Evidence Of Intrusion Research Articles

Seismic and aseismic slip evolution and deformation associated with the 2009-2010 northern Malawi earthquake swarm, East African Rift

SUMMARY Interferometric synthetic aperture radar (InSAR) measurements, field observations and elastic modelling of the 2009–2010 Karonga (northern Malawi) earthquake swarm reveal widespread coseismic and localized post-seismic deformation. In a period of about 1.5 months starting on November 5, 29 M ≥ 4 earthquakes struck the region, culminating in an Mw 6 peak event on December 19. The next few months were characterized by significant localized deformation with a very low seismic moment release. We find a very good agreement between InSAR and field observations of surface ruptures. Our best fitted coseismic models indicate dip-slip displacements on a fault dipping 40 ◦ to the southwest with maximum slip of about 120 cm at 3–5 km depth. Fault activity continued until 2010 August as shallow aseismic afterslip mostly above the maximum coseismic slip patches. Although the swarm occurred within a coastal plain covered by porous Quaternary sediments and a high water table, the effect of poroelastic relaxation on the post-seismic deformation was found to be negligibly small. In contrast with other recent earthquake swarms along the East African Rift, the Karonga swarm shows no evidence for dike intrusion.

TAPHONOMY OF THE UPPER EDIACARAN ENIGMATIC RIBBONLIKE FOSSIL SHAANXILITHES

Shaanxilithes ningqiangensis is an enigmatic ribbon-shaped fossil from the upper Ediacaran Gaojiashan Member of the Dengying Formation, southern Shaanxi Province, South China. This taxon has also been reported from Ediacaran successions in North China and possibly in Siberia, making it a potential index fossil for interregional biostratigraphic correlation of upper Ediacaran successions. At Gaojiashan, Shaanxilithes ningqiangensis is often preserved along bedding planes of phosphate-rich silty and calcareous shale, with no evidence of vertical intrusion into adjacent beds and containing little to no carbonaceous material. Here, through detailed microstructural and microchemical investigation using a combination of analytical techniques, taphonomic details of Shaanxilithes ningqiangensis and potentially related forms are revealed, showing that these enigmatic fossils are preserved as clay molds. Together with other taphonomic features, such as abruptly bent ribbons, overlapping but not crosscutting ribbons, and co-occurring discoidal structures interpreted as disarticulated sections of the original organism, the new data suggest that Shaanxilithes ningqiangensis is a body fossil consisting of serially arranged units that are discoidal, lensoidal, or crescentic in shape. Shaanxilithes ningqiangensis is not a trace fossil as some previous researchers have suggested.

Intrusion, integration and innovation on small and not‐so‐small islands with particular reference to Samoa

ABSTRACT Investigations on small Polynesian outliers have illustrated how difficult it can be to identify archaeological evidence of intrusion, or to interpret the effect of any intrusion on the resident populations. In Samoa, the still meagre amount of artefactual and faunal remains from archaeological excavations adds to these problems. A review of the known Samoan archaeological sequence finds little or no evidence of intrusion, apart from a probable post‐settlement introduction of pigs and dogs. This need not mean that Samoa was ever isolated from contacts with other islands.

Jet characterization in the upper troposphere/lower stratosphere (UTLS): applications to climatology and transport studies

Abstract. A method of classifying the upper tropospheric/lower stratospheric (UTLS) jets has been developed that allows satellite and aircraft trace gas data and meteorological fields to be efficiently mapped in a jet coordinate view. A detailed characterization of multiple tropopauses accompanies the jet characterization. Jet climatologies show the well-known high altitude subtropical and lower altitude polar jets in the upper troposphere, as well as a pattern of concentric polar and subtropical jets in the Southern Hemisphere, and shifts of the primary jet to high latitudes associated with blocking ridges in Northern Hemisphere winter. The jet-coordinate view segregates air masses differently than the commonly-used equivalent latitude (EqL) coordinate throughout the lowermost stratosphere and in the upper troposphere. Mapping O3 data from the Aura Microwave Limb Sounder (MLS) satellite and the Winter Storms aircraft datasets in jet coordinates thus emphasizes different aspects of the circulation compared to an EqL-coordinate framework: the jet coordinate reorders the data geometrically, thus highlighting the strong PV, tropopause height and trace gas gradients across the subtropical jet, whereas EqL is a dynamical coordinate that may blur these spatial relationships but provides information on irreversible transport. The jet coordinate view identifies the concentration of stratospheric ozone well below the tropopause in the region poleward of and below the jet core, as well as other transport features associated with the upper tropospheric jets. Using the jet information in EqL coordinates allows us to study trace gas distributions in regions of weak versus strong jets, and demonstrates weaker transport barriers in regions with less jet influence. MLS and Atmospheric Chemistry Experiment-Fourier Transform Spectrometer trace gas fields for spring 2008 in jet coordinates show very strong, closely correlated, PV, tropopause height and trace gas gradients across the jet, and evidence of intrusions of stratospheric air below the tropopause below and poleward of the subtropical jet; these features are consistent between instruments and among multiple trace gases. Our characterization of the jets is facilitating studies that will improve our understanding of upper tropospheric trace gas evolution.

