Atmospheric Environment (v.39, #18)

Formation potential of vehicle exhaust nucleation mode particles on-road and in the laboratory by Barouch Giechaskiel; Leonidas Ntziachristos; Zissis Samaras; Volker Scheer; Roberto Casati; Rainer Vogt (3191-3198).
A mobile laboratory equipped with gas analysers, a particle number counter and a scanning mobility particle sizer was employed to measure the exhaust particle size distributions of a diesel Euro III passenger car, chasing its exhaust plume on a high-speed track at 50, 100 and 120 km h−1. Emissions from the same vehicle were also measured in the laboratory under the same driving conditions, using a partial flow sampling system with constant sampling conditions. The vehicle was equipped with an oxidation catalyst and was operated on diesel fuel with 280 ppm wt. sulphur content. Similar results for the exhaust aerosol behaviour were found in both sampling environments, despite the different dilution ratio, sampling temperature and residence time of the aerosol in dilute conditions. A relatively constant soot particle mode was formed in all cases and, in addition, a nucleation mode started to form at 100 km h−1 and became more stable at 120 km h−1. No nucleation mode was observed at 50 km h−1 road load. The similar behaviour of nucleation mode particles both in the chasing and the laboratory tests indicated that such small volatile particles are a true vehicle emission component and not a dilution artefact. Additional measurements in the laboratory with varying engine load revealed that the nucleation mode formation is sensitive to exhaust gas temperature and its occurrence in increased temperature is repeatable and stable for long sampling times. The findings of this study indicate that nucleation mode particles are an actual emission component of diesel passenger cars and they need to be considered in relevant exhaust aerosol characterization studies.
Keywords: Nucleation mode; Exhaust plume; Particle number concentration; Particle size distribution; Chasing test;

Plant species effects on methane emissions from freshwater marshes by Weixin Ding; Zucong Cai; Haruo Tsuruta (3199-3207).
Wetland plants not only stimulate CH4 emissions from wetlands to the atmosphere by providing the gas conduit and releasing organic compounds through root exudation and debris to increase CH4 production, but also reduce CH4 emissions by delivering O2 into the underground to accentuate CH4 oxidation in the rhizosphere. The capacity of the plants Carex lasiocarpa, Carex meyeriana and Deyeuxia angustifolia to transport CH4 from freshwater marshes in Sanjiang plain, China to the atmosphere was measured. Their integrated effects on dissolved porewater CH4 concentration and CH4 emission were studied by clipping plants just above the water surface. The amount and fraction of CH4 emissions via intact plants C. lasiocarpa, C. meyeriana and D. angustifolia was 16.0, 20.8 and 8.0 μg CH4 stem−1  h−1, and 73–82%, 75–86% and 28–31%, respectively, indicating that when CH4 is released by the diffusive rather than pressurized transport through the aerenchyma system, cyperaceous plants have a significantly higher gas transport capacity than gramineous plants. After plants were clipped 3 cm above the water surface, CH4 emissions and CH4 concentrations in the C. lasiocarpa marsh with a standing water depth of ∼20 cm increased significantly; only slight increases were measured in the C. meyeriana marsh with a standing water layer of ∼15 cm. The redox potentials (E h) in the vertical profiles of both marshes were further lowered after plants were clipped. These results suggest that apart from being a conduit for gas transport, C. lasiocarpa made a greater contribution to CH4 oxidation than CH4 production, whereas C. meyeriana made nearly the same contribution to CH4 oxidation as to CH4 production. In contrast, a decrease in dissolved porewater CH4 concentration and a very limited increase in CH4 emission in the D. angustifolia marsh with a standing water depth of ∼5 cm indicated that D. angustifolia might make a greater contribution to CH4 production than to CH4 oxidation. Thereafter, as the standing water depth and the capacity of plants to transport CH4 from wetlands increased, plant integrated effect of stimulating CH4 production by releasing root exudates and debris minus accentuating CH4 oxidation by excreting O2 from roots not only decreased but also varied from increasing CH4 concentration for D. angustifolia to reducing CH4 concentration for C. lasiocarpa. The higher CH4 emission in the C. lasiocarpa marsh than in the D. angustifolia marsh was due to the high CH4 transport capacity of C. lasiocarpa rather than the stimulating effect of C. lasiocarpa on CH4 production. This finding suggests that when models are used for estimation of CH4 emissions from natural wetlands, difference in plant species effect on CH4 production and oxidation should be involved in.
Keywords: Carex lasiocarpa; Carex meyeriana; Deyeuxia angustifolia; Methane oxidation; Methane production; Plant mediated gas transport;

