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Detailed information on the chemical and physical properties of aerosols is important for assessing their role in air quality and climate. This work explores the origin and fate of continental aerosols transported over the Central Atlantic Ocean, in terms of chemical composition, number and size distribution, using chemistry-transport models, satellite data and in situ measurements. We focus on August 2005, a period with intense hurricane and tropical storm activity over the Atlantic Ocean. A mixture of anthropogenic (sulphates, nitrates), natural (desert dust, sea salt) and chemically aged (sulphate and nitrate on dust) aerosols is found entering the hurricane genesis region, most likely interacting with clouds in the area. Results from our modelling study suggest rather small amounts of accumulation mode desert dust, sea salt and chemically aged dust aerosols in this Atlantic Ocean region. Aerosols of smaller size (Aitken mode) are more abundant in the area and in some occasions sulphates of anthropogenic origin and desert dust are of the same magnitude in terms of number concentrations. Typical aerosol number concentrations are derived for the vertical layers near shallow cloud formation regimes, indicating that the aerosol number concentration can reach several thousand particles per cubic centimetre. The vertical distribution of the aerosols shows that the desert dust particles are often transported near the top of the marine cloud layer as they enter into the region where deep convection is initiated. The anthropogenic sulphate aerosol can be transported within a thick layer and enter the cloud deck through multiple ways (from the top, the base of the cloud, and by entrainment). The sodium (sea salt related) aerosol is mostly found below the cloud base. The results of this work may provide insights relevant for studies that consider aerosol influences on cloud processes and storm development in the Central Atlantic region.

A cas e of abrupt and significant increase of surface ozone was examined, which is recorded during the night of October 9 th , 2003, over the area of the Thriassion Plain – Greece, being acco mpanied by the occurrence of a wind outbreak during the same period. As this increase cannot be explained by the ordinary diurnal evolution of tropospheric ozone concentration, the possibility of downward stratospheric ozone transport was investigated. It was indeed found that the case is associated with tropopause folding and subsequent stratospheric air intrusion in the upper troposphere over Greece. With the aid of mesoscale modeling, being also supported by the hydraulic theory, it was found that the intense wind outbreak is associated with the development of disturbances in the lee side of the neighbouring mountain ranges. These disturbances contribute to the free troposphere – atmospheric boundary layer interaction, facilitating the transportation of stratospheric air close to the surface, thus resulting to the observed tropospheric ozone increase. Zusammenfassung

The predictions of O 3 , precursor species and the key transit species from an Air Quality Forecast Modeling System (AQFMS) are evaluated for July 2001 and January 2004 utilizing an extensive measurement data set from the PMTACS-NY “Supersite” program. The AQFMS, consisting of a chemical transport model coupled with a mesoscale meteorological forecasting model, operated routinely over the course of the six-year study. The domain-wide 8-h average O 3 predictions in the summer season have an average mean normalized bias (MNB) of 8.6%. The AQFMS captured the day-to-day variations of O 3 , CO, NO x (NO y ) and SO 2 at the Queens College (urban) and Pinnacle State Park (rural) sites during both summer and winter. During July 2001, the linear regressions of CO vs. NO x at Queens College and CO vs. NO y at Pinnacle State Park are in reasonable agreement with observations. However, during January 2004 the slopes of the linear regressions are significantly overestimated suggesting more uncertainties with the winter emission inventories. The Ozone Production Efficiency (OPE) is under predicted by 45% at Pinnacle State Park during July 2001 which may be caused by the underestimation of NO z removal and/or the underestimation of OH concentrations. Concentrations of HONO, a key transient species for OH production are significantly under predicted by the AQFMS indicating deficiencies in the chemical mechanism in the AQFMS. The underestimation of OH concentrations is much more significant during January 2004 which suggests larger uncertainties with chemical mechanism for winter conditions.

