Atmospheric Environment (v.36, #39-40)

Phase separation and regrowth of aerosol matter collected after size fractionation in an impactor by K. Wittmaack; N. Menzel; H. Wehnes; U. Heinzmann (5877-5886).
Aerosol matter in the size range <2 μm was collected in a Berner impactor and subsequently analysed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectrometry. Owing to the low electron beam energy of 5 keV (occasionally 10 keV), analysis was restricted to elements with atomic numbers ⩽20 (Ca). Sub-micrometer aerosol matter was found to contain mostly S, O, and C as well as some K and Ca. Nitrogen appeared to escape detection, probably due to bombardment-induced sublimation of NO3 and NH4. During sampling at low to moderate relative humidity (<60%) the sulphur-rich fraction of the aerosol matter (most likely sulphates) regrew in the form of microcrystals with sizes up to 10 times larger than the mean aerodynamic diameter of the respective impactor stage. By contrast, when sampling during periods in course of which the relative humidity exceeded 70%, the aerosol matter regrew in the form of extended amorphous agglomerates. The aerosol deposits also contained large numbers of carbon nanoparticles, well separated from the regrown sulphate-rich matter. The nanoparticles were similar in size (∼20–40 nm), much smaller than the equivalent aerodynamic diameter of the impacting particles (63 nm–2 μm). Presumably, the carbon nanoparticles constituted the core of larger air-borne particles covered with sulphates (as well as with nitrates and organic carbon). The regrown microcrystals disappeared rapidly under electron bombardment at high current density, an observation that indicates high volatility at elevated temperatures. Aerosol matter collected in the size range between 1 and 2 μm contained large fractions of particles made of O, Si, P, K, and Ca (oxides). These particles were highly resistant to electron bombardment (hard) and showed little or no evidence for agglomeration or regrowth. After removing the soluble (acidic) material from the collected aerosol matter, only carbon nanoparticles and hard coarse particles were left behind. The observation of agglomerated or crystallized “soft” aerosol matter in combination with phase separation of carbon nanoparticles lends further support to the assertion that it is not possible to collect useful quantities of fine and ultrafine aerosol particles with as-suspended morphology. Some implications for health-related research are discussed.
Keywords: Scanning electron microscopy; Atmospheric aerosols; Regrowth; Phase separation; Microcrystals; Carbon nanoparticles;

Annual growth rings sampled from three free-standing trees (Platanus hybrida sp.), grew in the metropolitan area of Palermo (Italy) and covering a 118 years time span (1880–1998), have been studied for their 13C/12C carbon isotope ratios. It has been found that the 13C/12C tree ring record, during the study time interval, decreased of −3.6‰, from −26.4‰ in 1880 to −30‰ in 1998. Such a progressive depletion has been attributed to the addition of anthropogenic 13C depleted carbon dioxide to the local atmosphere. The observed 13C/12C decrease has been used to infer some possible pathways of atmospheric CO2 change in the study urban area.
Keywords: Tree rings; Carbon dioxide; Urban environment; Environmental geochemistry; Dendrochemistry; Stable carbon isotopes;

In this theoretical investigation, we elucidated several factors governing the ability of organic, water-soluble vapours to participate into either the secondary organic aerosol formation or particle condensational growth in the atmosphere. The corresponding requirements for physico-chemical properties of the vapour were estimated. These estimates were also compared with the properties of several difunctional carboxylic acids identified in the atmosphere. Our analysis indicates that while many of the considered compounds are expected to contribute to the total particle mass in the atmosphere, their role in particle condensational growth process remains uncertain. This uncertainty emerges from the fact that the saturation ratio of an organic vapour does not alone determine its ability to act as a “nonvolatile” compound. Instead, the hygroscopic and chemical properties of the vapour, as well as the particle pH and size, have also to be taken into account.
Keywords: Secondary organic aerosol; Aerosol dynamics; Organic acids; Aqueous phase; Phase partitioning;

