Marine Boundary Layer Experiment Research Articles

Aerosol size distribution properties associated with cold-air outbreaks in the Norwegian Arctic

Abstract. The aerosol particles serving as cloud condensation and ice nuclei contribute to key cloud processes associated with cold-air outbreak (CAO) events but are poorly constrained in climate models due to sparse observations. Here we retrieve aerosol number size distribution modes from measurements at Andenes, Norway, during the Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) and at Zeppelin Observatory, approximately 1000 km upwind from Andenes at Svalbard. During CAO events at Andenes, the sea-spray-mode number concentration is correlated with strong over-ocean winds with a mean of 8±4 cm−3 that is 71 % higher than during non-CAO conditions. Additionally, during CAO events at Andenes, the mean Hoppel minimum diameter is 6 nm smaller than during non-CAO conditions, though the estimated supersaturation is lower, and the mean number concentration of particles that likely activated in-cloud is 109±61 cm−3 with no statistically significant difference from the non-CAO mean of 99±66 cm−3. For CAO trajectories between Zeppelin Observatory and Andenes, the upwind-to-downwind change in number concentration is the largest for the accumulation mode with a mean decrease of 93±95 cm−3, likely attributable primarily to precipitation scavenging. These characteristic properties of aerosol number size distributions during CAO events provide guidance for evaluating CAO aerosol–cloud interaction processes in models.

Simulating Mixed‐Phase Open Cellular Clouds Observed During COMBLE: Evaluation of Parameterized Turbulence Closure

AbstractMarine cold‐air outbreaks, or CAOs, are airmass transformations whereby relatively cold boundary layer (BL) air is transported over relatively warm water. To more deeply understand BL and mixed‐phase cloud properties during CAO conditions, the Cold‐Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) took place from late 2019 into early 2020. During COMBLE, the U.S. Department of Energy's first Atmospheric Radiation Measurement Mobile Facility (AMF1) was deployed to Andenes, Norway, far downstream (∼1,000 km) from the Arctic pack ice. This study examines the two most intense CAOs sampled at the AMF1 site. The observed BL structures are open cellular with high (∼3–5 km) and cold (−30 to −50 C) cloud tops, and they often have pockets of high liquid water paths (LWPs; up to ∼1,000 g ) associated with strong updrafts and enhanced turbulence. We use a high‐resolution mesoscale model to explore how well four turbulence closure methods represent open cellular clouds. After applying a radar simulator to model outputs for direct evaluation, cloud top properties agree well with AMF1 observations (within ∼10%), but radar reflectivity and LWP agreement is more variable. Results suggest that the turbulent Prandtl number may play an important role for the simulated BL and cloud properties. All simulations produce enhanced precipitation rates that are well‐correlated with a cloud transition. Finally, the eddy‐diffusivity/mass‐flux approach produces the deepest cloud layer and therefore the largest and most coherent cellular structures. We recommend the use of a non‐local turbulence closure approach to better capture turbulent processes in intense CAOs.

Characterizing Mesoscale Cellular Convection in Marine Cold Air Outbreaks With a Machine Learning Approach

AbstractDuring marine cold‐air outbreaks (MCAOs), when cold polar air moves over warmer ocean, a well‐recognized cloud pattern develops, with open or closed mesoscale cellular convection (MCC) at larger fetch over open water. The Cold‐Air Outbreaks in the Marine Boundary Layer Experiment provided a comprehensive set of ground‐based in situ and remote sensing observations of MCAOs at a coastal location in northern Norway. MCAO periods that unambiguously exhibit open or closed MCC are determined. Individual cells observed with a profiling Ka‐band radar are identified using a watershed segmentation method. Using self‐organizing maps (SOMs), these cells are then objectively classified based on the variability in their vertical structure. The SOM nodes contain some information about the location of the cell transect relative to the center of the MCC. This adds classification noise, requiring numerous cell transects to isolate cell dynamical information. The SOM‐based classification shows that comparatively intense convection occurs only in open MCC. This convection undergoes an apparent lifecycle. Developing cells are associated with stronger updrafts, large spectrum width, larger amounts of liquid water, lower surface precipitation rates, and lower cloud tops than mature and weakening cells. The weakening of these cells is associated with the development of precipitation‐induced cold pools. The SOM classification also reveals less intense convection, with a similar lifecycle. More stratiform vertical cloud structures with weak vertical motions are common during closed MCC periods and are separated into precipitating and non‐precipitating stratiform cores. Convection is observed only occasionally in the closed MCC environment.

