A novel methodology for identifying environmental exposures using GPS data

Aim: While studies using global positioning systems (GPS) have the potential to refine measures of exposure to the neighbourhood environment in health research, one limitation is that they do not typically identify time spent undertaking journeys in motorised vehicles when contact with the environment is reduced. This paper presents and tests a novel methodology to explore the impact of this concern.

Methods: Using a case study of exposure assessment to food environments, an unsupervised computational algorithm is employed in order to infer two travel modes: motorised and non-motorised, on the basis of which trips were extracted. Additional criteria are imposed in order to improve robustness of the algorithm.

Results: After removing noise in the GPS data and motorised vehicle journeys, 82.43% of the initial GPS points remained. In addition, after comparing a sub-sample of trips classified visually of motorised, non-motorised and mixed mode trips with the algorithm classifications, it was found that there was an agreement of 88%. The measures of exposure to the food environment calculated before and after algorithm classification were strongly correlated.

Conclusion: Identifying non-motorised exposures to the food environment makes little difference to exposure estimates in urban children but might be important for adults or rural populations who spend more time in motorised vehicles.     

Powerful unconfined turbidity current captured by cabled observatory on the Fraser River delta slope,British Columbia,Canada

Turbidity currents are one of the main sediment transport processes on Earth, yet are notoriously difficult to monitor directly. This article presents the first direct and high bandwidth observation of a turbidity current using a cabled sea floor observatory. On 5 June 2012, a platform on Ocean Networks Canada, located in 107 m of water on the Fraser River delta slope, was displaced downslope and severed from its data cable. The platform weighed ca 1000 kg in water. The event took place during high river discharge, high tides and rapid sediment accumulation on adjacent upslope areas of the sea floor. Data recorded as it tumbled downslope allow a reconstruction of the flow, which is inferred to have been an unconfined turbidity current. Lines of evidence indicate that the flow came in as a bed hugging wedge, and built up to between 1 m and 4 m in height as the head passed through. Comparison with laboratory data suggest that the flow was initially supercritical. While the adjacent slope offset to the north clearly exhibits change over an annual resurvey period, the bathymetry directly at the event location show no resolvable change over a period from seven months before the event to one month after. Sediment cores collected after the event were pervasively biototurbated and they contain no obvious deposit connected with this event. The remarkable aspects of this research follow. The flow was powerful enough to carry a 1 tonne platform and sever a heavily armoured cable. The current occurred on the unchannelized open slope. This powerful event failed to cause discernible seabed elevation change. The flow was triggered by tidal conditions. The event was detected by a purpose‐designed cabled observatory, thus providing high bandwidth data and also alerting researchers in real time to mount follow‐on investigations.     

Aegaeon (Saturn LIII), a G-ring object

Aegaeon (Saturn LIII, S/2008 S1) is a small satellite of Saturn that orbits within a bright arc of material near the inner edge of Saturn’s G-ring. This object was observed in 21 images with Cassini’s Narrow-Angle Camera between June 15 (DOY 166), 2007 and February 20 (DOY 051), 2009. If Aegaeon has similar surface scattering properties as other nearby small saturnian satellites (Pallene, Methone and Anthe), then its diameter is approximately 500 m. Orbit models based on numerical integrations of the full equations of motion show that Aegaeon’s orbital motion is strongly influenced by multiple resonances with Mimas. In particular, like the G-ring arc it inhabits, Aegaeon is trapped in the 7:6 corotation eccentricity resonance with Mimas. Aegaeon, Anthe and Methone therefore form a distinctive class of objects in the Saturn system: small moons in corotation eccentricity resonances with Mimas associated with arcs of debris. Comparisons among these different ring-arc systems reveal that Aegaeon’s orbit is closer to the exact resonance than Anthe’s and Methone’s orbits are. This could indicate that Aegaeon has undergone significant orbital evolution via its interactions with the other objects in its arc, which would be consistent with the evidence that Aegaeon’s mass is much smaller relative to the total mass in its arc than Anthe’s and Methone’s masses are.     