The Mediterranean Basin is highly heterogeneous with regard to its climatic and oceanographic properties. The appropriate approach for simulating the transport and transformations of Hg in the water compartment requires the use of a hydrodynamic model with additional modules for transport–dispersion and biogeochemistry. In this work, the PCFLOW3D model was upgraded with a biogeochemical module and used for simulation of mercury transport and transformation processes in the Mediterranean. The circulation for the four seasons due to wind, thermohaline forcing and inflow momentum of the main rivers and through the straits was calculated. The results were compared with measurements and the results of another model (POM — Princeton Ocean Model). An acceptable agreement was achieved. The seasonally averaged velocity fields obtained were used to simulate transport and dispersion of mercury. A new biogeochemical module dealing with the different mercury species: gaseous elemental (Hg0), divalent (Hg2+), and mono-methyl mercury (MMHg) in dissolved form and bound to particulate matter and plankton was introduced. Exchange of mercury at the boundaries (bottom sediment/water and water/atmosphere) and transformation processes such as methylation, demethylation, reduction and oxidation were taken into account. The transformation rates between the mercury species were described using simple equations, and thus the time and space variable reaction coefficients should be determined from in-situ measurements. Instead, machine-learning tools and classical statistical methods were used to connect the measured sets of geophysical/environmental parameters and concentrations of different Hg species. The provisional annual Hg mass balance established for the Mediterranean showed that exchange with the atmosphere is the most important source/sink of mercury for the water compartment. Therefore, the model was further upgraded with a gas exchange module for Hg0. To improve the results of the simulations the PCFLOW3D aquatic model was further linked to the RAMS–Hg atmospheric model which provided real-time meteorological data, deposition and concentrations of mercury in the atmosphere. Simulations with the integrated modelling tool were performed and the results were compared to the measurements. Acceptable agreement of the average concentrations down the water column for both total mercury (HgT) and elemental mercury (Hg0) was achieved. Agreement of Hg0 concentrations near the surface was good; thus exchange with the atmosphere can be simulated with relatively high reliability. Agreement of simulated MMHg concentrations with measurements was not satisfactory, which is probably due to poor understanding of the processes of MMHg formation and its dependence on environmental factors, which have, so far, not been taken into account in the modelling. In view of the satisfactory modelling results obtained for HgT and Hg0, a simulation of management scenarios, particularly the policy target (PoT) scenarios for 2010 and 2020, was performed. The results of these simulations were further used to establish the mass balance of HgT in the Mediterranean Sea.

Using the well-known Regional Atmospheric Modelling System (RAMS) version 4.3 an integrated system able to simulate the atmospheric mercury cycle has been developed. Basic processes of the mercury atmospheric cycle have been incorporated into the atmospheric model. The model deals with elemental Hg (Hg0), divalent gaseous Hg (Hg2) and particulate Hg (HgP). Wet deposition mechanisms used to describe the removal of Hg2 and HgP are merged with the detailed cloud microphysical scheme in order to provide better representation of the wet deposition processes. The advantages of this approach have been examined through results intercomparison with simulated Hg wet deposition using CMAQ-Hg from previous work for two evaluation periods: 4 April–2 May 1995, and 20 June–18 July 1995. An attempt to clarify the main parameters that affect wet deposition mechanism of mercury is also made.

The development of two comprehensive modelling systems and a test application to estimate the mercury budget for the State of New York is discussed in the present study. The modelling tools developed are based on two well known meteorological modelling systems namely, RAMS and SKIRON/Eta. Both models have been modified and the developed modules describing the physico-chemical processes of mercury have been incorporated in each modelling system. Model calculations and measurements have been compared during the 14 to 26 August 1997 simulation period. Two different simulations were performed, ‘scenario1’ where all available sources of mercury were used and ‘scenario2’ where the New York State mercury sources were excluded. An attempt was made to identify and quantify critical gaps in the current understanding of regional scale transport deposition and fate of mercury in New York State, improving modelling capabilities and understanding of mercury as an air pollutant, and provide a tool for policy makers in mitigating the impacts of mercury pollution.

The Mediterranean Atmospheric Mercury Cycle System (MAMCS) project was performed between 1998 and 2000 and involved the collaboration of universities and research institutes from Europe, Israel and Turkey. The main goal of MAMCS was to investigate dynamic processes affecting the cycle of mercury in the Mediterranean atmosphere by combining ad hoc field measurements and modelling tasks. To study the fate of Hg in the Mediterranean Basin an updated emission inventory was compiled for Europe and the countries bordering the Mediterranean Sea. Models were developed to describe the individual atmospheric processes which influence the chemical and physical characteristics of atmospheric Hg, and these were coupled to meteorological models to examine the dispersion and deposition of Hg species in the Mediterranean Basin. One intercomparison and four two-week measurement campaigns were carried out over a three-year period. The work presented here describes the results in general terms but focuses on the areas where definite conclusions were unforthcoming and thus highlights those aspects where, in spite of advances made in the understanding of Hg cycling, further work is necessary in order to be able to predict confidently Hg and Hg compound concentration fields and deposition patterns.

