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Atmospheric Environment (v.42, #23)

Editorial board (pp. i).

International Conference on Atmospheric Chemical Mechanisms: Special section in Atmospheric Environment by Anthony S. Wexler (pp. 5731-5732).

A study of VOC reactivity in the Houston-Galveston air mixture utilizing an extended version of SAPRC-99 chemical mechanism by Beata H. Czader; Daewon W. Byun; Soon-Tae Kim; William P.L. Carter (pp. 5733-5742).

The Houston-Galveston area is the region where National Ambient Air Quality Standard (NAAQS) for ozone is often violated. Emissions from industrial sources contribute significantly to elevated ozone concentrations and make air composition quite unique in comparison to typical urban composition. In this study, source-oriented reactivity of individual organic compounds, released in urban and industrial areas inside Houston-Galveston region, with regard to ozone formation is determined. The CMAQ model and an extended version of SAPRC-99 mechanism, representing 26 additional individual compounds explicitly, is employed for the model simulations. The incremental volatile organic compound (VOC) reactivities calculated for the Houston-Galveston conditions show variations with respect to the differences in the air compositions between urban and industrial areas as well as meteorological conditions. The compounds with the highest reactivities, such as 2-methyl-2-butene, trans-2-butene and cis-2-butene, were found to be most sensitive to the environment in which they were released. According to the reactivities weighted by VOC mixing ratios, ethene, propene and formaldehyde show the highest impact on ozone formation. Reactivities of most investigated compounds estimated for the Houston-Galveston air conditions are in good agreement with the ‘reference’ maximum incremental reactivity (MIR) scale.

Keywords: Photochemical ozone creation potential; Incremental reactivity; MIR; POCP; Ozone; Houston

Application of a Lagrangian Process Analysis tool to characterize ozone formation in Southeast Texas by Yosuke Kimura; Elena McDonald-Buller; William Vizuete; David T. Allen (pp. 5743-5759).

Ozone formation in the Houston area is more rapid and efficient than in many other urban areas; these features are due to the interaction of urban emissions with industrial plumes which are associated with both continuous and episodic industrial emissions. To examine the chemistry of interactions of the industrial plumes with urban emissions, a Lagrangian Process Analysis tool was embedded in a gridded photochemical model. The tool successfully isolated the plume so that, within the Process Analysis volume, the dominant process affecting ozone concentrations was chemical production. The analyses showed that the chemistry of the industrial plumes is dependent on emissions encountered downwind, and the extent of radical availability in the morning. For one episode, morning stagnation over the industrial region resulted in enhanced morning radical formation due to aldehyde photolysis. The enhanced morning reactivity led to high ozone concentrations as the plume interacted with urban emissions later in the day. In contrast, during a second episode, the dominant factor influencing ozone concentrations was high volatile organic carbon (VOC)/NO _x ratios late in the afternoon, as the plume advected over wooded areas. For emission events, the main perturbation of the ozone formation chemistry occurred during the first few hours of the event due to enhanced production of free radicals from aldehyde photolysis, and more efficient utilization of free radicals due to increased reactive hydrocarbon concentrations.

Keywords: Photochemical grid model; Atmospheric chemistry; Ozone; Lagrangian Process Analysis

A comparison of CMAQ HONO predictions with observations from the Northeast Oxidant and Particle Study by Golam Sarwar; Shawn J. Roselle; Rohit Mathur; Wyat Appel; Robin L. Dennis; Bernhard Vogel (pp. 5760-5770).

Predictions of nitrous acid from the Community Multiscale Air Quality modeling system are compared with the measurements from the 2001 Northeast Oxidant and Particle Study. Four different sources of nitrous acid were considered in the study: gas-phase reactions, direct emissions, a heterogeneous reaction, and a surface photolysis reaction. When only gas-phase reactions were considered in the model, the diurnally averaged mean bias, the normalized mean bias, the root mean square error, and the normalized mean error of the model were −1.01ppbv, −98%, 1.05ppbv, and 98%, respectively. However, the diurnally averaged mean bias, normalized mean bias, the root mean square error, and the normalized mean error of the model improved to −0.42ppbv, −41%, 0.45ppbv, and 41%, respectively, when all sources were considered. Model results suggest that the heterogeneous reaction and the surface photolysis reaction are the most important sources of nitrous acid in the atmosphere, accounting for about 86% of the predicted nitrous acid. Emissions and the gas-phase reactions were relatively minor sources and accounted for only 14% of the predicted nitrous acid. Model predictions suggest that the heterogeneous reaction is the most significant source of nitrous acid at night, while the surface photolysis reaction is the most significant source during the day. The addition of these sources increased the diurnally averaged hydroxyl radicals and ozone by 10% and 1.4ppbv, respectively.

