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Atmospheric Environment (v.44, #14)
Evaluation of the SAPRC-07 mechanism against CSIRO smog chamber data by Merched Azzi; Stephen J. White; Dennys E. Angove; Ian M. Jamie; Ajith Kaduewela (pp. 1707-1713).
The updated SAPRC-07 mechanism was evaluated against data from experiments performed in the CSIRO smog chamber. The mechanism predictions have been compared to experimental results as well as predictions by SAPRC-99.Experiments were performed using either toluene or m-xylene in the presence of NOx at sub-0.1 ppmv concentrations. For the majority of m-xylene experiments, the modelled Δ(O3–NO) concentration was within 20% of observed values for both SAPRC mechanisms. However during the oxidation of toluene the production of radicals was poorly predicted, with final Δ(O3–NO) concentration under-predicted by up to 60%. The predictions of major oxidants from isoprene oxidation were in good agreement with observed values. For the NOx-limited conditions however, the ozone concentration predicted by both mechanisms were under-predicted by approximately 20% in the five experiments tested.The performance of the SAPRC-07 mechanism was also evaluated against twelve evaporated fuel experiments. Two types of evaporative mode experiments were performed: headspace evaporated fuel and wholly evaporated fuel. The major difference was a significantly higher concentration of aromatic hydrocarbons and larger alkane products in wholly evaporated fuels. For headspace evaporated fuel experiments both SAPRC mechanisms were in good agreement with experimental results. For wholly evaporated experiments the average Δ(O3–NO) model error was −25% with SAPRC-07 compared to less than −5% for SAPRC-99. Updates to the photolysis data for dicarbonyls, the light source used and the experimental conditions under which these experiments were performed are possible causes for the discrepancy between SAPRC-99 and -07 predictions for wholly evaporated experiments.
Keywords: SAPRC-07; Smog chamber; Ozone; Evaporated fuel; Chemical mechanism
Comparison of calculation models for wind-driven rain deposition on building facades by B. Blocken; G. Dezsö; J. van Beeck; J. Carmeliet (pp. 1714-1725).
Wind-driven rain (WDR) is an important factor in the dry and wet deposition of atmospheric pollutants on building facades. In the past, different calculation models for WDR deposition on building facades have been developed and progressively improved. Today, the models that are most advanced and most frequently used are the semi-empirical model in the ISO Standard for WDR assessment (ISO), the semi-empirical model by Straube and Burnett (SB) and the CFD model by Choi. This paper compares the three models by applying them to four idealised buildings under steady-state conditions of wind and rain. In each case, the reference wind direction is perpendicular to the windward facade. For the CFD model, validation of wind-flow patterns and WDR deposition fluxes was performed in earlier studies. The CFD results are therefore considered as the reference case and the performance of the two semi-empirical models is evaluated by comparison with the CFD results based on two criteria: (1) ability to model the wind-blocking effect on the WDR coefficient; and (2) ability to model the variation of the WDR coefficient with horizontal rainfall intensity Rh. It is shown that both the ISO and SB model, as opposed to the CFD model, cannot reproduce the wind-blocking effect. The ISO model incorrectly provides WDR coefficients that are independent of Rh, while the SB model shows a dependency that is opposite to that by CFD. In addition, the SB model can provide very large overestimations of the WDR deposition fluxes at the top and side edges of buildings (up to more than a factor 5). The capabilities and deficiencies of the ISO and SB model, as identified in this paper, should be considered when applying these models for WDR deposition calculations. The results in this paper will be used for improvement and further development of these models.
Keywords: Driving rain; Atmospheric deposition; Computational Fluid Dynamics; Numerical simulation; Model intercomparison
Impacts of boundary layer mixing on pollutant vertical profiles in the lower troposphere: Implications to satellite remote sensing by Jin-Tai Lin; Michael B. McElroy (pp. 1726-1739).
Mixing in the planetary boundary layer (PBL) affects vertical distributions of air tracers in the lower troposphere. An accurate representation of PBL mixing is critical for chemical-transport models (CTMs) for applications sensitive to simulations of the vertical profiles of tracers. The full mixing assumption in the widely used global CTM GEOS-Chem has recently been supplemented with a non-local PBL scheme. This study analyzes the impact of the non-local scheme on model representation of PBL mixing, consequences for simulations of vertical profiles of air tracers and surface air pollution, and implications for model applications to the interpretation of data retrieved from satellite remote sensing. The non-local scheme significantly improves simulations of the vertical distributions for NO2 and O3, as evaluated using aircraft measurements in summer 2004. It also reduces model biases over the U.S. by more than 10 ppb for surface ozone concentrations at night and by 2–5 ppb for peak ozone in the afternoon, as evaluated using ground observations. The application to inverse modeling of anthropogenic NOx emissions for East China using satellite retrievals of NO2 from OMI and GOME-2 suggests that the full mixing assumption results in 3–14% differences in top–down emission budgets as compared to the non-local scheme. The top–down estimate combining the non-local scheme and the Lin et al. inverse modeling approach suggests a magnitude of 6.6 TgN yr−1 for emissions of NOx over East China in July 2008 and 8.0 TgN yr−1 for January 2009, with the magnitude and seasonality in good agreement with bottom–up estimates.