The online approach consists of the coupled treatment of chemical parameters, simultaneously with the meteorological parameters, in a single integrated modeling system that is referred to as chemical weather modeling. This approach offers the possibility to simulate the links and feedbacks between atmospheric processes that are traditionally neglected in air quality models. Both meteorological and chemical components are expected to benefit from this approach. Both approaches have advantages and disadvantages that make their use appropriate for different applications. This study discusses and evaluates the performance of the online Integrated Community Limited Area Modeling System (RAMS/ICLAMS) and the offline model, Comprehensive Air Quality Model with Extensions (CAMx), for a month long retrospective summertime text period, over Europe and the Greater Mediterranean Area (GMR). The implementation of the same chemical mechanisms, meteorological fields, emissions, initial and boundary conditions makes it easier to compare the results from the two models. However, there are some differences in the physical parameterizations utilized in the two models that are expected to result in differences between them. The feedback mechanisms are not activated in order to evaluate the performance of the two models regarding the advantages that the online approach offers (same projection, no interpolation in time and space, explicit calculation of the meteorological components, availability of the meteorological fields at each time step etc.). Results showed that the online approach gave better results regarding ground 1 h ozone and 24 h sulfate aerosol concentrations improving the main statistical parameters by roughly 20 % and increased correlation from 0.51 to 0.70. The differences may be mainly the outcome of the utilization of different methods for the calculation of the photolysis rates and the interpolation of the meteorological data for use in the offline model.

In this paper, the performance of two commonly used regional-scale Eulerian photochemical modeling systems, namely, RAMS/UAM-V and MM5/SAQM, from the regulatory or operational perspective, is examined. While the Urban Airshed Model with Variable Grid (UAM-V) is driven with the meteorological fields derived from the Regional Atmospheric Model System (RAMS), the San Joaquin Valley Air Quality Model (SAQM) used the meteorological fields derived from the Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). The model's performance in reproducing the observed ozone air quality over the eastern United States is evaluated for three typical high-ozone episodic events that occurred during 16–20 June, 12–16 July, and 30 July–2 August of 1995. The prevailing meteorological conditions associated with these three episodes are characterized by a slow eastward-moving high pressure system, westerly and southwesterly low-level jets, stable boundary layers, and the Appalach...

The present paper reports results of a study that attempted to elucidate the factors causing relatively high levels of particulate sulfate that have frequently been observed over central Israel. Aircraft research flights were performed some 70 km west of and parallel to the Israeli coastline during September 1993 and June 1994. Comparison between the two measurement periods revealed a distinctive difference between the two different sampled air masses. While both air masses were nearly homogeneous throughout the measurement period and along the 180 km flight path, the air mass sampled in September 1993 was much cleaner than the air mass sampled during June 1994. The concentrations of the air pollutants measured during the 1993 campaign averaged 0.7 ± 0.4 parts per billion by volume (ppbv) SO 2 , 1.0 ± 0.6 ppbv NO y , 39 ± 7 ppbv O 3 and 38 ± 7 nmol/m 3 particulate sulfate, whereas in the second period the levels averaged 3.0 ± 1.0, 3.9 ± 1.8, 48 ± 9, and 108 ± 63, respectively. These results suggest that the two air masses traveled different paths before reaching the eastern Mediterranean region. Further examination of the air mass sources and transport were performed using the Regional Atmospheric Modeling System for meteorological simulations and the Hybrid Particle and Concentration Transport Package for dispersion modeling. The model simulation showed that during the 1993 measurement period, the pollution sources in southern Europe and the Balkans did not effect the eastern coasts of the Mediterranean, while the synoptic conditions and simulation results for the June 1994 period indicated that the winds over the eastern Mediterranean tended to be northwesterly and thus forcing the polluted air masses toward the coast of Israel.

In the frame of this work, the sensitivity of a grid-based photochemical model on the emission inventory is examined. More precisely, the role of various emission categories on simulated oxidant formation is investigated and especially the biogenic ones. Special emphasis is given on the investigation of the role of the biogenic emissions on tropospheric ozone formation over areas with significant anthropogenic sources. The area of interest is the SE part of the Greek Peninsula where there are various types of anthropogenic pollutant sources and important biogenic sources from forest areas. Ozone air quality simulations were performed using the combined system of the atmospheric model RAMS and the photochemical model UAM. The simulations revealed that there is a significant increase of the calculated ozone concentrations over areas with significant precursor sources when biogenic emissions are taken into account. This increase is more pronounced during the days with a significant regional scale transport.

