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Atmospheric Environment (v.41, #31)

Editorial board (pp. i).

Evaluation of the UNC toluene-SOA mechanism with respect to other chamber studies and key model parameters by Di Hu; Richard M. Kamens (pp. 6465-6477).

In a companion paper by Hu et al. [2007. A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NO _x and natural sunlight. Atmospheric Environment,doi:10.1016/j.atmosenv.2007.04.025], a kinetic mechanism was developed from data generated in the University of North Carolina's (UNC) 270m³ dual outdoor aerosol smog chamber, to predict secondary organic aerosol (SOA) formation from toluene oxidation in the atmosphere. In this paper, experimental data sets from European Photoreactor (EUPHORE), smog chambers at the California Institute of Technology (Caltech), and the UNC 300m³ dual-outdoor gas phase chamber were used to evaluate the toluene mechanism. The model simulates SOA formation for the ‘low-NO _x’ and ‘mid-NO _x’ experiments from EUPHORE chambers reasonably well, but over-predicts SOA mass concentrations for the ‘high-NO _x’ run. The model well simulates the SOA mass concentrations observed from the Caltech chambers. Experiments with the three key toluene products, 1,4-butenedial, 4-oxo-2-pentenal and o-cresol in the presence of oxides of nitrogen (NO _x) are also simulated by the developed mechanism. The model well predicts the NO _x time–concentration profiles and the decay of these two carbonyls, but underestimates ozone (O₃) formation for 4-oxo-2-pentenal. It well simulates SOA formation from 1,4-butenedial but overestimates (possibly due to experimental problems) the measured aerosol mass concentrations from 4-oxo-2-pentenal. The model underestimates SOA production from o-cresol, mostly due to its under-prediction of o-cresol decay. The effects of varying temperature, relative humidity, glyoxal uptake, organic nitrate yields, and background seed aerosol concentrations, were also investigated.

Keywords: Toluene; Kinetic mechanism; Aerosol modelling; Secondary organic aerosol (SOA)

A kinetic mechanism for predicting secondary organic aerosol formation from toluene oxidation in the presence of NO _x and natural sunlight by Di Hu; Michael Tolocka; Qianfeng Li; Richard M. Kamens (pp. 6478-6496).

A kinetic mechanism to predict secondary organic aerosol (SOA) formation from the photo-oxidation of toluene was developed. Aerosol phase chemistry that includes nucleation, gas–particle partitioning and particle-phase reactions as well as the gas-phase chemistry of toluene and its degradation products were represented. The mechanism was evaluated against experimental data obtained from the University of North Carolina (UNC) 270m³ dual outdoor aerosol smog chamber facility. The model adequately simulates the decay of toluene, the nitric oxide (NO) to nitrogen dioxide (NO₂) conversion and ozone formation. It also provides a reasonable prediction of SOA production under different conditions that range from 15 to 300μgm⁻³. Speciation of simulated aerosol material shows that up to 70% of the aerosol mass comes from oligomers and polymers depending on initial reactant concentrations. The dominant particle-phase species predicted by the mechanism are glyoxal oligomers, ketene oligomers from the photolysis of the toluene OH reaction product 2-methyl-2,4-hexadienedial, organic nitrates, methyl nitro-phenol analogues, C7 organic peroxides, acylperoxy nitrates and for the low-concentration experiments, unsaturated hydroxy nitro acids.

Keywords: Toluene; Kinetic mechanism; Aerosol modeling; Secondary organic aerosol (SOA)

Effect of local and long-range transport emissions on the elemental composition of PM_10–2.5 and PM_2.5 in Beirut by Najat A. Saliba; Hovig Kouyoumdjian; Roumie Mohamad Roumié (pp. 6497-6509).

