Thermodynamic data are presented for hydrates of nitric acid: HNO 3 ⋅H 2 O, HNO 3 ⋅2H 2 O, HNO 3 ⋅3H 2 O, and a higher hydrate. Laboratory data indicate that nucleation and persistence of metastable HNO 3 ⋅2H 2 O may be favored in polar stratospheric clouds over the slightly more stable HNO 3 ⋅3H 2 O. Atmospheric observations indicate that some polar stratospheric clouds may be composed of HNO 3 ⋅2H 2 O and HNO 3 ⋅3H 2 O. Vapor transfer from HNO 3 ⋅2H 2 O to HNO 3 ⋅3H 2 O could be a key step in the sedimentation of HNO 3 , which plays an important role in the depletion of polar ozone.

Thermodynamic properties of solid phases containing H2O and HCl are examined for conditions expected over Antarctica in winter. It is shown that solid solutions with 0.02–0.035 mole fraction HCl in H2O ice will form in the stratosphere at temperatures between 193° and 190°K. Crystalline HNO3·H2O and/or HNO3·3H2O form in the same temperature range. Thus condensation of small quantities of H2O (<1 ppm) leads to nearly complete removal of both HCl and HNO3 from the gas phase. Formation of ice‐HCl solid solutions provides a favorable setting for the heterogeneous reaction of HCl with ClNO3, initiating a rapid sequence of reactions that converts solid‐phase HCl into gaseous ClNO3 or chlorine oxide radicals. Odd nitrogen species are simultaneously converted into solid‐phase nitrates. The cycle producing reactive chlorine gases is likely to be completed before particles grow large enough to fall from the stratosphere. Hence precipitation of particles from the polar stratospheric clouds is expected to remove odd nitrogen from the stratosphere efficiently, leaving chlorine gases behind. High concentrations of reactive chlorine oxide radicals are rapidly produced by heterogeneous reactions if the initial concentration of HCl exceeds a critical value, 0.5 times the concentration of NOx before onset of condensation. Production of unreactive ClNO3 and HOCl is favored if HCl levels are lower than this value. The onset of Antarctic ozone depletion in the late 1970s may in part reflect growth of HCl levels beyond this threshold.

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Phase changes in stratospheric aerosols were studied by cooling a droplet of sulfuric acid (H(2)SO(4)) in the presence of nitric acid (HNO(3)) and water vapor. A sequence of solid phases was observed to form that followed Ostwald's rule for phase nucleation. For stratospheric partial pressures at temperatures between 193 and 195 kelvin, a metastable ternary H(2)SO(4)-HNO(3) hydrate, H(2)SO(4) . HNO(3) . 5H(2)O, formed in coexistence with binary H(2)SO(4) . kH(2)O hydrates (k = 2, 3, and 4) and then transformed to nitric acid dihydrate, HNO(3) . 2H(2)O, within a few hours. Metastable HNO(3) . 2H(2)O always formed before stable nitric acid trihydrate, HNO(3).3H(2)O, under stratospheric conditions and persisted for long periods. The formation of metastable phases provides a mechanism for differential particle growth and sedimentation of HNO(3) from the polar winter stratosphere.

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A comprehensive investigation of the nitrogen cycle in the Delaware River was carried out using 15N tracers to measure rates for important transformations of nitrogen. Daily, depth-averaged 15N rates for the principal inorganic nitrogen species were consistent with rates derived from longitudinal profiles of concentration in the river.The data indicated that nitrification was a rapid, irreversible sink for NH4+, with export of the product N03-from the study area. Utilization of NO,-by primary producers was negligible, owing to low irradiance levels and to high NH,+ concentrations.The oxygen sag near Philadelphia was found to result from oxygen demand in the water column, with only minor benthic influence. Reaeration provided the major oxygen input. Nitrification accounted for about 1% of the net oxygen demand near Philadelphia but as much as 25% farther downstream.Many rivers and coastal waters act as treatment facilities for large inputs of primary-treated sewage. Microbial processes convert the organic material and inorganic nitrogen to more oxidized forms. Water quality declines owing to low oxygen concentrations, phytoplankton blooms, and high turbidity due to sewage-derived suspended particulate material. Investigation of the nitrogen cycle in such systems must account for the complexity and dynamic nature of the various nitrogen transformations and for the influence of light (Lipschultz et al. 198 5; Stanley and Hobbie 198 1;Dugdale and Goering 1967; Garside 198 l), dissolved oxygen content (Goreau et al. 1980; Lipschultz et al. 198 1;Lipschultz 1984), and flushing rates (Wofsy et al. 198 1).Most investigations of nitrogen cycling have been focused on a restricted subset of metabolic processes, principally phytoplankton uptake of NH,+ and N03-. Recent studies, however, have emphasized the need to expand the number of processes under investigation in order to understand the nitrogen cycle in a system (e.g. see Glibert et al. 1982;Lipschultz et al. 1985; Olson 198 1;McCarthy et al. 1984).We describe here 15N tracer experiments on the Delaware River intended to permit ' This work was funded by NSF grant BSR 83-16359 and by EPA grant R8 10219-01-O to Harvard University. identification of the important processes from an expanded set of nitrogen transformations and to relate the measured rates to observed changes in inorganic nitrogen concentration. Rates were integrated over depth and time to account for their sensitivity to light (Lipschultz et al. 198 5). This permitted quantitative comparisons and identification of the principal processes in the ecosystem. Net production (or loss) rates were then calculated for each inorganic nitrogen species at seven stations along the river near Philadelphia, and net production rates derived from 15N incubations were compared to net rates derived from river concentration data using a mass conservation model. We thank L. Kerkhof for assistance in the laboratory and fieldwork and M. Zahniser for help in design and use of the emission spectrometer.

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Sediments from the Mississippi River estuary were suspended in solutions with a range of salinities and various initial concentrations of phosphate. After 42 days the suspensions had nearly uniform values for the ion activity product of calcium times biphosphate, [Ca2+] [HP0,2-] x 10e9 M2. Similar values were observed for this ion product in the Mississippi River and in the upper estuary, suggesting that the concentration of soluble phosphorus nlay be controlled by an equilibrium with sedimentary material. The data are consistent with a Inechanism where soluble phosphorus is controlled by hydrolysis on the surface of hydroxyapatize particles:Ca,,(PO,),(OH), + 6H2O Z 4[Ca,HPO,(OH),],,,,, + 2Ca2+ + 2HPOd2-.Phosphorus levels in the lower estuary are controlled primarily by tlilution with low-nutrient waters from-the Gulf of Mexico.