Atmospheric concentrations and air–sea exchanges of phthalates in the North Sea (German Bight) by Zhiyong Xie; Ralf Ebinghaus; Christian Temme; Armando Caba; Wolfgang Ruck (3209-3219).
Phthalates, e.g. dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-butyl phthalate (DBP), benzylbutyl phthalate (BBP), and di-(2-ethylhexyl) phthalate (DEHP) were measured in the atmosphere and sea water of the North Sea (German Bight). The air and water samples were collected during an expedition cruise with German research ship ‘Gauss’ in the North Sea from 29th February to 10th March 2004. The concentrations of phthalates in the atmosphere ranged from below the method detection limit to 3.4 ng m−3. DBP, BBP, and DEHP were determined in the water phase with concentrations ranging from below the method detection limit to 6.6 ng L−1.Air–sea vapour exchange of DBP, BBP, and DEHP was estimated using the two-film resistance model based upon relative air–water concentrations. The average of air–sea exchange fluxes was −338 ng m−2  day−1 for DBP and −13 ng m−2  day−1 for BBP, which indicates a net deposition is taking place. The air–sea exchange fluxes of DEHP were ranging from −95 to +686 ng m−2  day−1. The average value of +53 ng m−2  day−1 for DEHP suggested a net volatilization from the North Sea. Moreover, the particle-associated fractions were calculated as 2%, 46%, 75% and 78% for DEP, DBP, BBP and DEHP, respectively. These results indicate that the air–sea vapour exchanges is an important process that intervenes in the mass balance of phthalates in the North Sea.
Keywords: Phthalate; Atmosphere; Sea water; Air–sea exchange;

Diurnal and seasonal cycles of ozone precursors observed from continuous measurement at an urban site in Taiwan by Kuang-Ling Yang; Chien-Chung Ting; Jia-Lin Wang; Oliver W. Wingenter; Chang-Chuan Chan (3221-3230).
Hourly measurement of 56 ozone precursors was conducted by a monitoring station located in a metropolitan area in central Taiwan. After nearly a year of continuous operation at this urban site, both diurnal and seasonal cycles of nonmethane hydrocarbons (NMHCs) were clearly observed, which was caused by the interplay between source, chemical loss, and meteorology. Selected species representing three different types of major sources namely the household fuel leakage, vehicular exhaust and gasoline evaporation, as well as biogenic emissions exhibit dramatic diurnal or seasonal cycles with each displaying its own unique characteristics.Ethane and propane, largely originated from leakage of natural gas or liquefied petroleum gases (LPG), showed concentrations elevating throughout the night and early morning, but began to decrease towards noon as the nocturnal temperature inversion elevated. Because of the lower chemical reactivity and somewhat more constant emissions than other measured target compounds, their diurnal cycles were presumably the direct reflection of the mixing height over the metropolitan area.For compounds originating from vehicular plus evaporative emissions such as benzene, which accounts for most of the monitored compounds, their diurnal cycles were also largely controlled by the variation in the height of temperature inversion.Of all the 56 species monitored, isoprene, an abundant biogenic species largely released by plants, showed distinct diurnal and seasonal cycles different from the other measured NMHCs. Its concentration usually peaked at noon in summer and fall when temperature and solar radiation reached their maximum level, demonstrating the close relationship of isoprene with photosynthesis.Seasonal variation was also clearly observed for the other NMHCs quantified. With the exception of isoprene, most species show higher average concentration in winter and lower in summer with the fall values being the intermediate, which presumably is caused by both the seasonal variation in HO abundance and the height of the temperature inversion.
Keywords: Volatile organic compounds (VOCs); Nonmethane hydrocarbons (NMHC); Liquefied petroleum gases (LPG); Temperature inversion;