Origin of salinity in a multilayered aquifer with high salinization vulnerability

AbstractThe Kaluvelly watershed is a coastal area (Tamil Nadu, India) where water abstraction has resulted in a dramatic fall in the level of the water table and a piezometric depression in the most exploited aquifer, the Vanur aquifer. In addition, intensification/mechanization of agriculture may have affected the quality of recharge water. An initial hydrodynamic study showed that the Vanur aquifer is highly vulnerable to salinization due to potential seawater intrusion, and our aim was to determine the source of salinity recorded in the groundwater of this multilayered aquifer. Our approach involved the use of existing boreholes and of a moderate number of samples, with the aim of developing appropriate water resource management techniques. Major element, 18O/16O, 2H/1H and 87Sr/86Sr, ratios were measured in rainwater, surface water and groundwater collected during five sampling campaigns over a 2‐year period. Geochemical data indicate that the Vanur aquifer is recharged and that small mixings between aquifers fluctuate according to monsoon intensity. There was no evidence of seawater intrusion. The range of recorded salinity originated mainly from water–rock interaction but a disconnection of some deeper parts of the aquifer was apparent. Strontium isotopic ratios in the recharge area suggest an anthropogenic influence, possibly related to fertilizer use. A high SO4/Cl ratio was observed in the aquifer; in the deeper parts, the influence of a formation containing lignite is hypothesized, whereas near the surface, sulphate may partly originate from fertilizer use and fossil fuel combustion. Water isotopic data suggest that the origin of precipitation in this region has been unchanged for several hundreds or thousands of years. Copyright © 2011 John Wiley & Sons, Ltd.

Geochemical and isotopic evidence for palaeo-seawater intrusion into the south coast aquifer of Laizhou Bay, China

This research aims to improve the current knowledge of groundwater salinisation processes in coastal aquifers using combined hydrochemical and isotopic parameters and inverse hydrochemical modelling. Field investigations were conducted in Laizhou Bay, which is the area most seriously affected by seawater intrusion in north China. During three sampling campaigns along a vertical transect in the ChangyiLiutuan area, 95 ground- and surface-water samples were collected for major ion and isotope analysis

The competing impacts of geology and groundwater on electrical resistivity around Wrightsville Beach, NC

Measurements of electrical resistivity have long been used to find freshwater resources below the earth's surface. Recently, offshore resistivity and electromagnetic techniques have been used to map occurrences of submarine groundwater originating from the offshore extension of terrestrial aquifers. In many cases, observations of a high resistivity (low conductivity) anomaly in the seafloor are sufficient to suggest the presence of fresh (and less conductive) pore waters. Data from offshore Wrightsville Beach, NC show highly variable resistivity structure, with moderately high resistivity at depths of ∼20 m subsurface that is at least in part caused by lithologic complexity in an underlying limestone unit, the Castle Hayne. These offshore results suggest caution in the interpretation of resistivity anomalies simply in terms of groundwater volumes. In contrast, low onshore resistivities show evidence for intrusion of saltwater into the subsurface beneath the beach, adjacent to areas of pumping for water supply.

Online Obsessive Relational Intrusion: Further Concerns About Facebook

Use of online social networking sites such as Facebook has burgeoned in the last 5 years. We examine these sites as facilitators of Online Obsessive Relational Intrusion (o-ORI)—a much-talked about, but relatively un-researched online phenomenon. We draw parallels between the types of behaviors conducted online and those identified in the literature on relational intrusion and its more extreme relative, stalking. We present a frequency analysis of students’ behavior on Facebook and find evidence of relational intrusion from both offenders and targets. The behaviors can be classified into five different categories, including: primary contact attempts, secondary contact attempts (i.e., contacting others connected to the target), monitoring or surveillance, expressions, and invitations. We conclude that Facebook facilitates behaviors that are indicative of obsessive relational intrusion and that such behaviors have implications for users’ privacy and security.

