hygroscopic particles will crystallize in the afternoon when the ambient RH is a minimum. During June and July of 2004 airborne measurements of size-resolved aerosol hygroscopic properties were made near Monterey, California. These were used to examine the change in soluble mass after the aerosol had been processed by cloud. The calculated change in soluble mass after cloud-processing ranged from 0.66 g m-3 to 1.40 g m-3. Model calculations showed these values to be within the theoretical bounds for the aerosols measured. Mass light-scattering efficiencies were calculated from both an averaged aerosol size distribution and from distributions modified to reflect the effects of cloud. These calculations show that the increase in mass light-scattering efficiency should be between 6% and 14%.

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