Processes driving the short-term variability of polycyclic aromatic hydrocarbons in the Baltimore and northern Chesapeake Bay atmosphere,USA

Polycyclic aromatic hydrocarbons (PAHs) were measured in the Baltimore and adjacent Chesapeake Bay in July 1997. Time series of 4- and 12-h samples were taken at two sites 15 km apart in order to evaluate the influence of a number of processes on the short-term variability of PAH in the Baltimore and northern Chesapeake Bay atmospheres. PAH concentrations were 2–3-fold higher in the Baltimore atmosphere than in the adjacent Chesapeake Bay atmosphere. For example, gas-phase phenanthrene and pyrene concentrations were 12.5 and 2.14 ng m⁻³ in the Baltimore site and 5.57 and 0.548 ng m⁻³ in the Chesapeake Bay, respectively. The influence of wind direction, wind speed and temperature was evaluated by multiple linear regressions which indicated that atmospheric gas-phase PAH concentrations over the Chesapeake Bay were significantly higher when the air mass was from the urban/industrial Baltimore area. Furthermore, the increase of gas-phase low-MW PAH concentrations with temperature and wind speed suggests that volatilization from the bay is an important source of pollutants to the atmosphere, at least when air masses are not influenced by the Baltimore urban and industrial area. Indeed, while on the long-term, the Chesapeake Bay is a receptor of atmospherically deposited PAHs, on the short-term and during appropriate meteorological conditions, the bay acts as a source of pollutants to the atmosphere. Aerosol-phase PAH concentrations and temporal trends showed a strong dependence on aerosol soot content due to the high affinity of PAHs to the graphitic structure of soot. These results confirm the important influence of urban areas as a source of pollution to adjacent aquatic environments and as a driving factor of the short-term variability, either directly by transport of urban-generated pollutants or by volatilization of previously deposited pollutants. Conversely, the complex diurnal trends of gas-phase PAHs at the Baltimore site suggests that degradation processes dominate the diurnal trends of PAHs in urban atmospheres. This conclusion is supported by estimated rate constants for PAH reaction with OH radicals which show good agreement with reported values within a factor of two.     

PAHs associated with the leaves of Quercus ilex L.: Extraction,GC–MS analysis,distribution and sources: Assessment of air quality in the Palermo (Italy) area

In this study, the leaves of Quercus ilex L. were selected as possible bioaccumulators of polycyclic aromatic hydrocarbons (PAHs). Quercus is an evergreen plant that occurs widely in both urban and rural areas. Several sites (urban roadside, urban, urban park, suburban and rural) in and around Palermo city were investigated.The purpose of this research was to optimize analytical method for quercus leaves, investigate the degree of contamination in the urban area of Palermo by comparing PAH concentration in leaves of quercus from the several sites, establish distribution patterns and relate them to possible sources of PAHs. To this aim, the 16 recommended as priority pollutants by the Environmental Protection Agency (EPA) and perylene were analyzed. PAHs were positively correlated to atmospheric particulate gravimetrically determined on filters aspiring a known volume of air in the various stations.The analyses have been performed by gas chromatography coupled to mass spectrometry (GC–MS) in selected ion monitoring (SIM) mode. The total PAH content in the samples ranged from 92 to 1454 μg kg⁻¹ d.w. The higher amounts of PAHs detected in leaves of quercus from the urban area of Palermo compared with the control site are diagnostic of air contamination, in particular in the zones with heavy traffic. The determination of PAHs in the leaves of quercus allows us, with very simple and fast procedures, to assess the quality of the air over a longer period, since PAHs are accumulated over the whole lifetime of the leaves, irrespective of atmospheric conditions at the moment of sampling.     

