Sea Breeze Conditions Research Articles

Lateral Baroclinic Forcing Enhances Sediment Transport from Shallows to Channel in an Estuary

We investigate the dynamics governing exchange of sediment between estuarine shallows and the channel based on field measurements at eight stations spanning the interface between the channel and the extensive eastern shoals of South San Francisco Bay. The study site is characterized by longitudinally homogeneous bathymetry and a straight channel, with friction more important than the Coriolis forcing. Data were collected for 3 weeks in the winter and 4 weeks in the late summer of 2009, to capture a range of hydrologic and meteorologic conditions. The greatest sediment transport from shallows to channel occurred during a pair of strong, late-summer wind events, with westerly winds exceeding 10 m/s for more than 24 h. A combination of wind-driven barotropic return flow and lateral baroclinic circulation caused the transport. The lateral density gradient was produced by differences in temperature and suspended sediment concentration (SSC). During the wind events, SSC-induced vertical density stratification limited turbulent mixing at slack tides in the shallows, increasing the potential for two-layer exchange. The temperature- and SSC-induced lateral density gradient was comparable in strength to salinity-induced gradients in South Bay produced by seasonal freshwater inflows, but shorter in duration. In the absence of a lateral density gradient, suspended sediment flux at the channel slope was directed towards the shallows, both in winter and during summer sea breeze conditions, indicating the importance of baroclinically driven exchange to supply of sediment from the shallows to the channel in South San Francisco Bay and systems with similar bathymetry.

Study of wind field under sea breeze conditions; an application of WRF model

Weather Research and Forecast (WRF) model is applied to study the wind field over the east coast of central Greece under typical summer conditions. This study aims at identifying the major features of the sea breeze circulation that is very frequently developing in the area during the warm period of the year and at verifying the model's ability to simulate the complex flow that is strongly influenced by the complicated coastline and very steep terrain. The simulation is carried out for a five-day period, characterized by sea breeze development. It is found that a complex meteorological phenomenon is evolving in the area, as two circulation systems are successively developing, the first over Pagasitikos Gulf, which is overridden by the system that develops later over the Aegean Sea. The major characteristics of the wind patterns and especially the sea breeze systems are identified and discussed. Additionally, the impact of topography and slope on sea breeze circulation is also commented. Model predictions agree fairly well with the observations taken at a near-surface meteorological station.

Simulation of equatorial wind field patterns with TAPM during the 1997 haze episode in Peninsular Malaysia

The large‐scale forest fires that occurred during the major El Niño event in 1997 can be counted among the worst environmental disasters in Southeast Asia. This study investigates atmospheric mesoscale features over Sumatra and the Klang Valley in Peninsular Malaysia during the resultant haze episode of September 1997 utilizing a limited area mesoscale three‐dimensional meteorological and dispersion model, The Air Pollution Model (TAPM).Mesoscale features that would not be highlighted by global numerical prediction models, such as the daily land and sea breeze conditions at the selected air pollution stations located near the west coast of Peninsular Malaysia, were predicted with an Index of Agreement of 0.3, which implied a moderate conformity between the predicted and observed values. Tracer analysis of air particles at a selected location in the Straits of Malacca verified the existence of the landward and shoreward movement of the air during the simulation of the low‐level wind field. Air pollutants from the burning areas of neighbouring Sumatra just across the straits were transported towards the Klang valley during the daytime and seawards during the nighttime, highlighting the recirculation features of aged and newer air particles over the seven days (13–18 September) of the model simulation. The presence of the central Main Range east of the Klang valley to a certain extent limited further eastward movement of the air particles. Near calm conditions at low levels were simulated from midnight to midday on 14 September, where the movement of the polluted air particles from the uncontrolled burning in Sumatra was confined within the Straits of Malacca. Turbulence within the planetary boundary layer in terms of the total kinetic energy was found to be weak from 14 to 15 September, congruent with the weak strength of low level winds that reduced the ability of the air to transport the pollutants during the period of severe smoke haze.Statistical evaluation showed that parameters such as the systematic RMSE (root mean square error) and unsystematic RMSE for the zonal wind component were slightly higher than for the meridional one, indicating higher errors between the observed and simulated zonal values. Otherwise, the equatorial meteorological parameters such as wind speed and temperature were successfully simulated by the model with comparatively high correlation coefficients, lower RMSEs and moderately high indices of agreement with observed values.

