Episodic Transport Events Research Articles

Sources of springtime surface black carbon in the Arctic: an adjoint analysis for April 2008

Abstract. We quantify source contributions to springtime (April 2008) surface black carbon (BC) in the Arctic by interpreting surface observations of BC at five receptor sites (Denali, Barrow, Alert, Zeppelin, and Summit) using a global chemical transport model (GEOS-Chem) and its adjoint. Contributions to BC at Barrow, Alert, and Zeppelin are dominated by Asian anthropogenic sources (40–43 %) before 18 April and by Siberian open biomass burning emissions (29–41 %) afterward. In contrast, Summit, a mostly free tropospheric site, has predominantly an Asian anthropogenic source contribution (24–68 %, with an average of 45 %). We compute the adjoint sensitivity of BC concentrations at the five sites during a pollution episode (20–25 April) to global emissions from 1 March to 25 April. The associated contributions are the combined results of these sensitivities and BC emissions. Local and regional anthropogenic sources in Alaska are the largest anthropogenic sources of BC at Denali (63 % of total anthropogenic contributions), and natural gas flaring emissions in the western extreme north of Russia (WENR) are the largest anthropogenic sources of BC at Zeppelin (26 %) and Alert (13 %). We find that long-range transport of emissions from Beijing–Tianjin–Hebei (also known as Jing–Jin–Ji), the biggest urbanized region in northern China, contribute significantly (∼ 10 %) to surface BC across the Arctic. On average, it takes ∼ 12 days for Asian anthropogenic emissions and Siberian biomass burning emissions to reach the Arctic lower troposphere, supporting earlier studies. Natural gas flaring emissions from the WENR reach Zeppelin in about a week. We find that episodic transport events dominate BC at Denali (87 %), a site outside the Arctic front, which is a strong transport barrier. The relative contribution of these events to surface BC within the polar dome is much smaller (∼ 50 % at Barrow and Zeppelin and ∼ 10 % at Alert). The large contributions from Asian anthropogenic sources are predominately in the form of chronic pollution (∼ 40 % at Barrow, 65 % at Alert, and 57 % at Zeppelin) on about a 1-month timescale. As such, it is likely that previous studies using 5- or 10-day trajectory analyses strongly underestimated the contribution from Asia to surface BC in the Arctic.

Pathways of PFOA to the Arctic: variabilities and contributions of oceanic currents and atmospheric transport and chemistry sources

Abstract. Perfluorooctanoic acid (PFOA) and other perfluorinated compounds are industrial chemicals in use for decades which resist degradation in the environment and seem to accumulate in polar regions. Transport of PFOA was modeled using a spatially resolved global multicompartment model including fully coupled three-dimensional ocean and atmosphere general circulation models, and two-dimensional top soil, vegetation surfaces, and sea ice compartments. In addition to primary emissions, the formation of PFOA in the atmosphere from degradation of 8:2 fluorotelomer alcohol was included as a PFOA source. Oceanic transport, delivered 14.8±5.0 (8–23) t a−1 to the Arctic, strongly influenced by changes in water transport, which determined its interannual variability. This pathway constituted the dominant source of PFOA to the Arctic. Formation of PFOA in the atmosphere led to episodic transport events (timescale of days) into the Arctic with small spatial extent. Deposition in the polar region was found to be dominated by wet deposition over land, and shows maxima in boreal winter. The total atmospheric deposition of PFOA in the Arctic in the 1990s was ≈1 t a−1, much higher than previously estimated, and is dominated by primary emissions rather than secondary formation.

Atmospheric transport of persistent semi-volatile organic chemicals to the Arctic and cold condensation in the mid-troposphere – Part 2: 3-D modeling of episodic atmospheric transport

Abstract. Two 3-dimensional global atmospheric transport models for persistent organic pollutants (POPs) have been employed to investigate the association between the large-scale atmospheric motions and poleward transports of persistent semi-volatile organic chemicals (SVOCs). We examine the modeled daily air concentration of α- and γ-hexachlorocyclohexane (HCH) over a period from 1997 through 1999 during which a number of episodic atmospheric transport events were detected in this modeling study. These events provide modeling evidence for improving the interpretation on the cold condensation effect and poleward atmospheric transport of SVOCs in the mid-troposphere. Two episodic transport events of γ-HCH (lindane) to the high Arctic (80–90° N), one from Asian and another from Eurasian sources, are reported in this paper. Both events suggest that the episodic atmospheric transports occurring in the mid-troposphere (e.g. from 3000 m to 5500 m height) are driven by atmospheric horizontal and vertical motions. The association of the transport events with atmospheric circulation is briefly discussed. Strong southerly winds, forced by the evolution of two semi-permanent high pressure systems over mid-high latitudes in the Northern Hemisphere, play an important role in the long-range transport (LRT) of HCHs to the high latitudes from its sources. Being consistent with the cold condensation effect and poleward atmospheric transport in a mean meridional atmospheric circulation simulated by a 2-D atmospheric transport model, as reported by the first part of this study, this modeling study indicates that cold condensation is likely occurring more intensively in the mid-troposphere where rapid declining air temperature results in condensed phase of the chemical over and near its source regions and where stronger winds convey the chemical more rapidly to the polar region during the episodic poleward atmospheric transport events.