The elemental composition of PM_10−2.5 and PM_2.5 were studied in winter, summer, stormy and non-stormy dates during a period extending from February 2004 till January 2005, in a populated area of Beirut. Results of PIXE analysis and enrichment factor (E.F.) calculation, using Si as a reference of crustal material, showed that crustal elements (E.F.<10) like Si, Ca, K, Ti, Mn and Fe were more abundant in PM_10−2.5 while enriched elements (E.F.>10) like S, Cu, Zn and Pb predominated in PM_2.5. In PM_10−2.5, concentrations of crustal elements increased during stormy episodes, all time high Ca concentrations were due to the abundance of calcite and limestone rocks in Lebanon, and increased Cl levels correlated with marine air masses. In PM_2.5, sulfur concentrations were more prominent in the summer due to the enhancement of photochemical reactions. Sources of sulfur were attributed to local, sea-water and long-range transport from Eastern Europe, with the latter being the most predominate. Anthropogenic elements like Cu and Zn were generated from worn brakes and tires in high traffic density area and spikes of Pb were directly linked to a southerly wind originated from Egypt and/or Israel as determined by the air trajectory HYSPLIT model. In brief, elemental variations depended on the regional variability of the transport pattern and the different removal rates of aerosols.

Keywords: Heavy elements; Enriched and non-enriched elements; Lebanon; PIXE; High Ca levels; Arabian and Saharan Desert storms

Simulations of stratospheric to tropospheric transport during the tropical cyclone Marlene event by J. Leclair De Bellevue; J.L. Baray; S. Baldy; G. Ancellet; R. Diab; F. Ravetta (pp. 6510-6526).

Enhanced ozone values observed in the upper troposphere near intense tropical cyclones have raised the question of the role of stratospheric–tropospheric exchange. The dynamical mechanisms involved in the enhanced ozone values of 6 April 1995 observed at Reunion and associated with the tropical cyclone Marlene could not be explained by ECMWF meteorological analysis with 1.125° horizontal resolution. A previous study based on the ECHAM model has demonstrated the impact of biomass burning, but of limited amplitude (<60–80ppbv max). In this paper, the upper tropospheric ozone enhancement on the periphery of Marlene has been studied with a mesoscale model (MESO-NH). This model is able to reproduce a stratospheric PV filament into the troposphere, crossing the isentropes to the 350K level. The ageostrophic circulation associated with divergence zones that have induced vertical movements has been shown. Further, the influence of vertical wind shear, evident in both the mesoscale analysis and in the idealized HURRICANE tropical cyclone model, also contributes to our understanding of this downward transport process.

Keywords: Stratospheric–tropopsheric exchange; Tropospheric ozone; Tropical cyclone; Mesoscale modelling

GEM fluxes and atmospheric mercury concentrations (GEM, RGM andHgp) in the Canadian Arctic at Alert, Nunavut, Canada (February–June 2005) by Frank D. Cobbett; Alexandra Steffen; Greg Lawson; Bill J. Van Heyst (pp. 6527-6543).

Five months of gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and particle bound mercury (Hgp) concentrations as well as fluxes of GEM were measured at Alert, Nunavut, Canada above the Arctic snow pack. The study spanned February to June of 2005 to capture the effects of polar night, the transition period between night and day as well as polar day on the behaviour of mercury in the near surface atmosphere.A micrometeorological approach was used to infer the flux of GEM using a continuous two-level sampling system to measure the GEM concentration gradient. The required turbulent transfer coefficients were derived from meteorological parameters measured on site. The flux of GEM was approximately zero during atmospheric mercury depletion events (AMDEs) demonstrating that mercury is not being deposited as GEM to the snow pack. Following AMDEs, there was no evidence of a net emission of GEM. The highest depositional fluxes of GEM occurred during polar night and the largest emission occurred when the tundra was first visible, followed by significant emission and deposition fluxes during the snow melt. Fluxes continued until the snow had completely melted (∼JD 170) before returning to near zero.Average concentrations of RGM (44.4±49.8pgm-3),Hgp (102.6±124.9pgm-3) and GEM (1.0±0.4ngm-3) were variable throughout the study due to the dynamic nature of atmospheric mercury during AMDEs. Increases inHgp preceded elevated levels of RGM during AMDEs by 60 days, yielding peak levels at 694 and344pgm-3, respectively. Elevated concentrations ofHgp typically occurred when the specific humidity dropped below0.75gkg-1, winds were light (<3ms-1) and the air temperature dropped below-20∘C. Increased levels of RGM were also noted when the winds were light (<3ms-1) but when the temperature increased above-10∘C and the specific humidity was in the range of 1 and3gkg-1. As different environmental conditions were observed for the elevated concentrations ofHgp versus RGM at Alert, it suggests that the formation mechanisms for each species may be different but tied to the atmospheric temperature and water content.Total mercury (TM) levels in fresh snow measured approximately 5–10ngl-1 during AMDEs and reached nearly80ngl-1 outside of depletion events, suggesting that wet deposition may not be a significant removal mechanism of GEM during depletion events. An unusually high concentration of TM was measured during a non-depletion event which coincided with the transitional period where atmospheric loading of RGM exceeded levels ofHgp.