Mesoscale model simulation of low level equatorial winds over Borneo during the haze episode of September 1997

The large-scale vegetation fires instigated by the local farmers during the dry period of the major El Niño event in 1997 can be considered as one of the worst environmental disasters that have occurred in southeast Asia in recent history. This study investigated the local meteorology characteristics of an equatorial environment within a domain that includes the northwestern part of Borneo from the 17 to 27 September 1997 during the height of the haze episode by utilizing a limited area three-dimensional meteorological and dispersion model, The Air Pollution Model (TAPM).Daily land and sea breeze conditions near the northwestern coast of Borneo in the state of Sarawak, Malaysia were predicted with moderate success by the index of agreement of less than one between the observed and simulated values for wind speed and a slight overprediction of 2.3 of the skill indicator that evaluates the standard deviation to the observed values. The innermost domain of study comprises an area of 24,193 km2, from approximately 109°E to 111°E, and from 1°N to 2.3°N, which includes a part of the South China Sea. Tracer analysis of air particles that were sourced in the state of Sarawak on the island of Borneo verified the existence of the landward and shoreward movements of the air during the simulation of the low level wind field. Polluted air particles were transported seawards during night-time, and landwards during daytime, highlighting the recirculation features of aged and newer air particles during the length of eleven days throughout the model simulation. Near calm conditions at low levels were simulated by the trajectory analysis from midnight to mid-day on the 22 of September 1997. Low-level turbulence within the planetary boundary layer in terms of the total kinetic energy was weak, congruent with the weak strength of low level winds that reduced the ability of the air to transport the pollutants.Statistical evaluation showed that parameters such as the systematic RMSE and unsystematic RMSE between the observed and simulated values indicated the modest skill of the model in simulating the low level winds. Otherwise, the equatorial meteorological parameters such as wind speed and temperature were successfully simulated by the model with comparatively high correlation coefficients, lower RMSEs and moderately high indices of agreement with observed values.

Evaluation of a Mesoscale Model with Different Surface Parameterizations and Vertical Resolutions for the Bay of Valencia

Abstract Two different setups of the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) are used and the results of 71 forecasts are evaluated with a focus on the accuracy of meteorological surface data including wind, temperature, dewpoint temperature, and pressure. The forecasts, which cover the Valencia coastal region with a 3-km grid, are nested into two coarser grids of 9- and 27-km mesh size. The integration time is 18 h. The ability to correctly simulate the sea breeze plays a major role for this area. Different model setups are used: in the M1 simulations 34 atmospheric vertical sigma levels combined with a five-layer land surface model (5L LSM) are applied for the first half of the year 2006. In total, 52 atmospheric vertical sigma levels and the Noah LSM are used for the second half of 2006 (setup M2). The accuracy improves for all meteorological parameters except pressure with the M2 setup. However, negative biases of temperature, dewpoint temperature, and wind speed worsen. Detailed analysis of results for 4 days simulated with all combinations of vertical resolutions and LSMs shows that the increase of vertical resolution is responsible for the improved forecast performance. The Noah LSM generally improves temperature performance, but reduces the performance of all other parameters, especially for days with sea-breeze conditions. The Noah LSM performs better than the 5L LSM scheme only for a day driven by synoptic changes.

Detailed chemical analysis of regional-scale air pollution in western Portugal using an adapted version of MCM v3.1

A version of the Master Chemical Mechanism (MCM) v3.1, refined on the basis of recent chamber evaluations, has been incorporated into a Photochemical Trajectory Model (PTM) and applied to the simulation of boundary layer photochemistry in the Portuguese west coast region. Comparison of modelled concentrations of ozone and a number of other species (NO x and selected hydrocarbons and organic oxygenates) was carried out, using data from three connected sites on two case study days when well-defined sea breeze conditions were established. The ozone concentrations obtained through the application of the PTM are a good approximation to the measured values, the average difference being ca. 15%, indicating that the model was acceptable for evaluation of the details of the chemical processing. The detailed chemistry is examined, allowing conclusions to be drawn concerning chemical interferences in the measurements of NO 2, and in relation to the sensitivity of ozone formation to changes in ambient temperature. Three important, and comparable, contributions to the temperature sensitivity are identified and quantified, namely (i) an effect of increasing biogenic emissions with temperature; (ii) an effect of increasing ambient water vapour concentration with temperature, and its influence on radical production; and (iii) an increase in VOC oxidation chain lengths resulting from the temperature-dependence of the kinetic parameters, particularly in relation to the stability of PAN and its higher analogues. The sensitivity of the simulations to the refinements implemented into MCM v3.1 are also presented and discussed.

