Atmospheric Environment (v.39, #25)

An empirical correlation between surface O3 and its factors by Bai Jianhui; Wang Gengchen; Wang Mingxing (4419-4423).
Observations were made of surface O3, NO x , UV radiation, and meteorological parameters at DBR, Guangdong province, and the statistical relationship of O3 to its factors (NO x , water, scattering, UV radiation) was studied. An empirical method was applied to estimate O3. The calculated value agrees well with that observed under different sky conditions, and averaged relative biases of daily O3 concentrations in clear and all sky conditions were ⩽7%. A good correlation was found between O3 and the ratio of NO2/NO in clear and all sky conditions, and correlation coefficients between O3 and NO2/NO were more than 0.90. This empirical method can be used as a tool to analyze the relationship between O3 and the factors affecting it. The analysis shows that O3 is more sensitive to the change of NO and NO2 than that of other factors.
Keywords: Surface O3; NO x ; UV radiation; Aerosol; Water vapor;

In October 1994, a tracer gas was released from a location in northwestern France and tracked at 168 sampling locations in 17 countries across Europe. This release, known as the European Tracer Experiment (ETEX), resulted in the collection of a wealth of data. This paper applies a previously described user-oriented measure of effectiveness (MOE) methodology to evaluate the predictions of 46 models against the long-range ETEX observations. The paper extends previous work by computing MOE values that are based on “true” areas (e.g., in square kilometers) and on actual European population distributions. In this way, assessments of model predictions of ETEX are placed in a possible operational context. The predictive performance of the models were assessed with nominal, area-, dosage-, and population-based MOE values and ranked using a few notional user-based scoring functions. This study finds that the rankings of some models—in particular, several of the model predictions that were ranked in the top 10—are relatively insensitive to the particular MOE technique used. This robust behavior, with respect to analysis assumptions, is regarded as an important feature of model performance.
Keywords: Model intercomparison; Transport and dispersion; Measure of effectiveness; European tracer experiment (ETEX);

Evaluation of the CALINE4 and CAR-FMI models against measurements near a major road by Joseph Levitin; Jari Härkönen; Jaakko Kukkonen; Juha Nikmo (4439-4452).
This study evaluates and inter-compares two atmospheric dispersion models against the results of a measurement campaign that was conducted near a major road at Elimäki in southern Finland from 15 September to 30 October 1995. The campaign was specifically designed for model evaluation purposes. The concentrations of NO x , NO2 and O3 were measured simultaneously at three locations, at three heights (3.5, 6 and 10 m) on both sides of the road. Traffic densities and relevant meteorological parameters were also measured on-site. The models evaluated were the Gaussian finite line source models California line source dispersion model, version 4 (CALINE4) and Contaminants in the Air from a Road—Finnish Meteorological Institute, version 3 (CAR-FMI); the latter was used in combination with the MPP-FMI meteorological pre-processing model. The agreement of measured and predicted datasets was good for both models, as measured using various statistical parameters. For instance, for the hourly NO x and NO2 data and predictions, analyzed separately at two distances from the road (17 and 34 m), each of these at two heights (3.5 and 6 m), the index of agreement values range from 0.77 to 0.88, and from 0.83 to 0.92 for the evaluations of the CAR-FMI and CALINE4 models, respectively. The performance of both models was better at a distance of 34 m, compared with that at a distance of 17 m. We also analyzed the difference between the model predictions and measured data in terms of the wind speed and direction. The performance of both models in most cases deteriorated as the wind speed decreased, and as the wind direction approached a direction parallel to the road.
Keywords: Dispersion; Traffic; Measurement campaign; Nitrogen oxide; Traffic wake; CALINE; CAR-FMI;

