Atmospheric Environment (v.35, #22)

An investigation into the traffic-related fraction of isoprene at an urban location by Agnès Borbon; Hervé Fontaine; M Veillerot; N Locoge; J.C Galloo; René Guillermo (3749-3760).
Continuous hourly measurements of isoprene and 30 other hydrocarbons were performed at an urban centre site in Lille, France, from May 1997 to April 1999. Parallel mass emissions of the same hydrocarbons from in-service passenger vehicles were determined from measurements made on a chassis dynamometer using the European MVEG driving cycle. On the one hand, descriptive statistics and principal component analysis revealed the strong traffic origin of isoprene in winter months and its double biogenic and anthropogenic origin during the summer. On the other hand, the emission measurements of individual hydrocarbons in exhaust gases confirmed the presence of isoprene in petrol fuelled (with or without catalytic converters) and diesel car exhausts. Finally, the isoprene/acetylene ratios, both of them derived from ambient concentrations and emission factors, were compared. No statistically significant difference was found in winter, indicating the strict traffic origin of isoprene during that period. For the winter period, a simple regression analysis was performed on daily isoprene concentrations vs. those of acetylene and three other exhaust gases tracers—propene, ethylene and 1,3-butadiene. The established regression equations, together with the four tracer concentrations, were used to estimate the vehicle exhaust fractions of isoprene. From November to March, vehicle exhaust explained the totality of isoprene levels. While traffic remained the major source of isoprene with a contribution greater than 50% during the growing season, it still constituted a non-negligible source of isoprene in summer, anti-correlated to temperature and fluctuating between 10% and 50%. The application with 1,3-butadiene gives the greatest estimation of the anthropogenic fraction of isoprene. Other sources of 1,3-butadiene, acetylene, ethylene and propene were suspected in addition to their known traffic origin.
Keywords: Non-methane hydrocarbons; Biogenic source; Temporal variations; Emission factor; Vehicle exhaust contribution;

Three experiments were conducted in which intercomparisons were made between the equilibrium concentration technique, developed at JTI, Sweden, and the integrated horizontal flux technique for measuring ammonia emissions following applications of urea fertiliser, cattle slurry and solid pig manure to land. Mean square prediction error analysis was used to compare the emission rates measured by the two techniques. There were no significant differences between the measurement techniques, although there was some evidence that emission rates were overestimated by the equilibrium concentration method relative to the integrated horizontal flux technique at higher emission rates (>400 g.N ha−1  h−1). The equilibrium concentration method provides a practical and relatively inexpensive technique for measuring emissions under ambient conditions from small plots but good sampler preparation, adequate replication of emission measurements and appropriate choice of duration of sampling periods are necessities for obtaining reliable results.
Keywords: Ammonia emission; Measurement techniques; Manure;

FT-IR product study on the photo-oxidation of dimethyl sulphide in the presence of NO x —temperature dependence by Cecilia Arsene; Ian Barnes; Karl H. Becker; Raluca Mocanu (3769-3780).
The products of the OH radical-initiated oxidation of dimethyl sulphide (DMS) have been investigated as a function of temperature (284, 295, and 306 K) and different initial NO x (NO+NO2) concentrations: initial NO was varied between 434 and 2821 ppb and NO2 between 135 and 739 ppb. The experiments were performed at 1000 mbar total pressure in synthetic air using the photolysis of H2O2 as the OH-radical source and FT-IR spectroscopy to monitor reactants and products. The major sulphur-containing products identified were SO2, dimethyl sulphoxide (DMSO), dimethyl sulphone (DMSO2), methane sulphonic acid (MSA), methane sulphonyl peroxynitrate (MSPN) and OCS. The variation of the product yields with temperature and NO x concentration are consistent with the occurrence of both addition and abstraction channels in OH radical-initiated oxidation of DMS. Distinct trends in the yields of the various products have been observed as a function of temperature, initial NO x conditions and also reaction time as NO is consumed in the system. Increasing the initial NO concentration was found to depress the DMSO, SO2 and OCS formation yields and enhance those of DMSO2, MSA and MSPN. The yield–time behaviour of DMSO2 is supportive of a formation mechanism involving addition of O2 to a (CH3)2SOH adduct, formed via the addition channel, followed by sequential reactions with NO and O2. The mechanisms controlling the concentration–time profiles of the individual products under the present experimental conditions are discussed in detail and consideration is given to possible implications for the photo-oxidation of DMS under ambient conditions.
Keywords: Organic sulphur; OH radical; Sulphur dioxide; Dimethyl sulphoxide; Troposphere;