Insights into the relation between vertical cloud structure and dynamics of three polar lows: Observations from COMBLE

AbstractThe vertical structure of three polar lows is described using profiling radar, lidar, and passive remote sensors deployed at a coastal site in northern Norway. The data were collected as part of the Cold‐air Outbreaks in the Marine Boundary Layer Experiment (COMBLE). These data are examined in the context of satellite imagery, operational weather radar reflectivity imagery, and output from the AROME Arctic model. A comparison to satellite images shows that AROME‐Arctic captured the polar lows well. All three polar lows form in marine cold‐air outbreaks and are surrounded by open‐cellular convection typical of such outbreaks. Two of the lows form in a forward shear environment and one under reverse shear. Initially, the three polar lows are comma shaped; two of them transition to be more spiraliform at their mature stage and have mostly cloud‐free warm cores. The warm cores are the result of a warm‐seclusion process. All three lows have stratiform precipitation bands, marked by little cloud liquid water, and rather high surface precipitation rate. The vertical drafts and turbulence in these stratiform clouds are generally weak. All three lows also have convective clouds, which have stronger vertical drafts, stronger turbulence, and pockets of high liquid water content.

Vertical Structure of Clouds and Precipitation During Arctic Cold‐Air Outbreaks and Warm‐Air Intrusions: Observations From COMBLE

AbstractThe Arctic is marked by deep intrusions of warm, moist air, alternating with outbreaks of cold air down to lower latitudes. The typical vertical structure of clouds and precipitation during these two synoptic weather extremes is examined at a coastal site at 69°N in Norway. The Norwegian Sea is a corridor for warm‐air intrusions (WAIs) and frequently witnesses cold‐air outbreaks (CAOs). This study uses data from profiling radar, lidar, and microwave radiometer, radiosondes and other probes that were collected during the CAOs in the Marine Boundary Layer Experiment (COMBLE) between 1 December 2019 and 31 May 2020. Marine CAOs are defined in terms of thermal instability relative to the sea surface temperature, and WAIs in terms of equivalent potential temperature stratification between the surface and 850 hPa. Cloud structures in CAOs are convective, driven by strong surface heat fluxes over a long fetch of open water, with cloud tops rarely exceeding 6 km above sea level. The mostly open‐cellular convection produces intermittent moderately‐heavy precipitation at the observational site, notwithstanding the low precipitable water vapor (PWV). In contrast, WAIs are marked by high values of PWV and integrated vapor transport. WAI clouds are synoptically driven, stratiform, with cloud tops often exceeding 5 km, sometimes layered, and generally producing persistent precipitation that can be heavier than in CAOs.

Surface-based observations of cold-air outbreak clouds during the COMBLE field campaign

Abstract. Cold-air outbreaks (CAOs) are characterized by extreme air–sea energy exchanges and low-level convective clouds over large areas in the high-latitude oceans. As such, CAOs are an important component of the Earth's climate system. The CAOs in the Marine Boundary Layer Experiment (COMBLE) deployment of the US Department of Energy Atmospheric Radiation Measurement (ARM) Mobile Facility (AMF) provided the first comprehensive view of CAOs using a suite of ground-based observations at the northern coast of Norway. Here, cloud and precipitation observations from 13 CAO cases during COMBLE are analyzed. A vertical air motion retrieval technique is applied to the Ka-band ARM Zenith-pointing Radar (KAZR) observations. The CAO cumulus clouds are characterized by strong updrafts with magnitudes between 2–8 m s−1, vertical extents of 1–3 km, and horizontal scales of 0.25–3 km. A strong relationship between our vertical air velocity retrievals and liquid water path (LWP) measurements is found. The LWP measurements exceed 1 kg m−2 in strong updraft areas, and the vertical extent of the updraft correlates well with the LWP values. The CAO cumulus clouds exhibit eddy dissipation rate values between 10−3 and 10−2 m2 s−3 in the lowest 10 km of the atmosphere, and using a radar Doppler spectra technique, evidence of secondary ice production is found during one of the cases.