Statistical and diagnostic evaluation of a new-generation urban dispersion modelling system against an extensive dataset in the Helsinki area

We have developed a modelling system for predicting the traffic volumes, emissions from stationary and vehicular sources, and atmospheric dispersion of pollution in an urban area. This paper describes a comparison of the NO_x and NO₂ concentrations predicted using this modelling system with the results of an urban air quality monitoring network. We performed a statistical analysis to determine the agreement between predicted and measured hourly time series of concentrations at four permanently located and three mobile monitoring stations in the Helsinki Metropolitan Area in 1996–1997 (at a total of ten urban and suburban measurement locations). At the stations considered, the so-called index of agreement values of the predicted and measured time series of the NO₂ concentrations vary between 0.65 and 0.82, while the fractional bias values range from −0.29 to +0.26. In comparison with corresponding results presented in the literature, the agreement between the measured and predicted datasets is good, as indicated by these statistical parameters. The seasonal variations of the NO₂ concentrations were analysed in terms of the relevant meteorological parameters. We also analysed the difference between model predictions and measured data diagnostically, in terms of meteorological parameters, including wind speed and direction (the latter separately for two wind speed classes), atmospheric stability and ambient temperature, at two monitoring stations in central Helsinki. The modelling system tends to overpredict the measured NO₂ concentrations both at the highest (u⩾6 m s⁻¹) and at the lowest wind speeds (u<2 m s⁻¹). For higher wind speeds, the modelling system overpredicts the measured NO₂ concentrations in certain wind direction intervals; specific ranges were found for both monitoring stations considered. The modelling system tends to underpredict the measured concentrations in convective atmospheric conditions, and overpredict in stable conditions. The possible physico-chemical reasons for these differences are discussed.     

Elevated atmospheric CO2 concentration and temperature across an urban–rural transect

The heat island effect and the high use of fossil fuels in large city centers are well documented, but by how much fossil fuel consumption is elevating atmospheric CO₂ concentrations and whether elevations in both atmospheric CO₂ and air temperature from rural to urban areas are consistently different from year to year are less well known. Our aim was to record atmospheric CO₂ concentrations, air temperature and other environmental variables in an urban area and compare it to suburban and rural sites to see if urban sites are experiencing climates expected globally in the future with climate change. A transect was established from Baltimore city center (Urban site), to the outer suburbs of Baltimore (suburban site) and out to an organic farm (rural site). At each site a weather station was set-up to monitor environmental variables for 5 years. Atmospheric CO₂ was consistently and significantly increased on average by 66 ppm from the rural to the urban site over the 5 years of the study. Air temperature was also consistently and significantly higher at the urban site (14.8 °C) compared to the suburban (13.6 °C) and rural (12.7 °C) sites. Relative humidity was not different between sites whereas the vapor pressure deficit (VPD) was significantly higher at the urban site compared to the suburban and rural sites. An increase in nitrogen deposition at the rural site of 0.6% and 1.0% compared to the suburban and urban sites was small enough not to affect soil nitrogen content. Dense urban areas with large populations and high vehicular traffic have significantly different microclimates compared to outlying suburban and rural areas. The increases in atmospheric CO₂ and air temperature are similar to changes predicted in the short term with global climate change, therefore providing an environment suitable for studying future effects of climate change on terrestrial ecosystems.     

