Atmospheric Environment (v.34, #10)

Aerosol mass size distributions of elemental carbon (EC) and organic carbon (OC) were measured to determine the extent to which carbon speciations within ambient aerosols differ between urban and over-water atmospheres. Samples were collected during twenty-one 12-h periods in urban Chicago and over Lake Michigan during the July 1994 and January 1995 sampling intensives. Total particle bound concentrations (sum of all size fractions) of elemental and organic carbon ranged from 0.15 to 0.96 and from 0.94 to 3.04 g m−3, respectively. On average, organic material (OM=1.4×OC) accounts for 18% of the total aerosol mass collected, but ranges from 10% (over-water, winter) to 39% (urban, summer). With regard to individual size fractions, organic matter ranges from 3% (>12 μm over-water, winter) to 49% (0.15–0.45 μm, urban, summer) of the particulate matter mass in the size class. Geometric mean aerodynamic equivalent diameters (GMDs) range from 0.72 to 2.4 μm for suspended particulate matter (Σ-PM), from 0.52 to 1.4 μm for EC, and from 0.60 to 1.9 μm for OC. Elemental and organic carbon GMDs are larger in the urban atmosphere than over the water during winter, while this trend is not observed in Σ-PM. Furthermore, geometric standard deviations are larger at the urban location in January for both EC and OC, indicating broader size distributions of both species under winter conditions due to greater quantities of EC and OC in large particles.
Keywords: Particulate matter; EC; OC; Chicago; Lake Michigan;

Influence of airmass back trajectory upon nitrogen compound composition by Roy M. Harrison; J.L. Grenfell; J.D. Peak; K.C. Clemitshaw; S.A. Penkett; J.N. Cape; G.G. McFadyen (1519-1527).
The speciation of nitrogen-containing compounds occurring in airmasses arriving at a site on the north Norfolk coast (UK) during 1993–1995 is examined. The data are disaggregated according to four types of back-trajectory, broadly categorised as characterising: (1) land (southern England) (2) land followed by a short stretch of shallow sea (3) the ocean (but encountering land during the last 100 km or so) and (4) the Atlantic Ocean or Arctic via the North Sea. We find clear differences in NO y , NO x /NO y , Fuchs surface area and total inorganic nitrate between the airmass types, although oxidised nitrogen speciation, i.e. nitrogen dioxide NO2, HONO, HNO3 and PAN remains broadly similar. For trajectories arriving off the land, summer samples reveal a greater degree of oxidation than those in winter. The percentage of total inorganic nitrate is enhanced and the NO x /NO y ratio appears to be suppressed in the more aged airmasses arriving over the sea. When data are disaggregated according to trajectory type, total NO y can be seen to relate to airmass age and clear compositional differences relating to airmass age and history appear, which are not seen in the analysis of the dataset as a whole. This suggests that the inference in some earlier published work that NO y composition is largely insensitive to airmass age may be incorrect if it is based on inadequate markers of airmass age. Concentrations of ammonia and ammonium are especially sensitive to back-trajectory, reflecting a strong source over the land and depletion in maritime air.
Keywords: Ammonia; Nitrogen speciation; Tropospheric chemistry; NO y ; NO x ; Nitrate; Airmass back trajectory;

The product formation from the OH-initiated oxidation of isoprene has been investigated in the presence as well as in the absence of NO x . The experiments were carried out in a 1080 l static quartz reactor under atmospheric relevant conditions (room temperature, 900 mbar). Irradiation of isoprene/H2O2/air mixtures was used to study the reaction of isoprene with OH radicals in the absence of NO x . Products were monitored in situ by means of FT-IR-spectroscopy and analysed by GC-FID and GC-MS. Two C5-unsaturated dihydroxy compounds, 2- and 3-methyl-3-butene-1,2-diol have been identified by comparison with authentic standards; the yields of these products were (4.7±1.4)% and (2.4±0.9)%, respectively. The identification of these diols is the first experimental evidence that the self- and cross-reactions of peroxy radicals derived from the isoprene+OH reaction follow the general peroxy radical reaction scheme. An estimate of the potential relevance of these products has been made. The molar yields measured for the well-known main products methacrolein, methyl vinyl ketone and formaldehyde confirm the results from earlier studies. In addition, methanol and methyl hydroperoxide have been observed as minor products in the absence of NO x . New mechanistic information on the OH-initiated oxidation of isoprene has been derived from this study, which helps to improve the understanding of tropospheric isoprene degradation mechanisms.
Keywords: Biogenic alkenes; Peroxy radicals; OH radical; Degradation mechanisms; Tropospheric ozone;

