Direct measurement of the hydration state of ambient aerosol populations

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. Throughout the study period the sampled aerosol had been exposed to a relative humidity (RH) greater than 80% before or soon after sunrise, while the ambient RH during sampling ranged from about 50 to 70%. To examine any possible size dependence, while also maximizing time resolution, 160 and 320 nm particles were analyzed. To assist in the interpretation of these data, a second, RH-scanning, TDMA was concurrently operated to directly characterize the deliquescence and crystallization properties of the aerosol. The hygroscopic growth of the more hygroscopic population of 160 and 320 nm particles was below that predicted for pure inorganics such as ammonium sulfate, suggesting the sampled particles were composed of mixtures of soluble and insoluble compounds. Although there were slight variations in the measured properties over the course of the study, those particles that were observed to exhibit hygroscopic growth hysteresis were almost always present as metastable aqueous solutions.