Air quality measurements in urban green areas – a case study

The influence of traffic-induced pollutants (e.g. CO, NO, NO₂ and O₃) on the air quality of urban areas was investigated in the city of Essen, North Rhine-Westphalia (NRW), Germany. Twelve air hygiene profile measuring trips were made to analyse the trace gas distribution in the urban area with high spatial resolution and to compare the air hygiene situation of urban green areas with the overall situation of urban pollution. Seventeen measurements were made to determine the diurnal concentration courses within urban parks (summer conditions: 13 measurements, 530 30 min mean values, winter conditions: 4 measurements, 128 30 min mean values). The measurements were carried out during mainly calm wind and cloudless conditions between February 1995 and March 1996. It was possible to establish highly differentiated spatial concentration patterns within the urban area. These patterns were correlated with five general types of land use (motorway, main road, secondary road, residential area, green area) which were influenced to varying degrees by traffic emissions. Urban parks downwind from the main emission sources show the following typical temporal concentration courses: In summer rush-hour-dependent CO, NO and NO₂ maxima only occurred in the morning. A high NO₂/NO ratio was established during weather conditions with high global radiation intensities (K>800 W m⁻²), which may result in a high O₃ formation potential. Some of the values measured found in one of the parks investigated (Gruga Park, Essen, area: 0.7 km²), which were as high as 275 μg m⁻³ O₃ (30-min mean value) were significantly higher than the German air quality standard of 120 μg m⁻³ (30-min mean value, VDI Guideline 2310, 1996) which currently applies in Germany and about 20% above the maximum values measured on the same day by the network of the North Rhine–Westphalian State Environment Agency. In winter high CO and NO concentrations occur in the morning and during the afternoon rush-hour. The highest concentrations (CO=4.3 mg m⁻³, NO=368 μg m⁻³, 30-min mean values) coincide with the increase in the evening inversion. The maximum measured values for CO, NO and NO₂ do not, however, exceed the German air quality standards in winter and summer.     

Validation of a hybrid forecasting system for the ozone concentrations over the Paris area

A simplified hybrid statistical-deterministic chemistry-transport model, is used in real time for the prediction of ozone in the area of Paris during Summer 1999. We present here a statistical validation of this experiment. We distinguish the forecasts in the urban area from forecasts in the pollution plume downwind of the city. The validation of model forecasts, up to 3 days ahead, is performed against ground based observations within and up to 50 km outside of Paris. In the urban area, ozone levels are fairly well forecast, with correlation coefficients between forecast and observations ranging between 0.7 and 0.8 and root mean square errors in the range 15–20 μg m⁻³ at short lead times. While the bias of urban forecast is very low, the largest peaks are somehow underestimated. The ozone plume amplitude is generally well reproduced, even at long lead times (root mean square errors of about 20–30 μg m⁻³), while the direction of the plume is only captured at short lead times (about 70% of the time). The model has difficulties in forecasting the direction of the plume under stagnant weather conditions. We estimate the model ability to forecast concentrations above 180 μg m⁻³, which are of practical relevance to air quality managers. It is found that about 60% of these events are well forecast, even at long lead times, while the exact monitoring station where the exceedance is observed can only be forecast at short lead times. Finally, we found that about half of the forecast error is due to the error in the estimation of the boundary conditions, which are forecast by a simple linear regression model here.     

Benzene,toluene, ozone,NO2 and SO2 measurements in an urban street canyon in Thessaloniki,Greece

Benzene, toluene, sulphur dioxide, ozone and nitrogen dioxide were measured at a mean level of 13.5 m above ground in a narrow, four-lane street canyon (height 30 m, width 20 m) in Thessaloniki, Greece during the period January–July 1997 by means of a commercial differential optical absorption spectrometer (OPSIS DOAS). Primary pollutant levels were found to be 2.5–4.4 times higher during the cold part of the year than during the warm part of the year, the winter/summer ratio increasing with the reaction rate constant with OH for each of the measured species. Ozone, on the other hand, exhibited a winter/summer ratio of 0.36. NO₂ originates from both primary and secondary sources; its winter/summer concentration ratio of 1.4 lies, therefore, between those of primary pollutants and ozone. Pollution levels were influenced considerably by wind speed, while for the street canyon under study wind direction did not influence pollutant levels considerably. While primary pollution was found to decrease with increasing wind speed, ozone increased. Benzene mean levels during the study period were around 6 ppb and hence much higher than the EU annual limit value of 5 μg m⁻³ (1.44 ppb at STP). Toluene mean levels were around 14 ppb and hence also several times above the WHO recommendation of 2 ppb for 24 h. The apportionment of traffic emissions in four time zones used in most inventories in urban airshed models was tested using benzene and toluene measurements at low (<1 m s⁻¹) wind speeds. The agreement between model emissions and calculated emissions apportionment into the four time zones was good, except for Zone D (23:00–1:59), where model inventory emissions were somewhat too low.     