The hygroscopic behaviour of NaCl, (NH4)2SO4, Na2SO4 and NH4NO3 particles in the size range of 0.1–20 μm was studied by environmental scanning electron microscopy (ESEM). This technique allows the in-situ observation of individual aerosol particles while changing the temperature and/or relative humidity (RH) in the sample chamber. The hygroscopic behaviour of these particles (e.g., deliquescence, adsorption of water on the particle surface) becomes directly observable with a lateral resolution of the order of 8–15 nm. The deliquescence relative humidities (DRH) of the different salts, the temperature dependence of the DRH for NH4NO3, and the growth factors (at increasing relative humidities) for NaCl were determined. Generally, a good agreement between the values obtained by ESEM and those found in literature was achieved. However, the DRH of NaCl determined by ESEM is systematically higher (approximately 2%, absolute) than the values obtained by other techniques, which can be explained by the observed strong absorption of water onto the crystal surface prior to droplet formation. The efflorescence behaviour of individual particles can be studied only qualitatively due to influences of the sample substrate. Furthermore, it is demonstrated that the activation of soot can be studied at high resolution by ESEM.
Keywords: Environmental scanning electron microscopy; Deliquescence; Relative humidity; Individual particle analysis; Hygroscopy;

Rate of pesticide volatilization from soil: an experimental approach with a wind tunnel system applied to trifluralin by Carole Bedos; Marie-France Rousseau-Djabri; Dominique Flura; Sylvie Masson; Enrique Barriuso; Pierre Cellier (5917-5925).
Pesticide volatilization to the atmosphere may be a major pathway of dissipation closely linked with environmental, physico-chemical and technical factors. Understanding the volatilization process requires systems that make it possible to control some of these factors. Wind tunnels meet to these criteria. The volatilization flux is determined from a mass balance, using the difference in atmospheric pesticide concentration between the entrance and the exit of the tunnel and the airflow rate. An experiment was carried out in June 2000 to study the repeatability of this technique. Volatilization of trifluralin was measured in three wind tunnels for 8 days with a sampling period varying between 3 h and 2 days. Pesticide concentration was determined by trapping by XAD-2 resin in a two-stage cartridge, solvent extraction and analysis by gas chromatography. Cumulated losses through volatilization reached 30% of the measured application dose after 8 days, with a variability of less than 20% between the three tunnels. Approximately 20% remained in the topsoil (0–2 cm), with a variability of 14% between the three tunnels. The decrease in the volatilization flux over time is coherent with the expected theoretical evolution for a volatile pesticide such as trifluralin and with previous experimental works.
Keywords: Emission; Soil/Atmosphere; Pesticide; Measurement; Wind tunnel;

Summer air quality over an artificial lake by W Kuttler; T Lamp; K Weber (5927-5936).
In the summer of 1998, the air quality (indicators: CO, NO, NO2, O3) above the water surface of the Lake Balderey (Essen, Ruhr area, North Rhine-Westphalia, Germany), an artificial lake used for recreation purposes, was measured using the Fourier transform infrared spectroscopy (FTIR) and differential optical absorption spectroscopy (DOAS) remote measurement methods. The lake, with an area of 3 km2 was created by damming the Ruhr and is surrounded by higher ground. In calm, bright weather conditions, this location results in a low-exchange situation (formation of temperature inversions, cold air dynamics) with a sustained impact on pollutant concentrations over the lake. The results of trace substance measurements (1/2 h mean values) were compared with values from comparison stations (suburban, high traffic and forest) located outside the area of the lake. In general, it was found that mean CO and NO concentrations over the lake were very low (0.3 ppm and 7.5 ppb, respectively). NO2 values (∼15 ppb) were some 3.5 times higher than those recorded at the forest station and O3 values, at 27 ppb, almost reached the same level as at the forest station (30 ppb). Mass flow densities as a function of wind direction, diurnal courses, differences between weekdays and weekends and comparisons with air quality standards are presented for the lake station.
Keywords: CO; NO; NO2; O3; SF6 measurements (FTIR, DOAS); Leisure area;