Volatile organic compounds (VOCs) of anthropogenic and biogenic origin play an important role in different photochemical processes in the atmosphere. In order to evaluate the lifetime of individual VOCs in the atmosphere and their impact on the photochemical processes, a detailed knowledge about their abundance in the atmosphere as well as kinetic parameters for the reactions with reactive species are required. Coding capabilities of different structural descriptors for the prediction of reactive rate constants of VOCs with ozone were studied. A diverse data set of 117 compounds with known reaction rate constants was taken from the literature. A 6-parameter MLR model was selected based on prediction capabilities of the model. The final prediction ability of the model was evaluated by 10-fold cross-validation procedure. The average root-mean squared error value for the prediction of log  k O 3 was 0.99.
Keywords: QSAR; Molecular descriptors; Prediction; Rate constant; O3; Ozone;

UV-B radiation is a driving factor for the chemistry of the polluted boundary layer. It is involved in the formation of radicals and consequently influences the formation and concentration of photo-oxidants. The 3-D mesoscale photochemical Metphomod model was employed to study the effect of changes in UV-B radiation on the concentration of photo-oxidants in the boundary layer over the Swiss Plateau. The model chemistry is based on the RACM mechanism and a two-stream approximation of radiative transfer. A summer (July) and a late winter (February) episode were simulated. All simulations were replicated with relatively large changes in the prescribed total ozone. The results for an increase in UV-B radiation show increases in PAN, HNO3, and ozone at noon in NO x -rich areas and a decrease in NO x . In NO x -poor areas in summer the effect on ozone is weak and has a negative sign, the main effect being an increase in H2O2. The spatial variability of NO x concentrations in the Swiss Plateau in the summer case is such that the effect of increased UV-B radiation on ozone is spatially variable. The effect on the ozone production rate in summer is strongest positive at the surface in the NO x -rich regions in the morning and strongest negative at some altitude above ground in NO x -poor regions in the early afternoon. In the winter episode, NO x -rich conditions are found almost everywhere on the Swiss Plateau, the effect of increased UV-B radiation on the ozone production rate is positive all day long and is largest at 300 m above ground at noon. In this case, in contrast to the summer case, the increase in ozone is carried over to the next day. The model results for ozone are in good agreement with results from a case study and a time series analysis of surface ozone measurements. We estimate the effect of day-to-day changes in total ozone on surface ozone peaks to range from 4 to 6 ppb at most.
Keywords: Photochemistry; Ozone; UV-B radiation; Modelling; Photolysis frequencies;

Resuspension of small particles from tree surfaces by Zitouni Ould-Dada; Nasser M. Baghini (3799-3809).
A detailed study of resuspension of 1.85 μm MMAD silica particles from five horizontal layers within a small scale spruce canopy was carried out in a wind tunnel in which saplings were exposed to a constant free stream wind speed of 5 m s−1. This provided quantitative estimates of the potential for a tree canopy contaminated with an aerosol deposit to provide (i) an airborne inhalation hazard within the forest environment and (ii) a secondary source of airborne contamination after an initial deposition event. Resuspension occurred with a flux of 1.05×10−7  g m−2  s−1 from spruce saplings initially contaminated at a level of 4.1×10−2  g m−2. An average resuspension rate (Λ) of 4.88×10−7  s−1 was obtained for the canopy as a whole. Values of Λ were significantly different (ANOVA, p<0.001) between canopy layers and Λ was markedly greater at the top of the canopy than lower down although there was a slight increase in Λ at the base of the canopy. The resuspended silica particles deposited onto the soil surface at an average rate of about 5.3×10−8  μg cm−2  s−1. It is concluded that resuspension under wind velocities similar to that used in the reported experiments is likely to pose a relatively small inhalation hazard to humans and a relatively minor source of secondary contamination of adjacent areas. Furthermore, resuspension rates are likely to diminish rapidly with time. The results are discussed in relation to the growing interest in the tree planting schemes in urban areas to reduce the impacts of air pollution.
Keywords: Aerosol particles; Spruce canopy; Wind tunnel; Resuspension rate; Inhalation hazard;