Identification,abundance and seasonal variation of anthropogenic organic aerosols from a mega-city in China

PM_2.5 aerosols were collected in Nanjing, a typical mega-city in China, during summer and winter 2004 and were characterized for aromatic and cyclic compounds using a GC/MS technique to understand the air pollution problem. They include polycyclic aromatic hydrocarbons (PAHs), hopanes, phthalates and hydroxy-PAHs (OH-PAHs). PAHs, hopanes and OH-PAHs presented higher concentrations in winter (26–178, 3.0–18, and 0.013–0.421 ng m⁻³, respectively) than in summer (12–96, 1.6–11, and 0.029–0.171 ng m⁻³, respectively) due to an enhanced coal burning for house heating and atmospheric inversion layers developed in the cold season. In contrast, phthalates are more abundant in summer (109–368 ng m⁻³, average 230 ng m⁻³) than in winter (33–390 ng m⁻³, average 170 ng m⁻³) due to an enhanced evaporation from plastics during the hot season and the subsequent deposition on the pre-existing particles. Generally, all the identified compounds showed higher concentrations in nighttime than in daytime due to inversion layers and increased emissions from heavy-duty trucks at night. PAHs, hopanes and phthalates in Nanjing aerosols are 5–100 times more abundant than those in Los Angeles, USA, indicating a serious air pollution problem in the city. Concentrations of OH-PAHs are 1–3 orders of magnitude less than their parent PAHs and comparable to those reported from other international cities. Source identification using diagnostic ratios of the organic tracers suggests that PAHs in Nanjing urban area are mainly derived from coal burning, whereas hopanes are more attributable to traffic emissions.     

Comparison between the atmospheric boundary layer in Paris and its rural suburbs during the ECLAP experiment

The ECLAP experiment has been performed during the winter of 1995 in order to study the influence of the urban area of Paris on the vertical structure and diurnal evolution of the atmospheric boundary layer, in situations favourable to intense urban heat island and pollution increase. One urban site and one rural site have been instrumented with sodars, lidars and surface measurements. Additional radiosondes, 100 m masts and Eiffel Tower data were also collected. This paper gives a general overview of this experiment, and presents results of the analysis of four selected days, characterized by various wind directions and temperature inversion strengths. This analysis, which consists in a comparison between data obtained in the two sites, has been focused on three parameters of importance to the ABL dynamics: the standard deviation of vertical velocity, the surface sensible heat flux, and the boundary layer height. The vertical component of turbulence is shown to be enhanced by the urban area, the amplitude of this effect strongly depending on the meteorological situation. The sensible heat flux in Paris is generally found larger than in the rural suburbs. The most frequent differences range from 25–65 W m^-2, corresponding to relative differences of 20–60%. The difference of unstable boundary layer height between both sites are most of the time less than 100 m. However, sodar and temperature data show that the urban influence is enhanced during night-time and transitions between stable and unstable regimes.     

Dry deposition fluxes and deposition velocities of PAHs at an urban site in Turkey