Research flights have been performed over the Greater Athens Area (GAA) and southwards over the Island of Aegina and east of Peloponnisos in order to investigate the evolution of an ozone episode over GAA and the transportation of the urban pollution plume southwards from the Athens region. During the 3 day period of 6 July to 8 July 1994, the GAA was under the influence of an ozone episode with ground-level noontime concentrations of more than 120 ppbv. Upper-air ozone concentrations measured during the flights were as high as 135 ppbv. The interaction of the weak synoptic conditions over the area along with the development of a mesoscale thermal circulation created poor dispersion conditions during the period of interest and resulted in elevated ozone levels. The primary pollutants emitted in the GAA during the night and early morning hours, were funneled out to the Saronic Gulf and southwards along the southwestern Aegean Sea, near the coast of east Peloponnisos. Under the influence of strong sunlight these primary pollutants continued to undergo photochemical reaction giving rise to elevated ozone levels tens of kilometers downwind of the pollution emission sources. Further evidence of the photochemically-aged air masses was the high correlation (R2 = 0.8) observed between NOy, and ozone. The ozone production efficiency in these transported air masses reached a value of close to six.

This study examines dynamical and microphysical features of convective clouds that affect mercury (Hg) wet scavenging and concentrations in rainfall. Using idealized numerical model simulations in the Regional Atmospheric Modeling System (RAMS), we diagnose vertical transport and scavenging of soluble Hg species in thunderstorms under typical environmental conditions found in the Northeast and Southeast United States (US). Three important environmental characteristics that impact thunderstorm morphology were studied: convective available potential energy (CAPE), vertical shear (0–6 km) of horizontal wind (SHEAR) and precipitable water (PW). We find that in a strong convective storm in the Southeast US that about 40% of mercury in the boundary layer (0–2 km) can be scavenged and deposited to the surface. Removal efficiencies are 35% or less in the free troposphere and decline with altitude. Nevertheless, if we assume that soluble Hg species are initially uniformly mixed vertically, then about 60% deposited mercury deposited by the thunderstorm originates in the free troposphere. For a given CAPE, storm morphology and Hg deposition respond to SHEAR and PW. Experiments show that the response of mercury concentration in rainfall to SHEAR depends on the amount of PW. For low PW, increasing SHEAR decreases mercury concentrations in high-rain amounts (>13 mm). However, at higher PW values, increasing SHEAR decreases mercury concentrations for all rainfall amounts. These experiments suggest that variations in environmental characteristics relevant to thunderstorm formation and evolution can also contribute to geographical difference in wet deposition of mercury. An ensemble of thunderstorm simulations was also conducted for different combinations of CAPE, SHEAR and PW values derived from radiosonde observations at five sites in the Northeast United States (US) and at three sites in the Southeast US. Using identical initial concentrations of gaseous oxidized mercury (GOM) and particle-bound mercury (HgP), from the GEOS-Chem model, the simulations predict higher mercury concentrations in rainfall from thunderstorms forming in the environmental conditions over the Southeast US compared to the Northeast US. Mercury concentrations in rainfall are also simulated for a typical stratiform rain event and found to be less than in thunderstorms forming in environments typical of the Southeast US. The stratiform cloud scavenges mercury from the lower ~4 km of the atmosphere, while thunderstorms scavenge up to ~10 km.

The causes of the poor air quality in Athens, Greece during the severe episode of 25–26 May 1990 has been studied, using a prognostic model (RAMS) and a three-dimensional Eulerian air quality model (CALGRID). The modeling effort indicates that the main urban area of Athens exhibited high concentrations of nitrogen oxides, the main sources of which are automobiles, while the NNE suburban area exhibited high ozone concentrations, the product of photochemical activity of the primary pollutants that were transported by the sea-breeze. The application of the models also demonstrated the need for an accurate emission inventory for improved predictions of the pollutant concentrations. It was also found that a 50% reduction of the nitrogen oxide emissions will increase the ozone levels in the downtown area substantially.

RAMS-Hg and CMAQ-Hg are two state-of-the-science integrated modelling systems developed to study the complex chemical transformation, transport and deposition of atmospheric mercury. In the present study model results using RAMS-Hg are both compared with observations from the Hg deposition network (MDN) and wet deposition of Hg results using CMAQ-Hg. A preliminary validation of the RAMS-Hg model as well as quantitative estimation of the advantages of the proposed approach on coupling mercury processes to an atmospheric modelling system is presented. Model validation indicated that the comprehensive model simulated reasonably well the wet deposition measurements of Hg at the MDN sites.