Mobile autonomous process sampling within coastal ocean observing systems

Predicting when and where key oceanic processes will be encountered is problematic in dynamic coastal waters where diverse physical, chemical, and biological factors interact in varied and rapidly changing combinations. Defining key processes often requires efficient sampling of specific water masses and prompt sample return for subsequent analyses. This compound challenge motivated our efforts to develop mobile autonomous process sampling (MAPS) for use with autonomous underwater vehicles (AUVs). With this system, features are recognized by artificial intelligence that integrates AUV sensor data to estimate probabilistic states for adaptive control of survey navigation and triggering of targeted water samplers. To demonstrate the utility of the MAPS/AUV system, we focused on intermediate nepheloid layers (INLs), episodic transport events that may play a role in zooplankton ecology. During multiple field tests in Monterey Bay, California, the MAPS/AUV system recognized, mapped, and sampled INLs. Invertebrate larvae contained in the water samples were subsequently characterized with molecular probes developed for high‐throughput screening. Preliminary results support the hypothesis that INLs function as vehicles for episodic larval transport. Applying MAPS within a greater coastal ocean observing system permitted description of regional oceanographic dynamics that influenced the patterns and scales of INL and larval transport.

Chemical speciation, transport and contribution of biomass burning smoke to ambient aerosol in Guangzhou, a mega city of China

Intensive measurements of aerosol (PM 10) and associated water-soluble ionic and carbonaceous species were conducted in Guangzhou, a mega city of China, during summer 2006. Elevated levels of most chemical species were observed especially at nighttime during two episodes, characterized by dramatic build-up of the biomass burning tracers levoglucosan and non-sea-salt potassium, when the prevailing wind direction had changed due to two approaching tropical cyclones. High-resolution air mass back trajectories based on the MM5 model revealed that air masses with high concentrations of levoglucosan (43–473 ng m −3) and non-sea-salt potassium (0.83–3.2 μg m −3) had passed over rural regions of the Pearl River Delta and Guangdong Province, where agricultural activities and field burning of crop residues are common practices. The relative contributions of biomass burning smoke to organic carbon in PM 10 were estimated from levoglucosan data to be on average 7.0 and 14% at daytime and nighttime, respectively, with maxima of 9.7 and 32% during the episodic transport events, indicating that biomass and biofuel burning activities in the rural parts of the Pearl River Delta and neighboring regions could have a significant impact on ambient urban aerosol levels.

Sediment and Wood Routing in Steep Headwater Streams: An Overview of Geomorphic Processes and their Topographic Signatures

Abstract Headwater streams in steep terrain pose a significant challenge for the development of best management practices (BMP) in forested watersheds. There is an incomplete understanding of the processes that govern the input, storage, and transport of sediment and wood, and these processes differ geographically. At the upstream extent of the channel network, headwater streams represent a transition from hillslope to channel processes. At the downstream extent, many of these channels transition from mass wasting to fluvial process dominance. Large-scale sediment routing processes typically consist of debris flows, earth flows, and/or gully erosion. In the interval between episodic transport events, headwater streams can interrupt the delivery of sediment from hillslopes to larger river systems by storing large volumes of sediment and wood. Forest management guidelines typically identify headwater streams as occurring upstream of the distribution of fish; however, a topographically based designation of headwater streams could prove useful for identifying which erosional processes are dominant in a particular area. Broad-scale terrain analysis based on unique topographic signatures can be used to identify the spatial domain of different geomorphic landforms that govern headwater stream processes, how they may be affected by forest management, and to infer the type and severity of downstream disturbance.

Linking Sediment Transport and Stratigraphy on the Continental Shelf

T H E GOAL OF THE SHELF sediment dynamics component of STRATAFORM is to link sediment transport processes active on the continental shelf to the formation and preservation of beds in shelf sediment deposits. An approach combining shelf sediment-transport models with high-resolution measurements of water-column and properties over periods from several months to several years allows us to make quantitative estimates of modification caused by sediment resuspension during episodic transport events. These modifications include erosion and deposition of material, formation of graded storm beds, and changes in small-scale surface morphology. The characteristics of the resulting event bed (thickness, grading, physical structures) are a function of flow and properties, depending on both temporal and spatial variations in sediment transport. Sediment transport on the continental shelf depends on surface-wave conditions, bottom-boundary-layer currents, fluid stratification, and characteristics, including grain size, density, porosity, and surface roughness. In general, sediment transport rates and depths of reworking are greatest when large, long-period waves occur simultaneously with strong, persistent currents. Thus understanding the magnitude and frequency of transport requires knowledge of the wave and current conditions on the shelf over sufficiently long periods of time to develop statistical characterizations of the wave and current fields and to document the resulting resuspension and modification. This information also provides critical