Constant volume balloons measurements in the urban Marseille and Fos–Berre industrial ozone plumes during ESCOMPTE experiment

ESCOMPTE programme aims at studying the emissions of primary pollutants in industrial and urban areas, their transport, diffusion and transformation in the atmosphere. This experiment, carried out in southeast France, can be used to validate and to improve meteorological and chemical mesoscale models. One major goal of this experiment was to follow the pollutant plumes, and to investigate its thermodynamic and physico-chemical time evolution. This was realized by means of constant volume balloons, located by global position satellite (GPS) and equipped with thermodynamic and ozone sensors, flying at constant density levels. During the two ESCOMPTE campaigns that took place in June and July 2000 and 2001, 40 balloons were launched, 17 of them equipped with ozone sensors during the day from 0800 to 1800 UTC. Balloons’ altitudes flight levels ranged between 400 and 1200 m altitude with Mistral (northerly synoptic flow) and Sea Breeze (southerly breeze) conditions. The atmospheric boundary layer (ABL) topography of the experimental domain is complex and varies strongly from day to day. Its depth presents a large gradient from the sea coast to the north part of the ESCOMPTE domain, and also more complex variability within the domain. The balloons’ trajectories describe the evolution of the pollutant plume emitted from the industrial area of Fos-Berre or from the Marseille urban area. Constant volume balloons give a good description of the trajectories of these two plumes. The balloons, which fly at an isopicnic level, cross different atmospheric layers chiefly depending on the ABL height in relation with the constant volume balloons flight level. Thus, each balloon flight is decomposed into different segments that correspond to the same atmospheric layer. In each segment, the ozone content variation is analyzed in relation to other thermodynamical parameters measured by the balloon and mainly to the vapor mixing ratio content. During ESCOMPTE campaign, the mean linear rate of chemical net ozone production at the top of the atmospheric boundary layer was found to be around 6 ppb h −1.

Photochemical ozone formation at Portugal West Coast under sea breeze conditions as assessed by master chemical mechanism model

Results on photochemical pollutants monitoring obtained in the field campaign CZCM-2001 were used as a database in photochemical trajectory model using the explicit mechanism Master Chemical Mechanism version 3 (MCM v3) aiming to simulate the formation of photochemical pollutants in air masses during the sea breeze circulation at Portugal West Coast. The modelling results were in good agreement with experimental data. The sensitivity analysis of MCM v3 revealed that under experimental sea breeze conditions, the initial concentrations of air pollutants such as NO x and VOCs at the coast are crucial factors influencing the level of ozone production in inland rural areas.

Photochemical pollution under sea breeze conditions, during summer, at the Portuguese West Coast

Photochemical pollution was studied at three sampling places located on the Portuguese West Coast along the coast-to-continent breeze direction, during the summer period. A first basic campaign (June–July 2001) covered mainly the monitoring of trace gases, volatile organic compounds (VOCs) and the vertical profiles of O 3. During a second auxiliary campaign (July 2002), the vertical profiles of pollutants (O 3 and VOCs) at the west coast point were further assessed. The important role of marine breezes on the formation and transport of photochemical pollutants has been confirmed. Vertical profiles of O 3 revealed a depletion of ozone at altitudes lower than 100–300 m during the night and the early morning and a good vertical mixture during the day in the sea breeze boundary layer. The analysis of vertical profiles of VOCs revealed that lower concentrations tended to exist at the top of the sea breeze layer. However some compounds, such as n-hexane, showed a higher concentration at higher levels. The concentration of O 3 in the diurnal boundary layer, resulting from photochemical reactions of both the anthropogenic and biogenic precursors, gradually increased with the sea breeze inland, ranging from around 30 ppb at midday on the coast, to 70 ppb in the inland rural zone. However, in all sampling places, the photochemical pollution episodes at 80–100 ppb level took place under particular conditions of high photochemical activity and see breeze circulation. As revealed from analyses of propylene equivalent concentrations, the photochemical formation of O 3 in the inland rural zone is strongly affected by biogenic VOCs (isoprene and monoterpenes), whereas a major contribution to the photochemical pollution with ozone in the coastal zone issued from anthropogenic VOCs (alkenes and aromatics).