Even though dry deposition and air–water exchange of semivolatile organic compounds (SOCs) are important for surfaces in and around the urban areas, there is still no generally accepted direct measurement technique for dry deposition. In this study, a modified water surface sampler (WSS) configuration, including a filter holder and an XAD-2 resin column, was employed to investigate the polycyclic aromatic hydrocarbon (PAH) dry deposition in an urban area. The measured total (particle+dissolved) PAH fluxes to the WSS averaged to be 34 960±16 540 ng m⁻² d⁻¹. Average particulate PAH flux, determined by analyzing the filter in the WSS, was about 8% of the total PAH flux. Temporal flux variations indicated that colder months (October–April) had the highest PAH fluxes. This increase could be attributed to the residential heating as well as meteorological effects including lower mixing height. A high volume air sampler was concurrently employed to collect ambient air concentrations. The average total (gas+particle) atmospheric PAH concentration (456±524 ng m⁻³) was within the range of previously measured values at different urban locations. PAH concentrations in urban areas are more than two orders of magnitude higher than those measured in pristine areas and this result may indicate that urban areas have major source sectors and greater deposition rates are expected near to these areas. The average contribution of particle phase was about 10% in total concentration. Simultaneous particulate phase dry deposition and ambient air samples were collected in this study. Then, particulate phase apparent dry deposition velocities were calculated using the fluxes and concentrations for each PAH compound and they ranged from 0.1 to 1.2 cm s⁻¹. These values are in good agreement with previously reported values.     

Examination of atmospheric ammonia levels near hog CAFOs,homes, and schools in Eastern North Carolina

Hog concentrated animal feeding operations (CAFOs) release ammonia (NH₃) in Eastern North Carolina (NC) to the atmosphere which is potentially hazardous for nearby human populations at community locations particularly homes and schools. We present NH₃ weekly average concentrations that were collected using passive diffusion tubes from October 2003 to May 2004 (20 sites) and from July 2004 to October 2004 (23 sites) near community locations in close proximity to hog CAFOs. The data for each phase of sampling was stratified by distance from the nearest hog CAFO. The mean Phase I levels were 16, 8, 7 and 5 ppb for distances <0.5, 0.5–1, 1–2, and 2 km or more, respectively. The mean levels for Phase II were 29, 16, and 11 ppb for distances <0.5, 0.5–1, and 1 km or more, respectively. The results of the distance stratification are the best results of this study and provide the strongest evidence that distance to one or more CAFOs is the key variable in controlling weekly NH₃ atmospheric concentration at the community level in Eastern NC. Statistical analyses confirmed that source terms such as distance to a hog CAFO and live weight per operation, as well as temperature, wind speed and wind direction were important predictors of atmospheric NH₃ at community locations. The results indicate potential zones of exposure for human populations who live or go to school near hog CAFOs.     

Variability in ultraviolet total optical depth during the Southern California Ozone Study (SCOS97)

Formation of photochemical air pollution is governed in part by the solar ultraviolet actinic radiation flux, but wavelength-resolved measurements of UV radiation in polluted urban atmospheres are rarely available. As part of the 1997 Southern California Ozone Study, cosine weighted solar irradiance was measured continuously at seven UV wavelengths (300, 306, 312, 318, 326, 333 and 368 nm) at two sites during the period 1 July to 1 November 1997. The first site was at Riverside (260 m a.s.l.) in the Los Angeles metropolitan area, which frequently experiences severe air pollution episodes. The second site was at Mt Wilson (1725 m a.s.l.), approximately 70 km northwest of Riverside, and located above much of the urban haze layer. Measurements of direct (i.e., total minus diffuse) solar irradiance were used to compute total atmospheric optical depths. At 300 nm, optical depths (mean±1 S.D.) measured over the entire study period were 4.3±0.3 at Riverside and 3.7±0.2 at Mt Wilson. Optical depth decreased with increasing wavelength, falling at 368 nm to values of 0.8±0.2 at Riverside and 0.5±0.1 at Mt Wilson. At all wavelengths, both the mean and the relative standard deviation of optical depths were larger at Riverside than at Mt Wilson. At 300 nm, the difference between the smallest and largest observed optical depths corresponds to over a factor 2 increase in the direct beam irradiance for overhead sun, and over a factor 7 increase for a solar zenith angle of 60°. Principal component analysis was used to reveal underlying factors contributing to variability in optical depths. PCA showed that a single factor (component) was responsible for the major part of the variability. At Riverside, the first component was responsible for 97% of the variability and the second component for 2%. At Mt Wilson, 89% of the variability could be attributed to the first component and 10% to the second. Dependence of the component contributions on wavelength allowed identification of probable physical causes: the first component is linked to light scattering and absorption by atmospheric aerosols, and the second component is linked to light absorption by ozone. These factors are expected to contribute to temporal and spatial variability in solar actinic flux and photodissociation rates of species including ozone, nitrogen dioxide, and formaldehyde.     