Three methods are currently in use for the determination of total N within rainwater and water-extracted aerosols—UV photo-oxidation, persulfate digestion, and high-temperature combustion. Among these methods, UV photo-oxidation has received scrutiny for its reported inability to digest “refractory” organic N compounds. In this manuscript we utilized inorganic and organic nitrogen standards to carefully assess the ability of a specific UV digestor, the Metrohm 705 UV digestor, to correctly digest organic N within dilute solutions (such as rainwater and atmospheric aerosol). We also discuss the negative aspects of the three methods listed above, comment on the photochemical reactions and chemical products produced by exposure to UV light, and make suggestions concerning the proper use of UV digestion technique when determining total N.Our tests indicate that at a digestion time of 2 h and a temperature of 85°C the system is satisfactory for determining the total amount of organic N within dilute solutions. The upper concentration limit using this UV system was found to be 50 μM N of organic N. Diluted samples were found to carry a larger burden of analytical error due to the combined effects of UV blanks (that can range from 0.1–0.6 μM N) and the uncertainty associated with the chromatographic ion analysis (for nitrate, nitrite, and ammonium). This error can often lead to negative values when reporting organic N. We suggest that negative values for organic N be included within the mean and standard deviation of organic N measurements rather than excluding them by rounding negative values to zero. Only by including all analytical error within measurements will true concentrations of organic N within rainwater and atmospheric aerosols and its relevance to total N budgets be known.
Keywords: Organic nitrogen; Total nitrogen methods; UV digestion; Rainwater; Atmospheric aerosol;

Do aerosols act as catalysts in the OH radical initiated atmospheric oxidation of volatile organic compounds? by M Sørensen; M.D Hurley; T.J Wallington; T.S Dibble; O.J Nielsen (5947-5952).
Smog chamber/FTIR techniques were used to study the relative reactivity of OH radicals with methanol, ethanol, phenol, C2H4, C2H2, and p-xylene in 750 Torr of air diluent at 296±2 K. Experiments were performed with, and without, 500–8000 μg m−3 (4000–50 000 μm2  cm−3 surface area per volume) of NaCl, (NH4)2SO4 or NH4NO3 aerosol. In contrast to the recent findings of Oh and Andino (Atmospheric Environment 34 (2000) 2901, 36 (2002) 149; International Journal of Chemical Kinetics 33 (2001) 422) there was no discernable effect of aerosol on the rate of loss of the organic compounds via reaction with OH radicals. Gas kinetic theory arguments cast doubt upon the findings of Oh and Andino. The available data suggest that the answer to the title question is “No”. As part of this work the rate constants for reactions of OH radicals with methanol, ethanol, and phenol in 750 Torr of air at 296 K were determined to be: k OH+CH3OH=(8.12±0.54)×10−13, k OH+C2H5OH=(3.47±0.32)×10−12 and k OH+phenol=(3.27±0.31)×10−11  cm3  molecule−1  s−1.
Keywords: Aerosol chemistry; Heterogeneous chemistry; Relative rate method; OH radical kinetics;

A simple semi-empirical model for predicting missing carbon monoxide concentrations by Kim N. Dirks; Murray D. Johns; John E. Hay; Andrew P. Sturman (5953-5959).
Carbon monoxide monitoring using continuous samplers is carried out in most major urban centres in the world and generally forms the basis for air quality assessments. Such assessments become less reliable as the proportion of data missing due to equipment failure and periods of calibration increases. This paper presents a semi-empirical model for the prediction of atmospheric carbon monoxide concentrations near roads for the purpose of interpolating missing data without the need for any traffic or emissions information. The model produces reliable predictions while remaining computationally simple by being site-specifically optimized. The model was developed for, and evaluated at, both a suburban site and an inner city site in Hamilton, New Zealand. Model performance statistics were found to be significantly better than other simple methods of interpolation with little additional computational complexity.
Keywords: Carbon monoxide; Urban air quality; Empirical modeling; Interpolation; Missing data;