A Novel Attack Graph Posterior Inference Model Based on Bayesian Network

Network attack graphs are originally used to evaluate what the worst security state is when a concerned net-work is under attack. Combined with intrusion evidence such like IDS alerts, attack graphs can be further used to perform security state posterior inference (i.e. inference based on observation experience). In this area, Bayesian network is an ideal mathematic tool, however it can not be directly applied for the following three reasons: 1) in a network attack graph, there may exist directed cycles which are never permitted in a Bayesian network, 2) there may exist temporal partial ordering relations among intrusion evidence that can-not be easily modeled in a Bayesian network, and 3) just one Bayesian network cannot be used to infer both the current and the future security state of a network. In this work, we improve an approximate Bayesian posterior inference algorithm–the likelihood-weighting algorithm to resolve the above obstacles. We give out all the pseudocodes of the algorithm and use several examples to demonstrate its benefit. Based on this, we further propose a network security assessment and enhancement method along with a small network scenario to exemplify its usage.

Evidence of magma intrusion at Fourpeaked volcano, Alaska in 2006–2007 from a rapid-response seismic network and volcanic gases

On September 17th, 2006, Fourpeaked volcano had a widely-observed phreatic eruption. At the time, Fourpeaked was an unmonitored volcano with no known Holocene activity, based on limited field work. Airborne gas sampling began within days of the eruption and a modest seismic network was installed in stages. Vigorous steaming continued for months; however, there were no further eruptions similar in scale to the September 17 event. This eruption was followed by several months of sustained seismicity punctuated by vigorous swarms, and SO 2 emissions exceeding a thousand tons/day. Based on observations during and after the phreatic eruption, and assuming no recent pre-historical eruptive activity at Fourpeaked, we propose that the activity was caused by a minor injection of new magma at or near 5 km depth beneath Fourpeaked, which remained active over several months as this magma equilibrated into the crust. By early 2007 declining seismicity and SO 2 emission signaled the end of unrest. Because the Fourpeaked seismic network was installed in stages and the seismicity was punctuated by discrete swarms, we use Fourpeaked to illustrate quantitatively the efficacy and shortcomings of rapid response seismic networks for tracking volcanic earthquakes.

Boosting performance in attack intention recognition by integrating multiple techniques

Recognizing attack intention is crucial for security analysis. In recent years, a number of methods for attack intention recognition have been proposed. However, most of these techniques mainly focus on the alerts of an intrusion detection system and use algorithms of low efficiency that mine frequent attack patterns without reconstructing attack paths. In this paper, a novel and effective method is proposed, which integrates several techniques to identify attack intentions. Using this method, a Bayesian-based attack scenario is constructed, where frequent attack patterns are identified using an efficient data-mining algorithm based on frequent patterns. Subsequently, attack paths are rebuilt by recorrelating frequent attack patterns mined in the scenario. The experimental results demonstrate the capability of our method in rebuilding attack paths, recognizing attack intentions as well as in saving system resources. Specifically, to the best of our knowledge, the proposed method is the first to correlate complementary intrusion evidence with frequent pattern mining techniques based on the FP-Growth algorithm to rebuild attack paths and to recognize attack intentions.

A 43-GHz VLA survey in the ELAIS N2 area

We describe a survey in the ELAIS N2 region with the Very Large Array at 43.4 GHz, carried out with 1627 independent snapshot observations in D-configuration and covering about 0.5 deg2. One certain source is detected, a previously catalogued flat-spectrum quasi-stellar object at z = 2.2. A few (<5) other sources may be present at about the 3σ detection level, as determined from positions of source-like deflections coinciding with blue stellar objects, or with sources from lower frequency surveys. Independently we show how all the source-like detections identified in the data can be used with a maximum-likelihood technique to constrain the 43-GHz source counts at a level of ∼7 mJy. Previous estimates of the counts at 43 GHz, based on lower frequency counts and spectral measurements, are consistent with these constraints, although the present results are suggestive of somewhat higher surface densities at the 7-mJy level. They do not provide direct evidence of intrusion of a previously unknown source population, although the several candidate sources need examination before such a population can be ruled out.

