Phase changes of ambient particles in the Southern Great Plains of Oklahoma

A new instrument, a 1 × 3 tandem differential mobility analyzer (1 × 3-TDMA), was deployed in June 2007 in the Southern Great Plains, Oklahoma, USA to study the phase of ambient particles. Its primary measurement, the irreversibility of the hygroscopic growth factor, is obtained by reversibly cycling relative humidity (RH) by ±8% and testing for irreversible changes in diameter. In 101 runs, efflorescence occurred 72% of the time for particles sampled at ambient RH. Deliquescence occurred in 13% of the runs. The more frequent occurrence of efflorescence compared to deliquescence was explained at least in part by the distribution of ambient RH, which had a median of 80% and quartiles of 65% and 93% RH. The deliquescence and efflorescence events were nearly exclusive from one another and could be separated by Min[RH Ambient, Inlet RH] <40% for deliquescence and Max[RH Ambient, Inlet RH] >50% for efflorescence. In outlook, the data set from the 1 × 3-TDMA regarding the phase and hence water content of ambient particles can be used for validating regional chemical transport models of particle phase.