Atmospheric gas-phase reactions of dimethylsulphoxide and dimethylsulphone with OH and NO3 radicals, Cl atoms and ozone by H. Falbe-Hansen; S. Sørensen; N.R. Jensen; T. Pedersen; J. Hjorth (1543-1551).
Kinetics and products of the gas-phase oxidation of dimethylsulphoxide (DMSO) and dimethylsulphone (DMSO2), two sulphur compounds produced in the gas-phase oxidation of dimethylsulphide, have been investigated. The reactions of DMSO and DMSO2 with some of the most important oxidising species in the atmosphere were investigated, which include reactions with OH and NO3 radicals, Cl atoms and ozone (O3). The experiments were carried out at room temperature and at (740±5) Torr in a 480 l reaction chamber. FT-IR spectrometry was used as the analytical technique.Using a relative rate reaction technique or a pseudo-first-order reaction rate technique, the following rate constants (expressed in cm3  molecule−1  s−1) have been determined: For the reactions of DMSO: k OH=(5.9±1.5)×10−11, k NO3 =(5.0±3.8)×10−13, k Cl=(7.4±1.0)×10−11 and k O3 <1×10−19. For the reactions of DMSO2: k OH<3×10−13, k NO3 <2×10−15, k Cl=(2.4±0.8)×10−14 and k O3 <1×10−19 (all values are given with 2 σ overall uncertainties). SO2, MSPN (methylsulphonyl peroxynitrate) and DMSO2 were identified as the main gas-phase sulphur-containing products of the oxidation of DMSO by Cl atoms, similar to what has previously been observed for the reaction with OH radicals. From the reaction between DMSO and the NO3 radical, DMSO2 was the only gas-phase sulphur-containing product observed. All of the reactions of DMSO2 were very slow and no sulphur-containing products were observed in the gas phase.The results obtained here indicate that in the atmosphere DMSO could be oxidised both by the gas-phase reaction with the OH radical leading to SO2 and DMSO2, and by transfer to the liquid phase, where the oxidation products are unknown. DMSO2 on the other hand will mainly be transferred to the liquid phase before undergoing further degradation.
Keywords: Kinetics; Products; DMSO; DMSO2; Troposphere; Hydroxyl radical; Nitrate radical; Chlorine; O3;

Wind tunnel experiments on how thermal stratification affects flow in and above urban street canyons by Kiyoshi Uehara; Shuzo Murakami; Susumu Oikawa; Shinji Wakamatsu (1553-1562).
The effects of atmospheric stability on flow in urban street canyons were studied using a stratified wind tunnel. We conducted experiments using a model that represented city streets with simply shaped block forms, while varying atmospheric stability across seven stages from stable (Rb=0.79) to unstable (Rb=−0.21). We used a laser Doppler anemometer (LDA) and a cold wire to measure the flow field and temperature within and above the street canyon. In addition to mean values of wind speed components and temperatures, we measured turbulence intensity, shear stress, and heat flux distribution. Our results led to the following conclusions: Cavity eddies that arose in the street canyon tended to be weak when the atmosphere was stable and strong when unstable. Stable atmospheric conditions led to a positive feedback effect in which the downward flow into the street canyon weakened due to buoyancy, which facilitated the formation of a more highly stable stratification. As a result, when stability exceeded a certain threshold (somewhere in the range of Rb=0.4–0.8), the wind speed in the street canyon dropped nearly to zero.
Keywords: Physical modeling; Buildings and streets; Cavity eddy; Air pollution; Atmospheric stability;

An automatic hydrocarbon oxidation system that uses structure activity relationships to automatically calculate how aliphatic hydrocarbons breakdown in the atmosphere is described. The program includes functions to calculate average reaction rates and products for use in a reduced mechanism based on functional groups. Chemical concentrations from the reduced mechanism compare well with results from the regional atmospheric chemistry mechanism, RACM, for clean and polluted boundary layer scenarios. The main advantage of this approach is the ease of generating different reduced mechanisms for different emissions scenarios, such as for different countries, or to take account of reductions in the emissions of particular VOCs.
Keywords: Air quality modelling; Lumped mechanism; Tropospheric ozone;

Lycopodium spore impacts onto surfaces by X Li; P.F Dunn; R.M Brach (1575-1581).
Experiments on oblique impact of Lycopodium spores with dry surfaces in the presence of adhesion were carried out. The experimental results are compared with those of stainless steel microsphere impact onto the same types of substrate surfaces. Three parameters are analyzed, the coefficient of restitution, the impulse ratio and the normalized kinetic energy loss and are used to represent the impact results. The comparisons show that the complex surface profile of a spore has a remarkable effect on impact response. The conjunction of the roughness of both the microparticle and the substrate make the impact mechanics even more complicated.
Keywords: Restitution coefficient; Impulse ratio; Oblique impact; Surface roughness; Spore surface impacts;

