Abstract. The deliquescence and efflorescence phase transitions of ammonium sulfate aerosols have been studied as a function of relative humidity (RH) over the temperature range from 234 K to 295 K. Polydisperse submicrometer ammonium sulfate particles produced by atomization were monitored in a temperature-controlled flow tube system using Fourier transform infrared spectroscopy. The relative humidity in the aerosol flow was controlled using a sulfuric acid bath conditioner and the addition of a known flow of dry nitrogen. The relative humidity was measured using a dew point hygrometer and infrared absorption features. The deliquescence transition was observed to be nearly independent of temperature, changing from 80% RH at 294.8 K to 82% RH at 258.0 K near the ice saturation line, in good agreement with previous results. The relative humidity at the efflorescence transition also increased slightly (32% to 39%) with decreasing temperature (294.8 K to 234.3 K). These results suggest that once a crystalline ammonium sulfate particle deliquesces, the droplet can exist as a metastable solution droplet over a broad region of temperature and water pressures under the conditions in the upper troposphere. The persistence of metastable ammonium sulfate solution droplets may have important implications for cirrus cloud formation and heterogeneous reaction rates in the upper troposphere.

Between January 12 and February 7, 1987, an outbreak of gastroenteritis affected an estimated 13,000 people in a county of 64,900 residents in western Georgia. Cryptosporidium oocysts were identified in the stools of 58 of 147 patients with gastroenteritis (39 percent) tested during the outbreak. Studies for bacterial, viral, and other parasitic pathogens failed to implicate any other agent. In a random telephone survey, 299 of 489 household members exposed to the public water supply (61 percent) reported gastrointestinal illness, as compared with 64 of 322 (20 percent) who were not exposed (relative risk, 3.1; 95 percent confidence interval, 2.4 to 3.9). The prevalence of IgG to cryptosporidium was significantly higher among exposed respondents to the survey who had become ill than among nonresident controls. Cryptosporidium oocysts were identified in samples of treated public water with use of a monoclonal-antibody test. Although the sand-filtered and chlorinated water system met all regulatory-agency quality standards, sub-optimal flocculation and filtration probably allowed the parasite to pass into the drinking-water supply. Low-level cryptosporidium infection in cattle in the watershed and a sewage overflow were considered as possible contributors to the contamination of the surface-water supply. We conclude that current standards for the treatment of public water supplies may not prevent the contamination of drinking water by cryptosporidium, with consequent outbreaks of cryptosporidiosis.

Fourier transform-infrared (FTIR) spectroscopy was used to examine films representative of stratospheric sulfuric acid aerosols. Thin films of sulfuric acid were formed in situ by the condensed phase reaction of SO 3 with H20. FTIR spectra show that the sulfuric acid films absorb water while cooling in the presence of water vapor. Using stratospheric water pressures, the most dilute solutions observed were >40 wt % before simultaneous ice formation and sulfuric acid freezing occurred. FTIR spectra also revealed that the sulfuric acid films crystallized mainly as sulfuric acid tetrahydrate (SAT). Crystallization occurred either when the composition was about 60 wt % H2SO 4 or after ice formed on the films at temperatures 1-4 K below the ice frost point. Finally, we determined that the melting point for SAT depended on the background water pressure and was 216-219 K in the presence of 4 x 10 'n Torr H20. Our results suggest that once frozen, sulfuric acid aerosols in the stratosphere are likely to melt at these temperatures, 30 K colder than previously thought.

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