Predicting the contents of BTEX and MTBE for the three types of tollbooth at a highway toll station via the direct and indirect approaches by Perng-Jy Tsai; Ching-Chang Lee; Mei-Ru Chen; Tung-Sheng Shih; Ching-Huang Lai; Saou-Hsing Liou (5961-5969).
This study was set out to assess the contents of five volatile organic compounds (VOCs), including BTEX (the acronym for benzene, toluene, ethylbenzene, and xylene) and methyl tertiary-butyl ether (MTBE), in three types of tollbooth (including the car lane/ticket-collecting, car lane/cash-collecting, and bus/truck lane tollbooths) at a highway toll station via the direct and indirect approaches. For the direct approach, VOC samples were collected from the breathing zone of booth attendants at all selected tollbooths during the three workshifts. For samples collected during the dayshift, we found VOC contents of BTEX and MTBE in both the car lane/ticket-collecting (=6.23, 21.93, 3.24, 8.56, and 5.63 ppb, respectively) and car lane/cash-collecting tollbooths (=5.98, 21.71, 3.25, 8.59, and 6.04 ppb, respectively) were quite comparable, but both were significantly higher than that in the bus/truck lane tollbooth (=3.13, 13.91, 2.05, 4.52, and 2.70 ppb, respectively). The same pattern can also be found for the other two workshifts. For the indirect approach, we conducted multivariate regression analyses to predict VOC contents for any given type of tollbooth by using the four independent variables of the vehicle flowrate, wind speed, relative humidity, and air temperature. We found that, except the vehicle flowrate, the other three factors did not have a significant effect on VOC contents in the three types of tollbooth. In addition, the magnitudes of the effect of the vehicle flowrate on VOC contents for the three types of tollbooth were: car lane/cash-collecting>bus/truck lane>car lane/ticket-collecting. All regression results yielded R 2-values in the range of 0.41−0.74 indicating that the developed indirect approach was able to predict VOC contents for three types of tollbooth.
Keywords: VOC contents; Tollbooth; Direct approach; Indirect approach;

The Main Geophysical Observatory 2D channel photochemical model is used to study the behavior of tropospheric OH within the 30–60°N zonal belt in relation to changing NO X and CO emissions. The changes of tropospheric OH as a function of the contributions by NO X and CO emissions during the period 1850–2050 are calculated. Our estimations show that the largest annual increment of total tropospheric OH within the belt considered occurs in the 1985–1995 period, about 0.27%  yr −1 . Based on scenarios of tropospheric pollution emissions in the first half of 21st century, the total tropospheric OH content will increase more slowly, by 0.12–0.15%  yr −1 . The maximum growth of OH concentration occurs close to air pollution locations—in the lower troposphere during 1850–1995 but in the upper troposphere in the 21st century when the NO X source from subsonic aircraft increases faster than the surface source.
Keywords: Photochemical source; Photochemical sink; Lifetime; Gas emission; North temperate zonal belt;

A series of experiments using bulk precipitation collectors of the type used in the UK precipitation chemistry network measured the amounts of NH4 +, SO4 2− and other ions that could be washed from funnels (diameter 15 cm) exposed to a wide range of NH3 and SO2 concentrations over periods from hours to days. In dry conditions, the average deposition flux of NH3 was between 50 and 120 nmol NH4 + funnel−1  d−1 (0.1–0.3 kg N ha−1  yr−1), and was independent of the concentration of NH3. Dry deposition of NH3 to wet funnels at small NH3 concentrations was almost 5 times that to dry funnels under the same conditions (average 240 nmol funnel−1  d−1; 0.7 kg ha−1  yr−1), and increased with increasing NH3 concentrations. The amount of NH4 + ions remaining on the funnel surface was inversely proportional to the vapour pressure deficit during the experiment. This result was interpreted as a dependence on the duration of surface wetness, with greater deposition of NH4 + when evaporation rates of surface water were small.The amount of SO2 deposited on funnel surfaces was closely related to the amount of NH3 deposited, in both wet and dry conditions, but was not strongly correlated with the SO2 concentration. At low NH3 and SO2 concentrations the average deposition to dry funnels was 70 nmol SO4 2− funnel−1  d−1 (0.5 kg ha−1  yr−1), and to wet funnels was approximately 2.5 times larger. The results are interpreted in terms of the balance between the rate of evaporation of surface water, and the rate of oxidation of SO2, which leads to the ‘fixing’ of NH4 + ions on the surface as involatile salts.It is predicted that dry deposition of NH3 to funnel surfaces across the UK Secondary Network could account for as much as one-half of the measured bulk wet deposition at sites where wet deposition of NH4–N is small. The amount of dry deposition depends on how long and how often funnel surfaces are wetted by rain or dew, and on the air concentrations of NH3. These predictions are based on funnels being wetted only once per day. More frequent wetting would increase the contribution from dry deposition, and the consequent overestimate of wet deposition of NH4–N across the UK by using data obtained from bulk collectors. To some extent this overestimate may be offset by microbial degradation and loss of NH4–N in weekly bulk precipitation samples during collection and storage.
Keywords: Acid rain; Precipitation chemistry; Rainfall networks; Wet-only collectors; Bulk deposition; Nitrogen deposition; Sulphur deposition;