A study on the distributions of the measured fluctuating wind velocity components by John Z Yim; Chun-Ren Chou; Wei-Po Huang (1583-1590).
Probability distributions of the three wind velocity components measured at a height of 26 m near the coast in Keelung, Taiwan were studied using statistical models found in the literature. Both the linear Gaussian distribution and a weakly nonlinear model were used. It is shown here that, for this specific site, the model based on the Gram–Charlier type A series expansions can be used to model all the three fluctuating velocity components more satisfactorily than the Gaussian model.
Keywords: Turbulent fluctuations; Probability distribution; Gaussian distribution; Gram–Charlier series expansions;

There is a need to apply detailed air quality models to examine the fate of air pollutant emissions over periods of a year or more so that issues which involve annual or even multi-year standards can be addressed. This is a difficult task due to computational and model input demands. In this study we examine multiple years of ozone and meteorological data for eastern Canada to determine how many warm seasons (May–September) would need to be modelled to ensure that the range of typical conditions in the warm season is included. We also evaluate how well individual warm seasons can match these conditions, assuming that if a single, typical warm season can be identified then it may be a good candidate for future modelling studies. We found that representation of the long-term average warm season O3 distribution, cumulative O3 statistics, surface weather conditions and frequency of synoptic-scale weather patterns with a combined absolute error of less than 20% requires, on average, five consecutive warm seasons. The number can be reduced to three through selection of specific consecutive warm seasons. The single most representative warm season matches the long-term conditions to within 26% in Ontario, 29% in Quebec and 30% in Nova Scotia and New Brunswick, while inclusion of a second year reduces these errors by 4, 7 and 2%, respectively. Beyond two years the reduction in error from adding more warm seasons occurs at an increasingly slower rate.
Keywords: Air quality modelling; Meteorological representativeness; Tropospheric ozone;

A modeling study of the impact of urban trees on ozone by David J Nowak; Kevin L Civerolo; S Trivikrama Rao; Gopal Sistla; Christopher J Luley; Daniel E. Crane (1601-1613).
Modeling the effects of increased urban tree cover on ozone concentrations (July 13–15, 1995) from Washington, DC, to central Massachusetts reveals that urban trees generally reduce ozone concentrations in cities, but tend to increase average ozone concentrations in the overall modeling domain. During the daytime, average ozone reductions in urban areas (1 ppb) were greater than the average ozone increase (0.26 ppb) for the model domain. Interactions of the effects of trees on meteorology, dry deposition, volatile organic compound (VOC) emissions, and anthropogenic emissions demonstrate that trees can cause changes in dry deposition and meteorology, particularly air temperatures, wind fields, and boundary layer heights, which, in turn, affect ozone concentrations. Changes in urban tree species composition had no detectable effect on ozone concentrations. Increasing urban tree cover from 20 to 40% led to an average decrease in hourly ozone concentrations in urban areas during daylight hours of 1 ppb (2.4%) with a peak decrease of 2.4 ppb (4.1%). However, nighttime (20:00–1:00 EST) ozone concentrations increased due to reduced wind speeds and loss of NO x scavenging of ozone from increased deposition of NO x . Overall, 8-hour average ozone concentration in urban areas dropped by 0.5 ppb (1%) throughout the day.
Keywords: Urban forestry; Air quality modeling; Photochemistry; Urban meteorology; Biogenic hydrocarbons; Dry deposition;

In recent years, there has been an increased use of prognostic meteorological models to assess current and future air quality related problems. Often, these meteorological models are applied in their forecasting mode with current land use/land cover patterns and data assimilation techniques to generate historical meteorological data for use in air quality models. In this study, we examined the sensitivity of land use/land cover on the predicted meteorological fields, and the implications for examining air quality in a future year. A community-based mesoscale meteorological model (MM5-Version 1) was applied to the northeastern US urban corridor under two scenarios, one with the existing land use/land cover (base case), and the second reflecting a hypothetical change in about 40% of the base case urban grid cells to deciduous forest. A comparison of the two meteorological fields reveals substantial localized differences in surface temperature and zonal wind speeds. These findings suggest reevaluation of the practice of using historical meteorological fields to assess future air quality, especially if one expects large changes in land use patterns.
Keywords: Meteorological modeling; Urban vegetation; Photochemistry; Land use;