Keywords: Arctic; Flux gradient; Gaseous elemental mercury; Mercury depletion events; Micrometeorology; Particle bound mercury; Reactive gaseous mercury

Scientific uncertainties in atmospheric mercury models II: Sensitivity analysis in the CONUS domain by C.-J. Che-Jen Lin; Pruek Pongprueksa; O. Russell Bullock Jr.; Steve E. Lindberg; Simo O. Pehkonen; Carey Jang; Thomas Braverman; Thomas C. Ho (pp. 6544-6560).

In this study, we present the response of model results to different scientific treatments in an effort to quantify the uncertainties caused by the incomplete understanding of mercury science and by model assumptions in atmospheric mercury models. Two sets of sensitivity simulations were performed to assess the uncertainties using modified versions of CMAQ-Hg in a 36-km Continental United States domain. From Set 1 Experiments, it is found that the simulated mercury dry deposition is most sensitive to the gaseous elemental mercury (GEM) oxidation product assignment, and to the implemented dry deposition scheme for GEM and reactive gaseous mercury (RGM). The simulated wet deposition is sensitive to the aqueous Hg(II) sorption scheme, and to the GEM oxidation product assignment. The inclusion of natural mercury emission causes a small increase in GEM concentration but has little impact on deposition. From Set 2 Experiments, it is found that both dry and wet depositions are sensitive to mercury chemistry. Change in model mercury chemistry has a greater impact on simulated wet deposition than on dry deposition. The kinetic uncertainty of GEM oxidation by O₃ and mechanistic uncertainty of Hg(II) reduction by aqueous HO₂ pose the greatest impact. Using the upper-limit kinetics of GEM–O₃ reaction or eliminating aqueous Hg(II)–HO₂ reaction results in unreasonably high deposition and depletion of gaseous mercury in the domain. Removing GEM–OH reaction is not sufficient to balance the excessive mercury removal caused by eliminating the HO₂ mechanism. Field measurements of mercury dry deposition, better quantification of mercury air-surface exchange and further investigation of mercury redox chemistry are needed for reducing model uncertainties and for improving the performance of atmospheric mercury models.

Keywords: Atmospheric mercury; Chemical transport modeling; Sensitivity analysis; Model uncertainty; CMAQ-Hg

Predicting residential indoor concentrations of nitrogen dioxide, fine particulate matter, and elemental carbon using questionnaire and geographic information system based data by Lisa K. Baxter; Jane E. Clougherty; Christopher J. Paciorek; Rosalind J. Wright; Jonathan I. Levy (pp. 6561-6571).

