Coastal Boundary Layer Research Articles

Transport and Mixing Between the Coastal and Offshore Waters in the Great Lakes: a Review

The Laurentian Great Lakes of North America have horizontal scales of hundreds of kilometers and depth scales of hundreds of meters. In terms of coastal dynamics, they behave much like inland seas and exhibit physical processes characteristic of the coastal oceans. The lakes are dynamically similar to the coastal ocean in that their horizontal dimensions are larger than the vertical dimensions, and the principal source of mechanical energy is the wind. The major difference in dynamical processes is that the lakes are enclosed basins and are not connected to the deep ocean. This paper presents an overview of some of the significant aspects of physical processes in the coastal zones of the Great Lakes. The review is based on examples ranging from lake-wide experiments like the International Field Year on the Great Lakes (IFYGL) to several process-oriented coastal boundary layer experiments. The basic circulations in the nearshore zone and coastal boundary layer are summarized. The review concludes with suggestions for future work on the understanding of the physical processes that would have a bearing on lake management in the coastal zones of the Great Lakes.

A numerical study of sea breeze circulation observed at a tropical site Kalpakkam on the east coast of India, under different synoptic flow situations

A simulation study of the sea breeze circulation and thermal internal boundary layer (TIBL) characteristics has been carried out at the tropical site Kalpakkam on the east coast of India, for operational atmospheric dispersion prediction. The community based PSU/NCAR MM5 Meso-scale meteorological model is used for the study. Three cases on typical days in summer (24 May 2003), southwest (SW) monsoon (1 July 2001) winter season (2 February 2003) with different large-scale flow pattern are studied. The MM5 model is used with 3 nested domains with horizontal grid resolutions 18 km, 6 km and 2 km and 26 vertical levels. The model is integrated for 24 hours in the above cases with initial and boundary conditions taken from NCEP-FNL analyses data. Observations of 10 meteorological stations and coastal boundary layer experiments conducted at Kalpakkam are used for comparison and validation of the simulation. The characteristics of simulated sea breeze and TIBL at Kalpakkam are seen to vary in the above cases according to the prevailing large-scale winds and surface fluxes. The sea breeze circulation is seen to develop early with larger strength and inland propagation in the summer case under the influence of moderate synoptic wind and strong heating conditions than in the SW monsoon and winter cases. The horizontal and vertical extents of TIBL are found to be larger in the summer case than in other cases. Although model parameters agree in general with observations, all the fine features are not clearly captured and some slowness in model sea breeze development is also seen. The results indicate the need to improve i) the initial conditions by assimilation of available surface/upper air observations to reduce model bias and ii) surface net radiation parameterisation. The model could predict the essential features of the local circulation and further improvement is expected with better initial condition data and incorporation of more realistic surface data.

Circulation and thermal structure in Lake Huron and Georgian Bay: Application of a nested-grid hydrodynamic model

A nested-grid hydrodynamic modelling system is used to study circulation and temperature distributions in Lake Huron (LH) and adjacent areas. This nested system is based on the three-dimensional, primitive-equation z-level ocean model. The nested system consists of two sub-components: a coarse-resolution outer model covering LH and Georgian Bay (GB) with a horizontal resolution of roughly 2.5 km, and the fine-resolution inner model covering eastern LH and northwestern GB with a horizontal resolution of roughly 900 m. Both the outer and inner models have 30 z-levels in the vertical. To assess the model performance, we simulate the three-dimensional circulation and temperature distributions of LH and GB in 1974–1975 and compare the model results with observations made in the lake. We demonstrate that outer model of the nested system simulates reasonably well the large-scale circulation and seasonal evolution of thermal stratifications in LH and GB, and the inner model produces reasonably well the three-dimensional flow and thermal structure over the coastal boundary layer close to the eastern shore of the lake.

Modelling coastal boundary layer flows over typical bed-forms

This paper presents details of hydrodynamics, turbulence characteristics, sediment concentrations and transport rates computed by a new three-dimensional numerical boundary layer model above typical coastal bed-forms. Comparisons are made against a large number of available laboratory and field measurements covering situations of current alone, wave alone, and combined wave and current, which demonstrate the model's ability to simulate the complex structure of flow interactions as well as the associated sediment transport processes with reasonably good accuracy. The model results are also parameterised by averaging over both bed-form length and wave period in an attempt to produce improved relations of the overall bed-form-induced roughness height and equivalent near-bed reference concentration for engineering morphological modelling. Verifications of these averaged results against some simple engineering predictors show realistic agreements along with some noticeable differences. Where differences are found, modifications are also suggested to the existing engineering formulae in order to improve their predictions for complex flows. Further model testing is also suggested to refine the constants used for the proposed modifications.