Particulate matter of anthropogenic and/or natural origin in the atmosphere is considered a parameter causing multiple effects on the environment of local and remote locations. The presence of absorbing and/or scattering aerosols in the lower troposphere can affect the atmospheric radiation through the modification of both short- and long-wave components. On a shorter term, aerosols can cause the modification of environmental conditions by influencing atmospheric temperature and dynamics. Atmospheric photochemistry is also affected by the presence of increased particle concentrations due to the alteration of UV and visible radiation fluxes and therefore the modification of the j -values (photolysis rates) of the photochemical reactions. Physiographic characteristics and climatic conditions in the Mediterranean Region are followed by excessive solar radiation leading to high photochemical activity in the Region. In addition long-range transport of fine particles is very common in the area. In this work we focus on the distribution of fine desert particles in the greater Mediterranean Region since Saharan dust is the desert responsible for many severe dust outbreaks that influence the Region. We examine the impact of increased desert dust concentration on the photochemistry of the region. For this purpose, advanced atmospheric and photochemical models are implemented with the aid of air pollutant measurements from stations in the region. The models used are the RAMS atmospheric model, the SKIRON/Eta atmospheric modeling system with the implementation of dust module, and the CAMx photochemical model. Sensitivity tests performed with and without the influence of dust load on the photolysis rates showed some interesting results in the formation and production of gases like ozone, nitrogen oxides, and nitric acid, and aerosols like sulfates and nitrates. The preliminary results of these simulations will be thoroughly discussed in this presentation.

The physical and chemical processes involved in the mercury cycle in the atmosphere are very complicated and need special treatment. At the framework of the EU/DG-XII project MAMCS a significant effort has been devoted for the development of appropriate models for studying the mercury cycle in the atmosphere. In addition, an improved emission inventory is created while monitoring data in various locations in Europe are selected and used for model calibration and inter-comparison. Our model development includes the incorporation of almost any type of source (point or area), gas and aqueous phase chemistry, gas-to-particle conversion, wet and dry deposition, air-water exchange processes etc. The development was performed within two well-known atmospheric modeling systems: the Regional Atmospheric Modeling System (RAMS) and the SKIRON/Eta. There are several reasons for performing the development of the mercury cycle modeling within these two models: The main reason for using RAMS is its unique capability of two-way interactive nesting of any number of grids which is considered as absolutely necessary for studying near-source dispersion of mercury species. Additional capabilities are the full microphysical parameterization for wet processes, the detailed parameterization of surface processes and the non-hydrostatic formulation. The main reason for using the SKIRON/Eta model for development is its unique capability of describing the dust cycle (uptake, transport, deposition) and the existence of a viscous sub-layer formulation which is necessary for description of mercury fluxes from the sea surface. In both models the mercury cycle formulation is called simultaneously at each time-step in order

In this paper an attempt was made to identify the in/out of state contributions of mercury sources to the total deposited mercury over the State of New York. The transport, transformation and deposition of mercury in New York State are also discussed.

The atmospheric part of the mercury cycle is considered as very complicated because of the various physicochemical processes involved. The temporal and spatial scales of various processes are varying according to mercury species. While Hg0 is considered as long-range transport pollutant, HgII is fast reacting and deposits quickly (wet and dry). HgP has behaviour similar to the other particulate in the atmosphere. There is enough evidence now about the various disturbances in what are considered as background quantities. The most important reasons are (i) the increase of emissions from sources like coal burning, waste incinerators, cement production, mining etc, (ii) the lack of understanding of important physicochemical processes like fluxes, transport, transformation and deposition. Because of these verified disturbances, during the last years, a considerable effort has been devoted to reduce the mercury emissions. At the framework of the EU/DG-XII project MAMCS a significant effort has been devoted at the development of appropriate models for studying the mercury cycle in the atmosphere. The model development is performed within the atmospheric models RAMS and SKIRON/Eta. In this development we tried to transfer and utilize the modeling techniques applied in conventional air pollution modelling studies. In addition, we had to develop new methodologies for processes like re-emissions from soil and water bodies and gas to particle formation. The developed modeling systems have been applied in the Mediterranean Region where the multi-scale atmospheric processes (thermal and mechanical circulations at regional and mesoscale) are considered as important, according to a number of past air pollution studies. Seasonal-type of simulation has been performed and annual deposition patterns have been estimated. As it was found, the regional-scale pattern and the trade wind systems (from North to South) and the photochemistry are the key factors for controlling the mercury deposition, especially the HgP.