Previous studies have identified associations between traffic-related air pollution and adverse health effects. Most have used measurements from a few central ambient monitors and/or some measure of traffic as indicators of exposure, disregarding spatial variability and factors influencing personal exposure-ambient concentration relationships. This study seeks to utilize publicly available data (i.e., central site monitors, geographic information system, and property assessment data) and questionnaire responses to predict residential indoor concentrations of traffic-related air pollutants for lower socioeconomic status (SES) urban households.As part of a prospective birth cohort study in urban Boston, we collected indoor and outdoor 3–4 day samples of nitrogen dioxide (NO₂) and fine particulate matter (PM_2.5) in 43 low SES residences across multiple seasons from 2003 to 2005. Elemental carbon (EC) concentrations were determined via reflectance analysis. Multiple traffic indicators were derived using Massachusetts Highway Department data and traffic counts collected outside sampling homes. Home characteristics and occupant behaviors were collected via a standardized questionnaire. Additional housing information was collected through property tax records, and ambient concentrations were collected from a centrally located ambient monitor.The contributions of ambient concentrations, local traffic and indoor sources to indoor concentrations were quantified with regression analyses. PM_2.5 was influenced less by local traffic but had significant indoor sources, while EC was associated with traffic and NO₂ with both traffic and indoor sources. Comparing models based on covariate selection using p-values or a Bayesian approach yielded similar results, with traffic density within a 50m buffer of a home and distance from a truck route as important contributors to indoor levels of NO₂ and EC, respectively. The Bayesian approach also highlighted the uncertanity in the models. We conclude that by utilizing public databases and focused questionnaire data we can identify important predictors of indoor concentrations for multiple air pollutants in a high-risk population.

Keywords: Indoor air; NO; ₂; PM; _2.5; EC; Geographic information system

Surface ozone in the Indian region by Moti L. Mittal; Peter G. Hess; S.L. Jain; B.C. Arya; C. Sharma (pp. 6572-6584).

Ozone(O3) is a dangerous air pollutant in terms of damage to crop plants and human health. It is phytotoxic to vegetation and a toxic agent for human health. Tropospheric ozone(O3) in the Indian sub-continent from Afghanistan in the west(60∘E) to parts of Southeast Asian countries in the east(105∘E) and parts of China in the north(45∘N) to Sri Lanka in the south(0∘N) is simulated with an episodic chemical transport model christened HANK for the spring and summer months (February–May 2000).O3 concentrations are mapped on the gridded region. It provides a general picture of the variations in the surface ozone concentrations in the different regions of the Indian sub-continent. SimulatedO3 is compared with the measurements at Delhi, Ahmedabad, and the Arabian Sea. AOT40—a parameter that represents the accumulated dose of ozone over a threshold of 40ppb is computed for the region. The Indo-Gangatic plain in the Northeast region of India has been found to have very high AOT40 and also much higher concentrations ofO3 and CO compared to the rest of the Indian region. There is substantial temporal and spatial variation inO3 across the region due to meteorological conditions and anthropogenic emissions of precursor gases.O3 concentrations are higher in March compared to other months during February–May period. Northern region of India has higher concentrations ofO3 then the southern region.

Keywords: Air pollution; Air quality modeling; Surface ozone; AOT40; HANK

An analysis of seasonal surface dust aerosol concentrations in the western US (2001–2004): Observations and model predictions by Kelley C. Wells; Marcin Witek; Piotr Flatau; Sonia M. Kreidenweis; Douglas L. Westphal (pp. 6585-6597).

Long-term surface observations indicate that soil dust represents over 30% of the annual fine (particle diameter less than 2.5μm) particulate mass in many areas of the western US; in spring and summer, it represents an even larger fraction. There are numerous dust-producing playas in the western US, but surface dust aerosol concentrations in this region are also influenced by dust of Asian origin. This study examines the seasonality of surface soil dust concentrations at 15 western US sites using observations from the Interagency Monitoring of PROtected Visual Environments (IMPROVE) network from 2001 to 2004. Average soil concentrations in particulate matter less than 10μm in diameter (PM₁₀) were lowest in winter and peaked during the summer months at these sites; however, episodic higher-concentration events (>10μgm⁻³) occurred in the spring, the time of maximum Asian dust transport to the western US. Simulated surface dust concentrations from the Navy Aerosol Analysis and Prediction System (NAAPS) suggested that long-range transport from Asia dominates surface dust concentrations in the western US in the spring, and that, although some long-range transport does occur throughout the year (1–2μgm⁻³), locally generated dust plays a larger role in the region in summer and fall. However, NAAPS simulated some anomalously high concentrations (>50μgm⁻³) of local dust in the fall and winter months over portions of the western US. Differences between modeled and observed dust concentrations were attributed to overestimation of total observed soil dust concentrations by the assumptions used to convert IMPROVE measurements into PM₁₀ soil concentrations, lack of inhibition of model dust production in snow-covered regions, and lack of seasonal agricultural sources in the model.

