Atmospheric Environment (v.44, #5)

Equilibration time scales of organic aerosol inside thermodenuders: Evaporation kinetics versus thermodynamics by Ilona Riipinen; Jeffrey R. Pierce; Neil M. Donahue; Spyros N. Pandis (597-607).
The interpretation of thermodenuder (TD) data often relies on the assumption that thermodynamic equilibrium is reached inside the instrument. We modeled the evaporation of three organic aerosol types (adipic acid, α-pinene SOA and aged OA) inside a thermodenuder with a mass transfer model, and calculated equilibration time scales for these systems at realistic conditions. The equilibrium times varied from less than a second to several hours, decreasing with increasing aerosol concentrations, decreasing particle sizes, decreasing volatilities and increasing mass accommodation coefficients. The results indicate that generally TDs measure particle evaporation rates rather than equilibria, and time-dependent modeling of the evaporation is usually needed to interpret the data. Measurements at varying residence times and temperatures, on the other hand, are desirable to investigate the equilibration of the studied aerosol and decouple the kinetic effects from the effects caused by the thermodynamic properties of the aerosol. Organic aerosol is likely to be further from equilibrium under typical field conditions compared with laboratory data. When determining the aerosol properties from TD data, assuming incorrectly equilibrium results in under-prediction of the vaporization enthalpy of the evaporating species. Similar under-estimation is predicted if multicomponent aerosols are approximated with single-component properties.
Keywords: Organic aerosol; Volatility; Vaporization enthalpy; Thermodenuder;

Simulating the fine and coarse inorganic particulate matter concentrations in a polluted megacity by Vlassis A. Karydis; Alexandra P. Tsimpidi; Christos Fountoukis; Athanasios Nenes; Miguel Zavala; Wenfang Lei; Luisa T. Molina; Spyros N. Pandis (608-620).
A three dimensional chemical transport model (PMCAMx) is applied to the Mexico City Metropolitan Area (MCMA) in order to simulate the chemical composition and mass of the major PM1 (fine) and PM1–10 (coarse) inorganic components and determine the effect of mineral dust on their formation. The aerosol thermodynamic model ISORROPIA-II is used to explicitly simulate the effect of Ca, Mg, and K from dust on semi-volatile partitioning and water uptake. The hybrid approach is applied to simulate the inorganic components, assuming that the smallest particles are in thermodynamic equilibrium, while describing the mass transfer to and from the larger ones. The official MCMA 2004 emissions inventory with improved dust and NaCl emissions is used. The comparison between the model predictions and measurements during a week of April of 2003 at Centro Nacional de Investigacion y Capacitacion Ambiental (CENICA) “Supersite” shows that the model reproduces reasonably well the fine mode composition and its diurnal variation. Sulfate predicted levels are relatively uniform in the area (approximately 3 μg m−3), while ammonium nitrate peaks in Mexico City (approximately 7 μg m−3) and its concentration rapidly decreases due to dilution and evaporation away from the urban area. In areas of high dust concentrations, the associated alkalinity is predicted to increase the concentration of nitrate, chloride and ammonium in the coarse mode by up to 2 μg m−3 (a factor of 10), 0.4 μg m−3, and 0.6 μg m−3 (75%), respectively. The predicted ammonium nitrate levels inside Mexico City for this period are sensitive to the physical state (solid versus liquid) of the particles during periods with RH less than 50%.
Keywords: Chemical transport model; Nitrate; Dust;