Positive matrix factorization (PMF) was used to infer the sources of PM2.5 observed at four sites in Georgia and Alabama. One pair of urban and rural sites in each state is used to examine the regional and urban influence on PM2.5 concentrations in the Southeast. Eight factors were resolved for the two urban sites and seven factors were resolved for the two rural sites. Spatial correlations of factors were investigated using the square of correlation coefficient (R 2) calculated from the resolved G factors. Fourier transform was used to define the temporal characteristics of PM2.5 factors at these sites. Factors were normalized by using aerosol fine mass concentration data through multiple linear regression to obtain the quantitative factor contributions for each resolved factor. Common factors include: (1) secondary sulfate dominated by high concentrations of sulfate and ammonium with a strong seasonal variation peaking in summer; (2) nitrate and the associated ammonium with a seasonal maximum in winter; (3) “coal combustion/other” factor with presence of sulfate, EC, OC, and Se; (4) soil represented by Al, Ca, Fe, K, Si and Ti; and (5) wood smoke with the high concentrations of EC, OC and K. The motor vehicle factor with high concentrations of EC and OC and the presence of some soil dust components is found at the urban sites, but cannot be resolved for the two rural sites. Among the other factors, two similar industry factors are found at the two sites in each state. For the wood smoke factor, different seasonal trends are found between urban and rural sites, suggesting different wood burning patterns between urban and rural regions. For the industry factors, different seasonal variations are also found between urban and rural sites, suggesting that this factor may come from different sources or a common source may impact the two sites differently. Generally, sulfate, soil, and nitrate factors at the four sites showed similar chemical composition profiles and seasonal variation patterns reflecting the regional characteristics of these factors. These regional factors have predominantly low frequency variations while local factors such as coal combustion, motor vehicle, wood smoke, and industry factors have high frequency variations in addition to low frequency variations.
Keywords: Receptor modeling; PM2.5; PMF; Urban; Rural; SEARCH; Factor contribution; Factor profiles; Time series analysis; Correlation coefficient;

This paper reports the results of using a Gaussian model to estimate the maximum inhalation doses (mSv), the spatial extent (km2) and radioactivity (Bq) of contamination within an urban area after the initial dispersion of 90Sr radiological dispersion device (dirty bomb) in a terrorist event. For modeling purposes, aerosol dispersion of approximately 7 g of 90Sr equaling 3.7×1013  Bq (1000 Ci) with effective release heights of 50 and 100 m above street level is estimated under varying local-scale atmospheric conditions. Maximum inhalation doses at the level of ⩾1, ⩾5, ⩾10, ⩾50 mSv are used as evaluative criteria to assess probable consequences. The intentional release of a relatively small amount of 90Sr using a conventional explosive has the potential to cause internal exposure to beta-radiation with relatively high maximum inhalation doses achieving hundreds of mSv, but the spatial extent of the area within which high exposures might occur is very small with most of the population receiving maximum inhalation doses between 1 and 10 mSv. The extent of radiation contamination (area and activity) is dependent on 90Sr particle size, the height of release, and local weather conditions.
Keywords: Strontuim-90; Dispersion; Contamination;

Seasonal and diurnal variations in methane emissions from Wuliangsu Lake in arid regions of China by Xiaonan Duan; Xiaoke Wang; Yujing Mu; Zhiyun Ouyang (4479-4487).
CH4 emissions have been widely studied in various wetlands, such as boreal peatlands, rice paddies, and tropical swamps. However, little investigation has been carried out for CH4 emissions from lakes or wetlands in arid regions where these freshwaters play a vital role in providing ecosystem services for local people. To quantify the spatial and temporal variations of CH4 flux and understand its key controlling factors in shallow lakes in arid regions of Western China, CH4 fluxes from Wuliangsu Lake were measured at different vegetation zones and water depths with a static chamber technique during a growing season from April to October in 2003. Results showed that the average emission flux of CH4 from submerged plant (Potamogeton pectinatus) growing zones was 2.16 mg CH4 m−2  h−1, which was 85.8% lower than that from emergent macrophyte (Phragmites australis) growing zones. CH4 emissions increased with increasing water depth in Phragmites Community. Significant seasonal and diurnal variations of CH4 emission were observed for P. australis during the plant growth stage, for P. pectinatus growing zones, however, the variations were minor. In addition to vegetation cover and water depth, bottom silt temperature and light intensity were also important factors influencing seasonal and diurnal variations of CH4 flux from Phargmites growing zone.
Keywords: Methane; Emission flux; Emergent plants; Submerged macrophyte;