Proliferation and demise of deep-sea corals in the Mediterranean during the Younger Dryas

Uranium-series and radiocarbon ages are reported for deep-sea corals Madrepora oculata, Desmophyllum dianthus, Lophelia pertusa and Caryophyllia smithii from the Mediterranean Sea. U-series dating indicates that deep-sea corals have persisted in the Mediterranean for over 480,000 years, especially during cool interstadial periods. The most prolific period of growth however appears to have occurred within the Younger Dryas (YD) period from 12,900 to 11,700 years BP followed by a short (~ 330 years) phase of post-YD coral growth from 11,230 to 10,900 years BP. This indicates that deep-sea corals were prolific in the Mediterranean not only during the return to the more glacial-like conditions of the YD, but also following the rapid deglaciation and transition to warmer conditions that followed the end of the YD. Surprisingly, there is a paucity Last Glacial Maximum (LGM) coral ages, implying they were largely absent during this period when cold-water conditions were more prevalent. Radiocarbon ages show that the intermediate depth waters of the Mediterranean generally had Δ 14C compositions similar to surface waters, indicating that these waters were extremely well ventilated. The only exception is a narrow period in the YD (12,500 ± 100 years BP) when several samples of Lophelia pertusa from the Ionian Sea had Δ 14C values falling significantly below the marine curve. Using a refined approach, isolation ages (τ isol) of 300 years to 500 years are estimated for these intermediate (800–1000 m) depth waters relative to surface marine waters, indicating a reduction or absence of deep-water formation in the Ionian and adjacent Adriatic Seas during the YD. Contrary to previous findings, we find no evidence for widespread intrusion of low Δ 14C Atlantic waters into the Mediterranean. Prolific growth of deep-sea corals in the Mediterranean ended abruptly at ~ 10,900 years BP, with many of the coral-bearing mounds on the continental slopes being draped in a thin veneer of mud. Their demise is attributed to a number of factors, including the direct loss of habitat due to high sedimentation that accompanied glacial meltwater pulses, together with rising temperatures that would have finally pervaded the deeper water of the Mediterranean following the onset of Holocene warming.

Transport by an intrusion generated by boundary mixing in a lake

A dye study was conducted to track an intrusion generated at the boundary of a small lake. Persistent turbid layers offshore presented evidence of possible intrusions from boundary mixing. After high winds, a streak of Rhodamine WT was injected at the boundary of the lake where the slope was between 5% and 10%. Both vertical profiles and longitudinal transects of the dye concentration were measured. The three‐dimensional dye mapping showed a distinct dye intrusion, ranging between 0.5 and 1 m thick, over 200 m in horizontal extent offshore, 1 day after the injection. Profiles of temperature microstructure measured soon after the injection both at the injection site and offshore showed an elevated eddy diffusivity near the boundary where the dye was injected, indicating that the intrusion resulted from boundary mixing. Because the dominant internal waves are subcritical, the mixing is most likely due to seiching currents interacting with the boundary. The propagation characteristics of the intrusion were predicted most closely by a formulation for a radially spreading intrusion governed by a balance between buoyancy and inertia. These results show that intrusion generation and propagation may be a significant process for mass transport in stratified lakes and reservoirs.

Geomorphology and accommodation space as limiting factors on tsunami deposition: Chatham Island, southwest Pacific Ocean

Chatham Island in the southwest Pacific Ocean is exposed on all sides to potential tsunami impact. In historical time, tsunamis are known to have inundated the coast on several occasions, with the largest event in 1868. Coastal dunes along the northeast coast of Chatham Island preserve sedimentary evidence of this and possibly earlier tsunami events, as localised gravel lags. However, these deposits lack a clear stratigraphic context and establishing their age is difficult. This study examines the sediment record in a freshwater wetland at Okawa Point, located directly landward of the dunes where apparent tsunami gravels occur. Sediment descriptions, pollen, foraminifera, chemical data and radiocarbon dates from cores are used to reconstruct the environmental history of the wetland. The record extends from ca. > 43 ka to the present and incorporates glacial, post-glacial and human-influenced phases. Throughout this time the wetland appears to have remained isolated from catastrophic marine inundation. The only evidence for saltwater intrusion is observed in the historic period, via geochemical, grain size and pollen data, which record a marine inundation event that forced the transport of a thin (cm-thick) deposit of dune and beach sand into the seaward edge of the wetland. This is interpreted as the signature of the 1868 tsunami. The lack of more widespread physical evidence for this and other tsunami events in the wetland is attributed to the morphological roughness afforded by coastal dunes and limited accommodation space for Holocene deposits.