A three-dimensional (3-D) computational fluid dynamics (CFD) model, coupled with a meteorological radiation and surface physics package, is used to model the mean flow field and tracer dispersion in the vicinity of an idealized cubical building. We first compare the simulations with earlier numerical studies as well as towing-tank laboratory experiments, where radiation effects were not included. Our simulations capture most of the features revealed by the towing-tank data, including the variation of the flow reattachment point as a function of Froude number and the induction of a prominent lee wave in the low Froude number regime. The simulated tracer concentration also compares very favorably with the data.We then assess the thermal effects due to radiative heating on the ground and building including shading by the building, on the mean flow and tracer dispersion. Our simulations show that convergence within and beyond the cavity zone causes a substantial lofting of the air mass downstream from the building. This lofting results from the combination of thermal heating of the ground and building roof, and vortex circulation associated with the horseshoe eddy along the lateral sides of the building. The specific effect of shading on the flow field is isolated by comparing simulations for which the radiative heating and shading patterns are kept constant, but the environmental wind direction is altered. It is found that the shading exerts local cooling, which can be combined into the overall thermodynamic interaction, described above, to effectively alter the circulation downstream from the building.
Keywords: Large eddy simulation; Dispersion; Transport; Urban; Building; Thermal effects;

Interactions between volatile organic compounds (VOCs) and vinyl flooring (VF), a relatively homogenous, diffusion-controlled building material, were characterized. The sorption/desorption behavior of VF was investigated using single-component and binary systems of seven common VOCs ranging in molecular weight from n-butanol to n-pentadecane. The simultaneous sorption of VOCs and water vapor by VF was also investigated. Rapid determination of the material/air partition coefficient (K) and the material-phase diffusion coefficient (D) for each VOC was achieved by placing thin VF slabs in a dynamic microbalance and subjecting them to controlled sorption/desorption cycles. K and D are shown to be independent of concentration for all of the VOCs and water vapor. For the four alkane VOCs studied, K correlates well with vapor pressure and D correlates well with molecular weight, providing a means to estimate these parameters for other alkane VOCs. While the simultaneous sorption of a binary mixture of VOCs is non-competitive, the presence of water vapor increases the uptake of VOCs by VF. This approach can be applied to other diffusion-controlled materials and should facilitate the prediction of their source/sink behavior using physically-based models.
Keywords: Building material; Emission; Indoor air; Microbalance; Sink; Sorption;

We demonstrated the effectiveness of a semiconductor ammonia sensor capable of performing diachronic measurements; its characteristics were checked in the laboratory by means including comparison with standard gases. We found as a result that the ammonia sensor's readings increased with increasing water vapor pressure. We compared sensor readings with values obtained by chemical analysis of samples collected in situ and checked sensor reading accuracy. Ammonia concentration was determined by combining ammonia sensor readings with measured values for water vapor pressure. In situ conditions were ammonia concentration of under 100 ppbv and water vapor pressure of 4–16 hPa. There was a good correlation with the concentration of samples trapped with boric acid and analyzed by indophenol colorimetry. We discerned a relationship between ammonia concentration and local meteorological conditions such as wind direction and speed. The estimated error of the ammonia sensor's measurements was ±9.7 ppbv when ammonia concentration as measured by acid sampling and colorimetry was regarded as correct. This demonstrated that it is possible to detect in situ fluctuations in low ammonia concentrations of about 10 ppbv, which was the background concentration in farming areas. We have shown a monitoring method for ammonia in situ that is both easy to operate and low-cost.
Keywords: Ammonia; Semiconductor sensor; Water vapor effect; Easy monitoring;

A method for deriving the site-specific and subgrid area wind speed and friction velocity from regional model output and detailed land type information is developed. The “subgrid velocity scale” is introduced to account for generation of turbulent fluxes by subgrid motions. The grid vector averaged wind speed is adjusted by adding the subgrid velocity scale. This is to account for the fact that the spatial average of the local wind speed is usually larger than the absolute value of the vector averaged velocity (| V
|), especially when there are different land or surface types within the spatial averaging area and when V
is small. The assumption of uu *=constant is then applied within a model grid area to obtain wind speed and friction velocity for specific sites and subgrid areas. Using this method, the site-specific and subgrid area wind speed and friction velocity can be estimated from grid-averaged model output. In addition, more realistic air pollutant dry deposition velocities for specific locations and subgrid areas can be calculated. Grid-averaged deposition velocity values calculated using this approach tend to be about 30% different (either larger or smaller) for HNO3 and sulphate and about 10% different for SO2 and O3 compared to values calculated by assuming a constant wind speed over the whole model grid area. These differences are found to be even larger at specific sites or over some subgrid areas. This method can be applied to determine a more realistic wind speed, friction velocity and pollutant dry deposition velocity at specific locations using gridded meteorological data.
Keywords: Dry deposition velocity; Spatially vector averaged velocity; Subgrid velocity scale;