An experimental study of aerosol distribution over a Mediterranean urban area

In this study an attempt is made to investigate the aerosol spatial and size distributions at different heights over the Greater Athens Area (GAA), Greece, under sea breeze conditions and clear sky and to further discuss possible implications for aerosol characteristics. The data used are airborne measurements of aerosol collected during two flights that were performed within the context of the 1997 STAAARTE experimental campaign. The aerosol measurements cover particle diameters from 0.1 to 45.5 μm. The horizontal and vertical distribution revealed that higher concentrations exist within or just above the atmospheric boundary layer, while greater concentrations are observed over the sea compared to land at high altitudes. At all altitudes the number size distributions show dominant diameter ranges between 0.1 and 0.3 μm at all altitudes. The volume distributions are characterised by two modes, one in the accumulation and the other in the coarse particle regime. At lower altitudes, fresh combustion emissions more likely cause the predominance of the size range 0.1–0.3 μm while enhanced physical and chemical processes that favour the growth of smaller particles to larger sizes could also act. The relative humidity does not seem to affect the observed number size distributions at low altitudes, where relative humidity is below 70% while at 4000 m the distributions seem to change over the sea where the humidity increases.

Ceilometer observations of aerosol layer structure above the Petit Lubéron during ESCOMPTE's IOP 2

A modified ceilometer has been used during the second Intensive Observation Period (IOP) of the “Expérience sur Site pour COntraindre les Modèles de Pollution atmosphériques et de Transport d'Émission” (ESCOMPTE) to perform continuous remote observations of aerosol accumulations in the first 3 km of the atmosphere. These observations encompassed an episode of intense particulate and photochemical pollution. The submicronic particles density, measured at an altitude of 600 m, went from a very low point of a few tens of particles per cubic centimeter (at the end of a Mistral episode in the free atmosphere) to a high point of more than 4500 particles per cubic centimeter (when pollutants were trapped by thermal inversions). The main result is that this instrument enables a fine documentation of the mixing layer height and of aerosol particles stratifications and circulation. Airborne aerosol measurements have been made above the mountainous region of Mérindol in order to validate in situ the remote sensing measurements. Ozone measurements near the summit of the mountains as well as in the valley were performed in order to correlate aerosol accumulation and ozone concentration. As a notable example, the two-layer aerosol stratification seen in the first 2 days of IOP 2b in that part of the ESCOMPTE domain confirms the results of another team which used backtrajectories. The low-altitude pollution for this timeframe had a local origin (the Fos industrial area), whereas above 500 m, the air masses had undergone regional-scale transport (from north-eastern Spain). The second major result is the highlighting of a pattern, in sea breeze conditions and in this part of the ESCOMPTE experiment zone, of nocturnal aerosol accumulation at an altitude of between 500 and 2000 m, followed by high ozone concentration the next day.

CO2 Flux Measurements over the Sea Surface by Eddy Correlation and Aerodynamic Techniques

The measurements of the vertical transport of CO2 were carried out over the Sea of Japan using the specially designed pier of Kyoto University on September 20 to 22, 2000. CO2 fluxes were measured by the eddy correlation and aerodynamic techniques. Both techniques showed comparable CO2 fluxes during sea breeze conditions: −0.001 to −0.08 mg m−2s−1 with the mean of −0.05 mg m−2s−1. This means that the measuring site satisfies the fetch requirement for meteorological observations under sea breeze conditions. Moreover, the eddy diffusivity coefficient used in the aerodynamic technique is found to be consistent with the coefficient used in the eddy correlation technique. The present result leads us to conclude that the aerodynamic technique may be applicable to underway CO2 flux measurements over the ocean and may be used in place of the bulk technique. The important point is the need to maintain a measuring accuracy of CO2 concentration difference of the order of 0.1 ppmv on the research vessels or the buoys.

Application of the VH‐TDMA technique to coastal ambient aerosols

A newly developed VH‐TDMA has been used for the first time to measure the volatile fractions and post volatilization hygroscopic growth factors of ambient aerosols in the coastal marine and urban environments. The results are compared with comparable data for laboratory generated aerosols of known composition. Measurements conducted on coastal Aitken mode particles showed volatilization behavior similar to laboratory generated aerosols composed of methane sulfonic acid and ammonium sulfate. Measurements conducted on 60 nm particles during nucleation events contained a greater fraction of material with similar volatility to ammonium sulfate than was found at other times. These particles were hygroscopic but less so than pure ammonium sulfate. Measurements conducted in the Brisbane central business district during sea breeze conditions show similar behavior to the coastal aerosol, but with additional low volatility species. This aerosol may originate from urban sources or from marine particles acquiring additional secondary aerosol species during transport.