Modeling of the Coastal Boundary Layer and Pollutant Transport in New England

Abstract Concentrations of ozone exceeding regulatory standards are regularly observed along the coasts of New Hampshire and Maine in summer. These events are primarily caused by the transport of pollutants from urban areas in Massachusetts and farther south and west. Pollutant transport is most efficient over the ocean. The coastline makes transport processes complex because it makes the structure of the atmospheric boundary layer complex. During pollution episodes, the air over land in daytime is warmer than the sea surface, so air transported from land over water becomes statically stable and the formerly well-mixed boundary layer separates into possibly several layers, each transported in a different direction. This study examines several of the atmospheric boundary layer processes involved in pollutant transport. A three-dimensional model [the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)] run on grids of 2.5 and 7.5 km is used to examine the winds, thermodynamic structure, and structure of tracer plumes emitted from Boston, Massachusetts, and New York City, New York, in two different real cases—one dominated by large-scale transport (22–23 July 2002) and one with important mesoscale effects (11–14 August 2002). The model simulations are compared with measurements taken during the 2002 New England Air Quality Study. The model simulates the basic structure of the two different episodes well. The boundary layer stability over the cold water is weaker in the model than in reality. The tracer allows for easy visualization of the pollutant transport.

Effect of coastal boundary resolution and mixing upon internal wave generation and propagation in coastal regions

A non-linear two-dimensional vertically stratified cross-sectional model of a constant depth basin without rotation is used to investigate the influence of vertical and horizontal diffusion upon the wind-driven circulation in the basin and the associated temperature field. The influence of horizontal grid resolution, in particular the application of an irregular grid with high resolution in the coastal boundary layer is examined. The calculations show that the initial response to a wind impulse is downwelling at the downwind end of the basin with upwelling and convective mixing at the opposite end. Results from a two-layer analytical model show that the initial response is the excitation of an infinite number of internal seiche modes in order to represent the initial response which is confined to a narrow near coastal region. As time progresses, at the downwind end of the basin a density front propagates away from the boundary, with the intensity of its horizontal gradient and associated vertical velocity determined by both horizontal and vertical viscosity values. Calculations demonstrate the importance of high horizontal grid resolution in resolving this density gradient together with an accurate density advection scheme. The application of an irregular grid in the horizontal with high grid resolution in the nearshore region enables the initial response to be accurately reproduced although physically unrealistic short waves appear as the frontal region propagates onto the coarser grid. Parameterization of horizontal viscosity using a Smagorinsky-type formulation acts as a selective grid size-dependent filter, and removes the short-wave problem although enhanced smoothing can occur if the scaling coefficient in the formulation is too large. Calculations clearly show the advantages of using an irregular grid but also the importance of using a grid size-dependent filter to avoid numerical problems.

A numerical study of the influence of synoptic flow on coastal meso-scale circulations on the East and West Coasts of India

The effect of synoptic flow on the coastal boundary layer and land sea breeze circulation on the west and east coasts has been investigated by a numerical simulation using PSU/NCAR-MM5 mesoscale model. A study on 24th May 2003 during ARMEX observation period shows occurrence of land-sea breeze circulation and formation of thermal internal boundary layer (TIBL) on both the coastal regions in the daytime but of some differences. An early development and accelerated propagation of sea breeze circulation is noticed on the west coast compared to the east coast under the influence of the prevailing southwesterly flow. The strength of the sea breeze is more on the west coast. The mixing heights during daytime in the coastal regions are reduced by the formation of TIBL. The horizontal extent of TIBL is very less (24 km) on the west coast under the complex topographic effect. The sea breeze frontal activity is more intensive on the east coast with vertical velocity of the order of 2-4 m/sec by the retarding influence of southwesterly synoptic flow.

