Atmospheric Environment (v.36, #32)

Recent decline of methyl bromide in the troposphere by Y Yokouchi; D Toom-Sauntry; K Yazawa; T Inagaki; T Tamaru (4985-4989).
Atmospheric methyl bromide (CH3Br) measured at a remote ground station in the Arctic (mid-1996 to early 2002) and in the free troposphere at mid-latitude (early 1999 to early 2002) showed a steady annual average decrease of 4–6%. The trend was consistent with a simulation of the response to the phase-out schedule of anthropogenic emissions under the Montreal Protocol and its amendments, suggesting that a decrease in CH3Br abundance in the Northern Hemisphere of as much as 40% from the level of the early 1990s would be possible by the completion date of the program.

Sensitivity of urban ozone formation to chlorine emission estimates by Sunghye Chang; Elena McDonald-Buller; Yosuke Kimura; Greg Yarwood; James Neece; Matthew Russell; Paul Tanaka; David Allen (4991-5003).
Recent evidence has demonstrated that chlorine radical chemistry can enhance tropospheric volatile organic compound oxidation and has the potential to enhance ozone formation in urban areas. In order to investigate the regional impacts of chlorine chemistry in southeastern Texas, preliminary estimates of atmospheric releases of atomic chlorine precursors from industrial point sources, cooling towers, water and wastewater treatment, swimming pools, tap water, reactions of chlorides in sea salt aerosols, and reactions of chlorinated organics were developed. To assess the potential implications of these estimated emissions on urban ozone formation, a series of photochemical modeling studies was conducted to examine the spatial and temporal sensitivity of ozone and a unique marker species for chlorine chemistry, 1-Chloro-3-methyl-3-butene-2-one (CMBO), to molecular chlorine emissions estimates. Based on current estimates of molecular chlorine emissions in southeastern Texas, chlorine chemistry has the potential to enhance ozone mixing ratios by up to 11–16 ppbv. Impacts varied temporally, with emissions from cooling towers primarily responsible for a morning enhancement in ozone mixing ratios and emissions from residential swimming pools for an afternoon enhancement. Maximum enhancement in CMBO mixing ratios ranged from 59 to 69 pptv.
Keywords: Chlorine; Ozone; Photochemical modeling; Urban air quality;

A pesticide emission model (PEM) Part I: model development by M.T. Scholtz; E. Voldner; A.C. McMillan; B.J. Van Heyst (5005-5013).
The application of pesticides to cultivated soil and crops is a major source of pesticides that are found in the atmosphere and which are transported and deposited to land and water surfaces over distances that range from local to global scales. In this first part of a two-part paper, a pesticide emission model (PEM) is proposed for estimating the exchange with the atmosphere of pesticides applied to soils and crops. The basis of PEM is a one-dimensional numerical solution of the dynamic equations describing the advection and diffusion of heat, moisture and pesticide within the soil column and exchange with the atmosphere through heat transfer, evapotranspiration and volatilization. The soil model is coupled with an atmospheric surface layer and a simple canopy model that includes: the interception of sprayed pesticide by the crop foliage; the partitioning of pesticide within a wet or dry canopy; and, the volatilization of pesticide to the atmosphere or the wash-off to the soil by precipitation. The finite-element technique used for solving the model equations is mass conservative and multi-year periods of simulation are possible while maintaining a proper mass balance of pesticide in the soil. The model is solved using 1200 s time-steps and 49 variably spaced levels in the soil to a depth of 2 m, with the highest vertical resolution (0.002 m spacing) near the soil surface. Similarity theory is used to parameterize the fluxes of heat, moisture and pesticide through the atmospheric surface layer with hourly meteorology being provided by either climate station observations or a meteorological model. In the second part to this paper, the results of an evaluation of PEM are reported.
Keywords: Finite element; Volatilization; Soil heat moisture solute; Canopy model; Atmospheric surface layer; Hourly emissions;