Keywords: Dust; Long-range transport; Particulate monitoring; Rural aerosol

Measurements of fine and ultrafine particles formation in photocopy centers in Taiwan by C.-W. Chia-Wei Lee; D.-J. Der-Jen Hsu (pp. 6598-6609).

This study investigates the levels of particulate matter smaller than 2.5μm (PM_2.5) and some selected volatile organic compounds (VOCs) at 12 photocopy centers in Taiwan from November 2004 to June 2005. The results of BTEXS (benzene, toluene, ethylbenzene, xylenes and styrene) measurements indicated that toluene had the highest concentration in all photocopy centers, while the concentration of the other four compounds varied among the 12 photocopy centers. The average background-corrected eight-hour PM_2.5 in the 12 photocopy centers ranged from 10 to 83μgm⁻³ with an average of 40μgm⁻³. The 24-h indoor PM_2.5 at the photocopy centers was estimated and at two photocopy centers exceeded 100μgm⁻³, the 24-h indoor PM_2.5 guideline recommended by the Taiwan EPA. The ozone level and particle size distribution at another photocopy center were monitored and indicated that the ozone level increased when the photocopying started and the average ozone level at some photocopy centers during business hour may exceed the value (50ppb) recommended by the Taiwan EPA. The particle size distribution monitored during photocopying indicated that the emitted particles were much smaller than the original toner powders. Additionally, the number concentration of particles that were smaller than 0.5μm was found to increase during the first hour of photocopying and it increased as the particle size decreased. The ultrafine particle (UFP, <100nm) dominated the number concentration and the peak concentration appeared at sizes of under 50nm. A high number concentration of UFP was found with a peak value of 1E+8 particlescm⁻³ during photocopying. The decline of UFP concentration was observed after the first hour and the decline is likely attributable to the surface deposition of charged particles, which are charged primarily by the diffusion charging of corona devices in the photocopier. This study concludes that ozone and UFP concentrations in photocopy centers should be concerned in view of indoor air quality and human health. The corona devices in photocopiers and photocopier-emitted VOCs have the potential to initiate indoor air chemistry during photocopying and result in the formation of UFP.

Keywords: Photocopy center; Particulate matter (PM); Ultrafine particle (UFP); Volatile organic compounds (VOCs); Corona charging

Simulation of the atmospheric concentrations of²¹⁰Pb and⁷Be and comparison with daily observations at three surface sites by P. Heinrich; O. Coindreau; Y. Grillon; X. Blanchard; P. Gross (pp. 6610-6621).

The atmospheric transport of²¹⁰Pb and⁷Be is simulated by the Laboratoire de Météorologie Dynamique general circulation model, LMDz, driven by ECMWF reanalyses. Daily averaged concentrations collected at three surface stations are compared with numerical results for a 1 year (2004) global simulation. The model, with a resolution of 3.75°×2.5° and 19 levels, succeeds in reproducing daily variations of²¹⁰Pb and⁷Be concentrations. Figures of merit in time (FMT), quantifying the overlapping of the predicted and observed time series over each month, range from 55% to 70%. Sensitivity studies as well as the analysis of numerical signals allow determining the main physical atmospheric processes characterizing the stations on a daily basis. Concentrations of²¹⁰Pb at the three stations are particularly sensitive to scavenging in convective updrafts.

Keywords: Atmospheric transport; Numerical modeling; Natural radionuclides; Particles rainout; LMDz model

Spatial variation of particle number and mass over four European cities by Arto Puustinen; Hameri Kaarle Hämeri; Juha Pekkanen; Markku Kulmala; Jeroen de Hartog; Kees Meliefste; Harry ten Brink; Gerard Kos; Klea Katsouyanni; Anna Karakatsani; Anastasia Kotronarou; Ilias Kavouras; Claire Meddings; Steve Thomas; Roy Harrison; Jon G. Ayres; Saskia van der Zee; Gerard Hoek (pp. 6622-6636).

