The yield of particle mass in secondary organic aerosol (SOA) formed by dark ozonolysis was measured for 0.3-22.8 ppbv of reacted alpha-pinene. Most experiments were conducted using a continuous- flow chamber, allowing nearly constant SOA concentration and chemical composition for several days. For comparison, some experiments were also conducted in batch mode. Reaction conditions were 25 degrees C, 40% RH, dry (NH4) SO4 seed particles, and excess 1-butanol. The organic particle loading was independently measured by an aerosol mass spectrometer and a scanning mobility particle sizer, and the two measurements agreed well. The observations showed that SOA formation occurred for even the lowest reacted alpha-pinene concentration of 0.3 ppbv. The particle mass yield was 0.09 at 0.15 mu gm(-3), increasing to 0.27 at 40 mu gm(-3). Compared to some results reported in the literature, the yields were 80 to 100% larger for loadings above 2 mu gm(-3). At lower loadings, the yields had an offset of approximately + 0.07 from those reported in the literature. To as low as 0.15 mu g m(-3), the yield curve had no inflection point toward null yield, implying the formation of one or several products having vapor pressures below this value. These observations of increased yields, especially for low loadings, are potentially important for accurate prediction by chemical transport models of organic particle concentrations in the ambient atmosphere.

A new instrument, a 1 x 3 tandem differential mobility analyzer (1 x 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 x 3-TDMA regarding the phase and hence water content of ambient particles can be used for validating regional chemical transport models of particle phase. Citation: Martin, S. T., T. Rosenoern, Q. Chen, and D. R. Collins (2008), Phase changes of ambient particles in the Southern Great Plains of Oklahoma, Geophys. Res. Lett., 35, L22801, doi: 10.1029/2008GL035650.