A pesticide emission model (PEM) Part II: model evaluation by M.T Scholtz; E Voldner; B.J Van Heyst; A.C McMillan; E Pattey (5015-5024).
In the first part of the paper, the development of a numerical pesticide emission model (PEM) is described for predicting the volatilization of pesticides applied to agricultural soils and crops through soil incorporation, surface spraying, or in the furrow at the time of planting. In this paper the results of three steps toward the evaluation of PEM are reported. The evaluation involves: (i) verifying the numerical algorithms and computer code through comparison of PEM simulations with an available analytical solution of the advection/diffusion equation for semi-volatile solutes in soil; (ii) comparing hourly heat, moisture and emission fluxes of trifluralin and triallate modeled by PEM with fluxes measured using the relaxed eddy-accumulation technique; and (iii) comparison of the PEM predictions of persistence half-life for 29 pesticides with the ranges of persistence found in the literature. The overall conclusion from this limited evaluation study is that PEM is a useful model for estimating the volatilization rates of pesticides from agricultural soils and crops. The lack of reliable estimates of chemical and photochemical degradation rates of pesticide on foliage, however, introduces large uncertainties in the estimates from any model of the volatilization of pesticide that impacts the canopy.
Keywords: Soil; Canopy; Heat moisture solute fluxes; Persistence; Hourly emissions; Relaxed eddy-accumulation; Triallate; Trifluralin;

Mathematical model for simulation of VOC emissions and concentrations in buildings by Tongbao Cheng; Yi Jiang; Ying Xu; Yinping Zhang (5025-5030).
A simple mathematical model is proposed to account for emissions of volatile organic compounds (VOC) from homogeneous materials. The model considers multi-emission sources and sinks. An important feature of the model is that all the parameters have clear physical meaning and are confirmed by previous data from experimental chambers or by direct measurement. The model can predict the emission rate of VOC from homogeneous materials into the air. It also can determine the relationship of the emission rates of VOC sources, the bulk air concentration, volumetric flow rate, and the sorption rates of VOC sinks.
Keywords: Mathematical model; Volatile organic compounds; Homogeneous materials; Multi-emission sources and sinks;

Correct prediction of the initial rise of a plume due to momentum and buoyancy effects is an important factor in dispersion modelling. A new plume rise scheme, based upon conservation equations of mass, momentum and heat, for the Lagrangian model, NAME, is described. The conservation equations are consistent with the well-known analytical plume rise formulae for both momentum- and buoyancy-dominated plumes. The performance of the new scheme is assessed against data from the Kincaid field experiment. Results show that the new scheme adds value to the model and significantly outperforms the previous plume rise scheme. Using data from assessments of atmospheric dispersion models using the Kincaid data set, it is shown that NAME is comparable to other models over short ranges.
Keywords: Dispersion modelling; Buoyancy; Conservation equations; Model evaluation; Turbulence;

Aerosol size distributions and visibility estimates during the Big Bend regional aerosol and visibility observational (BRAVO) study by J.L Hand; S.M Kreidenweis; D Eli Sherman; J.L Collett; S.V Hering; D.E Day; W.C Malm (5043-5055).
The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was conducted in Big Bend National Park in 1999. The park is located in a remote region of southwest Texas but has some of the poorest visibility of any Class 1 monitored area in the western US. The park is frequently influenced by air masses carrying emissions from Mexico and eastern Texas. Continuous physical, optical and chemical aerosol measurements were performed in an effort to understand the sources of and contributions to haze in the park. As part of this characterization, dry aerosol size distributions were measured over the size range of 0.05<D p<20 μm. Three instruments with different measurement techniques were used to cover this range. Complete size distributions were obtained from all of the instruments in terms of a common measure of geometric size using a new technique. Size parameters for accumulation and coarse particle modes were computed and demonstrate periods when coarse mode volume concentrations were significant, especially during suspected Saharan dust episodes in July and August. Study average (and one standard deviation) geometric volume mean diameters for the accumulation and coarse particle modes were 0.26±0.04 and 3.4±0.8 μm, respectively. Dry light scattering coefficients (b sp) were computed using measured size distributions and demonstrated periods when contributions to b sp from coarse particles were significant. The study average computed b sp was 0.026±0.016 km−1. Computed dry b sp values were highly correlated with measured values (r 2=0.97). Real-time sulfate measurements were correlated with accumulation mode volume concentrations (r 2=0.89) and computed dry light scattering coefficients (r 2=0.86), suggesting sulfate aerosols were the dominant contributor to visibility degradation in the park.
Keywords: Particle size distributions; Aerosol monitoring; Aerosol optical properties; Remote continental aerosol particles; Visibility;