Effects of Treflan injection on winter wheat growth and root clogging of subsurface drippers

Abstract The clogging of drippers caused by crop root intrusion has been a great concern of subsurface drip irrigation (SDI) systems. To attempt to solve the problem of root clogging of drippers, a series of field experiments were conducted in the growing seasons of 2006–2008. The goal was to investigate the effects of Treflan injection on dripper clogging by roots, and on root distribution, yield, and the quality of winter wheat (Triticum aestivum L.) under SDI. For each growing season, two Treflan injection dates (March 6 and April 15 for the 2006–2007 growing season, and March 6 and April 15 for the 2007–2008 growing season) and three injection concentrations of 0, 3, and 7 mg/l were arranged in a randomized block experimental design. During harvest, root length density (RLD) and Treflan concentration at different soil layers were measured using the auger-sampling method. Thirty-five drippers from each treatment were randomly chosen to observe evidence of root intrusion into the dripper flow passage in order to estimate root clogging. The experimental results showed that Treflan injection could effectively reduce root density in areas adjacent to drippers, thereby significantly decreasing the potential of root clogging. In 2007, 4 out of the 35 drippers were found with root intrusion problems in the control (without Treflan injection), while no root clogging existed any dripper in Treflan application treatments. In 2008, 6 drippers from the control but only 1 dripper from those treated with Treflan application showed root clogging. In addition, within the range of concentration used by the current experiment, Treflan concentrations had no significant effects on winter wheat root distribution, yield, and quality. Injection date, however, influenced the vertical root distribution significantly. Injection of Treflan late in the growing season influenced the root distribution only in the areas close to the drippers; the influenced areas increased if Treflan was injected early in the growing season.

The 2005 Ilan earthquake doublet and seismic crisis in northeastern Taiwan: evidence for dyke intrusion associated with on-land propagation of the Okinawa Trough

SUMMARY Northern Taiwan underwent mountain building in the early stage of the Taiwan orogeny but is currently subjected to post-collisional crustal extension. It may be related to gravitational collapse or to the rifting of the Okinawa Trough, which lies offshore northeastern Taiwan. The Ilan Plain, northeastern Taiwan, which is bounded by the normal fault systems and filled up with thick Pliocene–Pleistocene sedimentary sequences, formed under such an extension environment. Over there on 2005 March 5 two earthquakes with about the same magnitude (M L = 5.9) occurred within 68 s and produced intense aftershocks activity according to the records of Central Weather Bureau Seismic Network of Taiwan. We relocated the earthquake sequence by the three-dimension earthquake location algorithm with the newly published 3D Vp and Vp/Vs velocity model, and determined the first-polarity focal mechanisms of the earthquake doublet. One major cluster of aftershocks which trends E–W and dips steeply to the south can be identified and picked up as a potential fault plane. The focal mechanisms of the two main shocks are both classified as normal type by first-polarity but strike-slip by centroid moment tensor inversion; however two methods both yield consistent E–W strike. Static coseismic deformation was additionally determined from Global Positioning System (GPS) daily solutions at a set of continuous GPS stations and from strong-motion seismographs. These data show NW–SE extension at high angle to the fault plane, which cannot be explained from a simple strike-slip double-couple mechanism. On the other hand, the small vertical displacements and steep fault plane cannot be explained from a simple normal event as well. We present from elastic dislocation modelling that the geodetic data are best explained by significant component of tensile source with centimetre-scale of opening on a 15-km-long fault extending from 1 to 13 km depth. We therefore interpret the crisis as the result of dyke intrusion at the very tip of the Okinawa Trough, which is reasonably driven by backarc spreading.