Estimation of ammonia exchange at the land–ocean boundary condition of Sundarban mangrove, northeast coast of Bay of Bengal, India by H. Biswas; A. Chatterjee; S.K. Mukhopadhya; T.K. De; S. Sen; T.K. Jana (4489-4499).
The Sundarban mangrove forest (4264 km2) is about 3% of the total area of the world mangrove is interdependent on the coastal aquatic subecosystem (1781 km2). Ammonia concentration and flux measurements in the mangrove ecosystem and measurements in such tropical land–ocean boundary conditions are few and far between. A micrometeorological method was used to measure ammonia exchange over Sundarban mangrove forest and aquatic subecosystem. Results showed both emission and gaseous dry deposition in the forest area at a rate 1.79×103 and 2.26×103  kg N km–2  yr−1, respectively. Flux of ammonia in the aquatic subecosystem was found always from air to water with a rate of 775.7 kg N km–2  yr−1. The annual particulate dry and wet atmospherically deposited ammonia flux were calculated to be 69.0 and 21.5 kg N km–2  yr−1, respectively. Exchange of gaseous ammonia in the mangrove forest showed net deposition with a rate of 4.90×102  kg N km–2  yr−1. Annual imported flux from the proximate terrestrial source to this virgin forest was found to be 4.26×106  kg N yr−1 out of which 32.9% was deposited to the coastal water. Ammonia compensation point for mangrove vegetation was found to occur between 51 and 421 nmol m−3 and its variation was related to wind velocity, temperature, and humidity.
Keywords: Ammonia; Emission; Deposition; Mangrove system; Sundarban;

A total of 92 samples of street dust were collected in Luanda, Angola, were sieved below 100 μm, and analysed by ICP-MS for 35 elements after an aqua-regia digestion. The concentration and spatial heterogeneity of trace elements in the street dust of Luanda are generally lower than in most industrialized cities in the Northern hemisphere. These observations reveal a predominantly “natural” origin for the street dust in Luanda, which is also manifested in that some geochemical processes that occur in natural soils are preserved in street dust: the separation of uranium from thorium, and the retention of the former by carbonate materials, or the high correlation between arsenic and vanadium due to their common mode of adsorption on solid particles in the form of oxyanions. The only distinct anthropogenic fingerprint in the composition of Luanda's street dust is the association Pb–Cd–Sb–Cu (and to a lesser extent, Ba–Cr–Zn). The use of risk assessment strategies has proved helpful in identifying the routes of exposure to street dust and the trace elements therein of most concern in terms of potential adverse health effects. In Luanda the highest levels of risk seem to be associated (a) with the presence of As and Pb in the street dust and (b) with the route of ingestion of dust particles, for all the elements included in the study except Hg, for which inhalation of vapours presents a slightly higher risk than ingestion. However, given the large uncertainties associated with the estimates of toxicity values and exposure factors, and the absence of site-specific biometric factors, these results should be regarded as preliminary and further research should be undertaken before any definite conclusions regarding potential health effects are drawn.
Keywords: Risk; Exposure; Trace elements; Urban; Children;

Data collected from 1998 to 2001 clearly show that formaldehyde levels in ambient air of the city of Rio de Janeiro increased in 2001 (Corrêa et al., 2003, Atmospheric Environment 37, 23–29). In order to continue this study, samples were collected at the same site in the period from 2001 to 2002. In this work, we present the observed trends for formaldehyde and acetaldehyde levels from 1998 to 2002. Mean formaldehyde levels increased from 20 ppb in 1998 to 80 ppb in 2002, while acetaldehyde concentrations remained nearly unchanged. The formaldehyde/acetaldehyde ratio increased from 1.0 to 4.5 in the same period of time. These results may be explained by the increasing use of compressed natural gas by the vehicular fleet, in substitution of ethanol and gasohol (a mixture of gasoline and ethanol, 24% v/v). In order to confirm this hypothesis, some experiments were carried out to estimate the formaldehyde and acetaldehyde emissions from 20 automobiles powered by natural gas. The results showed a mean formaldehyde/acetaldehyde emission ratio of 3.42 for natural gas-fueled vehicles and of 0.24 when the same vehicles are fueled with gasohol. These high levels of formaldehyde may be attributed to the incomplete combustion of methane (80–90% of the natural gas) that is catalytically converted to formaldehyde in the exhaust pipe.
Keywords: Formaldehyde; Acetaldehyde; Vehicular emissions; Natural gas;

Impact of building configuration on air quality in street canyon by Xiaomin Xie; Zhen Huang; Jia-song Wang (4519-4530).
The objective of this study is to provide a simulation of emissions from vehicle exhausts in a street canyon within an urban environment. Standard, RNG and Chen–Kim k – ε turbulence models are compared with the wind tunnel measured data for optimization of turbulence model. In the first approach, the investigation is made into the effect of the different roof shapes and ambient building structures. The results indicate that the in-canyon vortex dynamics (e.g. vortex orientation) and the characteristics of pollutant dispersion are dependent on the roof shapes and ambient building structures strongly. A second set of calculations for a three-dimensional simulation of the street canyon setup was performed to investigate the influence of building geometry on pollutant dispersion. The validation of the numerical model was evaluated using an extensive experimental database obtained from the atmospheric boundary layer wind tunnel at the Meteorological Institute of Hamburg University, Germany (Studie on different roof geometries in a simplified urban environment, 1995). The studies give evidence that roof shapes, the ambient building configurations and building geometries are important factors determining the flow patterns and pollutant dispersion in street canyon.
Keywords: Building configuration; Street canyon; Numerical simulation; Wind tunnel data;