Keywords: PBL mixing; Non-local scheme; Full-mixing; Satellite remote sensing
Near-road air pollutant concentrations of CO and PM2.5: A comparison of MOBILE6.2/CALINE4 and generalized additive models by Kai Zhang; Stuart Batterman (pp. 1740-1748).
The contribution of vehicular traffic to air pollutant concentrations is often difficult to establish. This paper utilizes both time-series and simulation models to estimate vehicle contributions to pollutant levels near roadways. The time-series model used generalized additive models (GAMs) and fitted pollutant observations to traffic counts and meteorological variables. A one year period (2004) was analyzed on a seasonal basis using hourly measurements of carbon monoxide (CO) and particulate matter less than 2.5 μm in diameter (PM2.5) monitored near a major highway in Detroit, Michigan, along with hourly traffic counts and local meteorological data. Traffic counts showed statistically significant and approximately linear relationships with CO concentrations in fall, and piecewise linear relationships in spring, summer and winter. The same period was simulated using emission and dispersion models (Motor Vehicle Emissions Factor Model/MOBILE6.2; California Line Source Dispersion Model/CALINE4). CO emissions derived from the GAM were similar, on average, to those estimated by MOBILE6.2. The same analyses for PM2.5 showed that GAM emission estimates were much higher (by 4–5 times) than the dispersion model results, and that the traffic-PM2.5 relationship varied seasonally. This analysis suggests that the simulation model performed reasonably well for CO, but it significantly underestimated PM2.5 concentrations, a likely result of underestimating PM2.5 emission factors. Comparisons between statistical and simulation models can help identify model deficiencies and improve estimates of vehicle emissions and near-road air quality.
Keywords: Traffic counts; CO; PM; 2.5; MOBILE6.2; CALINE4; Generalized additive model; Dispersion modeling
Kinetic study of the OH reaction with some hydrochloroethers under simulated atmospheric conditions by Pablo R. Dalmasso; Raúl A. Taccone; Jorge D. Nieto; Pablo M. Cometto; Silvia I. Lane (pp. 1749-1753).
Using the relative rate technique, rate constants for the gas-phase reactions of hydroxyl radicals with 2-chloroethyl methyl ether ( k1), 2-chloroethyl ethyl ether ( k2) and bis(2-chloroethyl) ether ( k3) have been measured. Experiments were carried out at (298 ± 2) K and atmospheric pressure using synthetic air as bath gas. Using n-pentane and n-heptane as reference compounds, the following rate constants were derived: k1 = (5.2 ± 1.2) × 10−12, k2 = (8.3 ± 1.9) × 10−12 and k3 = (7.6 ± 1.9) × 10−12, in units of cm3 molecule−1 s−1. This is the first experimental determination of k2 and k3 under atmospheric pressure. The rate constants obtained are compared with previous literature data and the observed trends in the relative rates of reaction of hydroxyl radicals with the ethers studied are discussed. The atmospheric implications of the results are considered in terms of lifetimes and fates of the hydrochloroethers studied.
Keywords: Hydroxyl radicals; Hydrochloroethers; Rate constants; Tropospheric chemistry
Kinetics of the reactions of soot surface-bound polycyclic aromatic hydrocarbons with the OH radicals by Yuri Bedjanian; Mai Lan Nguyen; Georges Le Bras (pp. 1754-1760).