The number of ultrafine particles may be a more health relevant characteristic of ambient particulate matter than the conventionally measured mass. Epidemiological time series studies typically use a central site to characterize human exposure to outdoor air pollution. There is currently very limited information how well measurements at a central site reflect temporal and spatial variation across an urban area for particle number concentrations (PNC).The main objective of the study was to assess the spatial variation of PNC compared to the mass concentration of particles with diameter less than 10 or 2.5μm (PM₁₀ and PM_2.5).Continuous measurements of PM₁₀, PM_2.5, PNC and soot concentrations were conducted at a central site during October 2002–March 2004 in four cities spread over Europe (Amsterdam, Athens, Birmingham and Helsinki). The same measurements were conducted directly outside 152 homes spread over the metropolitan areas. Each home was monitored during 1 week. We assessed the temporal correlation and the variability of absolute concentrations.For all particle indices, including particle number, temporal correlation of 24-h average concentrations was high. The median correlation for PNC per city ranged between 0.67 and 0.76. For PM_2.5 median correlation ranged between 0.79 and 0.98. The median correlation for hourly average PNC was lower (range 0.56–0.66). Absolute concentration levels varied substantially more within cities for PNC and coarse particles than for PM_2.5. Measurements at the central site reflected the temporal variation of 24-h average concentrations for all particle indices at the selected homes across the urban area. A central site could not assess absolute concentrations across the urban areas for particle number.

Keywords: Particle number concentration; PM; _2.5; PM; ₁₀; Spatial variation; Exposure assessment

Measurements and analyses of nitrogen oxides and ozone in the yard and on the roof of a street-canyon in Suzhou by F. Costabile; I. Allegrini (pp. 6637-6647).

The concentrations of air pollutants such as nitrogen oxides and ozone characterised by very fast chemical reactions can significantly vary within urban street-canyon due to the short distances between sources and receptor. With the primary objective to analyse this issue, NO, NO₂, NO _x, O₃, BTX, and wind flow field were continuously measured for 1 week at two heights (a street-level yard and a 25-m-high rooftop) in an urban canyon in Suzhou (China). The yard ozone concentrations were found to be up to six times lower than on the roof. Different frequency distributions (FD), dynamical and chemical processes of the pollutant variations from yard to roof are discussed to explain the findings. The predominant factors for the dissimilar pollutant vertical diffusion at the two measurement locations were associated to dissimilar fluid-dynamic and heterogeneous removal effects that likely induced dissimilar ozone chemical processes relative to NO _x and BTX precursors.

Keywords: Air pollution; Spatial distribution; Street-canyon; Yard; Ozone; Nitrogen oxides

Origin of low-molecular-weight dicarboxylic acids and their concentration and size distribution variation in suburban aerosol by L.-Y. Li-Ying Hsieh; S.-C. Su-Ching Kuo; C.-L. Chien-Lung Chen; Ying I. Tsai (pp. 6648-6661).

The concentrations and size distributions of low molecular weight dicarboxylic acids in suburban particulate matter collected in early and mid-autumn 2002 and early and mid-summer 2003 in Tainan, Taiwan, were analyzed. PM_2.5 contained, on average, 449.3ngm⁻³ oxalic acid, 53.0ngm⁻³ malic acid, 45.5ngm⁻³ maleic acid, 29.6ngm⁻³ succinic acid, 20.8ngm⁻³ malonic acid, and 11.6ngm⁻³ tartaric acid. Bar tartaric acid, concentrations were higher during the day, indicating that these acids are photochemical products. Furthermore, the malonic acid–succinic acid ratio of 0.79 during daytime and 0.60 during nighttime demonstrates that more succinic acid is converted to malonic acid during daytime, and that aerosol dicarboxylic acids predominantly originate from photochemical oxidation during daytime. The concentration peak of oxalic acid occurred in the condensation and droplet modes (0.32–1.0μm), as did that of sulfate. In early summer, succinic acid, malonic acid, and oxalic acid major concentration peaks occurred at 0.32–0.54μm, indicative of the relationship created by photochemical decomposition of succinc acid into malonic acid into oxalic acid. This photochemical decomposition accelerated in mid-summer such that most concentration peaks for succinic and malonic acids also occurred at 0.32–1.0μm. Mid-summer is also the wettest period of the four in Tainan, with 85% RH. As a result of hygroscopic reactions in mid-summer, malonic acid and oxalic acid major concentration peaks shifted from 0.32–0.54μm or 0.54–1.0μm to 1.0–1.8μm, thus extending the range in which these species were found to larger particle sizes, and this shift was highly correlated with a shift in succinic acid size distribution. This latter observation offers additional evidence that succinic acid is photochemically decomposed into malonic acid and oxalic acid and that the presence of malonic and oxalic acids in the wet mid-summer atmosphere is made more obvious via hygroscopic growth. Close correlation between succinic acid and Na⁺ and succinic acid and NO₃⁻ in the coarse mode is related to sea spray.