Cloud Morphology Evolution in Arctic Cold‐Air Outbreak: Two Cases During COMBLE Period

AbstractCloud feedbacks play an important role in Arctic warming. Cloud morphology, for example, cloud size and spatial distributions, is among key factors that directly impact their radiative effects. In this work, we use two cases observed during the Cold‐air Outbreak (CAO) in the Marine Boundary Layer Experiment (COMBLE) to study the evolution of cloud size distributions as an air mass is advected from the Arctic over a comparatively warm ocean and cloud mesoscale organization changes from rolls to cells. Cloud objects are identified from Moderate Resolution Imaging Spectroradiometer (MODIS) reflectance images through an object segmentation procedure and roll breakup is identified by homogeneities in cloud water path (CWP). Roll breakup is found to be accompanied by a local minimum in wind shear and local maxima in cloud size and marine cold air outbreak index. The mean cloud horizontal aspect ratio has weak fetch dependency and is around 2 in roll, transition, and cell regimes. Regardless of distance from the ice edge, smaller clouds (<10 km2) dominate the population number but not cloud cover. Cloud size distributions show bimodality in transition and cell regimes. For clouds with comparable sizes, mean nearest neighbor distances normalized by equivalent cloud radius converge to a single value for all regimes and for all but the smallest clouds, suggesting that clouds of comparable sizes in CAOs are separated by distance proportional to their sizes. The presented statistical results pave the way to evaluating model simulated cloud organizations during CAO events.

The COMBLE Campaign: A Study of Marine Boundary Layer Clouds in Arctic Cold-Air Outbreaks

Abstract One of the most intense air mass transformations on Earth happens when cold air flows from frozen surfaces to much warmer open water in cold-air outbreaks (CAOs), a process captured beautifully in satellite imagery. Despite the ubiquity of the CAO cloud regime over high-latitude oceans, we have a rather poor understanding of its properties, its role in energy and water cycles, and its treatment in weather and climate models. The Cold-Air Outbreaks in the Marine Boundary Layer Experiment (COMBLE) was conducted to better understand this regime and its representation in models. COMBLE aimed to examine the relations between surface fluxes, boundary layer structure, aerosol, cloud, and precipitation properties, and mesoscale circulations in marine CAOs. Processes affecting these properties largely fall in a range of scales where boundary layer processes, convection, and precipitation are tightly coupled, which makes accurate representation of the CAO cloud regime in numerical weather prediction and global climate models most challenging. COMBLE deployed an Atmospheric Radiation Measurement Mobile Facility at a coastal site in northern Scandinavia (69°N), with additional instruments on Bear Island (75°N), from December 2019 to May 2020. CAO conditions were experienced 19% (21%) of the time at the main site (on Bear Island). A comprehensive suite of continuous in situ and remote sensing observations of atmospheric conditions, clouds, precipitation, and aerosol were collected. Because of the clouds’ well-defined origin, their shallow depth, and the broad range of observed temperature and aerosol concentrations, the COMBLE dataset provides a powerful modeling testbed for improving the representation of mixed-phase cloud processes in large-eddy simulations and large-scale models.

An Atmospheric Hydraulic Jump in the Santa Barbara Channel

AbstractAs part of the Precision Atmospheric Marine Boundary Layer Experiment, the University of Wyoming King Air sampled an atmospheric environment conducive to the formation of a hydraulic jump on 24 May 2012 off the coast of California. Strong, northwesterly flow rounded the Point Arguello–Point Conception complex and encountered the remnants of an eddy circulation in the Santa Barbara Channel. The aircraft flew an east–west vertical sawtooth pattern that captured a sharp thinning of the marine boundary layer and the downstream development of a hydraulic jump. In situ observations show a dramatic rise in isentropes and a coincident sudden decrease in wind speeds. Imagery from the Wyoming Cloud Lidar clearly depicts the jump feature via copolarization and depolarization returns. Estimations of MBL depth are used to calculate the upstream Froude number from hydraulic theory. Simulations using the Weather Research and Forecasting Model produced results in agreement with the observations. The innermost domain uses a 900-m horizontal grid spacing and encompasses the transition from supercritical to subcritical flow south of Point Conception. Upstream Froude number estimations from the model compare well to observations. A strongly divergent wind field, consistent with expansion fan dynamics, is present upwind of the hydraulic jump. The model accurately resolves details of the marine boundary layer collapse into the jump. Results from large-eddy simulations show a large increase in the turbulent kinetic energy field coincident with the hydraulic jump.