Keywords: Nitrous acid; Homogeneous reaction; Heterogeneous reaction; Surface photolysis reaction; Emissions

A study of organic nitrates formation in an urban plume using a Master Chemical Mechanism by Roberto Sommariva; Michael Trainer; J.A. Joost A. de Gouw; J.M. James M. Roberts; Carsten Warneke; Elliot Atlas; Frank Flocke; P.D. Paul D. Goldan; W.C. William C. Kuster; A.L. Aaron L. Swanson; F.C. Fred C. Fehsenfeld (pp. 5771-5786).

Secondary organic chemistry inside a typical urban plume in the North-East of the United States has been studied using a highly detailed chemical model, based upon the Master Chemical Mechanism (MCM). The model results have been qualitatively compared to measurements taken during three flights of the NOAA WP-3D aircraft, which sampled a plume from the New York City area during the NEAQS 2004 campaign. The model has been used to study the formation processes and photochemical evolution of alkyl nitrates.While long-chain (C5) alkyl nitrates are produced for 90% or more from the oxidation of a single parent alkane, short-chain(

Keywords: Urban plume; Photochemistry; MCM; Alkyl nitrates; NEAQS

An adjoint sensitivity analysis and 4D-Var data assimilation study of Texas air quality by Lin Zhang; E.M. Constantinescu; A. Sandu; Y. Tang; T. Chai; G.R. Carmichael; D. Byun; E. Olaguer (pp. 5787-5804).

In this paper, we discuss the theory of adjoint sensitivity analysis and the method of 4D-Var data assimilation in the context of the Sulfur Transport Eulerian Model (STEM). The STEM atmospheric Chemical Transport Model (CTM) is used to perform adjoint sensitivity analysis and data assimilation over the State of Texas.We first demonstrate the use of adjoint sensitivity analysis for a receptor located at ground level in the Dallas Fort Worth (DFW) area. Simulations are carried out for three 36-h intervals in July 2004. One set of simulations focuses on a passive tracer, and illustrates the influence of meteorological conditions. The other results show the areas of influence associated with DFW ground level ozone, i.e. the areas where changes in precursors (O₃, NO₂, and HCHO) have the maximum impact on DFW ozone.Next, we employ data assimilation to optimize initial conditions of chemical fields and ground level emissions. We optimize the initial conditions for two episodes on 1 and 16 July 2004. Data assimilation makes use of AirNow ground level observations (for both episodes) and SCHIAMACHY NO₂ and HCHO observations from ENVISAT (for the 16 July episode). The re-analyzed chemical fields show considerable improved agreement with AirNow observations for non-initial conditions.We also perform inverse modeling of ground level emissions using data assimilation under an additional smoothness constraint. The results indicate higher NO₂ emission levels than in the current emission inventory in the DFW area, and lower emission levels in eastern Texas. The formaldehyde emissions are found to be larger than reported in a localized area near the Gulf Coast, and about at the reported level elsewhere. While the results obtained with the current state-of-the-art tools can help guide tuning of emission inventories, better constraints on the inverse problem are needed to obtain more rigorous quantitative estimates.

Keywords: Chemical transport model; Adjoint sensitivity analysis; Data assimilation

Effects of using the CB05 vs. SAPRC99 vs. CB4 chemical mechanism on model predictions: Ozone and gas-phase photochemical precursor concentrations by D.J. Luecken; S. Phillips; G. Sarwar; C. Jang (pp. 5805-5820).