HNO3 fluxes to a deciduous forest derived using gradient and REA methods by S.C Pryor; R.J Barthelmie; B Jensen; N.O Jensen; L.L Sørensen (5993-5999).
Summertime nitric acid concentrations over a deciduous forest in the midwestern United States are reported, which range between 0.36 and 3.3 μg m−3. Fluxes to the forest are computed using the relaxed eddy accumulation technique and gradient methods. In accord with previous studies, the results indicate substantial uncertainties in the gradient-based calculations. The relaxed eddy accumulation (REA) derived fluxes are physically reasonable and are shown to be of similar magnitude to dry deposition estimates from gradient sampling. The REA derived mean deposition velocity is approximately 3 cm s−1, which is also comparable to growing season estimates derived by Meyers et al. for a similar deciduous forest. Occasional inverted concentration gradients and fluxes are observed but most are not statistically significant. Data are also presented that indicate substantial through canopy penetration of nitric acid.
Keywords: Nitric acid; Atmosphere–surface exchange; Relaxed eddy accumulation;

A new approach for the estimation of trace metal emissions in Vilnius city was implemented, using vertical concentration profiles in the urban boundary layer and road tunnel measurement data. Heavy metal concentrations were examined in fine and coarse particle fractions using a virtual impactor (cut-off size diameter 2.5 μm). Negative vertical concentration gradients were obtained for all metals (Ba, Pb, V, Sb, Zn) and both fractions. It was estimated that the vertical concentration gradient was formed due to emissions from an area of about 12 km2. Road tunnel measurements indicated that trace metal concentrations on fine particles were lower than those on coarse particles, which suggested that re-emitted road dust was highly enriched in trace metal due to historic emissions within the tunnel. Emission rates of different pollutants in the road tunnel were calculated using pollutant concentration differences at the tunnel entrance and exit and traffic flow data. Heavy metal emission rates from the area of Vilnius city were estimated using the vertical gradient of heavy metal concentrations and the coefficient of turbulent mixing, as derived from meteorological measurement data. The emission values calculated by the two different methods coincided reasonably well, which indicated that the main source of airborne trace metals in Vilnius city is traffic. The potential of the vertical concentration gradient method for the direct estimation of urban heavy metal emissions was demonstrated.
Keywords: Heavy metals; Carbon monoxide; Vertical concentration gradient; Urban aerosol; Tunnel; Emission;

Levels and composition of volatile organic compounds on commuting routes in Detroit, Michigan by Stuart A Batterman; Chung-Yu Peng; James Braun (6015-6030).
Vehicle emissions can constitute a major share of ambient concentrations of many volatile organic compounds (VOCs) and other air pollutants in urban areas. Especially high concentrations may occur at curbsides, vehicle cabins, and other microenvironments. Such levels are not reflected by monitoring at fixed sites. This study reports on measurements of VOCs made from buses and cars in Detroit, MI. A total of 74 adsorbent tube samples were collected on 40 trips and analyzed by GC-MS for 77 target compounds. Three bus routes, selected to include residential, commercial and heavily industrialized areas, were sampled simultaneously on four sequential weeks during morning and afternoon rush hour periods. Nineteen compounds were regularly detected and quantified, the most prevalent of which included hexane/2-methyl pentane (15.6±5.8 μg m−3), toluene (10.2±7.9 μg m−3), m,p-xylene (6.8±4.7 μg m−3), benzene (4.5±3.0 μg m−3), 1,2,4-trimethylbenzene (4.0±2.6 μg m−3), o-xylene (2.2±1.6 μg m−3), and ethylbenzene (2.1±1.5 μg m−3). VOC levels in bus interiors and outdoor levels along the roadway were similar. Despite the presence of large industrial sources, route-to-route variation was small, but temporal variation was large and statistically significant. VOC compositions and trends indicate the dominance of vehicle sources over the many industrial sources in Detroit with the possible exceptions of styrene and several chlorinated VOCs. In-bus levels exceeded concentrations at fixed site monitors by a factor of 2–4. VOC concentrations in Detroit traffic are generally comparable to levels measured elsewhere in the US and Canada, but considerably lower than measured in Asia and Europe.
Keywords: Benzene; Commuting; Exposure; Traffic; Vehicles; Volatile organic compounds;