Interannual Variation in Photosynthetically Significant Optical Properties and Water Quality in a Coastal Blackwater River Plume

Surface water optical characteristics, nutrients, and planktonic chlorophyll a concentrations were analyzed in the Cape Fear River (CFR) plume over a 2-year period. CFR discharge during the dry year (109 ± 105 m³s⁻¹) was only 25% of the wet year discharge (429 ± 337 m³s⁻¹). Partitioning the contributions of phytoplankton pigments, non-pigmented particles, and colored dissolved organic matter (CDOM) to the absorption of photosynthetically active radiation (PAR) indicated that CDOM was the dominant contributor to PAR absorption. Particulate absorption was relatively greater during the dry year. Pigment absorption was minor and varied little among stations or between years. Chlorophyll a concentrations were reduced at the most plume-influenced stations during the wet year, despite lower turbidity and higher nitrate concentrations. Ammonium and orthophosphate concentrations were not different between years. CDOM absorption [a _CDOM (412)] ranged from 0.05 to 8.25 m⁻¹ with highest values occurring near the CFR mouth. Our results suggest that for coastal ecosystems with significant blackwater river inputs, CDOM may exert a major limiting influence over near-shore primary production.     

Trophic Relationships of a Marsh Bird Differ Between Gulf Coast Estuaries

Much of North America’s tidal marsh habitat has been significantly altered by both natural and man-made processes. Thus, there is a need to understand the trophic ecology of organisms endemic to these ecosystems. We applied carbon (δ ¹³C) and nitrogen (δ ¹⁵N) stable isotope analysis, along with isotope mixing models, to egg yolk, liver, and muscle tissues of clapper rails (Rallus longirostris) and their likely prey items. This analysis enabled us to explore variation in trophic niche and diet composition in this important marsh bird in two northern Gulf of Mexico tidal marshes that are river and ocean-dominated. For the river-associated estuary, δ ¹³C and δ ¹⁵N of egg yolks, liver, and pectoral muscle tissue samples provided evidence that clapper rails maintained a similar diet during both the winter and the breeding season. A trophic link between C₃ primary productivity and the clapper rail’s diet was also indicated as the δ ¹³C of clapper rail egg yolks related negatively with the aerial cover of C₃ macrophytes. Clapper rails from the ocean-dominated estuary had a narrower trophic niche and appeared to be utilizing marine resources, particularly, based on modeling of liver stable isotope values. Variation in stable isotope values between egg yolk and liver/muscle in both systems suggests that endogenous resources are important for egg production in clapper rails. These results demonstrate that diet composition, prey source, and niche width of clapper rails can vary significantly across different estuaries and appear to be influenced by hydrological conditions.     

The effects of rerouting aircraft around the arctic circle on arctic and global climate

Climate data suggest greater warming over the Arctic than lower latitudes, and the most abundant direct source of black carbon and other climate-relevant pollutants over the Arctic is cross-polar flights by international aviation. A relevant question is whether rerouting cross-polar flights to circumnavigate the Arctic Circle reduces or enhances such warming. To study this issue, a model accounting for subgrid exhaust plumes from each individual commercial flight worldwide was used with 2006 global aircraft emission inventories that treated cross-polar flights and flights rerouted around the Arctic Circle (66.56083 °N), respectively. Rerouting increased fuel use by 0.056?% in the global average, mostly right outside the Arctic Circle, but most of the associated black carbon and other emissions were removed faster because they were now over latitudes of greater precipitation and lesser stability. Rerouting also reduced fuel use and emissions within the Arctic Circle by 83?% and delayed pollutant transport to the Arctic. The Arctic reduction in pollutants, particularly of black carbon, decreased Arctic and global temperature and increased Arctic sea ice over 22?years. Although the slight increase in total CO₂ emissions due to rerouting may dampen the benefit of rerouting over more decades, rerouting or even partial rerouting (allowing cross-polar flights during polar night only) may delay the elimination of Arctic sea ice, which will otherwise likely occur within the next 2–3 decades due to global warming in general. Rerouting may increase worldwide fuel plus operational costs by only ~$99 million/yr, 47–55 times less than an estimated 2025 U.S.-alone cost savings due to the global warming reduction from rerouting.     