Elevated NH3 and NO2 air concentrations and nitrogen deposition rates in the vicinity of a highway in Southern Bavaria by Manfred Kirchner; Gert Jakobi; Ernst Feicht; Markus Bernhardt; Anton Fischer (4531-4542).
A transect study consisting of air concentration and deposition measurements of nitrogen compounds was performed to estimate the potential influence of car emissions on the nitrogen input to ecosystems. Therefore, two transects each consisting of 4 plots, the first in a coniferous forest and the second one in an extensively farmed grassland, were installed perpendicular to a highway south of Munich (Bavaria). Both profiles were influenced mainly by car emissions and showed only small local influences caused by agricultural activities. In the framework of a pilot study based upon denuder measurements we found a strong temporal dependency of both nitrogen dioxide (NO2) and ammonia (NH3) concentrations on traffic density. In the main study air concentrations of NO2 and NH3 were measured by passive samplers; they used as the basis for the estimation of dry deposition. These estimations have been compared with the results of analyses from simultaneously conducted canopy throughfall deposition and open air bulk measurements of nitrate (NO3 ) and ammonium (NH4 +). Additionally, within the forest transect the variety of different soil vegetation species was recorded and quantified. We obtained a strong gradient of gas concentrations along both profiles. Whereas the bulk deposition remained quite constant along the non-forested transect, the nitrogen throughfall deposition rate diminished substantially with the distance from the highway. The deposition rate at the forest edge was twice of that inside. The nitrogen load estimated for the examined forest in the vicinity of the highway was comparable to other forest ecosystems situated near diffuse emission sources from agriculture. It could be shown that changes in soil composition and soil vegetation along the forest transect are caused by decreasing nitrogen deposition with distance from the highway. The application of road salt in winter leads to further impacts.
Keywords: Road traffic; Emissions; Transect; Forest edge; Ground vegetation;

Receptor modeling of source contributions to atmospheric hydrocarbons in urban Kaohsiung, Taiwan by C.H. Lai; K.S. Chen; Y.T. Ho; Y.P. Peng; Youn-Min Chou (4543-4559).
Data of 60 hydrocarbon species in C2–C15 from previous measurements in Kaohsiung (Lai et al., 2004. Atmospheric Environment 38, 1997–2011) were analyzed by applying a chemical mass balance receptor model to determine the major source contributions to atmospheric hydrocarbons. Five major source groups were identified using factor analysis, and 11 source profiles were selected based on sensitivity test results to yield best model performance. Chemical mass balance (CMB) receptor model analyses show that motor vehicle exhaust is the primary source of ambient hydrocarbons during the morning (07–10) and evening (18–21) periods corresponding to the rush hours, but drops in the afternoon non-rush-hour (13–16) period. Furthermore, industrial processes and/or organic solvents contribute most to ambient hydrocarbons during non-rush hours. Contributions from boiler combustion and sewage treatment tanks are always minor. Wind direction analyses reveal that a relatively clean sea breeze tends to reduce the afternoon hydrocarbon concentrations below those during any other period of the day.
Keywords: Receptor model; Chemical mass balance; Atmospheric hydrocarbons;

The influence of size-dependent droplet composition on pollutant processing by fogs by K.M. Fahey; S.N. Pandis; J.L. Collett; P. Herckes (4561-4574).
The effects of size-dependent fog chemistry and physics on the pre-fog aerosol size/composition distribution are examined for a San Joaquin Valley fog. In this paper, we compare the results of a size-resolved fog model with measurements from Colorado State University's five-stage and bulk cloud collectors. The model reproduces the amounts and trends of the observed bulk fog water concentrations and size-dependent composition of the major species (sulfate, ammonium, nitrate, sodium, chloride, calcium). We examine the size-dependent evolution of the measured species and compare sulfate concentrations predicted by the highly size-resolved fog model with the variable size resolution model (VSRM) [Fahey and Pandis, 2001. Atmospheric Environment 35, 4471–4478]. It is shown that for lengthy fog events in relatively clean environments deposition of fog droplets is the most important process for the evolution of the size/composition distribution of aerosols over the course of fog processing. The results indicate a need for a larger number of measurements of deposition fluxes for individual species and the need for aqueous-phase concentration measurements from the early formation stage of fogs.
Keywords: Fog chemistry; VSRM; SO2 oxidation; Sulfate; San Joaquin Valley;