Atmospheric boundary layer observations on the East Coast at Kalpakkam using Doppler mini-sodar during different seasons

A phased array Doppler Mini-Sodar has been installed at Anupuram near Kalpakkam nuclear site during the Coastal Atmospheric Boundary Layer Experiments (CABLE) carried out on the east coast of India. Mini-Sodar was used to measure the wind profile and investigate the coastal boundary layer development due to land-sea breeze circulation for the study of coastal dispersion of pollutants. The atmospheric boundary layer, its variability during SW (August 13, 14 : ARMEX I and September 3, 7, 11, 14) and pre-monsoon period (April 27, 28 : ARMEX II) 2003 has been studied. Large wind direction change and high vertical velocity variance were observed during the onset of sea breeze. The Thermal Internal Boundary Layer (TIBL) height has been found to increase progressively with season: summer - 170 m, SW monsoon - 210 m and NE monsoon - 325 m whereas the Nocturnal Boundary Layer (NBL) height decrease progressively during the corresponding period: summer - 370 m, SW monsoon - 340 m and NE monsoon - 210 m.

The cycling and fate of terrestrially-derived sediments and nutrients in the coastal zone of the Great Barrier Reef shelf

The coastal zone of the Great Barrier Reef shelf, with an area of 30,000 km 2 and a water volume of 300 km 3, receives an average annual input of sediment on the order of 14–28 Mt y −1––an estimated two- to fourfold increase since European settlement. There is considerable concern about the impact and ultimate fate of terrestrially-derived nutrients entering the Great Barrier Reef World Heritage Area (GBRWHA). Analysis of current data suggests that microbial communities in coastal waters and in unconsolidated sediments metabolise nutrients equivalent to the entire dissolved and particulate nutrient load debouched from land. River-derived nutrients account for 40–80% of the carbon, 13–30% of the nitrogen, and 2–5% of the phosphorus necessary to support the observed rates of benthic and pelagic mineralisation in Princess Charlotte Bay in the far north, and in Rockingham Bay and Missionary Bay in the central section, of the GBRWHA. Nearly all nitrogen is ultimately returned to the atmosphere via denitrification. There is little net burial of nutrients in subtidal sediments. These budget estimates are based on a sparse data set, but it is clear that marine sources of nutrients (N-fixation by pelagic and benthic cyanobacteria) must be important, but the magnitude of these sources is poorly known and likely to be highly variable in space and time. Data from sediment trap deployments suggest that, despite significant re-suspension, sedimentation fluxes are sufficient to balance benthic mineralisation rates. Most organic material deposited to the benthos appears to be flocculent or colloidal aggregates, perhaps formed via microbial mediation and exudation of extra-cellular material. The geophysical dynamics of the coastal boundary layer plays an important role in concentrating biological and biogeochemical activity within a shallow, narrow coastal zone. Mangroves and tidal flats are small in area, but trap, transform, and store a disproportionate amount of sediment and organic matter within the GBRWHA. The highly efficient use of terrestrially-derived nutrients by benthic and pelagic microbes in the coastal zone helps to explain why coral reefs on the middle and outer shelf have remained relatively unscathed despite a significant increase in sediment delivery.

Review of Mechanics of Coastal Sediment Transport by Jorgen Fredsoe and Rolf Deigaard

Sedimentation processes in coastal areas are fascinating. The analysis of sediment transport under steady flow seems complex enough that one can only welcome this analytical treatment of sediment transport under wind and tidal waves in coastal areas. The book is the third volume of a set of four titles published by World Scientific in the broad area of Advanced Series on Ocean Engineering. The first volume is entitled The Applied Dynamics of Ocean Surface Waves by C.C. Mei. The second volume is Water Wave Mechanics for Engineers and Scientists by R.G. Dean and R.A. Dalrymple. The fourth volume is entitled Coastal Bottom Boundary Layers by P. Nielsen. This third volume is divided into 12 chapters: (1) “Basic concepts and potential wave theory”; (2) “Wave boundary layers”; (3) “Bed friction and turbulence in wave-current motion”; (4) “Waves in the surf zone”; (5) “Wave-driven currents”; (6) “Current velocity distribution in the surf zone”; (7) “Basic concepts of sediment transport”; (8) “Vertical distribution of suspended sediment waves and current over a plane bed”; (9) “Current-generated bed waves”; (10) “Wave-generated bed forms”; (11) “Cross-shore sediment transport and coastal profile development”; and (12) “Long-shore sediment transport and coastline development”. The title of this text is very appropriate. The book is intended for graduate students and describes the processes of sediment transport and how to represent them in mathematical models. The book is divided into two main parts. The first relates to hydrodynamic theory and the second to sediment transport and morphological development. The hydrodynamic part first reviews the elementary theory of water waves, followed with chapters on the turbulent wave boundary layer and turbulent interaction between waves and currents, and finally covers surf zone hydrodynamics and wave-driven currents. The part on sediment transport introduces the basic concepts (critical bed shear stress, bed load, suspended load and sheet layer, near-bed concentration and sloping bed); it treats suspended sediment in waves and current and in the