Size distributions of airborne microbes in moisture-damaged and reference school buildings of two construction types by T. Meklin; T. Reponen; M. Toivola; V. Koponen; T. Husman; A. Hyvärinen; A. Nevalainen (6031-6039).
Any risk assessment of moisture-damaged buildings requires an accurate characterization of the factors contributing to the human exposure. In this study, the size distributions of indoor air viable fungi and bacteria and average mean diameters of the most common fungi in school buildings were determined. One special focus was to analyze how the microbial size distributions are affected by the building frame (either wooden or concrete) and moisture damage in the building. The study was performed in 32 school buildings classified as moisture-damaged (index) and non-damaged (reference) schools according to technical building investigations. Sampling for indoor air microbes was carried out using a cascade impactor that collects particles on six stages (range from 0.65 to >7 μm) according to their aerodynamic diameters. Both wooden and concrete schools had their highest fungal levels in the size range of 1.1–4.7 μm. However, the concentrations of fungi in all size classes were higher in wooden schools than in concrete schools. Moisture damage-associated differences in size distribution, in the particle size range of 1.1–2.1 μm, were seen in concrete schools but not in wooden schools. In general, the average geometric mean diameter (d g,ave) of total viable fungi was smaller in wooden schools than in concrete schools, and smaller in index schools of both construction types than in their reference schools. Variation in particle size, however, by genus was observed. No differences in particle size distributions of viable airborne bacteria were found. Our results on the dependency of the particle size on the building type and presence of moisture damage provide an interesting point to be considered in assessing the complex issue of indoor-related bioaerosol exposures.
Keywords: Schools; Moisture damage; Building material; Particle sizes; Bioaerosols;

This work explores the utility of time-of-flight static secondary-ion mass spectrometry (TOF-SIMS) for the analysis of the surface organic layer on individual atmospheric aerosol particles. The surface sensitivity and minimal fragmentation available with TOF-SIMS suggest that it can be a powerful tool for the examination of the organic and inorganic species on the surface of individual particles. Cascade impactors were used to collect aerosol from summer 2000 Montana forest fires, winter snowmobile samples in Yellowstone National Park, Hawaiian lava and sea salt, from an Asian Dust event reaching Salt Lake City, Utah in April 2001 and from Salt Lake Valley summer urban aerosol. TOF-SIMS analysis and multivariate statistical techniques combined gave chemical and morphological information about the particles. Surfaces of the aerosol from forest fires, snowmobile exhaust, and sea salt were all dominated by aliphatic hydrocarbons and their amphiphilic derivatives. Each source showed a different organic chemical signature. The extent and composition of the organics layer which typically covers the surface of atmospheric particles are expected to effect all of the surface related aerosol properties such as health effects, the ability of the particle to activate and form cloud droplets, and the aggregation of particles as well as reactions between the particle and gas phase species.
Keywords: Aerosol; Particle; Surface analysis; TOF-SIMS;

In situ measurement of isoprene in the marine air and surface seawater from the western North Pacific by Sou Matsunaga; Michihiro Mochida; Takuya Saito; Kimitaka Kawamura (6051-6057).
Isoprene (2-methyl-1,3-butadiene) was measured on board of R/V Mirai for eight air samples and 14 seawater samples collected in the western North Pacific during ACE-Asia campaign (from 18 to 26 May 2001). The measurements were conducted in situ using a cryo-focus/gas chromatography/mass spectrometry (Cryo/GC/MS). Concentrations of isoprene ranged from 7.2 to 110 parts-per-trillion (pptv) in the marine air, and ranged from below 12 to 94 pmol l−1 in the seawater. Based on these results, sea-to-air fluxes of isoprene were calculated to be 184 and 300 nmol m−2  day−1 for two samples, and the upper limits of the fluxes were also calculated to be from 32 to 300 nmol m−2  day−1. Atmospheric isoprene concentrations cannot be explained only by the flux from the seawater. Thus, the concentrations of isoprene in the marine air in western North Pacific should be significantly affected by terrestrial vegetational emission and subsequent long-range atmospheric transport of isoprene.
Keywords: Sea-to-air flux; VOCs; Long-range transport; Cryo-focus; Liquid CO2;