Thin-window electron probe X-ray microanalysis of individual atmospheric particles above the North Sea by J. de Hoog; J. Osán; I. Szalóki; K. Eyckmans; A. Worobiec; C.-U. Ro; R. Van Grieken (3231-3242).
A recently developed, automated, standardless, quantitative analytical method, based on electron probe X-ray microanalysis using thin-window energy-dispersive X-ray detector technology, has proven to be a valuable tool for single particle analysis with the determination of the concentrations of light and heavier elements. The combined knowledge of low-Z element concentrations and air mass backward trajectories could give us information on the speciation of environmentally important chemical compounds and their reactions in the atmosphere. Individual marine aerosol particles collected over the North Sea with a nine-stage Berner cascade impactor were analysed using this new method, which resulted in a more profound characterization of the analysed aerosol samples. According to air mass backward trajectory analysis, it is inferred that the aerosol compositions of two samples (S1–S2) should be mostly under the marine influence, whereas the other two samples (S4–S5) under the continental influence. This inference is undoubtedly confirmed by the results from the single particle analysis. For the more marine samples (S1–S2), “pure” and “aged” sea salt particles are major chemical species encountered on all stage samples. Various types of particles from continental sources, such as aluminosilicates, ammonium sulfate, organic, iron oxide particles are mostly observed on all impactor stages for the more continental samples (S4–S5). This kind of detailed information on chemical compositions of aerosol samples which is provided by this analytical technique can be very much valuable for the study of atmospheric reactions of marine aerosols, which is linked to many environmental issues like e.g. nutrient deposition and marine eutrophication.
Keywords: Thin-window EPMA; Atmospheric particles; North Sea; Air mass backward trajectories;

Numerical modelling of the flow through a simplified domain representing regular cube arrays of three different packing densities was carried out, using seven-equation Reynolds stress turbulence modelling (RSM) with a high-resolution mesh. The cases considered were cube arrays of area density 0.0625, 0.16 and 0.44 in a simulated atmospheric boundary layer, as studied previously in experiments by Macdonald et al. (Measurements of mean velocity and turbulence in simple obstacle arrays at 1:200 scale, 2000) and LES by Hanna et al. (Atmos. Env. 36 (2002) 5067). Profiles of mean velocity and individual turbulence components were examined and found to be acceptable for the purposes of examining the flow field within the canopy and the processes of exchange at its top. The time-averaged flow field in sheltered regions was found to be dominated by large vortical flow patterns whose form varied with array packing density. It is suggested that the form of these vortices may influence the effectiveness of upward mixing within the canopy. The concept of exchange velocity, and the analytical model of Bentham and Britter (Atmos. Env. 37 (2003) 2037) were summarized, and values for the exchange velocities at the canopy top were estimated from the CFD solution. The exchange velocity, u E is an attempt to quantify the exchanges of mass, momentum or energy across the top of a canopy. This velocity was estimated to be around 1 % of the wind velocity at 2.5 obstacle heights for the two lower-density arrays, and around 0.3 % for the densest array. These values are thought to be lower bounds on the real magnitude of exchanges, as any in-canopy variations in pressure and momentum flux across a repeat unit of the canopy may lead to non-negligible advective fluxes through the canopy top. These fluxes are relevant to the removal of polluted air and heat from urban canopies, where spatial geometrical variations exist, and where the flow may not be fully adjusted to the roughness.
Keywords: Exchange velocity; Urban canopy; Wind flow in urban area; Turbulence in urban area; Force-momentum balance;