Coastal Boundary Layer Influence on Pollutant Transport in New England

AbstractAir pollution episodes in northern New England often are caused by transport of pollutants over water. Two such episodes in the summer of 2002 are examined (22–23 July and 11–14 August). In both cases, the pollutants that affected coastal New Hampshire and coastal southwest Maine were transported over coastal waters in stable layers at the surface. These layers were at least intermittently turbulent but retained their chemical constituents. The lack of deposition or deep vertical mixing on the overwater trajectories allowed pollutant concentrations to remain strong. The polluted plumes came directly from the Boston, Massachusetts, area. In the 22–23 July case, the trajectories were relatively straight and dominated by synoptic-scale effects, transporting pollution to the Maine coast. On 11–14 August, sea breezes brought polluted air from the coastal waters inland into New Hampshire.

Direct evidence for coastal iodine particles from Laminaria macroalgae – linkage to emissions of molecular iodine

Abstract. Renewal of ultrafine aerosols in the marine boundary layer may lead to repopulation of the marine distribution and ultimately determine the concentration of cloud condensation nuclei (CCN). Thus the formation of nanometre-scale particles can lead to enhanced scattering of incoming radiation and a net cooling of the atmosphere. The recent demonstration of the chamber formation of new particles from the photolytic production of condensable iodine-containing compounds from diiodomethane (CH2I2), (O'Dowd et al., 2002; Kolb, 2002; Jimenez et al., 2003a; Burkholder and Ravishankara, 2003), provides an additional mechanism to the gas-to-particle conversion of sulphuric acid formed in the photo-oxidation of dimethylsulphide for marine aerosol repopulation. CH2I2 is emitted from seaweeds (Carpenter et al., 1999, 2000) and has been suggested as an initiator of particle formation. We demonstrate here for the first time that ultrafine iodine-containing particles are produced by intertidal macroalgae exposed to ambient levels of ozone. The particle composition is very similar both to those formed in the chamber photo-oxidation of diiodomethane and in the oxidation of molecular iodine by ozone. The particles formed in all three systems are similarly aspherical. When small, those formed in the molecular iodine system swell only moderately when exposed to increased humidity environments, and swell progressively less with increasing size; this behaviour occurs whether they are formed in dry or humid environments, in contrast to those in the CH2I2 system. Direct coastal boundary layer observations of molecular iodine, ultrafine particle production and iodocarbons are reported. Using a newly measured molecular iodine photolysis rate, it is shown that, if atomic iodine is involved in the observed particle bursts, it is of the order of at least 1000 times more likely to result from molecular iodine photolysis than diiodomethane photolysis. A hypothesis for molecular iodine release from intertidal macroalgae is presented and the potential importance of macroalgal iodine particles in their contribution to CCN and global radiative forcing are discussed.

Processes influencing wind-induced current profiles in near coastal stratified regions