A three-dimensional air quality model is used to examine the magnitude and spatial distribution of differences in predictions among three chemical mechanisms that are used for regulatory and research modeling. The Carbon Bond mechanism CB05 is compared to an earlier version, CB4, to assess how much changes due to an update might potentially affect previous model conclusions on ozone concentrations and behavior. SAPRC-99 is compared to identify differences that might be expected between two more recent mechanisms, namely CB05 and SAPRC-99. The predicted ozone concentrations are similar for most of the United States, but statistically significant differences occur over many urban areas and the central US. SAPRC-99 predicts higher concentrations than CB05 on average, and both predict higher ozone than CB4. The difference in ozone predictions depends on location, the VOC/NO _x ratio and concentrations of precursor pollutants. We highlight where the largest differences occur, give some explanation for why they occur, and discuss the effect of differences on model applications.

Keywords: CB05; CB4; Chemical mechanism; Atmospheric chemistry

Comparison of the carbon bond and SAPRC photochemical mechanisms under conditions relevant to southeast Texas by Maedeh Faraji; Yosuke Kimura; Elena McDonald-Buller; David Allen (pp. 5821-5836).

Gridded, regional photochemical models use simplified photochemical reaction mechanisms, and two commonly used mechanisms are the SAPRC and the carbon bond (CB) mechanism. Versions of the mechanisms currently in use include SAPRC99 and the CB-IV mechanism. For most urban areas, the CB-IV and SAPRC mechanisms yield similar results, but for the modeling done of the summer of 2000 in southeast Texas, the SAPRC mechanism leads to concentrations of ozone that are 30–45ppb higher than with CB-IV. The differences are due to differences in both reaction rate/stoichiometry parameters and condensation methods in the mechanisms. Major reasons for the differences are: (1) the products of the reactions of aromatics with hydroxyl radical, which are more reactive in the SAPRC formulation, (2) the overall balancing of radical generation and termination reactions, which lead to higher radical concentrations in the SAPRC formulation, and (3) the production of higher aldehydes, which is greater in the SAPRC formulation. The differences between the mechanisms is particularly relevant for evaluating attainment with the National Ambient Air Quality Standard (NAAQS) for ozone concentrations averaged over 8h.

Keywords: Photochemical mechanisms; Atmospheric chemistry; Ozone; Air quality models

Failures and limitations of quantum chemistry for two key problems in the atmospheric chemistry of peroxy radicals by Theodore S. Dibble (pp. 5837-5848).

Quantum chemical calculations have obtained significant and useful information about chemical reactions relevant to atmospheric chemistry. This article reviews two problems for which it would seem that such calculations could readily provide a great deal of helpful information, but for which that potential has not been fully realized. The first is the formation of alkylnitrates (RONO₂^{) from the reaction of peroxy radicals (ROO}) with NO. Several studies of this process have been carried out, but most clearly yielded unphysical results due to methodological pitfalls. The one study which appears valid [Zhao, Y., Houk, K.N., Olson, L.P., 2004. Mechanisms of peroxynitrous acid and methyl peroxynitrite, ROONO (R=H, Me), rearrangements: a conformation-dependent homolytic dissociation. Journal of Physical Chemistry A 108, 5864–5871] suggests an indirect route between ROONO and RONO₂, and implies that statistical rate theory will be insufficient to understand the yields of RONO₂. The second problem discussed here is the competition between radical production and disproportionation in the self-reaction of organic peroxy radicals, for which only one quantum chemical study has found a plausible mechanism connecting reactants with products.

Keywords: Peroxy radical; Alkylnitrate; Disproportionation; Quantum chemistry

A theoretical investigation of nitrooxyalkyl peroxy radicals from NO₃-initiated oxidation of isoprene by Jun Zhao; Renyi Zhang (pp. 5849-5858).

Density functional theory and ab initio molecular orbital calculations have been employed to determine the structures and energetics of the nitrooxyalkyl peroxy radicals arising from the NO₃-initiated oxidation of isoprene. Geometry optimizations of the peroxy radicals are performed using density functional theory at the B3LYP/6-31G(d,p) level and single-point energies are computed using second-order Møller-Plesset perturbation theory and the coupled-cluster theory with single and double excitations including perturbative corrections for the triple excitations (CCSD(T)). The zero-point corrected energies of the nitrooxyalkyl peroxy radicals are 37–43kcalmol⁻¹ more stable than the separated NO₃, O₂ and isoprene reactants at the CCSD(T)/6-31G(d)+CF level. The rate constants for the addition of O₂ to the NO₃–isoprene adducts are calculated using the canonical variational transition state theory (CVTST), with an overall rate constant of 3.8×10⁻¹²cm³molecule⁻¹s⁻¹. The results provide the isomeric branching ratios between eight nitrooxyalkyl peroxy radicals.

