Inhibition of efflorescence in mixed organic-inorganic particles at temperatures less than 250 K.

It is now well recognized that mixed org.-inorg. particles are abundant in the atm. While there have been numerous studies of efflorescence of mixed org.-inorg. particles close to 293 K, there are only a few at temps. less than 273 K. Understanding the efflorescence properties of these particles at temps. less than 273 K could be esp. important for predicting ice nucleation in the upper troposphere. The efflorescence properties were studied of mixed citric acid-ammonium sulfate particles as a function of temp. to better understand the efflorescence properties of mixed org.-inorg. particles in the middle and upper troposphere. Data for 293 K illustrate that the addn. of citric acid decreases the efflorescence relative humidity of ammonium sulfate, which is consistent with the trends obsd. with other systems contg. highly oxygenated org. compds. At low temps. the trend is qual. the same, but efflorescence can be inhibited by smaller concns. of citric acid. For example at temps. <250 K an org. mass/(org. mass + sulfate mass) of only 0.33 is needed to inhibit efflorescence of ammonium sulfate. In the upper troposphere the org. mass/(org. mass + sulfate mass) can often be larger than this value. As a result, particles in the upper troposphere may be more likely to remain in the liq. state than previously thought and solid ammonium sulfate may be less likely to participate in heterogeneous ice nucleation in the upper troposphere. Addnl. studies are required on other model org. systems. [on SciFinder(R)]