Pilot study using SPOT satellite imagery over Apalachicola national forest to determine appropriate spatial scale for area-wide aggregation of surface fluxes

Summary High resolution radiances from SPOT satellite imagery converted to Normalized Difference Vegetation Indices (NDVI) over a 16×16 km² mixed ground cover study-area in the Apalachicola National Forest in northwest Florida, along with in situ measurements from a Bowen ratio surface flux monitoring system and physical modeling techniques, are used to determine the length manifold beyond which degraded resolution satellite imagery fails to capture flux variability over the scene. The investigation is relevant to an understanding of how bias error is generated in methods designed to produce scale-invariant surface flux estimates from satellite measurements. Error estimates are based on assigning characteristic NDVI values to the four predominant types of ground cover found within the study-area. An open site near the center of the study-area, which satisfies the conditions for surface flux monitoring, is used for obtaining input data for a biosphere-atmosphere exchange model designed to calculate representative fluxes for the different ground covers. Continuous 6-minute meteorological and surface flux measurements were made at the monitoring site for a period of 22 days. These measurements are used in conjunction with surface layer theory to provide surface layer profile estimates of wind speed, temperature, and relative humidity at the tops of the forested sites. The measured and derived meteorological parameters, together with representative biophysical parameters, are used as input to the biosphere-atmosphere exchange model. By representing sensible and latent heat flux distributions due to the variable ground cover with characteristic NDVI values at 20-m resolution, baseline area-wide sensible and latent heat flux quantities are calculated. Error-growth curves as a function of spatial resolution for the fluxes are found by degrading the resolution of the SPOT radiances used to calculate NDVI, and rationing the associated area-wide fluxes to the baseline values. The point at which an error-growth curve becomes invariant represents the edge of a length manifold beyond which the satellite input no longer contains information on surface flux variability, even though NDVI variability continues at all scales up to that of the complete SPOT scene. The error-growth curves are non-linear, with all the error build-up taking place between 20 m and 1.6 km. Decreasing the spatial resolution of the NDVI information down to or below 1.6 km, introduces bias errors in the area-wide surface flux estimates of 10% for sensible heat and 8% for latent heat. The underlying assumptions and modeling produce uncertainty in estimating the manifold limits, however, the principal objective is to show that in using satellite data for scale-invariant surface flux retrieval, there is an optimal spatial resolution factor that can be objectively quantified.With 10 Figures     

Estimation of hydroxyl radical concentrations in the marine atmospheric boundary layer using a reactive atmospheric tracer

Hydroxyl radical (OH) concentrations in the atmospheric boundary layer over a number of remote ocean locations are calculated from the measured diurnal variation in atmospheric dimethylsulfide (DMS). By using averaged DMS data sets from extended periods, the calculation yields OH concentrations averaged over periods from several days to weeks. These average OH concentrations range from 7×10⁵ to 2.9×10⁶ molecules cm^-3, corresponding to midday maxima of 3 to 12×10⁶ molecules cm^-3. The lowest values correspond to studies with the lowest light intensity (Antarctic summer and South Atlantic winter), and the highest values to regions with probable anthropogenic influence. In addition to the long term averages, daily average OH levels can be calculated for most days in a two week period from a cruise in the tropical eastern Pacific. These calculations are in good argeement with global average OH levels derived from other tracers, and are consistent with model OH calculations when allowance is made for variation in ambient ozone levels between the studies. Estimates of gas exchange made from the diurnal variation of DMS suggest that either the gas exchange coefficient of DMS or the boundary layer mixing depth may have been overestimated in past analyses.