Model predictions of coastal winds in a small scale

Details of surface winds across a flat and non-curved coastline have been considered, mostly in the 1–10 km scale, by looking at some published observed cases and simulating with a high-resolution numerical model. Stability conditions were emphasized by having the land and sea at slightly different temperatures, but sea breeze conditions were excluded.A warm surface generally enhances momentum transfer and wind speed, and reduces the cross-isobar angle. This transforms to rapid acceleration and downstream veering of offshore winds as they enter a warm, smooth sea. Cold sea, on the other hand, strongly inhibits vertical momentum transfer. This leads to offshore winds temporarily backing downstream off the coast, accelerating only slowly and even decreasing in speed in some cases. These observed facts were well simulated by the model.Onshore flow on to cold land brought along rapid deceleration and strong inland backing of winds. The deceleration was weaker and more gradual over warm land with downstream veering instead. Smaller geostrophic wind speed accentuated these and all other stability effects as the wind shears then were weaker relative to the buoyancy effects (i.e. the absolute values of the local Richardson numbers were larger).For large-scale winds along the shore with land on the right, the induced coastal convergence and rising motion appears only above about 100 m height in the cold sea simulations (typical of early summer). Below, there is divergence and hence relatively weak coastal wind. The case is the opposite for winds with land on the left (e.g. westerlies along a south coast): then the coastal surface winds are relatively strong along a cold sea. For a warm sea (typical of autumn) these low-level features disappear and coastal 10 m winds tend to be close to the geostrophic wind in speed and direction for all onshore directions.

The ESCOMPTE program: an overview

In this paper, the “Expérience sur Site pour COntraindre les Modèles de Pollution atmosphérique et de Transport d'Emissions” (ESCOMPTE) program is presented. The ESCOMPTE program is used to produce a relevant set of data for testing and evaluating regional pollution models. It includes high-resolution (in space and time) atmospheric emission inventories and field experiments, and covers an area of 120×120 km, centered over the Marseilles-Berre area in the southeast of France during Summer 2001. This region presents a high occurrence of photochemical pollution events, which result from numerous industrial and urban sources of primary pollutants. From the dynamical characteristics of the area, sea-breeze circulation and channeling effects due to terrain features highly influence the location of the pollutant plumes. ESCOMPTE will provide a highly documented framework for dynamics and chemistry studies. Campaign strategies and experimental set up are described. During the planning phase, existing modeling results helped defining the experimental design. The campaign involved surface measurement networks, remote sensing, ship-borne, balloon-borne, and airplane measurements. Mean standard meteorological parameters and turbulent fluxes, ozone, ozone precursors, photochemically active trace gases, and aerosols were measured. Five intensive observation periods (IOPs) were documented using a wide spectrum of instruments, involving aircraft (7) (one of them equipped with a Doppler lidar, the others for in situ meteorological and chemical measurements), constant volume balloons (33), ozone lidars (5), wind profilers (15 sodars and radars), Doppler scanning lidar (1), radiosonde systems (at 4 locations), instrumented ships (2). In addition to the air quality networks from environmental agencies, 15 supplementary ground stations equipped for chemistry and/or meteorology and/or surface flux measurements, were operational. All instruments were calibrated and compared during a Quality Control/Quality Assurance (QC/QA) week, at the very beginning of the campaign. Fifteen days were intensively documented during five IOPs, referenced as 1, 2a, 2b, 3, and 4. High pollution levels were encountered during sea-breeze conditions observed during IOPs 2b and 3, whereas IOPs 2a and 4 corresponded to moderate wind, and channeled plume regimes. In addition, hourly emissions inventories for all IOPs were established to complete data sets and to finalize the ESCOMPTE database (EDB). Two other projects were associated to ESCOMPTE: urban boundary layer (UBL) and tropospheric water vapor content by GPS tomography (GPS/H 2O). They took advantage of the scientific environment provided by ESCOMPTE.