Synthesis of Observations from the Precision Atmospheric Marine Boundary Layer Experiment (PreAMBLE)

Abstract Research flights during the Precision Atmospheric Marine Boundary Layer Experiment (PreAMBLE) in Southern California during May–June 2012 focused on three main features found in the nearshore marine boundary layer (MBL): the coastal jet (10 flights), the Catalina eddy (3 flights), and the initiation of a southerly surge (1 flight). Several topics were examined with case studies, but results from individual events may not represent typical conditions. Although these flights do not constitute a long-term set of data, observations from PreAMBLE are used to find common features. Two main topics are addressed: the MBL collapse into the expansion fan, and the subsequent transition into the Santa Barbara Channel (SBC). The midmorning to late afternoon flights occur during moderate to strong northerly wind. Slope of the MBL in the expansion fan varies and wave perturbations can be embedded within the expansion fan. As the cool MBL flow turns into the SBC, it moves underneath a deeper and warmer MBL that originates from the southeast over the warmer ocean. The temperature inversion between the cool and warm MBL erodes toward the east until there is only the inversion between the warm MBL and free troposphere. The dissipation of the lower layer into the SBC observed by the aircraft differs from previous conceptual models that depict a continuous MBL that thins and then deepens again in the SBC, which was inferred from sparse observations and numerical simulations. Only one flight within the SBC detected a hydraulic jump from 100 to 200 m above the surface.

Observations of Large Wind Shear above the Marine Boundary Layer near Point Buchon, California

Abstract Particularly strong winds along the coast of Southern California on 24 May 2012 were measured by the Wyoming King Air research aircraft during the Precision Atmospheric Marine Boundary Layer Experiment (PreAMBLE). The fast flow is bounded laterally by the coastal topography and vertically by a pronounced temperature inversion separating the cool, moist air in the marine boundary layer (MBL) from the warm, dry air aloft. Many studies have investigated the response of this two-layer flow to changes in the coastline by invoking hydraulic theory, which explains the essential characteristics including changes in MBL depth and the attendant wind. Processes occurring just above the MBL are important to the low-level thermodynamic and kinematic structure. Observations on this day demonstrate how the large shear above the MBL can impact the lower atmosphere. A typical two-layer system was observed north of Point Buchon, which was supercritical. Around Point Buchon, the depth of the MBL decreased and wind increased, characteristic of an expansion fan. As a result, the Richardson number becomes reduced and favors shear instability that breaks down into turbulence. Observations indicate that a secondary well-mixed layer develops above the MBL that is bounded by narrow layers of high stability separating the secondary layer from the MBL below and the free troposphere above. It is hypothesized that the secondary layer develops as a result of Kelvin–Helmholtz instability, although more targeted observations are needed to confirm or reject that hypothesis.

Aircraft Observations and Numerical Simulations of the Developing Stage of a Southerly Surge near Southern California

Abstract Summertime low-level winds in the marine boundary layer off the California coast are predominantly from the north. This pattern is interrupted periodically by southerly winds and low stratus that can propagate for hundreds of kilometers northward along the coast. These events have been termed coastally trapped disturbances, coastally trapped wind reversals, and southerly surges; their forcing has been the subject of extensive study and debate. Southerly surges remain difficult to forecast, yet have a significant impact on coastal activities. The beginning stage of a southerly surge on 16 June 2012 was explored during the Precision Atmospheric Marine Boundary Layer Experiment. Measurements of the horizontal wind and pressure field in the marine layer offshore from Cape Arguello hours prior to the onset of the surge were made using the University of Wyoming King Air research aircraft. Airborne measurements show that a horizontal pressure field is established with higher pressure to the south just prior to the surge, supporting southerly ageostrophic winds and a south-to-north movement of marine stratus. Aircraft soundings and lidar returns confirm the existence of offshore flow of warm, continental air north of Point Arguello that alters the pressure field adjacent to the coast. The southerly surge originates near Point Arguello and propagates northward past San Francisco during the early morning hours on 17 June 2012. Results from the Weather Research and Forecasting Model are consistent with the King Air observations. Analyses and model output presented here confirm that the large-scale environment is critical to the initiation of these wind reversals.