Keywords: PM; _2.5; Size distributions; Oxalic acid; Malonic acid; Succinic acid

Seasonal and diel variation of atmospheric mercury concentrations in the Reno (Nevada, USA) airshed by Jelena Stamenkovic; Seth Lyman; Mae S. Gustin (pp. 6662-6672).

This paper describes total gaseous mercury (TGM) concentrations measured in Reno, Nevada from 2002 to 2005. The 3-year mean and median air Hg concentrations were 2.3 and 2.1ngm⁻³, respectively. Mercury concentrations exhibited seasonality, with the highest concentrations in winter, and the lowest in summer and fall. A well-defined diel pattern in TGM concentration was observed, with maximum daily concentrations observed in the morning and minimum in the afternoon. A gradual increase of TGM concentration was observed in the evening and over night. The early morning increase in TGM was likely due to activation of local surface emission sources by rising solar irradiance and air temperature. The subsequent decline and afternoon minimum in TGM were likely related to increased vertical mixing and the buildup of atmospheric oxidants during the day resulting in increased conversion to oxidized species that are quickly deposited, coupled with weakening of the surface emissions processes. The described diel pattern was seasonally modulated with the greatest amplitude in variation of TGM concentrations occurring in the summer. It is suggested based on the comparison of diel TGM pattern with other gaseous pollutants that natural source surface emissions are a dominant source of TGM in the study area.

Keywords: Mercury; Urban airshed; Temporal variability; Boundary layer; Oxidants

Temporal trends in the concentration of arsenic, chromium, copper, iron, nickel, vanadium and zinc in mosses across Europe between 1990 and 2000 by Harry Harmens; David A. Norris; Georgia R. Koerber; Alan Buse; Eiliv Steinnes; Å. Åke Rühling (pp. 6673-6687).

The European heavy metals in mosses biomonitoring network provides data on the concentration of 10 heavy metals in naturally growing mosses and is currently coordinated by the UNECE ICP Vegetation (United Nations Economic Commission for Europe International Co-operative Programme on Effects of Air Pollution on Natural Vegetation and Crops). The technique of moss analysis provides a surrogate, time-integrated measure of metal deposition from the atmosphere to terrestrial systems. It is easier and cheaper, less prone to contamination and allows a much higher sampling density than conventional precipitation analysis. Moss surveys have been repeated at five-yearly intervals and in this paper we report on the temporal trends in the concentration of arsenic, chromium, copper, iron, nickel, vanadium and zinc between 1990 and 2000. Maps were produced of the metal concentration in mosses for 1990, 1995 and 2000, showing the mean concentration per metal per50km×50km EMEP grid square. Metal- and country-specific temporal trends were observed. Although the metal concentration in mosses generally decreased with time for all metals, only the decreases for arsenic, copper, vanadium and zinc were statistically significant. The observed temporal trends were compared with emission trends for Europe reported by EMEP (Co-operative Programme for Monitoring and Evaluation of the Long-range Transmission of Air Pollutant in Europe).

Keywords: Biomonitoring; EMEP maps; Heavy metal; Metal deposition; Moss

Corrigendum to “Developing intake fraction estimates with limited data: Comparison of methods in Mexico City” by Gretchen Stevens; Benjamin de Foy; J. Jason West; Jonathan I. Levy (pp. 6688-6689).

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Atmospheric Environment (v.41, #31)