Keywords: Isoprene; Nitrate radical; Ozone; Troposphere

Atmospheric chemistry of alkanes: Review and recent developments by Roger Atkinson; Janet Arey; Sara M. Aschmann (pp. 5859-5871).

Alkanes comprise a significant fraction of gasolines and of the non-methane volatile organic compounds present in ambient air in urban areas. In this work, we have extended our previous studies to investigate the formation of 1,4-hydroxycarbonyls (and other products) from the reactions of OH radicals with C₅–C₈ n-alkanes in the absence of NO, and have investigated the rates of conversion of the commercially available 4,5-dihydro-2-methylfuran (DHMF) into 5-hydroxy-2-pentanone under various conditions in a ∼6900l Teflon chamber in order to assess whether this conversion is a homogenous gas-phase or a heterogeneous wall process. We observe that the same 1,4-hydroxycarbonyls are formed in the absence of NO as in the presence of NO, but with formation yields approximately a factor of 10 lower as expected from the reaction mechanisms involved. The results of the experiments to investigate the conversion of 4,5-dihydro-2-methylfuran to 5-hydroxy-2-pentanone indicate that this conversion is not a homogeneous gas-phase process, but is a heterogeneous reaction at the chamber wall which is probably acid-catalyzed. The current understanding of the mechanisms and products of the atmospheric degradation reactions of alkanes is also presented.

Keywords: Alkanes; Atmospheric chemistry; Reaction products; Reaction mechanism

The water-soluble fraction of carbon, sulfur, and crustal elements in Asian aerosols and Asian soils by R.M. Duvall; B.J. Majestic; M.M. Shafer; P.Y. Chuang; B.R.T. Simoneit; J.J. Schauer (pp. 5872-5884).

We quantified the water-soluble species in 24h average TSP (Dunhuang and Gosan) and PM1.0 (Gosan only) samples associated with the Spring 2001 Asian Dust season. Samples were analyzed for water-soluble organic carbon, water-soluble sulfur, and water-soluble crustal elements, as well as their bulk chemical composition. Water-soluble organic carbon in Gosan accounted for 28–83% (average=63%) of the particle-phase TSP total organic carbon, and 1–69% (average=23%) of the particle-phase PM1.0 organic carbon. Water-soluble sulfur, primarily in the form of sulfate, accounted for 2–22% of the TSP mass in Gosan, and 0.9–11% of the TSP mass in Dunhuang. The absolute concentrations and the soluble fraction of crustal elements in TSP samples collected at Gosan were found to correlate with the air mass source region as determined by back-trajectory analysis. For example, elevated levels of water-soluble sodium, potassium, and calcium were observed during dust events. These observations are likely the result of differences in anthropogenic sources, mineralogical composition of resuspended crustal materials, and atmospheric processing of the aerosols. Experiments were conducted using Asian soil samples to study the impact of acidification by nitric acid vapor on the solubility of crustal elements present in Asian desert and non-desert dusts. These experiments demonstrated that gaseous nitric acid attack leads to significant increases (>100% increase) in water-soluble calcium, magnesium, aluminum, manganese, and iron, while little or no increases in water-soluble sodium and potassium were observed in the soils.

Keywords: ACE-Asia; China Loess; Atmospheric aerosols; Soluble metals; Kosa; Acidification

Dispersion and transfer of passive scalars in and above street canyons—Large-eddy simulations by X.-M. X.-M. Cai; J.F. Barlow; S.E. Belcher (pp. 5885-5895).

