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.

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.

The cloud condensation nucleus (CCN) activity of organic-sulfate particles was investigated using a steadystate environmental chamber. The organic component consisted of secondary organic aerosol (SOA) generated in the dark from 24 +/- 2 ppb alpha-pinene for conditions of 300 +/- 5 ppb ozone, 40 +/- 2% relative humidity, and 25 +/- 1 degrees C, with the organic mass loading in the chamber ranging from 23 to 37 mu g m(-3). CCN analysis was performed for 80-to 150-nm particles having variable organic-sulfate volume fractions, which were estimated from the diameter of the organic-sulfate particle relative to that of the seed as well as independently from mass spectra. Critical supersaturation, which increased for greater SOA volume fraction and smaller particle diameter, was well predicted by a Kohler model having two components, one for ammonium sulfate and another for SOA. The entire data set could be successfully modeled by a single suite of effective chemical parameters for SOA. The results suggest that the effects of limited organic solubility in mixed SOA-sulfate particles may be reliably omitted in the treatment of cloud droplet formation.

The heterogeneous reaction of NO2 on NaCl particles has been investigated with the new sample preparation and the mode of gas-solid free diffusion. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is used to characterize the adsorbed products in situ, combined with Ion Chromatographic (IC), X-ray Photoelectron Spectroscopy (XPS) and Scan Electron Microscopy (SEM). Our results indicate that the reaction is not limited to the surface of the NaCl particles, but penetrated into the upper layers. Surface reactive sites determine the reaction. Kinetic measurements show that nitrate formation on sodium chloride is second order in NO2 concentration and reactive uptake coefficient is (1.54 +/- 0.70) x 10(-5)