Measurements of OVOC fluxes by eddy covariance using a proton-transfer-reaction mass spectrometer – method development at a coastal site

We present here vertical fluxes of oxygenated volatile organic compounds (OVOCs) measured with eddy covariance (EC) during the period of March to July 2012 near the southwest coast of the United Kingdom. The performance of the proton-transfer-reaction mass spectrometer (PTR-MS) for flux measurement is characterized, with additional considerations given to the homogeneity and stationarity assumptions required by EC. Observed mixing ratios and fluxes of OVOCs (specifically methanol, acetaldehyde, and acetone) vary significantly with time of day and wind direction. Higher mixing ratios and fluxes of acetaldehyde and acetone are found in the daytime and from the direction of a forested park, most likely due to light-driven emissions from terrestrial plants. Methanol mixing ratio and flux do not demonstrate consistent diel variability, suggesting sources in addition to plants. We estimate air-sea exchange and photochemical rates of these compounds, which are compared to measured vertical fluxes. For acetaldehyde, the mean (1 sigma) mixing ratio of 0.13 (0.02) ppb at night may be maintained by oceanic emission, while photochemical destruction out-paces production during the day. Air-sea exchange and photochemistry are probably net sinks of methanol and acetone in this region. Their nighttime mixing ratios of 0.46 (0.20) and 0.39 (0.08) ppb appear to be affected more by terrestrial emissions and long-distance transport, respectively.

Development and validation of a shipboard system for measuring high-resolution vertical profiles of aqueous dimethylsulfide concentrations using chemical ionization mass spectrometry

A sampling and analytical system has been developed for shipboard measurements of high-resolution vertical profiles of the marine trace gas dimethylsulfide (DMS). The system consists of a tube attached to a CTD with a peristaltic pump on deck that delivers seawater to a membrane equilibrator and atmospheric pressure chemical ionization mass spectrometer (Eq-APCIMS). This allows profiling DMS concentrations to a depth of 50 m, with a depth resolution of 1.3-2 m and a detection limit of nearly 0.1 nmol L-1. The seawater is also plumbed to allow parallel operation of additional continuous instruments, and simultaneous collection of discrete samples for complementary analyses. A valve alternates delivery of seawater from the vertical profiler and the ship�s underway intake, thereby providing high-resolution measurements in both the vertical and horizontal dimensions. Tests conducted on various cruises in the Mediterranean Sea, Atlantic, Indian, and Pacific Oceans show good agreement between the Eq-APCIMS measurements and purge and trap gas chromatography with flame photometric detection (GC-FPD) and demonstrate that the delivery of seawater from the underway pump did not significantly affect endogenous DMS concentrations. Combination of the continuous flow DMS analysis with high-frequency hydrographic, optical, biological and meteorological measurements will greatly improve the spatial/temporal resolution of seagoing measurements and improve our understanding of DMS cycling.

Atmospheric pressure chemical ionisation liquid chromatography/mass spectrometry of the ultraviolet screening pigment scytonemin: characteristic fragmentations

Atmospheric pressure chemical ionisation reversed-phase high-performance liquid chromatography/multistage mass spectrometry has been used to study the mass spectral fragmentation of the cyanobacterial sheath pigment scytonemin and its reduced counterpart. The two pigments exhibit characteristic fragment ions in their MS2 and MS3 spectra that are of value in confirming the identification of the structures in extracts from natural environments.

Atmospheric pressure chemical ionisation liquid chromatography/mass spectrometry of bacteriochlorophylls from Chlorobiaceae: characteristic fragmentations

Atmospheric pressure chemical ionisation liquid chromatography/mass spectrometry/mass spectrometry (APCI-LC/MS/MS) has been applied to the study of bacteriochlorophylls c, d, and e of phototrophic prokaryotes. Cultures of Chlorobiaceae containing bacteriochlorophyll c, d or e were examined using a high-resolution high-performance liquid chromatography (HPLC) method and APCI-LC/MS/MS employing post-column addition of formic acid. The results reveal complex distributions of bacteriochlorophyll homologues, with some closely eluting species giving isobaric protonated molecules. On-line LC/MS/MS studies reveal characteristic fragment ions for bacteriochlorophylls c, d, and e. Fragmentations involving loss of the extended alkyl substituents that are unique to bacteriochlorophylls c, d and e and their derivatives have been rationalised by studying the phaeophorbides and the results applied to the direct study of the bacteriochlorophylls.

Evidence of mass failure in the Hess Deep Rift from multi-resolutional bathymetry data

New regional swath and near-bottom bathymetric data provide constraints on shallow structures at the Hess Deep Rift, an oceanic rift that exposes the crust and upper mantle of fast-spreading oceanic lithosphere created at the East Pacific Rise. These data reveal the presence of a lobate structure with a length of ~ 4 km and a width of ~ 6 km south of an Intrarift Ridge, north of Hess Deep. The lobe consists of a series of concentric benches that are widest in the center of the lobe and narrower at the edges, with a dominant bench separating two distinct morphologic regions in the lobe. There are two end-member possible interpretations of this feature: 1) the lobate structure represents a mass failure with little translation that contains coherent blocks that preserve rift-related lineaments; or 2) it represents degraded tectonic structures, and the lobate form is accounted for by, for example, two intersecting faults. We favor the slump interpretation because it more readily accounts for the lobate form of the feature and the curved benches and based on the presence of other similar lobes in this region. In the slump model, secondary structures within the benches may indicate radial spreading during or after failure. The large lobate structure we identify south of the Intrarift Ridge in Hess Deep is one of the first features of its kind identified in an oceanic rift, and illustrates that mass failure may be a significant process in these settings, consistent with the recognition of their importance in mid-ocean ridges, oceanic islands, and continental rifts. Understanding the structure of the Hess Deep Rift is also important for reconstructing the section of fast-spreading oceanic crust exposed here.