Cross sectional and single point models of wind-induced currents in a stratified sea coastal region are used to determine the role of internal waves due to various wind periods upon mixing and current profiles. The limitations of a point model in reproducing current profiles are also examined. Calculations are performed with both fixed vertical diffusion coefficients and those derived from a turbulence closure model. Wind forcing at sub-inertial, inertial and super-inertial frequencies together with a wind impulse are considered. Results show that the offshore extent of the coastal boundary layer where mixing occurs depends upon the frequency of the wind forcing. The extent of this region and offshore propagation of internal waves influences the offshore variability of current profiles and the extent to which they can be reproduced by a single point model. Calculations with periodic wind forcing show that within the coastal boundary layer a point model cannot reproduce the current profile due to the internal pressure gradients associated with the internal wave field. However outside this region, current profiles from a point model are in good agreement with those computed with the cross section model when a barotropic pressure gradient forcing proportional to the local wind forcing is included. In the case of transient wind forcing, the inertial period dominates and determines the offshore extent of the coastal boundary layer. The transient nature of the response is such that although the single point model, outside the coastal region can reproduce the surface current due to direct wind forcing, the current at depth is not reproduced. The offshore extent of the coastal boundary layer computed with the turbulence energy model is comparable to that found with fixed diffusion coefficients although in some cases there are important differences which influence current profiles.

OH in the coastal boundary layer of Crete during MINOS: Measurements and relationship with ozone photolysis

Abstract. Hydroxyl radical (OH) concentrations were measured in August 2001 at Finokalia Station on the northeastern coast of Crete during the Mediterranean Intensive Oxidant Study (MINOS). OH was measured based on selected ion chemical ionization mass spectrometry (SI/CIMS) with a time resolution of 30 sec and signal integration of 5 min. The corresponding accuracy, precision, and detection limit were 20% (1σ), 11% (1σ), and 2.4 x 105 molecules cm-3 (2σ), respectively. OH levels showed a strong diurnal variability with high maxima (approximately 2 x 107 molecules cm-3) occurring around 13:30 LT (10:30 UTC) and nighttime values below the detection limit. Daily 24-hour average concentrations varied between 3.6-6.7 x 106 cm-3. For the total measurement period (6-21 August) the mean and standard deviation were 4.5 ± 1.1 x 106 cm-3. The OH data set is analyzed based on a classification into three periods: I: Aug 6-8, II: Aug 9-11, III: Aug 13-18. For each of the three periods the measured OH concentrations are described by the empirical function [OH] = a J(O1D)b, with J(O1D) being the ozone photolysis frequency and a = 1.4 x 1010 s cm-3, 1.7 x 1010 s cm-3, 2.2 x 1010 s cm-3, and b = 0.68, respectively. Taking into account the estimated precision of the OH measurements this empirical function using three values for a and one value for b explains 99% of the observed variance of OH. A detailed sensitivity analysis using a CH4-CO box model was performed to interpret this relationship, in particular the meanings of the pre-exponential factor a and the exponent b. It was found that the value of b which represents the total logarithmic dependence of [OH] on J(O1D) includes the individual contributions from the photolysis of O3, NO2, HCHO, HONO, and H2O2 which could be determined using the box model. For the conditions prevailing during the MINOS campaign the exponent b was found to be dominated by the contributions from O3- and NO2-photolysis. For the individual functional dependences between [OH] and J(O1D), [OH] and J(NO2), and J(NO2) and J(O1D) the partial logarithmic derivatives were determined to be 0.5, 0.6, and 0.3, respectively. Overall, the box model yields a value of 0.70 for the exponent b in very good agreement with the corresponding value derived from the empirical analysis of the measurements. This empirical approach in which the chemical air mass characteristics influencing the OH radical balance and thereby, the self-cleansing efficiency of the atmosphere, are represented by only two parameters which are constant over quite substantial time periods may be used in future experiments to test and compare OH measurements made in different atmospheric environments.