Ozone uptake by an evergreen Mediterranean Forest (Quercus ilex) in Italy. Part I: Micrometeorological flux measurements and flux partitioning by Giacomo Gerosa; Marcello Vitale; Angelo Finco; Fausto Manes; Antonio Ballarin Denti; Stanislaw Cieslik (3255-3266).
Ozone, water and energy fluxes have been measured over a Mediterranean evergreen forest in Central Italy from August to October 2003 with the eddy-correlation technique in order to evaluate the amount of ozone taken up by plants in dry summer and in mild autumn conditions. The stomatal ozone fluxes have been calculated using the analogy with water vapor fluxes inside the stomata, which are easily measurable. The total ozone dose was obtained by integrating the stomatal fluxes over time.Stomatal flux resulted a minor part (31.5%) of the total ozone flux over the forest ecosystem. The main part of ozone deposition follows non-stomatal pathways. Chemical sink seems to play a relevant role in the morning non-stomatal deposition. Stomatal uptake is enhanced by water availability but, on the average, it does not exceed the 34.4% of the total ozone flux.A comparison between the cumulated stomatal ozone fluxes and the currently used AOT40 exposure index highlighted important distortions introduced by this index. AOT40, which do not take into account plant physiology, lead to substantial overestimation of ozone risk, particularly when water supply is limited, as occurs frequently in Southern European and Mediterranean areas.
Keywords: Ozone deposition; Stomatal uptake; Mediterranean forest ecosystem; Holm oak;

Ozone (O3) induces deleterious effects on plants by its oxidising capacity. Efforts have aimed at defining O3 critical loads which are based on O3 penetrating into the plant through the stomata, being an important mechanism of O3 removal. Recently, papers have proposed the use of process-based models for estimating the O3 dose at the community level. These models should define the stomatal O3 flux (F st) on vegetation, correlating it to O3-induced injury and hence to improve the data-base available for decision makers. A process-based model has been utilised for the quantification of O3 flux (F) toward a Holm oak forest and its repartition in F st and non-stomatal flux (F nost) during daylight. The model outputs have been compared with eddy covariance measurements. Simulation of O3 fluxes under the climatic limitations of summer 2003 showed that the mean values of F st represented only 28.9% of F, which was similar to eddy covariance (31.5%). The use of LAIeffective to link the leaf F st to the canopy scale O3 stomatal flux (F effectjve; on ground unit) is shown to be useful not only for the model validation but also for upscaling purposes from leaf to canopy. Forest acts as a LAI = 1 , i.e. a “real” big leaf for canopy O3 uptake. Changes in O3 fluxes were more closely related to the factors that control O3 deposition than to the factors controlling O3 concentration. Simulations showed that Holm oak forest was most effective at taking up O3 under moderate to high irradiance and high physiological activity, but not in limiting environmental conditions. The model used here does not require a great number of input variables and it is based on simple assumptions with respect to other more recognised models. So, the use of this model can have useful applications for the risk assessment of level II for the forests.
Keywords: Critical loads; Gas exchange; Holm oak forest; Leaf area index; Modelling;

A neural network forecast for daily average PM10 concentrations in Belgium by Jef Hooyberghs; Clemens Mensink; Gerwin Dumont; Frans Fierens; Olivier Brasseur (3279-3289).
Over the past years, the health impact of airborne particulate matter (PM) has become a very topical subject. In the environmental sciences a lot of research effort goes towards the understanding of the PM phenomenon and the ability to forecast ambient PM concentrations. In this paper, we describe the development of a neural network tool to forecast the daily average PM10 concentrations in Belgium one day ahead. This research is based upon measurements from ten monitoring sites during the period 1997–2001 and upon ECMWF simulations of meteorological parameters. The most important input variable found was the boundary layer height. A model based on this parameter currently operational online serves to monitor the daily average threshold of 100 μg m−3. By extending the model with other input parameters we were able to increase the performance only slightly. This brings us to the conclusion that day-to-day fluctuations of PM10 concentrations in Belgian urban areas are to a large extent driven by meteorological conditions and to a lesser extend by changes in anthropogenic sources.
Keywords: Particulate matter; Prediction; Neural networks; Boundary layer height; Air pollution;