Emission characteristics of particulate matter and heavy metals from small incinerators and boilers by Jong-Ik Yoo; Ki-Heon Kim; Ha-Na Jang; Yong-Chil Seo; Kwang-Seol Seok; Ji-Hyung Hong; Min Jang (5057-5066).
The characteristics of particulate matter (PM) emission such as the estimation of emission factors, size distributions and of heavy metal emission from small-size incinerators and boilers have been investigated. In PM-10 emission, a fine mode was found in the formation of sub-micron PM by growth of nucleated aerosol of metal vapor, having a bimodal particle size distribution in overall size range. The emission ratios of PM-10 to TPM (total PM) from boilers and incinerators ranged from 29% to 62% and 10% to 84%, respectively, which resulted in more and larger sized PM emission due to poorer combustion from solid waste incinerators than boilers. The targeted metals were copper, cadmium, manganese, chromium, magnesium, lead, zinc and copper, and their contents in bottom ash, fly ash and dust (PM) were compared. More volatile metals such as cadmium, lead and zinc showed higher enrichment in PM emitted through stack than bottom ashes. Cadmium, copper, lead and zinc on the fine PM under 2.5 μm accounted for approximately 90% of the total mass of each metal in PM-10. The effects of chlorine concentration and temperature on such metals emission were also observed due to their volatility changes.
Keywords: Particle size distribution; PM10; PM2.5; Heavy metals; Incinerator; Emission factor;

Comparisons of model simulations with observations of mean flow and turbulence within simple obstacle arrays by S.R Hanna; S Tehranian; B Carissimo; R.W Macdonald; R Lohner (5067-5079).
A three-dimensional numerical code with unstructured tetrahedral grids, the finite element flow solver (FEFLO), was used to simulate the mean flow and the turbulence within obstacle array configurations consisting of simple cubical elements. Model simulations were compared with observations from a hydraulic water flume at the University of Waterloo. FEFLO was run in large eddy simulation mode, using the Smagorinsky closure model, to resolve the larger scales of the flow field. There were four experiment test cases consisting of square and staggered arrays of cubical obstacles with separations of 1.5 and 0.5 obstacle heights. The mean velocity profile for the incoming neutral boundary layer was approximated by a power law, and the turbulent fluctuations in the approach flow were generated using a Monte Carlo model. The numerical simulations were able to capture, within 40% on average, the general characteristics of the mean flow and the turbulence, such as the strong mean wind shears and the maximum turbulence at the elevation of the obstacles and the nearly constant mean wind and the 50% reduction in the turbulent velocity within the obstacle canopy. As expected, the mean wind speeds were significantly decreased (by about a factor of two or three) in the array with closer obstacle packing. It was found that, a “street canyon” effect was more obvious for the square arrays, with higher flow speeds in between the obstacles, than for the staggered arrays.
Keywords: Urban canopy; Wind flow in urban area; Turbulence in urban area; Building effects;

3D-air quality model evaluation using the Lidar technique by O Duclaux; E Frejafon; H Schmidt; A Thomasson; D Mondelain; J Yu; C Guillaumond; C Puel; F Savoie; P Ritter; J.P Boch; J.P Wolf (5081-5095).
This paper reports on a model investigation of a particular episode of tropospheric ozone formation in the city of Lyon, France. A large-scale measurement campaign involving ground-based analyzers, sampling, Sodars and Lidars has been used to validate the model results. Based on validated meteorological data and primary pollutant concentrations, the numerical model has been run to obtain 3D ozone concentration profiles during the whole campaign (22–25 June 1999). The results are compared to the ozone Lidar vertical profiles. Good agreement between Lidar data and model predictions is first obtained on 22 June (but not on the following days). On 23 and 24 June, ozone concentrations are significantly underestimated by the model. The ozone Lidar measurements allowed identifying large import processes from high altitudes that explain the difference. In a second model simulation, these imports are taken into account as new boundary conditions. This yielded good agreement between the experimental data and the predicted ozone concentrations over the whole period. The evidence of high altitude ozone intrusion is confirmed by back-trajectories calculations.
Keywords: LIDAR; Photochemistry; Model evaluation; Ozone; Aerosol;