Multi-zonal air flow rates in residences in Boston,Massachusetts

In spite of the importance of interzonal air flow for indoor air quality assessment, few studies have characterized these flows. As part of the Boston Exposure Assessment in Microenvironments (BEAM) Study, air flow rates were estimated within 45 residences in the Boston area, most over two seasons. Thirty-five residences had basements, 11 of which also had attached garages, and 10 other residences had common apartment hallways. Air flow rates between zones were calculated using tracer gases (PFTs and SF6) and mass-balance models. Mean air flow rates from the basement to the occupied zone were significantly higher in the winter (174 m³ h⁻¹) than in the summer (67 m³ h⁻¹). The mean percent of the total air flow within the occupied zone of the residence from the basement was 26% (SD=34%) in the summer and 47% (SD=26%) in the winter while the mean percent from apartment hallways was 22% (SD=33%). Residences with garages attached to the basement had higher air flow rates to the adjacent zone (means from 50 to 887 m³ h⁻¹) than those with garages attached directly to the occupied zone (means from 1 to 65 m³ h⁻¹). These data provide a basis for modeling the contribution of indoor sources to concentrations in occupied zones.     

Experimental study of the air pollution transport by synchronised monitoring of atmospheric aerosols

Particulate pollution transport is estimated by means of cross-correlation and regression analyses. The aerosol particle size number spectrum in the form of 12 fraction concentrations is measured in units cm⁻³ (particles per cm³) every 10 minutes during three approximately 20-day measurement campaigns simultaneously in two measurement points in Estonia. The distance between these points is approximately 100 km for two campaigns, and 7 km for the third campaign. Two electrical aerosol spectrometers designed at Tartu University, having a wide particle diameter range (10 nm–10 μm), are used. The spectrometer's record is the mean particle spectrum for the 10 min measurement time. The air pollution transport is investigated during the time intervals when the air mass moves from one measurement point towards the other. The time series of aerosol size fraction concentrations for both locations are prewhitened to eliminate auto-correlation and to achieve stationary series of the ARIMA residuals. Then the cross-correlation function of these two series of residuals is calculated. The time lag corresponding to the mode of this function is treated as the mean time of pollution transport from the windward measurement point to the leeward one. For the submicron aerosol fraction (d=60 nm–1 μm) 3–5 h time lags are found. Mostly these time lags coincide with the mean wind velocities on some of the pressure levels (ground, 850 and 700 hPa) available in the study. In cross-wind cases the fraction concentrations measured in two points separated by 100 km were uncorrelated, but for the two points separated by 7 km there was quite a high correlation with zero time lag. The part of local and distant sources in the formation of the particle concentration in the leeward location is estimated by regression analysis.     

Measurements of ozone and nonmethane hydrocarbons at Chichi-jima island,a remote island in the western Pacific: long-range transport of polluted air from the Pacific rim region

Chichi-jima island is located in the Pacific about 1000 km from the Japanese main island and is an ideal remote observatory from which to assess the long-range transport of polluted air from East Asia. The ozone concentration was measured from August 1997 to August 1998. Owing to the air mass change, the seasonal variation of ozone shows a distinct character: low concentration (about 13 ppbv) for the maritime air mass during the summer, and high concentration (about 40 ppbv) for the continental air mass during the winter. To assess the contribution of the long-range transport of polluted air during winter, nonmethane hydrocarbons were also measured in December 1999. Using backward trajectory analysis, the transport time of the air mass from the source area in the Pacific rim region was calculated for each sample. The concentration of hydrocarbons shows a clear negative correlation against the transport time. This analysis clearly shows the transport of polluted air, emitted in East Asia, to the Pacific during the winter. The plots of suitable hydrocarbon pairs showed that the decrease of hydrocarbon concentrations during winter is mainly caused by the mixing with clean background air.     