Nitric acid dry deposition to conifer forests: by H Sievering; T Kelly; G McConville; C Seibold; A Turnipseed (3851-3859).
The dry deposition velocity of nitric acid, V d(HNO3), over a 12-m (mean height) spruce–fir forest at Niwot Ridge, Colorado was estimated during 13 daytime periods using the flux-gradient approach. Turbulence intensity at this site is high (mean u∗ of 0.65 m s−1 with u of 2.9 m s−1) and contributed to the large observed V d(HNO3). The overriding contributor is identified to be the small aerodynamic needle width of the conifer trees. Two cases had inflated V d(HNO3) due to height-differentiated nitric acid loss to soil-derived particle surfaces. Not considering these cases, the mean V d(HNO3) was 7.6 cm s−1. The mean laminar boundary layer resistance (R b) was found to be 7.8 s m−1 (of similar magnitude to that of the aerodynamic resistance, 8.5 s m−1). The data-determined R b is bracketed by two theoretical estimates of the mean R b, 5.9 and 8.6 s m−1, that include consideration of the small canopy length scale (aerodynamic needle width), 1 mm or less, at this conifer forest. However, the poor correlation of data-determined R b values with both sets of theoretical estimates indicates that measurement error needs to be reduced and/or improved formulations of theoretical R b values are in order. The large observed V d(HNO3) at this conifer forest site is attributed to high turbulence intensity, and, especially, to small aerodynamic needle width.
Keywords: Nitric acid; Dry deposition; Conifer forests; Spruce; Fir; Pine;

Ambient aerometric data were used to predict whether ozone formation at specific times and locations in central California was limited by the availability of volatile organic compounds (VOC) or oxides of nitrogen (NO x ). The predictions were compared with differences between mean weekday and weekend peak ozone values. The comparison with weekend and weekday ozone levels provided a means for empirically investigating the effects of VOC and NO x reductions on ozone formation, because the relative proportions and levels of ozone precursor species were significantly different on weekends than on weekdays. Weekend NO x levels averaged 27 percent lower than weekday levels at the time of the peak ozone hour. Daytime weekend levels of VOC species were also consistently lower than weekday values throughout the region, though the differences between weekends and weekdays were not always statistically significant (p<0.05). Site-to-site differences between weekend and weekday mean peak hourly ozone were related to whether ozone formation was VOC- or NO x -limited.
Keywords: Peak ozone; Weekend ozone; VOC emissions; NO x emissions; Ozone response;

Assessment of exposure to traffic related air pollution of children attending schools near motorways by Nicole A.H Janssen; Patricia H.N van Vliet; Francée Aarts; Hendrik Harssema; Bert Brunekreef (3875-3884).
To assess exposure to air pollution from traffic of children attending schools near motorways, traffic related air pollution (PM2.5,NO2 and benzene) was measured in and outside 24 schools located within 400  m of motorways in the Netherlands. Reflectance of PM2.5 filters was measured as a proxy for elemental carbon (EC). The relationship between this proxy and measurements of EC was studied in a sub-sample and a high correlation was established. In both indoor and outdoor air, concentrations of PM2.5 and “soot” significantly increased with increasing truck traffic density and significantly decreased with increasing distance. Indoor NO2 concentrations significantly increased with increasing car traffic. The percentage of time that the school was downwind of the motorway during the measurements was significantly associated with “soot” and NO2, but not with PM2.5 and benzene. Estimated yearly averaged concentrations, calculated after standardising for differences in the background concentrations during the measurements, showed an about 2.5 fold range in “soot”, benzene (indoors and outdoors) and NO2 (indoors) concentrations. For PM2.5 (indoors and outdoors) and NO2 outdoors the range was smaller (1.4–1.7). Standardised concentrations were highly correlated with the results of two other approaches that were used to order the exposures at the schools. This study has shown that concentrations of air pollutants in and outside schools near motorways are significantly associated with distance, traffic density and composition, and percentage of time downwind. These variables can therefore be used to assess exposure to traffic related air pollution of subjects living near motorways. Furthermore, the yearly averaged concentrations of PM2.5, “soot”, NO2 and benzene can be used as a more direct measure of long-term exposure in epidemiological studies of the children attending the 24 schools.
Keywords: Traffic pollution; Particles; Soot; Nitrogen dioxide; Benzene; Roads;