A new approach for the fractionation of water-soluble organic carbon in atmospheric aerosols and cloud drops by Antonella Andracchio; Catia Cavicchi; Domenica Tonelli; Sergio Zappoli (5097-5107).
A novel approach is described for the fractionation of water-soluble organic carbon (WSOC) in atmospheric aerosols and cloud drops. The method is based on the preliminary adsorption of the sample, acidified at pH 2, on a polymeric styrene-divinylbenzene resin (XAD-2) and subsequent elution with a series of solvents, which leads to the fractionation of the sample into three classes of compounds. The method was set up using synthetic mixtures of organic compounds and then applied to selected samples of atmospheric aerosols and cloud drops. All samples and collected fractions were analysed using size exclusion chromatography (SEC). This method proved particularly useful both in providing information on the organic content of the samples and for the characterisation of the macromolecular compounds (MMCs) in the samples. Synthetic samples were prepared using humic, fulvic and tannic acid to simulate naturally occurring MMCs. In the first fraction, eluted with HCl, only the most soluble organic compounds (oxalic acid, formic acid and acetic acid) were collected. In the second fraction, eluted with methanol, the major part of the organic material was collected together with the more hydrophilic constituents of the humic substances. In the third fraction, it was possible to separately recover the more hydrophobic component of the humic substances. A large number of atmospheric samples (fog, aerosol, cloud) were then analysed using SEC. Most of these samples evidenced a noteworthy chromatogram at 254 nm. Moreover, the chromatographic area evidenced a clear linear correlation with the total organic carbon (TOC) values. The fractionation method on XAD-2 was finally applied to selected atmospheric samples, yielding three classes of organic compounds. In each sample, a non-negligible amount of compounds with dimensional and chemical properties similar to humic substances were collected in the third fraction. The carbon content in this latter fraction was estimated both by TOC and by means of the correlation between TOC and SEC area.
Keywords: Carbonaceous aerosol; Humic-like substances; Organic carbon; Size exclusion chromatography; XAD resin;

The concentration of polychlorinated dibenzo-p-dioxins and furans (PCDD/Fs) and particle mass distribution were measured in size-segregated airborne particulate matter using a six-stage high-volume cascade impactor in urban and suburban areas in the summer and fall. The distribution of particle concentration and PCDD/Fs concentration with respect to particle size was presented by using various methods. More than 50% of particles were in the <1.4 μm size class and the particles showed the bimodal size-distribution pattern characteristic of urban particles. More than 60% of the PCDD/Fs were associated with particles of d ae<0.41 μm, and 90% of the PCDD/Fs were found on the particles d ae<2.1 μm in both sites. The distribution of PCDD/Fs varied over the particle size range 1.4–2.1 μm, regardless of sampling site. The less-chlorinated dioxin/furans (tetra-, penta-CDD/Fs) were mainly distributed among large particles (d ae>2.1 μm), whereas high-chlorinated dioxin/furans (hexa-, hepta-, octa-CDD/Fs) were mainly found among fine particles (d ae<2.1 μm). The fraction of less-chlorinated dioxin/furans increased with increasing particle size; however, the opposite was true for high-chlorinated dioxin/furans. These results are similar to the findings for polyaromatic hydrocarbons in previous reports.
Keywords: PCDD/Fs; Particle size; Distribution;

The surface ocean—lower atmosphere study (SOLAS) by Robert A Duce; Peter S Liss (5119-5120).