Investigation of air pollution distribution in Linz: case studies to evaluate a K-type diffusion model coupled with a mass-consistent wind model

An Eulerian diffusion model coupled with a refined mass consistent wind model is developed for the operational forecasting of pollution distribution in complex terrain. The model is evaluated for a city situated in complex terrain. The study is carried out for a 20×20 km² area surrounding Linz, one of the industrial cities of Austria. The models are initialized with routinely measured meteorological parameters and topology information derived from the Geographical Information Systems (GIS). Four case studies, representative for major pollution episodes, are presented for model evaluation. The case studies include presence of a thermally induced wind system, presence of cold front an easterly southeasterly flow and a westerly–northwesterly flow. In presence of thermally induced wind systems, the flow field is most complex and existence of a shallow mixed layer with residual layer aloft enhances the pollution levels inside the city. Second case is used to study the development of pollution distribution inside the city in presence of low-level inversions and associated with low surface wind speeds. The low wind speeds in the surface layer lead to less mechanical generation of turbulence and lateral transport. The case studies of easterly and westerly flow fields are carried out to assess the capability of model under most frequently observed meteorological situations. The model is able to reproduce the pollution distribution near the slopes. There were over prediction inside the city in presence of thermally induced wind systems and is attributed to inadequate model physics during convective case. The present model setup is found to be a useful tool for the routine forecasting of pollution and could also be tested for other complex terrains.     

A study of seasonal and yearly modulation of carbon dioxide sources and sinks,with a particular attention to the Boreal Atlantic Ocean

With the intention of identifying and monitoring space and time patterns of carbon dioxide sources and sinks, the seasonal fields of atmospheric CO₂ concentration over an area covering Europe, the Boreal Atlantic, and North Africa have been computed by using CO₂ observations measured at one or two remote sites in conjunction with the backward air trajectories crossing the same observation sites.The air trajectories have been calculated by means of the wind speed fields provided by the ECMWF (European Centre of Medium-range Weather Forecast, of Reading, UK) analyses (T213/L31 model) on a regular grid, while the atmospheric CO₂ concentrations have been measured at two alpine European stations, located in the free atmosphere, far from the influence of local industrial pollution.A modified version of the statistical receptor-to-source-oriented-model (hereafter, source-oriented model) of Stohl (Atmos. Environ. 30 (1998) 947), using the above-mentioned air trajectories, has then been applied to reconstruct the spatial distribution fields of atmospheric CO₂.This source-oriented methodology belongs to a family of models which are simpler and easier to use than the more powerful and widespread inverse models and can allow a reliable deduction of the location of sources and sinks of gas tracers.We have applied this kind of model in order to identify source and sink macro-regions of CO₂ over the above-mentioned area in the period 1993–1998. The CO₂ observing stations of Plateau Rosà (3480 m a.s.l., in the western Alps) and Zugspitze (2937 m, in the eastern Alps) have been considered particularly fit for this purpose, because of their location in high orography areas, allowing to monitor values of atmospheric CO₂ concentrations representative of fairly well-mixed air, not affected by some local influences (industries, urban emissions, etc.). In this way, it can be assumed that possible maxima or minima observed in the trend of measured gas concentration can be due to contaminations of the air mass during its whole travel, at some specific locations identified by the source-oriented model.The most interesting result obtained in this study is the seasonal cycle of the atmospheric CO₂ concentration found over the mid- and sub-tropical latitudes of the Boreal Atlantic Ocean and evident in all simulations. This cycle appears to be clearly related to the seasonal trend of the SST, particularly in the tropical and subtropical Atlantic regions, and is particularly evident during the warmest months (during Spring, Autumn and particularly in Summer).     