Aircraft Observations of the Marine Boundary Layer Adjustment near Point Arguello, California

AbstractNortherly winds set up by synoptic conditions are persistent in the marine boundary layer (MBL) off the California coast from late spring through summer. Wind, pressure, and MBL height are modulated as the low-level flow impinges on the points and capes along the California coast. The Precision Atmospheric Marine Boundary Layer Experiment was conducted in May and June of 2012 with the primary goal to directly measure the dynamics responsible for the wind field near Point Arguello and Point Conception. Detailed measurements of the horizontal pressure field within the MBL were made using the University of Wyoming King Air research aircraft. Airborne measurements made during cases of strong northerly wind show an abrupt adjustment of the MBL near Point Arguello, including a modulation of the horizontal pressure gradient force and a near collapse of the MBL. Airborne lidar measurements complement measurements of the horizontal pressure field and help to elucidate the large changes in the MBL height in the vicinity of Point Arguello. The Weather Research and Forecasting Model was used to simulate the 20 May 2012 case at a high resolution. Model results showed large-amplitude height perturbations near Point Arguello, similar to those observed from the airborne platform. In this case, the offshore flow played an important role in the local forcing.

Airborne Observations of a Catalina Eddy

Abstract Summertime low-level winds over the ocean adjacent to the California coast are typically from the north, roughly parallel to the coastline. Past Point Conception the flow often turns eastward, thereby generating cyclonic vorticity in the California Bight. Clouds are frequently present when the cyclonic motion is well developed and at such times the circulation is referred to as a Catalina eddy. Onshore flow south of the California Bight associated with the eddy circulation can result in a thickening of the low-level marine stratus adjacent to the coast. During nighttime hours the marine stratus typically expands over a larger area and moves northward along the coast with the cyclonic circulation. A Catalina eddy was captured during the Precision Atmospheric Marine Boundary Layer Experiment in June of 2012. Measurements were made of the cloud structure in the marine layer and the horizontal pressure field associated with the cyclonic circulation using the University of Wyoming King Air research aircraft. Airborne measurements show that the coastal mountains to the south of Los Angeles block the flow, resulting in enhanced marine stratus heights and a local pressure maximum near the coast. The horizontal pressure field also supports a south–north movement of marine stratus. Little evidence of leeside troughing south of Santa Barbara, California, was observed for this case, implying that the horizontal pressure field is forced primarily through topographic blocking by the coastal terrain south of Los Angeles, California, and the ambient large-scale circulation associated with the mean flow.

Measurements and modelling of molecular iodine emissions, transport and photodestruction in the coastal region around Roscoff

Abstract. Iodine emissions from the dominant six macroalgal species in the coastal regions around Roscoff, France, have been modelled to support the Reactive Halogens in the Marine Boundary Layer Experiment (RHaMBLe) undertaken in September 2006. A two-dimensional model is used to explore the relationship between geographically resolved regional emissions (based on maps of seaweed beds in the area and seaweed I2 emission rates previously measured in the laboratory) and in situ point and line measurements of I2 performed respectively by a broadband cavity ringdown spectroscopy (BBCRDS) instrument sited on the shoreline and a long-path differential optical absorption spectroscopy (LP-DOAS) instrument sampling over an extended light path to an off-shore island. The modelled point and line I2 concentrations compare quantitatively with BBCRDS and LP-DOAS measurements, and provide a link between emission fields and the different measurement geometries used to quantify atmospheric I2 concentrations during RHaMBLe. Total I2 emissions over the 100 km2 region around Roscoff are calculated to be 1.7×1019 molecules per second during the lowest tides. During the night, the model replicates I2 concentrations up to 50 pptv measured along the LP-DOAS instrument's line of sight, and predicts spikes of several hundred pptv in certain conditions. Point I2 concentrations up to 50 pptv are also calculated at the measurement site, in broad agreement with the BBCRDS observations. Daytime measured concentrations of I2 at the site correlate with modelled production and transport processes. However substantial recycling of the photodissociated I2 is required for the model to quantitatively match measured concentrations. This result corroborates previous modelling of iodine and NOx chemistry in the semi-polluted marine boundary layer which proposed a mechanism for recycling I2 via the formation, transport and subsequent reactions of the IONO2 reservoir compound. The methodology presented in this paper provides a tool for linking spatially distinct measurements to inhomogeneous and temporally varying emission fields.

