Sampling for Purgeable Organic Compounds Using Positive-Displacement Piston and Centrifugal Submersible Pumps: A Comparative Study

Positive-displacement piston pumps that minimize sample agitation have no apparent advantage over centrifugal submersible pumps when used to collect ground water samples for analysis of low concentrations of purge-able organic compounds. Analytical uncertainties inherent in laboratory environments appear to influence analytical results of low-concentration purgeable organic compound samples more than either pump type or sampling team. Centrifugal submersible pumps are at least equally efficient as positive-displacement piston pumps in the recovery of carbon tetrachloride, 1,1,1-trichloroethane, trichloroethylene, and chloroform after sampling and analytical influences are made constant.     

Cation ordering of orthopyroxenes from the Skaergaard Intrusion: implications for the subsolidus cooling rates and permeabilities

 We have determined the quenched cation ordering states of five orthopyroxene crystals collected from the marginal border group and the lower zone a and b of the Skaergaard intrusion, and modeled these data to retrieve their closure temperatures (T _c) of Fe–Mg ordering and cooling rates. According to existing thermal models for the Skaergaard pluton, conductive cooling dominated the high and low temperature regimes, which were separated by an intermediate temperature regime in which the cooling was controlled primarily by convective fluid circulation. The cooling rates retrieved from the quenched cation ordering states of the orthopyroxene crystals strictly apply to temperatures around the closure temperatures of the ordering states, ∼340–400^° C, which fall at the transition from convective to the lower temperature conductive cooling. The cooling rates obtained from the cation ordering states of orthopyroxene vary from ∼1 to 270 K/ka. These results agree well with a thermal model calculated using an assumed average permeability of 10^-12 cm² for the pluton, but not completely with a model calculated on the basis of an average permeability of 10^-13 cm², although both values produced shifts of δ¹⁸O that are comparable to those observed in the pluton. Received: 27 February 1995/Accepted: 20 July 1995     

The construction of self-consistent models for outflowing stellar envelopes

The article presents self-consistent computations of the spherically symmetric envelope of a massive (M=20M _⊙) star whose envelope is flowing outward under the action of continuum radiation pressure. A system of equations describing the out flow for arbitrary optical depth, degree of ionization, and opacity κ are obtained. A numerical solution is obtained for the case of constant κ and degree of ionization via approaching integrations from the singularities and smooth joining in the intermediate region. Asymptotic solutions are found in the vicinity of the sonic point and the singularity at infinity.     

Measurements of solar occulation: the error in a naive retrieval if the constituent's concentration changes

The stratospheric concentrations of many minor constituents change rapidly at sunrise or sunset. If this happens, there is an inherent error when retrieving the vertical profiles of the constituents from measurements of their absorption of sunlight. For retrievals of NO at sunset the error can be estimated from in-situ measurements alone, without appeal to a model of stratospheric photochemistry. Below 20 km this error can approach 100% so that the retrieved NO is zero. But at 40 km, and at 25 km when the absorption is strong and Lorentzian, it can be less than 20%. Precise calculations of the error, even if small, require model calculations of the sunset and sunrise changes. With a model, we have calculated the error for NO, NO₂, OH and ClO.