We considered conjugate mass transfer during absorption of a gas by a falling droplet with internal circulation. Gaseous phase is assumed to contain inert admixtures, and resistance to mass transfer in both phases is taken into account. Mass flux is directed from a gaseous phase to a droplet, and the interfacial shear stress causes a fluid flow inside the droplet. Droplet deformation under the influence of the interface shear stress is neglected. Absorbate accumulation in the bulk of dispersed phase is taken into account. The problem is solved in the approximations of a thin concentration boundary layer in the dispersed and continuous phases. The bulk of a droplet, beyond the diffusion boundary layer is completely mixed, and concentration of absorbate is homogeneous and time-dependent in the bulk. The thermodynamic parameters of a system are assumed constant. By combining the generalized similarity transformation method with Duhamel's theorem, the system of transient conjugate equations of convective diffusion for absorbate transport in liquid and gaseous phases with time-dependent boundary conditions is reduced to Volterra integral equation of the second kind which is solved numerically. Theoretical results are compared with the available experimental data.
Keywords: Diffusion; Gas absorption; Droplet; Mass transfer; Inert admixtures; Internal circulation;

The solar photodegradation of 16 polycyclic aromatic hydrocarbons (PAHs), sorbed on surfaces of pine [Pinua thunbergii] needles was investigated. The PAHs were produced by combustion of polystyrene and exposed onto the surfaces of pine needles. The disappearance of PAHs sorbed on the pine needle surfaces is mainly caused by volatilization and photolysis, with photolysis playing a major role. The volatilization rates correlate with PAH molecular weight significantly. The photolysis of the 16 PAHs follows first-order kinetics and their photolysis half-lives (t 1/2,P) range from 12.9 h for naphthalene to 65.4 h for fluorene. The PAHs have similar half-lives whether they are sorbed on spruce or pine needles. Compared with water, the cuticular waxes of pine needles can stabilize photolysis of PAHs and facilitate accumulation of PAHs. t 1/2,P for selected PAHs correlate with semi-empirically calculated energy of the highest occupied orbital (E HOMO). Photochemical behaviors of PAHs are dependent not only on their molecular structures but also the physical–chemical properties of the substrate on which they are adsorbed.
Keywords: PAH; Pine needle; Photolysis; Solar irradiation;

Effect of ambient NH3 levels on PM2.5 composition in the Great Smoky Mountains National Park by Kenneth J. Olszyna; Solomon T. Bairai; Roger L. Tanner (4593-4606).
Continuous measurements were made of gaseous NH3, gaseous HNO3, PM2.5 sulfate, aerosol mass, air temperature, and relative humidity (RH), and 24-h filter samples of PM2.5 aerosols were analyzed to determine the effects of ambient NH3 levels on PM2.5 aerosol composition. These measurements were conducted from 2 to 15 August 2002, at Look Rock, TN, at an air monitoring station located on a mountain ridge (elevation ∼800 m MSL) on the southwestern edge of the Great Smoky Mountains National Park. A strong diurnal pattern was observed for gaseous NH3 levels at this site with higher daytime and much lower NH3 during the night hours. The diurnal variability of both gaseous NH3 concentrations and RH suggests that conditions controlling aerosol nitrate formation and acidic aerosol neutralization are complex. The observations during this 2-week long sampling period showed that measurable NH4NO3 aerosol was not present under the ambient conditions encountered. However, modest perturbations from observed ambient conditions could have enabled NH4NO3 aerosol production. The extent of neutralization of sulfate aerosol collected on filters indicates that there was insufficient regional NH3 to neutralize the acidic sulfate aerosols to the degree predicted from local NH3 and RH conditions. Incomplete neutralization of acidic sulfate could also result if neutralization proceeds more slowly under field conditions compared to laboratory conditions or model predictions. Continuous measurements of aerosol ammonium and/or acidity are needed to determine if ammonia availability or kinetic limitations (or both) restrict neutralization of acidic sulfate aerosols, a key factor for modeling and exposure studies.
Keywords: Ammonia; NH4NO3; PM2.5 sulfate aerosol; Acid sulfate aerosol; Neutralization;

New directions: How dangerous is ozone? by Tomislav Cvitaš; Leo Klasinc; Nenad Kezele; Sean P. McGlynn; William A. Pryor (4607-4608).