Air pollution records on selenite in the urban environment by M Del Monte; P Ausset; P Forti; R.A Lèfevre; M Tolomelli (3885-3896).
Black crusts (BC) on selenite, i.e. gypsum stone used in buildings and monuments in the city of Bologna (northern Italy) have been studied.The crusts have been compared with genetically identical ones encountered on gypsum outcrops in the area surrounding the same city, in a relatively clean rural environment.Because of gypsum's high solubility, also in the past, BC began to form and embed particles of atmospheric aerosol as soon as the stone was used in construction, even though the SO2 levels in the air were probably much lower than today. Unlike other types of stone (sandstone, limestone, marbles), selenite is currently undergoing a deceleration of dissolution processes, following the acidification of the environment caused by industrialisation. For this reason the BC on selenite are older (up to 580±45 yr), thicker (up to 4 cm) and far more stable than those encountered on other types of building stones used in the same area.The BC are stratified: at the base, in contact with the stone, the crust contains numerous prismatic black particles originating from wood combustion; immediately above the base layer these particles progressively increase; finally, in the outer part of the crust one encounters the characteristic black, spongy spherical fly-ash originating from the combustion of mineral oils.From the chemical point of view, a comparison between urban and rural crusts shows that elements such as C, Si, K, Fe, V, Cr, Ni, Zn, Ga, As and Pb are due to present-day and/or past pollutant sources within the city itself.Moreover, the chemical data from the stratigraphy of the BC show that, while C and K were more abundant in the past than now, elements such as Fe, V, Cr, (Mn, Ni), Cu, (Zn), (Ga), As, Pb characterise recent and current urban atmospheres.
Keywords: Air pollution records; Black crusts; Urban environment; Selenite;

The influence of soluble compounds leached from real atmospheric aerosol particles (size range D ae: 0.17–1.6 μm) and dissolved NO2 on S(IV) oxidation in aqueous solution is presented. Experiments were conducted with aerosol particles of two different origins (i.e., urban and industrial) and at concentrations of trace gases in the gas mixtures (SO2/air and SO2/NO2/air) typical for a polluted atmosphere. During the introduction of SO2/air into the aqueous aerosol suspensions under dark conditions at pH 4, the formation of SO4 2− was very slow with a long induction period. However, in the presence of NO2 the oxidation rate of dissolved SO2 in suspensions of aerosols from both origins increased substantially (about 10 times). The results suggest that soluble compounds eluted from atmospheric aerosols have not only a catalytic (e.g. Fe, Mn), but also a pronounced inhibiting effect (e.g., oxalate, formate, acetate, glycolate) on S(IV) autoxidation. When NO2 was also introduced into the aerosol suspensions, the inhibition was not so highly expressed. An explanation for this is that the radical chain mechanism is mainly initiated by the interaction of dissolved NO2 and HSO3 . Therefore, at conditions typical for a polluted atmosphere dissolved NO2 can have a significant influence on the secondary formation of SO4 2−.
Keywords: Aerosols; Transition metals; Organic compounds; S(IV) oxidation; NO2;

During investigations of the formation of volatile organic compounds (VOCs) in leaves, we observed C5 VOCs during leaf drying, senescence, and following freeze–thaw damage. VOCs were quantified by proton-transfer-reaction mass spectrometry (PTR-MS). In freeze-damaged leaves, VOC products were verified with a gas chromatography PTR-MS system, showing that a variety of plants produced 1-penten-3-ol and 1-penten-3-one with smaller amounts of 2(Z)-penten-1-ol and pentenals; similar VOCs have been detected in soybean seed homogenates (Gardner et al., J. Agric. Food Chem. 44 (1996) 882). Most plants wounded in this way also released hexenals and hexanal, and clover also released methylbutanals. The formation of the C5 products was oxygen–dependent, consistent with the involvement of the enzyme lipoxygenase, and pentenone appeared to form independent of an alcohol dehydrogenase reaction; the latter is apparently disrupted by the freeze–thaw treatment. In parallel with these laboratory experiments, on-line PTR-MS measurements of ambient air were conducted at the Sonnblick Observatory in the Austrian Alps (3106 m a.s.l.). Following a hard freeze in central Austria, substantial amounts of C5 VOCs, ranging from 300 pptv to 6 ppbv and including 1-penten-3-ol, methylbutanals and probably pentenone, were detected at this site for several days peaking after midnight. Factor analysis supported their biogenic origin. We speculate that these VOCs were derived from freeze-damaged local vegetation by processes similar to those seen in laboratory freezing studies. If confirmed, these results suggest that leaf-freezing events in forests will give rise to the release of substantial levels of reactive C5 and C6 VOCs that can contribute to regional tropospheric chemistry.
Keywords: Biogenic; Volatile organic compounds; Leaf wounding; Leaf senescence; Sonnblick Observatory; Troposphere;