This study applies a large-eddy simulation (LES) model to a street canyon in order to derive the fields of wind, turbulence, scalar concentration, concentration fluctuations, and scalar flux across the roof level. The wind blows at a right angle to the canyon axis and the emission is specified either as a line source with a constant emission rate along the street or as an area source with a constant concentration on the street and/or building surface, for which a wall function is introduced. Results have been compared with wind tunnel experiments. For the cases with a line source, the results of 2D spatial distribution of mean concentration and standard deviation demonstrate the promising capability of the LES model. Quantitative comparisons of mean concentration on the two walls provide convincing evidence that the LES model captures the main features of transport and dispersion processes in a street canyon. For the cases with an area source, simulations have been conducted for different canyon aspect ratios of H/W=1/3, 1/2, 2/3, 1/1, 3/2, and 2/1, where H is building height and W is street width. LES results of spatial and temporal-mean scalar-flux-at–the-roof-level (SFRL) as a function of H/ W agree fairly well with those of wind tunnel data. Profiles of temporal-mean SFRL across the canyon are indicative of flow regimes recommended by Oke [1988. Street design and urban canopy layer climate. Energy and Buildings 11, 103–113.]. Results of the cases with double facet source provide evidence that the LES model with an appropriate scalar boundary condition is promising for further LES studies of heat transfer inside a street canyon.

Keywords: Street canyon turbulence; Dispersion of passive scalars; Large-eddy simulation; Transfer velocity; Exchange coefficient

The properties of dust aerosol and reducing tendency of the dust storms in northwest China by Peijian Fu; Jianping Huang; Chunwei Li; Sharon Zhong (pp. 5896-5904).

Since 2004 the dust aerosol observation has been carried out in Minqin Weather Station, located on the boarder of the source region of Asian dust storms (ADS). We obtained current data throughout the consecutive 3 years of observation, from January 2004 to December 2006, with advanced equipment (a particulate mass concentration of PM-10 monitor, a FD12 visibility meter, an integrating nephelometer and a robotic photometer) and also the regular data on dust storms collected in the past 50 years from 168 weather stations of northwest China to analyze the averaged mass concentration of dust aerosol and its optical properties for both dust storms and clear days. We defined the intensity index of dust storms (IIDS) to describe the intensity of a dust storm and TIIDS to describe for yearly intensity of dust storms. The temporal and spatial distribution of the indices show a decreasing trend of the intensity of dust storms in the past 20 years. This decreasing trend was correlated with the temperature anomalies in the same period in northwest China. The conclusion is that rising temperature depresses intensity of the cold flows from North Mongolia, which often trigger dust storms in Gobi Desert.

Keywords: Asian dust storms; Northwest China; Mass concentration and optical properties of the dust aerosol; Climate change

Predicting personal exposure of Windsor, Ontario residents to volatile organic compounds using indoor measurements and survey data by Corinne Stocco; Morgan MacNeill; Daniel Wang; Xiaohong Xu; Mireille Guay; Jeff Brook; Amanda J. Wheeler (pp. 5905-5912).

As part of a multi-year personal exposure monitoring campaign, we collected personal, indoor, and outdoor levels of 188 volatile organic compounds (VOCs). In 2005, data were obtained for 48 non-smoking adults from Windsor, Ontario in order to assess their exposure to VOCs based on their daily routines and characteristics of their homes. During the 8-week winter and summer sampling sessions, five repeated 24-h measurements were obtained for each home. This paper focuses on the analysis of 18 VOCs: 11 have been declared toxic as defined under the Canadian Environmental Protection Act, [1999. Statutes of Canada. Act assented to September 14, 1999. Ottawa: Queen's Printer. Available at Canada Gazette (Part III) 22(3): (Chapter 33).http://canadagazette.gc.ca/partIII/1999/g3-02203.pdf], and seven are commonly found in household and personal care products. Results of mixed effects models indicate that personal exposure to these VOCs can be largely predicted by indoor concentrations, with models including indoor concentrations found to have an r² value for the fixed effects ranging from 58.4% to 87.2% for the CEPA toxic VOCs and from 41.7% to 90.1% for the commonly found VOCs. Given that people spend the majority of their time inside their home, characteristics of the home such as air exchange rates, type of garage, and type of stove have a greater potential to impact personal exposures.

Keywords: Personal exposure; Volatile organic compounds; Indoor air; Canadian toxic substances; Mixed models

Wet and dry deposition fluxes of trace elements in Tokyo Bay by Masahiro Sakata; Yukinori Tani; Tomoharu Takagi (pp. 5913-5922).