On dynamics and mixing of inflowing saltwater in the Arkona Sea

Time series measurements of temperature, salinity and current at the sills and the rim of the Arkona Basin have been performed during the winter seasons 1998/1999 and 1999/2000 together with ship‐based measurements of stratification and currents covering the whole Arkona Basin. The well‐mixed brackish surface was separated from the stratified saline bottom water by a halocline at a mean depth of 30 m. The saline bottom water was contained in an up to 15 m thick pool filling the central part of the Arkona Basin. This stratification resulted in a 6 km wide Rossby radius and a phase velocity of 0.7 m/s for the first baroclinic mode. The mean stratification was disturbed by cyclonic geostrophic eddies with a characteristic radius equal to the first baroclinic Rossby radius and up‐ and downwelling processes in the coastal boundary layer. 90% of the variability of the vertical current profile can be described by the first and second EOF. The most pronounced feature of the current profile is the existence of an Ekman bottom friction layer found in both modes. The thickness of the Ekman layer of the barotropic mode is 10–15 m and that of the baroclinic mode is about 5m. The baroclinic circulation consists of a pulsating cyclonic motion in the bottom layer and an anticyclonic motion in the upper layer along the rim of the Arkona Basin, driven by the spilling of saline bottom water over the sills into the Arkona Basin. The saline bottom water is flowing as gravity current from the sills into the Arkona Basin which is in a first order balanced by gravity and the Coriolis force, whereas friction is smaller than the other two forces involved. The friction imposes a spiral motion into the center of the Arkona Basin where it contributes to the dense bottom water pool. The front of the dense bottom water moves with a phase speed of about 30 cm/s at the westerly rim and slows down to about 10 cm/s along the southerly and easterly rim of the Arkona Basin. Evidence was found that the dense bottom water plume mixes with the ambient water by wind induced entrainment in the vicinity of the sill. The main mixing mechanism of a propagating saltwater plume is mixing of the water ahead of its front with the bottom water behind the front by differential advection.

CONSIDERATIONS IN ESTIMATING LARVAL DISPERSAL DISTANCES FROM OCEANOGRAPHIC DATA

Determination of larval dispersal distances and larval origins is a central challenge in contemporary marine ecology. In this work, the larval dispersal problem is discussed from the perspective of oceanography. Following formulation of the advection- diffusion model, the importance of scale is argued. When considering dispersion parameters at the appropriate population scales, advection is usually weaker than initially anticipated (and often used), and diffusion is stronger than typically used in model studies. Focusing attention on coastal populations, the importance of retention zones is described, and the more general existence of a coastal boundary layer is discussed. The coupling of cross- shore and alongshore dispersion results in a nonlinear relation between alongshore dispersal distance and larval planktonic period for dispersion in a sheared flow. Thus, small changes in cross-shore dispersal, whether due to environmental differences or larval behavior result in significant differences in alongshore dispersal. Finally, the interplay between advection and diffusion is explored, showing the importance of adequately representing the diffusive effects that mitigate alongshore advection. In most cases, diffusion acts to prevent ''wash- out'' of a population and allows for more flexibility in the size and spacing of effective marine reserves. Future challenges must bring oceanographers and ecologists together around specific dispersal problems if there is to be a significant improvement in the notable absence of hard data in this field of enquiry.

Zooplankton from the Maracajaú Reefs, Northeastern Brazil.

This study was carried out to assess the diversity and the abundance of the reef zooplankton in the Maracajaú reefs. Samples were collected with two plankton nets (65 and 300 micrometers mesh size, each) from February to June/2000 at three stations in a transect perpendicular to coast. Plankton biomass in the 65 µm fraction varied from 45.19 mg.m-3 to 316.45 mg.m-3. In the 300 µm fraction biomass varied from 9.33 mg.m-3 to 43.16 mg.m-3. Zooplankton presented 61 taxa. Copepods were the most diverse taxa with 23 species. Euterpina acutifrons, Oithona oswaldocruzi, Oithona hebes, Oithona sp., Parvocalanus crassirostris, Acartia lilljeborgi, Bivalvia and Gastropoda larvae, Trilocularina sp., Balanus sp. larvae, Spirillina sp., Polychaeta larvae, Brachyura (zoea) and Oikopleura longicauda occurred in more than 75% of the samples. The zooplankton followed a seasonal pattern, with the highest numbers occurring during the rainy season (February). A high density of microzooplankton was found which varied from 1,917 ind.m-3 to 47,620 ind.m-3. The macrozooplankton presented lower densities with a minimum of 92.4 ind.m-3 and a maximum of 1,617 ind.m-3. Species diversity was intermediate (~2 bits.ind-1) and eveness relatively high (~0.78). Samples could be joined into 3 groups, mainly influenced by rainfall. Species similarity presented 5 groups: oceanic, neritic, a mixture of neritic and oceanic, reef and estuarine. Our data suggest a dynamic relationship between the zooplankton communities characteristic of inshore water trapped by the coastal boundary layer and more offshore plankton communities. Key words: Zooplankton, reefs, distribution, diversity

Gas‐aerosol relationships of H2SO4, MSA, and OH: Observations in the coastal marine boundary layer at Mace Head, Ireland