Despite vigorous attempts to control the ozone problem during the past three decades, ozone levels in many areas over the Eastern United States continue to exceed the National Ambient Air Quality Standards. Until recently, photochemical models were applied to simulate historical ozone episodic events to examine the future ozone non-attainment problem. When the model’s ability to reproduce the observed ozone air quality was deemed acceptable, control measures needed to meet and maintain the ozone standards were evaluated using projected emissions inventories and historical episodic meteorological conditions. Since the episodic meteorological events under which the model has performed best may never occur in the future, there is an inherent uncertainty in the controls identified as required to comply with the ozone standards.

(1990). Some Evaluations of the Effect of Ambient Temperature on Plume Rise. Journal of the Air & Waste Management Association: Vol. 40, No. 7, pp. 1020-1022.

Many urban areas in the Eastern United States have been classified to be in non-attainment for ozone, placing a high priority on finding cost-effective emission control measures for improving ambient ozone air quality. Recognizing the complexities associated with the nation’s ozone non-attainment problem, the 1990 Clean Air Amendments mandated the use of grid-based photochemical models for evaluating emission control strategies in urban areas having a serious or higher designation. Given the influx of elevated concentrations of ozone and its precursors into the urban-scale modeling domains (regional-scale transport), many states in the Eastern U.S. were unable to demonstrate ozone attainment for urban areas in their 1994 State Implementation Plans (SIPs) submittal using the urban-scale models. The 1994 SIPs were based on the UAM-IV photochemical model (Morris et al., 1990), which is an urban-scale model that reflects the state-of-science of the late 1980’s. Systems Applications International (SAI) recently developed the UAM-V, a regional-scale ozone air quality model, which contains some new features over the UAM-IV such as multi-scale modeling capability, grid nesting, plume-in-grid (PiG) treatment for point sources, etc. (SAI, 1995). Of particular interest is this model’s treatment of subgrid-scale processes relating to the transport, transformation, and interaction of elevated plumes with the ground-level plume.

[1] The detection of high ozone levels over large inland areas in Israel during the early, mid and late summer triggered an analysis of air mass back-trajectories. This, in turn, pointed to the transportation system in the metropolitan coastal Tel Aviv region as the possible origin of the ozone’s precursors. To link the daily dynamics of rush hour transportation emissions to inland air pollution, in general, and airborne ozone measurements, in particular, an interdisciplinary modeling system was established. The simulations of transportation-to-inland air pollution integrated transportation, emission factor, atmospheric, transport/diffusion, and photochemical models. The modeling results elucidated a spatial and temporal overlap between the ozone precursors and ozone production. The model simulations indicated east to southeasterly dispersion of the pollution cloud. The results agreed well with both spatial and temporal ozone levels as recorded by aircraft over central Israel, as well as with ground-based monitoring station observations. The impact of the Tel Aviv metropolitan area as well as the Gaza Strip, as pivotal coastal transportation sources for inland air pollution in general and ozone formation in particular, is discussed. The synoptic analysis identified the conditions prevailing when elevated air pollution, and especially high ozone levels, exists over central Israel. The analysis showed that this season features a shallow mixed layer and weak zonal flow, which leads to poor ventilation rates and inhibit efficient dispersion of this secondary pollutant. These poor ventilation rates result in the slow transport of ozone precursors, enabling their photochemical transformation under intense solar radiation during their travel from the coast inland. Under these conditions, model results showed that traffic emissions during the morning rush hour from the Tel Aviv metropolitan area contribute about 60% to the observed ozone concentrations. INDEX TERMS: 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); KEYWORDS: air pollution, numerical atmospheric modeling, transportation model, emission factors, photochemical model, ozone, photochemical aged air mass

In this study, an extreme ozone episode that occurred during the period 12-15 July 1995 over the north-eastern United States is investigated through model simulations. This paper is focused on the meteorological conditions prevailing during this ozone episode. The analysis is based on nested grid simulations performed with the meteorological model CSU-RAMS (Colorado State University - Regional Atmospheric Modelling System) and on the available surface and upper air observations. The definition of the model nests seemed very important in order to capture the prevailing atmospheric circulations which led to the build up of the air pollution episode.