This paper investigates the hypothesis that cloud and fog processes produce fine organic particulate matter in the atmosphere. The evidence provided suggests that cloud and fog processes could be important contributors to secondary organic aerosol formation, and the contribution of this formation pathway should be further investigated. This conclusion is based on the following observations: (1) many organic vapors present in the atmosphere are sorbed by suspended droplets and have been measured in cloud and fog water, (2) organics participate in aqueous-phase reactions, and (3) organic particulate matter is sometimes found in the size mode attributed to cloud processing (i.e. the droplet mode). Specific compounds identified as potential precursors include aldehydes (e.g. formaldehyde, acetaldehyde, and propionaldehyde), acetone, alcohols (e.g. methanol, ethanol, 2-propanol, and phenol), monocarboxylic acids, and organic peroxides. Carboxylic acids (e.g. diacids and oxo-acids), glyoxal, esters, organosulfur compounds, polyols, amines and amino acids are potential products of cloud and fog processing.

In atmospheric waters, oxidation of Cr(III) by Mn(III/II) species has been considered to be the major pathway for Cr(III) to become Cr(VI), but the finding of light-induced Cr(III) oxidation in the presence of Fe(III) implicates a new possible pathway. This study further tests the occurrence of this pathway in simulated atmospheric water containing organic acids and Mn(II). Our results showed that in simulated atmospheric water, the light and Fe(III)-induced Cr(III) oxidation occurred in the presence of the organic acids of low Fe(III) photoreductivity (LFPR) such as acetate and succinate and in the presence of very low levels of the organic acids of high Fe(III) photoreductivity (HFPR) such as citrate and tartarate (e.g., [Citrate]0  : [Cr(III)]0<∼1 : 10). This oxidation became very limited in the presence of very high levels of the LFPR acids (e.g., [Acetate]0  : [Cr(III)]0>∼7.5 : 1) because of competition of the acids with Cr(III) for ·OH radicals as well as in the presence of mild levels of the HFPR acids (e.g., [Citrate]0:[Cr(III)]0>∼1 : 10) because of rapid reduction of the formed Cr(VI) by organic radicals from photolysis of complexes of Fe(III) with these acids. This oxidation was substantially limited at pH 〈2.5 or〉 4.5 in the presence of the organic acids. The presence of Mn(II) led to decreases in the Cr(III) oxidation presumably resulting from the competition of Mn(II) with Cr(III) for ·OH radicals. This study thus suggests that the light and Fe(III)-induced Cr(III) oxidation could be a potential important pathway for Cr(III) to be converted to toxic Cr(VI) in atmospheric waters where organic acids such as acetate are frequently predominant.
Keywords: Free radicals; Heavy metal; Photochemistry; Redox; Tropospheric chemistry;

Biomass burning is an important global source of aerosol particles to the atmosphere. Aerosol particles were collected in plumes of tropical forest and cerrado biomass burning fires in the Amazon Basin during August–September, 1992. Fine (d p<2 μm, where d p is the aerodynamic diameter of the particle) and coarse (2 μm<d p<10 μm) aerosol particles were collected using stacked filter units. Up to 19 trace elements were determined using particle-induced X-ray emission analysis. Ion chromatography was used to determine up to 11 water-soluble ion components. The dominant species were black carbon, K+, Cl, and SO4 2−. Organic matter represents in average 70–92% of the fine mode particle mass. The composition of the emitted particles in cerrado fires presents a well-defined pattern related to both the combustion phase and cerrado categories, which is not observed in the case of forest fires. Higher concentrations relative to the fine particulate mass were observed during the flaming emissions compared to the smoldering ones, for almost all experiments. Global emission flux estimates showed that biomass burning could be an important source of heavy metals and black carbon to the atmosphere. Estimates showed that savanna and tropical forest biomass burning could be responsible for the emission of about 1 Gg yr−1 of copper, 3 Gg yr−1 of zinc and 2.2 Tg yr−1 of black carbon to the atmosphere. In average, these values correspond to 2, 3 and 12%, respectively, of the global budget of these species.
Keywords: Biomass burning; Aerosol particles; Black carbon; Trace elements; Amazonia;

Estimation of total circulation within a plume in a crosswind by Eric Savory; Norman Toy (1655-1658).
The present paper describes an approach which may be adopted for determination of the total circulation associated with the large-scale streamwise vorticity within a plume deflected by a crosswind. The method requires a much smaller experimental data set than would otherwise be necessary in order to define the variation of circulation within the plume with distance from the source. Such data may be used to validate numerical models for the prediction of the near-field flow regime close to a pollutant source.
Keywords: Aerodynamics; Vorticity; Near-field behaviour;