Establishing ozone flux–response relationships for winter wheat: Analysis of uncertainties based on data for UK and Polish genotypes by Ignacio Gonzalez–Fernandez; Agnieszka Kaminska; Mahmadali Dodmani; Eleni Goumenaki; Steve Quarrie; Jeremy D. Barnes (621-630).
The work outlined in this paper had three objectives. The first was to explore the effects of ozone pollution on grain yield and quality of commercially-grown winter wheat cultivars. The second was to derive a stomatal ozone flux model for winter wheat and compare with those already developed for spring wheat. The third was to evaluate exposure- versus flux–response approaches from a risk assessment perspective, and explore the implications of genetic variation in modelled ozone flux.Fifteen winter wheat cultivars were grown in open-top chambers where they were exposed to four levels of ozone. During fumigation, stomatal conductance measurements were made over the lifespan of the flag leaf across a range of environmental conditions. Although significant intra-specific variation in ‘ozone sensitivity’ (in terms of impacts on yield) was identified, yield was inversely related (R 2 = 0.63, P < 0.001) to the accumulated hourly averaged ozone exposure above 40 ppb during daylight hours (AOT40) across the dataset. The adverse effect of ozone on yield was principally due to a decline in seed weight. Algorithms defining the influence of environmental variables on stomatal uptake were subtly different from those currently in use, based on data for spring wheat, to map ozone impacts on pan-European cereal yield. Considerable intra-specific variation in phenological effects was identified. This meant that an ‘average behaviour’ had to be derived which reduced the predictive capability of the derived stomatal flux model (R 2 = 0.49, P < 0.001, 15 cultivars included). Indeed, given the intra-specific variability encountered, the flux model that was derived from the full dataset was no better in predicting O3 impacts on wheat yield than was the AOT40 index. The study highlights the need to use ozone risk assessment tools appropriate to specific vegetation types when modelling and mapping ozone impacts at the regional level.
Keywords: Risk assessment; Modelling ozone uptake; Grain yield; Quality; Dose–response relationships;

This paper describes the further development and application of the Edinburgh–Lancaster Model for Ozone (ELMO). We replace straight-line back-trajectories with trajectories and associated meteorology supplied by the US National Oceanic and Atmospheric Administration Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) service to allow more realistic modelling of specific UK ozone episodes. We call this ELMO-2. Model performance is rigorously tested against observed ozone concentrations for two episodes recorded across 14 rural UK monitoring stations during the spring and summer of 1995. For both episodes, the afternoon concentrations (usually coinciding with the daily maxima) are captured well by the model and the diurnal ozone cycle is reproduced, although the amplitude in concentrations is generally smaller than the observed. The summer episode is investigated further through indicator species analysis and source attribution, and found to be mainly VOC-limited. European emissions account for the majority of ozone production. We demonstrate how improved modelling leads to better understanding of regional and local ozone production across the UK under episodic conditions.
Keywords: ELMO; HYSPLIT; Source attribution; Episode sensitivity;

Contribution of residential wood combustion to PM10 levels in Portugal by C. Borrego; J. Valente; A. Carvalho; E. Sá; M. Lopes; A.I. Miranda (642-651).
Wood is commonly used in residential combustion for heating purposes; however, it can be a major source of air pollutants, namely fine particles, volatile organic compounds and carbon monoxide. Since 2004, the PM10 daily limit value has been surpassed in Portugal, and the European Commission has stated that plans and programs must be designed in order to reduce these levels. In Portugal, 18% of PM10 emissions are due to residential wood combustion, which may deeply impact the PM10 levels in the atmosphere. The main aim of this study is to investigate the impact of residential wood combustion on the air quality in Portugal. The air quality modelling system MM5/CHIMERE was applied over Portugal for a winter month, for the following three scenarios: the reference scenario, considering the actual emissions of PM10; scenario 1, where residential wood combustion emissions are not considered; and scenario 2, which takes into account a complete conversion from traditional fireplaces to certified appliances (with a 90% reduction in PM emissions). The residential wood combustion contribution to PM10 air quality concentration values during January 2007 ranges from 0 to 14 μg m−3, with a mean contribution of 10 μg m−3 in the Lisboa area and 6 μg m−3 in the Porto region. Concerning the legislated values, the area where the daily average limit value (50 μg m−3) is exceeded decreases by 46% in the simulation when residential combustion is not considered. The modelling results for scenario 2 are not significantly different from those for scenario 1. In summary, the regulation of the residential wood combustion sector is as an effective way to reduce the PM10 levels in the atmosphere as regards air quality plans and programs.
Keywords: Residential combustion; PM10 emissions; PM10 concentrations; Air quality modelling;