Atmospheric concentrations of gaseous sulfuric acid (H2SO4), methane sulfonic acid (MSA), and hydroxyl radicals (OH) were measured by chemical ionization mass spectrometry (CIMS) during the second New Particle Formation and Fate in the Coastal Environment (PARFORCE) campaign in June 1999 at Mace Head, Ireland. Overall median concentrations in marine background air were 1.5, 1.2, and 0.12 × 106 cm−3, respectively. H2SO4 was also present at night indicating significant contributions from nonphotochemical sources. A strong correlation was found between daytime OH and H2SO4 levels in clean marine air suggesting a fast local production of H2SO4 from sulfur precursor gases. Steady state balance calculations of ambient H2SO4 levels agreed with measured concentrations if either very low H2SO4 sticking coefficients (0.02–0.03) or sources in addition to the SO2 + OH reaction were assumed. Overall, variations in ambient H2SO4 levels showed no correlation with either the tidal cycle or ultrafine particle (UFP) concentrations. However, on particular days an anticorrelation between H2SO4 and UFP levels was occasionally observed providing evidence for the contribution of H2SO4 to new particle formation and/or particle growth. Gaseous MSA concentrations were inversely correlated with dew point temperature reflecting a highly sensitive gas‐particle partitioning equilibrium of this compound. The present observations seriously question the general use of MSA as a conservative tracer to infer the relative production yield of H2SO4 from dimethylsulfide (DMS) oxidation. MSA/H2SO4 concentration ratios typically ranged between 0.06 and 1.0 in marine air at ground level. Measured diel OH profiles showed a significant deviation from concurrent variations of the ozone photolysis frequency. They also showed up to 1 order of magnitude lower values compared to OH concentrations calculated with a simple photochemical box model. These differences were most pronounced during particle nucleation events occurring on sunny days around noon and at low tide. The present results suggest that both the oxidation capacity and the particle formation potential in the coastal boundary layer were significantly affected by reactions of unknown compounds prevailing in this type of environment.

An Algorithm for the Calculation of the Time-Dependent Mixing Height in Coastal Sites

Abstract This paper describes a simple algorithm for the calculation of the time-dependent mixing height h(t) in coastal sites using wind, temperature, momentum flux, and heat flux time series as input data. A stationary expression for the coastal internal boundary layer height is used to avoid the solution of a partial differential equation for the evolution of h as a function of time and fetch, thus reducing the problem to a direct integration in time. The algorithm has been implemented in a previously developed scheme that allows a continuous calculation over the diurnal cycle. The calculated evolution of h(t) is compared with sodar estimates in a coastal site over a few days.

Influence of micrometeorological features on coastal boundary layer aerosol characteristics at the tropical station, Trivandrum

Characteristics of aerosols in the Atmospheric Boundary Layer (ABL) obtained from a bistatic CW lidar at Trivandrum for the last one decade are used to investigate the role of ABL micro-meteorological processes in controlling the altitude distribution and size spectrum. The altitude structure of number density shows three distinct zones depending on the prevailing boundary layer feature; viz, the well-mixed region, entertainment region and upper mixing region. In the lower altitudes vertical mixing is very strong (the well-mixed region) the upper limit of which is defined as aerosol-mixing height, is closely associated with the low level inversion. The aerosol mixing height generally lies in the range 150 to 400 m showing a strong dependence on the vertical eddy mixing processes in ABL. Above this altitude, the number density decreases almost exponentially with increase in altitude with a scale height of 0.5–1.5 km. The aerosol mixing height is closely associated with the height of the Thermal Internal Boundary Layer (TIBL). Sea-spray aerosols generated as a result of the interaction of surface wind with sea surface forms an important component of mixing region aerosols at this location. This component shows a non-linear dependence on wind speed. On an average, depending on the season, the mixing region contributes about 10–30% of the columnar aerosol optical depth (AOD) at 0.5Μm wavelength. A long term increasing trend (∼ 2.8% per year) is observed in mixing region AOD from 1989 to 1997. A study on the development of the aerosols in the nocturnal mixing region shows that the convectively driven daytime altitude structure continues to persist for about 4–5 hrs. after the sunset and thereafter the altitude structure is governed by vertical structure of horizontal wind. Stratified aerosol layers associated with stratified turbulence is very common during the late night hours.