Night-time radical chemistry during the NAMBLEX campaign

Abstract. Night-time chemistry in the Marine Boundary Layer has been modelled using a number of observationally constrained zero-dimensional box-models. The models were based upon the Master Chemical Mechanism (MCM) and the measurements were taken during the North Atlantic Marine Boundary Layer Experiment (NAMBLEX) campaign at Mace Head, Ireland in July–September 2002. The model could reproduce, within the combined uncertainties, the measured concentration of HO2 (within 30–40%) during the night 31 August–1 September and of HO2+RO2 (within 15–30%) during several nights of the campaign. The model always overestimated the NO3 measurements made by Differential Optical Absorption Spectroscopy (DOAS) by up to an order of magnitude or more, but agreed with the NO3 Cavity Ring-Down Spectroscopy (CRDS) measurements to within 30–50%. The most likely explanation of the discrepancy between the two instruments and the model is the reaction of the nitrate radical with inhomogeneously distributed NO, which was measured at concentrations of up to 10 ppt, even though this is not enough to fully explain the difference between the DOAS measurements and the model. A rate of production and destruction analysis showed that radicals were generated during the night mainly by the reaction of ozone with light alkenes. The cycling between HO2/RO2 and OH was maintained during the night by the low concentrations of NO and the overall radical concentration was limited by slow loss of peroxy radicals to form peroxides. A strong peak in [NO2] during the night 31 August–1 September allowed an insight into the radical fluxes and the connections between the HOx and the NO3 cycles.

Single-Particle Detection Efficiencies of Aerosol Time-of-Flight Mass Spectrometry during the North Atlantic Marine Boundary Layer Experiment

During the North Atlantic marine boundary layer experiment (NAMBLEX) sampling campaign at Mace Head, Ireland, both continental and maritime air masses were sampled. Aerosol was characterized both with a TSI 3800 time-of-flight mass spectrometer (ATOFMS) and a MOUDI microorifice impactor, and particle number counts were measured independently with an aerodynamic particle sizer. The data have been analyzed in order to elucidate factors determining the particle detection efficiencies of the ATOFMS. These are broken down according to the efficiency of the inlet system, the hit efficiency on particles which enter the sensing zone of the instrument and the sensitivity of the measured ion signal to the chemical species. A substantial matrix effect depending on the chemical composition of the aerosol sampled at the time was found, which is reflected in variations in the hit efficiency of particles entering the sensing zone of the instrument with the main desorption-ionization laser. This is in addition to the strong inverse power-law dependence of inlet transmission efficiency on particle diameter. The variation in hit efficiency with particle type is likely attributable to differences in the energetics of laser energy absorption, ablation, and ion formation. However, once variations in both inlet transmission and hit efficiencies are taken into account, no additional matrix dependence of ATOFMS response is required to obtain a linear relationship between the ion signal and the concentration of a particular chemical species. The observations show that a constant mass of material is ionized from each particle, irrespective of size. Consequently the integrated ion signal for a given chemical component and particle size class needs to be increased by a factor related to the cube of particle diameter in order to correlate with the airborne mass of that component.

Ambient formaldehyde measurements made at a remote marine boundary layer site during the NAMBLEX campaign – a comparison of data from chromatographic and modified Hantzsch techniques

Abstract. Ambient formaldehyde concentrations are reported from the North Atlantic Marine Boundary Layer Experiment (NAMBLEX) campaign at Mace Head on the west coast of Eire during August 2002. The results from two techniques, using direct determination via gas chromatography and the Hantzsch technique, show similar trends but a significant off set in concentrations. For westerly air flows characteristic of the marine boundary layer, formaldehyde concentrations from the gas chromatographic and Hantzsch technique ranged from 0.78–1.15 ppb and 0.13–0.43 ppb, respectively. Possible reasons for the discrepancy have been investigated and are discussed, however, no satisfactory explanation has yet been found. In a subsequent laboratory intercomparison the two techniques were in good agreement. The observed concentrations have been compared with previous formaldehyde measurements in the North Atlantic marine boundary layer and with other measurements from the NAMBLEX campaign. The measurements from the Hantzsch technique and the GC results lie at the lower and upper ends respectively of previous measurements. In contrast to some previous measurements, both techniques show distinct diurnal profiles with day maxima and with an amplitude of approximately 0.15 ppb. Strong correlations were observed with ethanal concentrations measured during NAMBLEX and the ratio of ethanal to formaldehyde determined by the gas chromatographic technique is in good agreement with previous measurements. Some simple box modelling has been undertaken to investigate possible sources of formaldehyde. Such models are not able to predict absolute formaldehyde concentrations as they do not include transport processes, but the results show that oxygenated VOCs such as ethanal and methanol are very significant sources of formaldehyde in the air masses reaching Mace Head.