Mono- and dicarboxylic n-alkyl acids were extensively investigated in downtown Rome, Italy, and in Montelibretti, ∼30 km NE of the city, during 2005–2007. Congeners ranging from lauric to mellisic, and from succinic to α,ω-docosanedioic acids were evaluated as well as phthalic, palmitoleic and oleic acids, by solvent extraction of airborne particulates followed by derivatization with propanol in the presence of boron trifluoride, and gas chromatographic-mass spectrometric analysis. Shorter measurements were made in Milan, in Taranto, at suburban and rural sites of Italy, and in the polar regions, from 1996 to 2005. The predominance of palmitic and stearic acids observed elsewhere was confirmed, and the behaviour of azelaic and phthalic acids resulted strongly dependent upon the year season. In the urban sites, among the long-chain compounds, the lignoceric acid was usually the most abundant, while the cerotic, montanic and mellisic homologues cumulatively never exceeded 8% of the total. Unlike other contaminants, the concentrations of organic acids remained fairly invariant over the last decade, suggesting that more attention must be paid to them in the future.
Keywords: Atmospheric particulate matter; Fatty acids; Dicarboxylic acids; GCMS;

WRF-Chem simulation of East Asian air quality: Sensitivity to temporal and vertical emissions distributions by Xueyuan Wang; Xin-Zhong Liang; Weimei Jiang; Zhining Tao; Julian X.L. Wang; Hongnian Liu; Zhiwei Han; Shuyan Liu; Yuyan Zhang; Georg A. Grell; Steven E. Peckham (660-669).
This study develops fine temporal (seasonal, day-of-week, diurnal) and vertical allocations of anthropogenic emissions for the TRACE-P inventory and evaluates their impacts on the East Asian air quality prediction using WRF-Chem simulations in July 2001 at 30-km grid spacing against available surface measurements from EANET and NEMCC. For NO2 and SO2, the diurnal and vertical redistributions of emissions play essential roles, while the day-of-week variation is less important. When all incorporated, WRF-Chem best simulates observations of surface NO2 and SO2 concentrations, while using the default emissions produces the worst result. The sensitivity is especially large over major cities and industrial areas, where surface NO2 and SO2 concentrations are reduced by respectively 3–7 and 6–12 ppbv when using the scaled emissions. The incorporation of all the three redistributions of emissions simulates surface O3 concentrations higher by 4–8 ppbv at night and 2–4 ppbv in daytime over broad areas of northern, eastern and central China. To this sensitivity, the diurnal redistribution contributes more than the other two.
Keywords: WRF-Chem; Emissions; Diurnal cycle; Vertical redistribution; Air quality modeling; East Asia;

Global scale emission and distribution of sea-spray aerosol: Sea-salt and organic enrichment by E. Vignati; M.C. Facchini; M. Rinaldi; C. Scannell; D. Ceburnis; J. Sciare; M. Kanakidou; S. Myriokefalitakis; F. Dentener; C.D. O'Dowd (670-677).
The chemical composition of marine aerosols as a function of their size is an important parameter for the evaluation of their impact on the global climate system. In this work we model fine particle organic matter emitted by sea spray processes and its influence on the aerosol chemical properties at the global scale using the off-line global Chemistry-Transport Model TM5. TM5 is coupled to a microphysical aerosol dynamics model providing size resolved information on particle masses and numbers. The mass of the emitted sea spray particles is partitioned between water insoluble organic matter (WIOM) and sea salt components in the accumulation mode using a function that relates the emitted organic fraction to the surface ocean chlorophyll-a concentrations. The global emission in the sub-micron size range of organic matter by sea spray process is 8.2 Tg yr−1, compared to 24 Tg fine yr−1 sea-salt emissions. When the marine sources are included, the concentrations of modelled primary particulate organic matter (POM) increase mainly over the oceans. The model predictions of WIOM and sea salt are evaluated against measurements carried out at Mace Head (Northern Hemisphere) and Amsterdam Island (Southern Hemisphere), showing that in clean marine conditions WIOM marine emissions contribute significantly to POM values.
Keywords: Modelling; Marine aerosols; Organics; Emissions;