The annual wet and dry deposition fluxes of As, Cd, Cr, Cu, Hg, Mn, Ni, Pb and V were measured on the basis of data obtained from 2 years of observation (December 2003–November 2005) at three sites in the Tokyo Bay area, which is highly affected by anthropogenic sources. Moreover, the concentrations of these trace elements in the air and bay sediments were determined concurrently. Using these data, we evaluated the factors affecting wet and dry deposition fluxes and the contribution of the atmospheric input of each element to the supply of trace elements to the bay. The order of the average ratios of dry/wet deposition fluxes at three sites was Cr, 6.9>As, 5.9>Mn, 4.7>V, 4.5>Cu, 4.4>Ni, 3.7>Cd, 2.9>Pb, 2.0>Hg, 1.0. Thus, the atmospheric deposition of the trace elements except Hg in Tokyo Bay is predominantly dry deposition. For such trace elements, the wet and dry deposition fluxes within the bay were higher than those inland. Except in the case of some elements, the differences in wet and dry deposition fluxes between these sites were primarily due to the differences in scavenging ratio (=concentration in precipitation/concentration in air) and dry deposition velocity (=deposition flux/concentration in air), respectively, rather than in atmospheric concentration. Also, a significant correlation ( r²=0.76, P<0.001) was observed between the scavenging ratios and dry deposition velocities of the trace elements. These suggest that large particles from sources in the coastal regions contribute largely to the wet and dry depositions of trace elements to the bay. The contributions of the atmospheric input of the trace elements to the bay ranged from 5% to 26%. Atmospheric deposition seems to have no significant role in the supply of trace elements to Tokyo Bay, suggesting the importance of river and effluent discharges.

Keywords: Atmospheric input; Scavenging ratio; Dry deposition velocity; Anthropogenic sources; Water surface sampler

A method to quantify organic functional groups and inorganic compounds in ambient aerosols using attenuated total reflectance FTIR spectroscopy and multivariate chemometric techniques by Charity Coury; Ann M. Dillner (pp. 5923-5932).

An attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopic technique and a multivariate calibration method were developed to quantify ambient aerosol organic functional groups and inorganic compounds. These methods were applied to size-resolved particulate matter samples collected in winter and summer of 2004 at three sites: a downtown Phoenix, Arizona location, a rural site near Phoenix, and an urban fringe site between the urban and rural site. Ten organic compound classes, including four classes which contain a carbonyl functional group, and three inorganic species were identified in the ambient samples. A partial least squares calibration was developed and applied to the ambient spectra, and 13 functional groups related to organic compounds (aliphatic and aromatic CH, methylene, methyl, alkene, aldehydes/ketones, carboxylic acids, esters/lactones, acid anhydrides, carbohydrate hydroxyl and ethers, amino acids, and amines) as well as ammonium sulfate and ammonium nitrate were quantified. Comparison of the sum of the mass measured by the ATR-FTIR technique and gravimetric mass indicates that this method can quantify nearly all of the aerosol mass on sub-micrometer size-segregated samples. Analysis of sample results shows that differences in organic functional group and inorganic compound concentrations at the three sampling sites can be measured with these methods. Future work will analyze the quantified data from these three sites in detail.

Keywords: Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR); PM; ₁; Size-segregated particulate matter; MOUDI; Partial least squares (PLS); Organic carbon

Determining the spatial scale for analysing mobile measurements of air pollution by Christy Lightowlers; Trisalyn Nelson; Eleanor Setton; C. Peter Keller (pp. 5933-5937).

When dealing with spatial data or modelling in a geographical context, identifying an appropriate scale for analysis is a critical precursor; however, it is difficult to determine due to limited availability of data at an adequate spatial resolution. This paper describes a mobile monitoring method to collect spatially representative measurements of woodsmoke particulates in support of spatial modelling. A geostatistical technique is described to characterize the spatial scale of woodsmoke particulates collected for 19 evenings over two heating seasons in Victoria, British Columbia, Canada. Semivariograms were applied to 20 data sets (19 evenings and a combined data set) to characterize the appropriate spatial-analysis scale as defined by the semivariogram range, the maximum distance of spatial dependence. Typically, the semivariogram range occurred at 2673m. This method can be used to identify an optimal sampling interval for woodsmoke data collection, to define the neighbourhood size for performing spatial analyses, and to produce robust model variables and parameters by characterizing the degree of spatial autocorrelation in the data set.

Keywords: Spatial autocorrelation; Semivariogram; Air pollution; Spatial scale; Mobile monitoring

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Atmospheric Environment (v.42, #23)