The significance of natural sources in the concentrations of particles smaller than 10 μm (PM10) in Europe is addressed. When considering only European anthropogenic emissions, chemistry-transport model simulations underestimate the PM10 concentrations by 30–50%, using the current knowledge about aerosol physics and chemistry. Along this article we hypothesize that the missing mass originates from natural sources like erosion dust entrainment and resuspension. The methodology consists in testing these hypotheses in the CHIMERE regional chemistry-transport model, and comparing the results with long series of PM10 measurements in Europe.It is demonstrated that the introduction of background Saharan dust boundary conditions greatly improves the model simulation over Southern Europe, and to a smaller extent also over northern Europe. However to accurately simulate acute episodes of Saharan dust transport time-resolved boundary conditions need to be used. Local erosion occurring over European regions is also considered. The introduction of a simplified bulk scheme for online calculation of mineral dust emission makes the model skill improve everywhere in Europe, indicating that this process can be significant in Europe.We finally assume that resuspension of material available on the ground, explains most of the remaining part of the missing part of PM10 average load. A simplified scheme for the corresponding emission, which depends on turbulence near the ground, is proposed. It also makes the model improve substantially, especially over Northern Europe. However sensitivity tests show that such an improvement can also be obtained by simply increasing the model secondary organic matter in aerosols by a factor of 3. We conclude that our results are consistent with the existence of a strong biogenic resuspension aerosol source, but more experimental work is required to ascertain this hypothesis.
Keywords: Aeolian dust emission; Resuspension; Soil erosion; Saharan dust; CHIMERE model;

Polycyclic aromatic hydrocarbon emissions from joss paper furnaces by Hsi-Hsien Yang; Ray-Chen Jung; Ya-Fen Wang; Lien-Te Hsieh (3305-3312).
The emissions of polycyclic aromatic hydrocarbons (PAHs) were quantified for two joss paper furnaces burning two kinds of joss papers (recycled paper made and virgin bamboo made). A cyclone and a wet scrubber were installed in series on one of the two furnaces. Particulate and gaseous PAHs were collected with a sampling system meeting the criteria of U.S. EPA Modified Method 5. Twenty-one species of PAH were analyzed by GC/MS. Individual PAH emission factors vary from less than 1 mg kg−1 fuel to several tens of mg kg−1 fuel. The total (sum of 21 compounds) and the carcinogenic PAH (benz[a]anthracene, chrysene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, indeno[1,2,3,-cd]pyrene, dibenz[a,h]anthracene) emission factors were not statistically different for the two furnaces and averaged 71.0 and 3.2 mg kg−1, respectively. The PAH profiles showed a predominance of naphthalene (58.1%), phenanthrene (11.7%) and fluorene (7.5%). Of the two joss papers examined, bamboo-made joss paper showed less emission in both particulate and gaseous PAHs. For particulate and gaseous PAHs, the removal efficiencies of total PAHs by the air pollution control devices were 42.5% and 11.7%, respectively. PAH emission factors in high airflow conditions were generally lower than those in low airflow condition.
Keywords: Joss paper; Emission profile; Characteristic ratio; Emission factor; Carcinogen;