VOC and O3Distributions over the Densely Populated Area of Greater Athens, Greece

The horizontal and vertical distributions of volatile organic compounds (VOCs) and ozone (O 3) concentrations within the lower troposphere over the greater Athens area, Greece, under sea-breeze conditions were studied. Furthermore, an attempt was made to explain the dynamic and chemical mechanisms responsible for the formation of these distributions. Measurements were collected using a specially instrumented Falcon 20-E5 research aircraft, ground-based meteorological instrumentation, and a network of air quality monitoring stations within the context of the Scientific Training and Access to Aircraft for Atmospheric Research Throughout Europe (STAAARTE) 1997 experimental campaign. Relatively high ozone values (55‐100 ppbv) were identified within the first 300‐ 400 m above ground, and significantly reduced values were found over the depth of the atmospheric boundary layer. High values of VOC concentrations [150‐350 ppbCarbon (C)] were observed near the ground as well as within the first 300‐400 m above ground. At higher altitudes, of 1400‐1600 m, VOC concentrations remained relatively high (100‐200 ppbC). It was demonstrated that the sea-breeze circulation plays a major role in the formation of the above-mentioned concentration levels and that chemical transformations explain specific characteristics.

Ozone distribution in coastal central Taiwan under sea-breeze conditions

Sea-breeze circulation influences ozone concentration differently for two types of synoptic winds across the westcentral coastal plain of Taiwan. During summer months, when the synoptic flow is southerly, a strong westerly sea-breeze confined to the lower 700–800 m produces a northeastward flow. Ozone concentration is low, with a core centered at about 100 m, leading to moderate ozone concentrations in the low population density northeastern foothills. In autumn (and spring), northerly synoptic winds of about the same strength combine with less energetic westerly sea breeze to produce a southeastward flow that carries higher ozone levels into the heavily populated Taichung basin. Ozone levels are high from 80 to 400 m, with a core up to 120 ppb from about 150 to 300 m, contributing to serious ozone episodes at the southern (downwind) end of the basin. Analyzing the backward trajectories and ozone concentration showed that the weak southeastward breeze is the dominant factor affecting the occurrence of high ozone events in the region. The horizontal distribution is based on 3 yr data obtained from a network of air-pollution monitoring sites in the study area, while the vertical data comes from two 2-day tethersonde experiments, measuring wind-speed, direction, temperature and humidity, NO, NO 2, NMHC and O 3, conducted during August and November 1999.

Coupling of mesoscale meteorological model with particle trajectory model to study the atmospheric dispersion under sea breeze conditions

A mesoscale meteorological model coupled with a particle trajectory model is used to study the atmospheric dispersion of 41Ar released through a stack of 100 m height continuously at a rate 1 Bq/s under sea breeze conditions. In a trial study, a mesoscale model successfully simulated the sea breeze phenomena observed at Kalpakkam on 22nd September 1992; the isodose and isoconcentration contours revealed that coupling of the mesoscale meteorological model and the particle trajectory model gives realistic simulation of atmospheric dispersion under such conditions.

Internal boundary layer structure under sea-breeze conditions in Hong Kong

A three-dimensional atmospheric model is used for modelling the airflow pattern and internal boundary layer (IBL) development over the Hong Kong region that comprises hilly terrain and complex coastline. Observations used to verify the model are drawn from 32 meteorological stations and a ground-based lidar system. It is found that the modelled results are in good agreement with the observed airflow field and IBL development. Due to the interaction of complex terrain and sea-breeze circulations, several confluence zones of wind fields are found in different locations, depending on the background wind velocity and the differential-heating rate between land and sea. Subsequently, these kinds of wind field patterns give rise to a three-dimensional dome-shaped IBL distribution that forms an impediment to air-pollutant dispersion outside of the layer.

Case studies of spectral atmospheric transmittance in the ultraviolet and visible regions in Athens, Greece: I. Total transmittance

This work accounts for an investigation about the diurnal variation of total spectral transmittance of solar irradiance under dominant wind conditions as case studies. Such a work is carried out in Athens for the first time. The spectral transmittance values estimated were derived using ground-based spectral measurements of beam irradiance in the range 310–575 nm (UV and VIS). The data were recorded by a system consisting of an automatic solar tracker and a spectrometer. All data were recorded under clear-sky conditions in the city center of Athens and the spectral total atmospheric transmittance was estimated towards zenith to avoid optical mass effects. The comparison reveals that the total atmospheric transmittance is higher under the influence of strong Etesian compared to sea-breeze conditions. The influence of low-pressure systems also plays a depollution role in the basin. Various features of diurnal variation are discussed with respect to emission sources, topographic peculiarities and wind pattern.