Coarse atmospheric aerosol: size distributions of trace elements

A sampler, employing nine single stage impactors placed in parallel within a portable wind tunnel, has been used to determine the metal content of coarse atmospheric aerosol. The wind tunnel maintains a constant flow environment for the collectors housed inside it, so that representative sampling conditions are achieved compared to the varied ambient wind conditions. At a flow rate of 8 m s⁻¹ the 50% cut-off diameters of the impactors ranged from 7.8 to 38.8 μm. Measurements were conducted at a rural and urban site near Colchester in south east England. The samplers were analysed by PIXE for P, K, Ca, Fe, Ti, Mn, Cu, V, Co, Cr, Br, Zn, Ni, Sc and Pb. It is found that the sampler can be employed to quantitatively characterise the elemental mass size distribution for aerosol larger than 10 μm. The results indicate that a small fraction of the above earth and trace elements’ metal mass is present in particles greater than 10 μm. This fraction for earth metals (Ca, K, Ti) is comparatively greater in the rural site than the urban site, while for trace metals (Mn, V, Cu, Cr) this fraction constitutes a more significant part of the coarse mass at the urban site. Trace element concentrations were of a similar order of magnitude to earlier literature reports. Although the number of measurements was limited it can be concluded that the size distributions obtained were characteristic of an unpolluted area.     

PM10 regional transport pathways in Thessaloniki,Greece

In this study, the most dominant regional transport pathways for the city of Thessaloniki, Greece were identified and linked to air quality issues with respect to particulate matter (PM). Using air mass trajectories, cluster analysis techniques and PM10 measurements of a background-urban station of the greater Thessaloniki area during 2001–2004, it was found that north-eastern and southern flows were the most frequent in appearance with high potential to influence the city of Thessaloniki, especially when coinciding with biomass burning or Saharan dust events correspondingly. These incidents appeared to occur mostly during summer adding to a PM10 monthly mean up to 10 μg m^?3. High concentrations of surface PM10 related to north-eastern flows were in most cases accompanied with high aerosol columnar optical depths implying that particulate matter transport from the North-East was multi-layered. South-southwesterly flows originating from N. Africa, though less frequent, seemed to affect decisively Thessaloniki's aerosol budget especially during transition seasons. These flows were related with an increase of the monthly PM10 average up to 20–30 μg m^?3 for the time period studied. Finally, northerly flows were found to transport rather clean air masses that did not seem to contribute to the air quality deterioration of the city.     

Urban scale modeling of particle number concentration in Stockholm

A three-dimensional dispersion model has been implemented over the urban area of Stockholm (35×35 km) to assess the spatial distribution of number concentrations of particles in the diameter range 3–400 nm. Typical number concentrations in the urban background of Stockholm is 10 000 cm⁻³, while they are three times higher close to a major highway outside the city and seven times higher within a densely trafficked street canyon site in the city center. The model, which includes an aerosol module for calculating the particle number losses due to coagulation and dry deposition, has been run for a 10-day period. Model results compare well with measured data, both in levels and in temporal variability. Coagulation was found to be of little importance in terms of time averaged concentrations, contributing to losses of only a few percent as compared to inert particles, while dry deposition yield particle number losses of up to 25% in certain locations. Episodic losses of up to 10% due to coagulation and 50% due to deposition, are found some kilometers downwind of major roads, rising in connection with low wind speed and suppressed turbulent mixing. Removal due to coagulation and deposition will thus be more significant for the simulation of extreme particle number concentrations during peak episodes.The study shows that dispersion models with proper aerosol dynamics included may be used to assess particle number concentrations in Stockholm, where ultrafine particles principally originate from traffic emissions. Emission factors may be determined from roadside measurements, but ambient temperature must be considered, as it has a strong influence on particle number emissions from vehicles.