A systematic study of individual precipitation (wet-only) events ( n = 91 ), collected for three years (2000–2002) during the period of SW-monsoon (June–September) from a semi-arid region (Ahmedabad, 23°03′N, 72°38′E) in India, reveals that rainwater is characteristically alkaline in nature (pH range: 5.2–8.2, Av.=6.7). The ionic ratio of Cl and Na+ in each event closely follows that of sea-salt composition. The measured abundances of Ca2+ (5.5–1057 μeq l−1), Mg2+ (1.5–251 μeq l−1) and HCO3 (8–836 μeq l−1); non-sea-salt (nss) components of Ca2+ and Mg2+ varying as 94–99% and 0–84%; and linear regression parameters among (nss-Ca2++nss-Mg2+) and HCO3 (slope=0.71, r 2 = 0.96 ) all tend to suggest that crustal sources dominate the chemical composition of precipitation. The concentrations of acidic constituents SO2− 4 and NO3 vary over a wide range, from 4.8 to 431 and 1.0 to 115 μeq l−1, respectively; and that of NH4 + varied from 1.7 to 220 μeq l−1 and K+ as 0.6–53 μeq l−1. A significant impact of anthropogenic sources on rainwater composition is evident based on the nss-fraction of SO2− 4 (67–99%, VWM=86%) and equivalent ratio of nss-SO4 2−/NO3 (0.8–6.7, VWM=2.5). With the exception of NH4 +, concentration of an individual species in a rain event is largely controlled by the precipitation amount. The inter-annual variations in the volume-weighted-mean composition are not significantly pronounced; making this a representative data set for the study region.
Keywords: Precipitation chemistry; Urban site; Alkaline rainwater; Acidic constituents; Neutralization;

Indoor and outdoor elemental carbon (EC) and organic carbon (OC) concentrations were measured from September 2001 through January 2002 at 20 residential sites and a local high school in western Riverside County, CA. The correlation (R 2) between indoor vs. outdoor EC and indoor vs. outdoor OC were 0.63 and 0.47, respectively, while the correlation of EC to OC outdoors and indoors was 0.58 and 0.23, respectively. The average OC content of PM2.5 was 0.25 and 0.55 for outdoor and indoor PM2.5, respectively. It was concluded that there were no significant indoor sources of EC while indoor OC sources contributed significantly to indoor PM2.5. Home with smokers had significantly higher TC and OC than homes without. Schoolrooms generally had less EC and OC due to the schools HVAC system.
Keywords: Organic carbon; Elemental carbon; Indoor air quality; Environmental tobacco smoke;

Short-term cycle of eolian dust (Kosa) recorded in Lake Kawaguchi sediments, central Japan by Tomohiro Kyotani; Satoshi Koshimizu; Hiroshi Kobayashi (3335-3342).
The fluctuation during the last 100 yr of the eolian dust (Kosa aerosol) originating from arid and semi-arid areas of China has been reconstructed by using the sediments from Lake Kawaguchi, central Japan with high temporal resolution. The quantification of Kosa contribution to the sediments was carried out by a new method using scanning electron microscopy-energy dispersive X-ray microanalysis (SEM-EDX) proposed by us. The correlation plot of (Na2O+K2O) contents against SiO2 was used for individual Si-rich particles having SiO2 content over 80%. The Kosa fraction of Si-rich particles in Lake Kawaguchi sediments during the last 100 yr is approximately 10–30%. The fluctuation of the Kosa fraction during the last 100 yr does not coincide with that of the total amount of Si-rich particles, because detrital components from Japanese igneous rocks control the fluctuation of the total number of Si-rich particles. The discrimination method based on single particle analysis is more effective than that of bulk analysis for the lake sediments formed by complex matrix components. We can first show a short-term (approximately 10–20 yr scale) cycle in Kosa aerosol fluctuation. Higher sedimentation rates (5–10 yr-cm) of the Lake Kawaguchi sediments and the new analytical method using SEM-EDX revealed a remarkable fluctuation pattern of Kosa aerosol, suggesting climate cycles much shorter than glacial–interglacial. Such short-term cycles may be related to sun-spots. The number of days of Kosa events during the last 30 yr, obtained by visual observation by Meteorological Agency of Japan, also supports the presence of such a short-term cycle.
Keywords: Kosa aerosol; Si-rich particles; Short-term cycle; Climate change; Lake Kawaguchi sediment; SEM-EDX;