Analysis of the effects of combustion emissions and Santa Ana winds on ambient ozone during the October 2007 southern California wildfires by Andrzej Bytnerowicz; Dan Cayan; Philip Riggan; Susan Schilling; Philip Dawson; Mary Tyree; Lynn Wolden; Robert Tissell; Haiganoush Preisler (678-687).
Combustion emissions and strong Santa Ana winds had pronounced effects on patterns and levels of ambient ozone (O3) in southern California during the extensive wildland fires of October 2007. These changes are described in detail for a rural receptor site, the Santa Margarita Ecological Reserve, located among large fires in San Diego and Orange counties. In addition, O3 changes are also described for several other air quality monitoring sites in the general area of the fires. During the first phase of the fires, strong, dry and hot northeasterly Santa Ana winds brought into the area clean continental air masses, which resulted in minimal diurnal O3 fluctuations and a 72-h average concentration of 36.8 ppb. During the second phase of the fires, without Santa Ana winds present and air filled with smoke, daytime O3 concentrations steadily increased and reached 95.2 ppb while the lowest nighttime levels returned to ∼0 ppb. During that period the 8-h daytime average O3 concentration reached 78.3 ppb, which exceeded the federal standard of 75 ppb. After six days of fires, O3 diurnal concentrations returned to pre-fire patterns and levels.
Keywords: Air pollution; Ozone; Wildland fires; Meteorology; Santa Ana winds;

Comparison of land-use regression models between Great Britain and the Netherlands by D. Vienneau; K. de Hoogh; R. Beelen; P. Fischer; G. Hoek; D. Briggs (688-696).
Land-use regression models have increasingly been applied for air pollution mapping at typically the city level. Though models generally predict spatial variability well, the structure of models differs widely between studies. The observed differences in the models may be due to artefacts of data and methodology or underlying differences in source or dispersion characteristics. If the former, more standardised methods using common data sets could be beneficial. We compared land-use regression models for NO2 and PM10, developed with a consistent protocol in Great Britain (GB) and the Netherlands (NL).Models were constructed on the basis of 2001 annual mean concentrations from the national air quality networks. Predictor variables used for modelling related to traffic, population, land use and topography. Four sets of models were developed for each country. First, predictor variables derived from data sets common to both countries were used in a pooled analysis, including an indicator for country and interaction terms between country and the identified predictor variables. Second, the common data sets were used to develop individual baseline models for each country. Third, the country-specific baseline models were applied after calibration in the other country to explore transferability. The fourth model was developed using the best possible predictor variables for each country.A common model for GB and NL explained NO2 concentrations well (adjusted R 2 0.64), with no significant differences in intercept and slopes between the two countries. The country-specific model developed on common variables for NL but not GB improved the prediction.The performance of models based upon common data was only slightly worse than models optimised with local data. Models transferred to the other country performed substantially worse than the country-specific models. In conclusion, care is needed both in transferring models across different study areas, and in developing large inter-regional LUR models.
Keywords: Land-use regression; Exposure modelling; Air pollution; Geographic Information Systems (GIS);

Homogeneous and heterogeneous reactions of phenanthrene with ozone by Yang Zhang; Bo Yang; Junwang Meng; Shaokai Gao; Xinyu Dong; Jinian Shu (697-702).
The reactions of gas-phase phenanthrene and suspended phenanthrene particles with ozone were conducted in a 200l chamber. The secondary organic aerosol formation was observed in the reaction of gas-phase phenanthrene with ozone and simultaneously the size distribution of the secondary organic aerosol was monitored with a scanning mobility particle sizer during the formation process. The particulate ozonation products from both reactions were analyzed with a vacuum ultraviolet photoionization aerosol time-of-flight mass spectrometer. 2,2′-Diformylbiphenyl was identified as the dominant product in both homogeneous and heterogeneous reactions of phenanthrene with ozone. GC/MS analysis of ozonation products of phenanthrene in glacial acetic acid was carried out for assigning time-of-flight mass spectra of reaction products formed in the homogeneous and heterogeneous reactions of phenanthrene with ozone.
Keywords: Phenanthrene; Ozonation; Homogeneous reaction; Heterogeneous reaction; Aerosol mass spectrometer;

