Grazing-angle Fourier transform infrared reflection absorption
spectroscopy was used to monitor the free OH
stretch (or “dangling bond”) in vapor-deposited
H2O−ice films between 94 and 120 K. Ice film
thicknesses
and the sensitivity of our instrument to water−ice molecules were
determined by optical interference using
a helium−neon laser. These calibrations indicate that the
dangling bond signals observed in the present
study are indicative of the surfaces of micropores present within the
amorphous ice bulk. The largest dangling
bond signal (corresponding to the largest number of micropores) was
observed at 94 K under conditions of
fast ice growth while the smallest signal was observed at 120 K under
conditions of slow growth. The
temperature and pressure dependence of the dangling bond signal during
film growth was used to estimate
a barrier to diffusion (E
dif) for
H2O on amorphous ice. We measured an upper limit of
E
dif = 4.2 (±0.5) kcal
mol-1, consistent with a theoretically
derived value of E
dif = 2.5−3 kcal
mol-1. The decay of the
dangling
bond over time (corresponding largely to the collapse of the
micropores) was monitored in ice films roughly
100 nm thick. With initial deposition rates of 2 nm
s-1, the decay took 125 and 175 min at 118
and 112 K,
respectively. Faster deposition rates and colder temperatures
decreased the decay rate.
Cloudwater samples have been collected for the first time at a high-elevation site in the US interior Southwest. Cloud samples were collected at the summit of Mt. Elden near Flagstaff, Arizona. The samples were analyzed for pH, ionic composition, trace metals, organic carbon content, and volatile organic compounds. All of the samples showed high pH values (5.12-6.66), which appear to be the result of soil/crustal acid-neutralizing components. Ammonium and nitrate were the dominant ionic species. Organic carbon concentrations ranged from 3 to 18 mg/l. Volatile aromatic compounds (toluene, ethylbenzene, and xylenes) were detected, although they did not contribute significantly to the dissolved organic matter (<1% of dissolved organic carbon). Still, their aqueous-phase concentrations were substantially higher than equilibrium partitioning from the gas phase would suggest. Metal concentrations were high when compared to other cloud studies in remote areas. Overall, with the exception of pH, the cloud chemistry showed marked inter-event variability. The source of the variability was investigated using NOAA HYSPLIT dispersion calculations. Like the cloud composition, the air mass back trajectories differed widely from event to event, and consistently, air masses that passed over highly urbanized areas had higher trace metal, organic, and ion concentrations than more pristine air masses.
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