Discussion of “Fatigue Crack Initiation Mechanism for Cast 319-T7 Al Alloy”

Jang and co-workers present interesting results for fatigue tests taken from a gravity sand cast aluminum alloy cylinder block casting. They find a wide spectrum of failure modes, including zero failure (fatigue runouts), initiation from pores, and observations of extensive coarse transgranular facets on the fracture surfaces. They conclude that the transgranular facets are slip planes, and the oxides observed in places on these planes are the result of fretting damage during crack propagation. I would like to draw the attention of the authors to the fact that facets on fracture surfaces of tensile and fatigue specimens are common on many kinds of cast alloys, including Al alloys, cast irons, and Ni-base superalloys. [1] The mechanism is simply that of straightening oxide bifilms (the necessarily double oxide films caused by surface turbulence during casting) by the advance of the dendrites during solidification. This dendrite pushing process creates large planar areas, which are separated by the planar unbonded interface between the two films, thus forming extensive transgranular, and sometimes intergranular, cracks. The transgranular cracks exhibit the accurately planar crystallography defined by the dendrites, whereas the bifilms pushed into intergranular planes exhibit the typical curved surfaces of grain boundaries. Critically, Cox and co-workers [2] only observed facets on the fracture surfaces of Ni-base superalloys that had been cast turbulently. Those that had been cast with carefully designed filling systems that avoided the entrainment of the oxide from the liquid surface were free from facets. The observation by the authors of oxides in some patches of the facets but not in others is typical of oxide films entrained into the bulk metal from the liquid surface. Such films are irregular and messy, thick in places, but often so thin (perhaps 20 nm) as to be difficult to detect by conventional microanalysis. If the facets had been the result of a slip band mechanism, some evidence of extrusions and intrusions

A new perspective on MST radar observations of stratospheric intrusions into‐troposphere associated with tropical cyclone

This paper presents the first ever evidence of stratospheric intrusion into troposphere during the passage of tropical cyclone using mesosphere‐stratosphere‐troposphere (MST) radar located at Gadanki (13.5°N, 79.2°E). The most significant and new observation is the enhanced signal‐to‐noise ratio (SNR) of thickness ∼1 km, extended from 16–13 km, when the cyclone is located close to the radar site. Small‐scale turbulence and strong downdraft followed by weak updraft are also observed from spectral‐width and vertical velocity, respectively. Further analysis of global data shows low out‐going long wave radiation (&lt;170 W m−2), low tropopause temperature (&lt;191 K), enhanced potential vorticity (∼1 PVU) in the upper/middle troposphere, and low specific humidity (∼2 × 10−4 kg/kg) at 300 hPa pressure level, occurs over the radar site. These observations clearly indicate the evidence of stratospheric intrusion into troposphere during the passage of tropical cyclone. The results are presented and discussed in the light of present understanding.

Synoptic influences on springtime tropospheric O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and CO over the North American export region observed by TES

Abstract. The relationship between synoptic circulation patterns over the western North Atlantic Ocean in spring (March, April, and May) and tropospheric O3 and CO was investigated using retrievals from the Tropospheric Emission Spectrometer (TES) for 2005 and 2006. Seasonal composites of TES retrievals reprocessed to remove the artificial geographic structure added from the a priori revealed a channel of slightly elevated O3 (&gt;55 ppbv) and CO (&gt;115 ppbv) at the 681 hPa retrieval level between 30° N and 45° N extending from North America out over the Atlantic Ocean. Ozone and CO in this region were correlated at r=0.22 with a slope value of 0.13 mol mol−1 indicative of the overall impact of photochemical chemical processes in North American continental export. Composites of TES retrievals for the six predominant circulation patterns identified as map types from sea level pressure fields of the NCEP FNL analyses showed large variability in the distribution of tropospheric O3. Map types MAM2 and MAM3 featuring cyclones near the US east coast produced the greatest export to the lower free troposphere with O3&gt;65 ppbv and a relatively well-defined O3-CO correlation (slope values near 0.20 mol mol−1). The ensembles of HYSPLIT backward trajectories indicated that the high O3 levels were possibly a result of pollutants lofted to the free troposphere by the warm conveyor belt (WCB) of a cyclone. An important finding was that pollutant export occurred in the main WCB branch to the east of the cyclone and in a secondary branch circling to the back of the cyclone center. Conversely, a map type featuring a large anticyclone dominating the flow over the US east coast (MAM6) restricted export with O3 levels generally &lt;55 ppbv and CO levels generally &lt;110 ppbv. There was also evidence of stratospheric intrusions particularly to the north of 45° N in the 316 hPa composites predominately for MAM1 which featured a large cyclone near Newfoundland. However, the concurrence of these intrusions with pollutant export, specifically in the southwestern North Atlantic Ocean, made it difficult to delineate their respective contributions to the 681 hPa O3 composites.