Inferring episodic atmospheric iron fluxes in the Western South Atlantic by Heitor Evangelista; Juan Maldonado; Elaine A. dos Santos; Ricardo H.M. Godoi; Carlos A.E. Garcia; Virginia M.T. Garcia; Erling Jonhson; Kenya Dias da Cunha; Carlos Barros Leite; René Van Grieken; Katleen Van Meel; Yaroslava Makarovska; Diego M. Gaiero (703-712).
Iron (Fe) and other trace elements such as Zn, Mn, Ni and Cu are known as key-factors in marine biogeochemical cycles. It is believed that ocean primary productivity blooms in iron deficient regions can be triggered by iron in aeolian dust. Up to now, scarce aerosol elemental composition, based on measurements over sea at the Western South Atlantic (WSA), exist. An association between the Patagonian semi-desert dust/Fe and chlorophyll-a variability at the Argentinean continental shelf is essentially inferred from models. We present here experimental data of Fe enriched aerosols over the WSA between latitudes 22°S–62°S, during 4 oceanographic campaigns between 2002 and 2005. These data allowed inferring the atmospheric Fe flux onto different latitudinal bands which varied from 30.4 to 1688 nmolFe m−2 day−1 (October 29th–November 15th, 2003); 5.83–1586 nmolFe m−2 day−1 (February 15th–March 6th, 2004) and 4.73–586 nmolFe m−2 day−1(October 21st–November 5th, 2005).
Keywords: Patagonia; Iron deposition; Western South Atlantic; Aerosols;

Influence of ship emissions on ozone concentrations around coastal areas during summer season by Sang-Keun Song; Zang-Ho Shon; Yoo-Keun Kim; Yoon-Hee Kang; In-Bo Oh; Chang-Hoon Jung (713-723).
The influence of ship emissions on ozone (O3) concentrations in a coastal area (CA) including Busan port, Korea was examined based on a numerical modeling approach during a high O3 episode. The analysis was performed by two sets of simulation scenarios: (1) with ship emissions (e.g., TOTAL case) and (2) without ship emissions (e.g., BASE case). A process analysis (PA) (the integrated processes rate (IPR) and integrated reaction rate (IRR) analyses) was used to evaluate the relative contributions of individual physical and chemical processes in O3 production in and around the CA (e.g., sites of Dong Sam (DS) and Dae Yeon (DY)). The model study suggested the possibility that pollutant gases emitted from the ships traversing Busan port can exert a direct impact on the O3 concentration levels in the CA. Largest impacts of ship emissions on the O3 concentrations were predicted at the coast (up to 15 ppb) and at inland locations (about 5 ppb) due to both the photochemical production of pollutant gases emitted from the ships and meteorological conditions. From the PA, the photochemical production of O3 (P(O3)) due to ship emissions in the CA was found to increase by a mean of 1.5 ppb h−1 (especially by ≥10 ppb h−1 at the DS site) during the day.
Keywords: Ship emission; O3; Photochemical production; Meteorological conditions; Process analysis;

A total of 134 aerosol samples (d p < 2.5 μm) were collected at one rural site and one urban site in Texas from November 2005 to July 2006 to investigate the different sources that contribute to the ambient levels of different compounds. In particular, saccharide compounds were studied as potential tracers to track aerosols of biologically derived origin. The ambient concentration, seasonal variation, and urban/rural comparison of major saccharides and other organic compounds including normal alkanes, hopanes, and carboxylic acids were determined and analyzed relative to characterizing sources of PM2.5. Saccharides, together with other known molecular markers, were analyzed by a positive matrix factorization model and eight source factors were isolated that provide meaningful interpretation of aerosol sources. Three isolated factors were characterized by the dominance of different saccharide compounds and were attributed to wood smoke, sucrose rich bio-aerosols, and fungal spore derived bio-aerosols. It was estimated that wood smoke and primary biologically derived carbon sources contributed 22% and 14% to the measured ambient PM2.5 mass at San Augustine and 16% and 5% to the measured ambient PM2.5 mass at Dallas. The relative PM contribution from other resolved sources were also calculated.
Keywords: Saccharides; Fine particulate matter; Eastern Texas; Molecular markers;

Keywords: Electric car; CO2; Electric vehicle efficiency; Lifecycle CO2 emissions; CO2 emission factor; Tesla; Volt;