The North Atlantic Marine Boundary Layer Experiment(NAMBLEX). Overview of the campaign held at Mace Head, Ireland, in summer 2002

Abstract. The North Atlantic Marine Boundary Layer Experiment (NAMBLEX), involving over 50 scientists from 12 institutions, took place at Mace Head, Ireland (53.32° N, 9.90° W), between 23 July and 4 September 2002. A wide range of state-of-the-art instrumentation enabled detailed measurements of the boundary layer structure and atmospheric composition in the gas and aerosol phase to be made, providing one of the most comprehensive in situ studies of the marine boundary layer to date. This overview paper describes the aims of the NAMBLEX project in the context of previous field campaigns in the Marine Boundary Layer (MBL), the overall layout of the site, a summary of the instrumentation deployed, the temporal coverage of the measurement data, and the numerical models used to interpret the field data. Measurements of some trace species were made for the first time during the campaign, which was characterised by predominantly clean air of marine origin, but more polluted air with higher levels of NOx originating from continental regions was also experienced. This paper provides a summary of the meteorological measurements and Planetary Boundary Layer (PBL) structure measurements, presents time series of some of the longer-lived trace species (O3, CO, H2, DMS, CH4, NMHC, NOx, NOy, PAN) and summarises measurements of other species that are described in more detail in other papers within this special issue, namely oxygenated VOCs, HCHO, peroxides, organo-halogenated species, a range of shorter lived halogen species (I2, OIO, IO, BrO), NO3 radicals, photolysis frequencies, the free radicals OH, HO2 and (HO2+Σ RO2), as well as a summary of the aerosol measurements. NAMBLEX was supported by measurements made in the vicinity of Mace Head using the NERC Dornier-228 aircraft. Using ECMWF wind-fields, calculations were made of the air-mass trajectories arriving at Mace Head during NAMBLEX, and were analysed together with both meteorological and trace-gas measurements. In this paper a chemical climatology for the duration of the campaign is presented to interpret the distribution of air-mass origins and emission sources, and to provide a convenient framework of air-mass classification that is used by other papers in this issue for the interpretation of observed variability in levels of trace gases and aerosols.

Peroxy radical chemistry and the control of ozone photochemistry at Mace Head, Ireland during the summer of 2002

Abstract. Peroxy radical (HO2+ΣRO2) measurements, using the PEroxy Radical Chemical Amplification (PERCA) technique at the North Atlantic Marine Boundary Layer EXperiment (NAMBLEX) at Mace Head in summer 2002, are presented and put into the context of marine, boundary-layer chemistry. A suite of other chemical parameters (NO, NO2, NO3, CO, CH4, O3, VOCs, peroxides), photolysis frequencies and meteorological measurements, are used to present a detailed analysis of the role of peroxy radicals in tropospheric oxidation cycles and ozone formation. Under the range of conditions encountered the peroxy radical daily maxima varied from 10 to 40 pptv. The diurnal cycles showed an asymmetric shape typically shifted to the afternoon. Using a box model based on the master chemical mechanism the average model measurement agreement was 2.5 across the campaign. The addition of halogen oxides to the model increases the level of model/measurement agreement, apparently by respeciation of HOx. A good correlation exists between j(HCHO).[HCHO] and the peroxy radicals indicative of the importance of HCHO in the remote atmosphere as a HOx source, particularly in the afternoon. The peroxy radicals showed a strong dependence on [NO2] with a break point at 0.1 ppbv, where the radicals increased concomitantly with the reactive VOC loading, this is a lower value than seen at representative urban campaigns. The HO2/(HO2+ΣRO2) ratios are dependent on [NOx] ranging between 0.2 and 0.6, with the ratio increasing linearly with NOx. Significant night-time levels of peroxy radicals were measured up to 25 pptv. The contribution of ozone-alkenes and NO3-alkene chemistry to night-time peroxy radical production was shown to be on average 59 and 41%. The campaign mean net ozone production rate was 0.11±0.3 ppbv h-1. The ozone production rate was strongly dependent on [NO] having linear sensitivity (dln(P(O3))/dln(NO)=1.0). The results imply that the N(O3) (the in-situ net photochemical rate of ozone production/destruction) will be strongly sensitive in the marine boundary layer to small changes in [NO] which has ramifications for changing NOx loadings in the European continental boundary layer.