Ozone removal by diesel particulate matter by T.A. Metts; S.A. Batterman; G.I. Fernandes; P. Kalliokoski (3343-3354).
The most significant removal mechanisms for tropospheric ozone (O3) include dry deposition, photolysis, and photochemical reactions. This study examines another mechanism potentially important in urban areas: sorption and removal on diesel particulate matter (DPM). Tests were performed to determine O3 breakthrough and the amount of O3 removed by the DPM generated by a heavy-duty diesel engine. Teflon filters loaded with 0.7–1.8 mg of DPM were exposed to a test atmosphere of humidified and ozonated air designed to represent realistic ambient air conditions. In addition, soot samples with the organic fraction removed were tested to determine whether the organic or elemental fraction contributed to O3 removal. For comparison, activated carbon (AC) samples were also tested. The DPM-loaded filters removed 5.6±1.8 wt% of O3. Considerably more ozone, 31±4 wt %, was removed by the DPM after removal of its soluble organic fraction. Removal capacities of DPM were small relative to AC, which removed >38±3 wt% of O3. Of the Lagergren pseudo-first-order, pseudo-second-order, Elovich, and Ritchie chemisorption models tested, the Ritchie model provided the best fit to the breakthrough data. Preliminary estimates drawn from laboratory results suggest that diesel soot present at typical urban levels will remove only a small portion of O3 from urban or tropospheric air. In air cleaning applications, DPM-loaded filters are also expected to remove only a small portion of indoor O3.
Keywords: Air pollution; Diesel; Filter; Ozone; Particulate matter; Soot;

Mechanisms of mercury removal by O3 and OH in the atmosphere by Jack G. Calvert; Steve E. Lindberg (3355-3367).
The mechanisms of the reactions of gaseous Hg atoms with O3 and OH radical are evaluated from current kinetic and enthalpy data. The reaction, O3+Hg→HgO+O2, is considered to be an unlikely pathway for atmospheric conditions. Considerations given here suggest that the reaction may occur with initial formation of a metastable HgO3 molecule that in laboratory experiments is the source of the HgO product observed to accumulate on the walls of the reactor (HgO3→HgO(s)+O2). Laboratory studies of the gas phase reaction, Hg+OH→HgOH (2), have been reported using relative rate measurements initiated by photodissociation of an organic nitrite in mixtures of Hg vapor with NO, air and various reference hydrocarbons. Computer simulations of this reaction system suggest that the use of reactive reference gases (e.g., cyclohexane) leads to the generation of significant ozone in these NO x –RH–air mixtures, and the resulting O3–Hg reaction can result in an over-estimate of the rate of reaction (2). Also the apparent rate coefficients for reaction (2) are highly dependent on the assumed rate coefficients of its competitive reactions of dissociation in HgOH→Hg+OH (3), and association of HgOH molecule with other free radicals present in the system: HgOH+XXHgOH (4); X=OH, HO2, RO, RO2, NO, NO2. Reaction (4) competes successfully with HgOH decomposition for the laboratory conditions employed, and the kinetic measurements relate to the rate determining reaction, Hg+OH→HgOH in this case. However, the use of these laboratory measurements of k 2 to determine the extent of Hg removal by OH in the troposphere will greatly over-estimate the importance of Hg removal by this reaction.
Keywords: Mechanism of atmospheric reactions of Hg; O3 reaction with atmospheric Hg; OH reaction with atmospheric Hg;

The need to develop and use data assimilation procedures in the emergency response systems (ERS) dealing with the atmospheric dispersion of pollutants is discussed.
Keywords: Atmospheric dispersion; RODOS;

Size distribution of sea-salt emissions as a function of relative humidity by K. Max Zhang; Eladio M. Knipping; Anthony S. Wexler; Prakash V. Bhave; Gail S. Tonnesen (3373-3379).
This note presents a straightforward method to correct sea-salt-emission particle-size distributions according to local relative humidity. The proposed method covers a wide range of relative humidity (0.45–0.99) and its derivation incorporates recent laboratory results on sea-salt properties. The formulas are given as functions of relative humidity and compared with growth factors derived from earlier theoretical work. Application of this method to several common open-ocean and surf-zone sea-salt-particle source functions is described. The resulting sea-salt-emission aerosol distributions can be used directly in atmospheric model simulations without further correction.
Keywords: Sea-salt; Emission; Size distribution; Size correction; Relative humidity; Atmospheric modeling; Aerosol thermodynamics;