Quantitative structure-activity relationships (QSARs) are developed relating initial and final pseudo-first-order disappearance rate constants of 45 halogenated aromatic hydrocarbons in anoxic sediments to four readily available molecular descriptors: the carbon-halogen bond strength, the summation of the Hammett sigma constants of the additional substituents, the summation of the inductive constants of these substituents, and the steric factors of the additional substituents.Comparison of the "initial" and "final" QSARs suggests that the same agent is involved as the reductant in both processes. The rate constants for reduction of seven haloaromatics obtained in 10 different sediment samples are correlated with the organic carbon content of the samples. The correlations are enhanced by taking into account the fraction of the compounds sorbed to the solid phase.

Quantitative structure‐activity relationships (QSARs) are developed relating initial and final pseudo‐first‐order disappearance rate constants of 45 halogenated aromatic hydrocarbons in anoxic sediments to four readily available molecular descriptors: the carbon‐halogen bond strength, the summation of the Hammett sigma constants of the additional substituents, the summation of the inductive constants of these substituents, and the steric factors of the additional substituents. Comparison of the “initial” and “final” QSARs suggests that the same agent is involved as the reductant in both processes. The rate constants for reduction of seven haloaromatics obtained in 10 different sediment samples are correlated with the organic carbon content of the samples. The correlations are enhanced by taking into account the fraction of the compounds sorbed to the solid phase. Biotransformation

The degradation of several classes of halogenated aromatic hydrocarbons in methanogenic sediment‐water systems was examined in laboratory studies. Several transformation processes were shown to occur, leading to formation of a variety of products. In the study a clear distinction could be made between transformation processes taking place immediately upon incubation of the compounds and processes taking place after a characteristic period of varying length. γ‐Irradiation was used to quench biological reactions and thus assess the impact of those processes on the overall rates of degradation of the studied compounds. The kinetics of both the initial and the final processes were pseudo‐first‐order, with half‐lives ranging from <0.46 d to no detectable degradation for a number of multiply halogenated benzenes. Compounds were selected with the aim of obtaining a maximum variation in chemical reactivity and physical properties. The sediments were selected with the aim of assessing several environmental factors influencing the kinetics of reduction.

Abstract. In this paper we present the continued development of the miniDOAS, an active differential optical absorption spectroscopy (DOAS) instrument used to measure ammonia concentrations in ambient air. The miniDOAS has been adapted for use in the Dutch National Air Quality Monitoring Network. The miniDOAS replaces the life-expired continuous-flow denuder ammonia monitor (AMOR). From September 2014 to December 2015, both instruments measured in parallel before the change from AMOR to miniDOAS was made. The instruments were deployed at six monitoring stations throughout the Netherlands. We report on the results of this intercomparison.Both instruments show a good uptime of ca. 90 %, adequate for an automatic monitoring network. Although both instruments produce 1 min values of ammonia concentrations, a direct comparison on short timescales such as minutes or hours does not give meaningful results because the AMOR response to changing ammonia concentrations is slow. Comparisons between daily and monthly values show good agreement. For monthly averages, we find a small average offset of 0.65 ± 0.28 µg m −3 and a slope of 1.034 ± 0.028, with the miniDOAS measuring slightly higher than the AMOR. The fast time resolution of the miniDOAS makes the instrument suitable not only for monitoring but also for process studies.

Rates of photodegradation of 13 structurally related 1,3-di-and 1,3,5-trisubstituted halogenated benzene derivatives in dilute aqueous solution were measured in the laboratory at wavelengths ranging from 250 to 360 nm. As shown by mathematical simulations of the reaction rate constants involved, photohydrolysis of all compounds studied is the main transformation process, generally accounting for over 95% of the amount of starting material removed. Quantum yields of this process were calculated using 3-chlorophenol as a reference, taking into account both the absorption characteristics of the compounds studied and the spectral energy distribution of the light source used. In an attempt to develop a structure-reactivity relationship, measured quantum yields of photohydrolysis were correlated to a number of readily available molecular descriptors. As shown by statistical analysis, the best correlation was obtained using a combination of the following descriptors: (1) the carbon-halogen bond strength and (2) the summation of the steric factors of all substituents.