The kinetics of the heterogeneous reaction of OH radicals with 15 polycyclic aromatic hydrocarbons (PAHs) present in laboratory generated simulated kerosene combustion soot was studied at T = 290 K in a low pressure discharge-flow reactor combined with an electron-impact mass spectrometer. The kinetics of soot-bound PAH consumption in reaction with OH were monitored using off-line HPLC measurements of their concentrations in soot samples as a function of time of exposure to OH. Concentration of OH radicals in the gas phase was measured by mass spectrometry. The first-order rate constants measured for the individual PAH at T = 290 K ranged from 0.02 to 0.04 s−1 and were found to be independent of the OH concentration ([OH] = (0.34–2.5) × 1012 molecule cm−3) and of the molecular structure of the PAH. In addition, the uptake coefficient of OH on soot surface and the diffusion coefficient of OH in He were measured to be 0.19 ± 0.03 (calculated with geometric surface area) and (615 ± 80) Torr cm2 s−1, respectively. Comparison of the results on the PAH + OH reaction to those from previous studies carried out on different carbonaceous substrates, indicates probable dependence of the heterogeneous reactivity of PAH toward OH on the substrate nature. Rapid reaction with OH can be an important potential pathway of the atmospheric degradation of non-volatile PAH present mainly in the particulate phase in the atmosphere.
Keywords: Polycyclic aromatic hydrocarbons; Soot; OH; Heterogeneous reactivity; Uptake coefficient
Identification of weather patterns impacting 24-h average fine particulate matter pollution by Scott Beaver; Ahmet Palazoglu; Angadh Singh; Su-Tzai Soong; Saffet Tanrikulu (pp. 1761-1771).
Methods are presented to extract intra-seasonal meteorological patterns at three scales to explain 24-h fine particulate matter (PM2.5) pollution events: evolving large-scale meteorological scenarios, synoptic regimes driving diurnal variability near the surface, and localized meteorological triggers. The methods were applied to understand how winter weather conditions impacted PM2.5 around the San Francisco Bay Area (SFBA). Analyzing data across 12 winters (November–March) ensured robust characterization of the SFBA conditions. SFBA 24-h PM2.5 exceedances (35μg m−3) required several simultaneous characteristics: a ridge of aloft high pressure moving over SFBA, providing weak surface pressure gradients over Central California; persistent easterly flows through SFBA extending vertically to around the 925-hPa pressure level; orographically channeled winds resulting from stability; enhanced nocturnal cooling under clear-sky conditions providing for enhanced drainage flows off the Central California slopes; and at least two consecutive days of these conditions.
Keywords: Air pollution; Meteorological cluster analysis; Vertical dispersion; Conceptual model development; San Francisco Bay Area of California
Use of Calluna vulgaris to detect signals of nitrogen deposition across an urban–rural gradient by S.A. Power; C.M. Collins (pp. 1772-1780).
Densely populated cities can experience high concentrations of traffic-derived pollutants, with oxides of nitrogen and ammonia contributing significantly to the overall nitrogen (N) budget of urban ecosystems. This study investigated changes in the biochemistry of in situ Calluna vulgaris plants to detect signals of N deposition across an urban–rural gradient from central London to rural Surrey, UK. Foliar N concentrations and δ15N signatures were higher, and C/N ratios lower, in urban areas receiving the highest rates of N deposition. Plant phosphorus (P) concentrations were also highest in these areas, suggesting that elevated rates of N deposition are unlikely to result in progressive P-limitation in urban habitats. Free amino acid concentrations were positively related to N deposition for asparagine, glutamine, glycine, phenylalanine, isoleucine, leucine and lysine. Overall, relationships between tissue chemistry and N deposition were similar for oxidised, reduced and total N, although the strength of relationships varied with the different biochemical indicators. The results of this study indicate that current rates of N deposition are having substantial effects on plant biochemistry in urban areas, with likely implications for the biodiversity and functioning of urban ecosystems.
Keywords: Bioindicator; Foliar N concentration; N:P ratios; Amino acids; Delta; 15; N; Transect study
Impact of dry deposition of semi-volatile organic compounds on secondary organic aerosols by Bertrand Bessagnet; Christian Seigneur; Laurent Menut (pp. 1781-1787).
Dry deposition of semi-volatile organic compounds (SVOC) is not currently treated in most chemical transport models of air quality and this omission has been identified as a possible major source of uncertainty. The effect of dry deposition of SVOC on the concentration of secondary organic aerosols (SOA) is investigated in summertime with the chemical transport model CHIMERE that simulates SOA concentrations by means of molecular SOA surrogate species. Omitting dry deposition could overestimate SOA concentrations by as much as 50%. This overestimation is larger during nighttime due to higher relative humidity.
Keywords: Dry deposition; Secondary organic aerosols; Modeling
Mercury speciation in Pacific coastal rainwater, Monterey Bay, California by Christopher H. Conaway; Frank J. Black; Peter Weiss-Penzias; Melanie Gault-Ringold; A. Russell Flegal (pp. 1788-1797).
We measured mercury speciation in coastal rainwater samples from Monterey Bay, California in 2007–2008 to investigate the source of monomethylmercury (MMHg) in rainwater and determine the relative importance of wet atmospheric deposition of MMHg to coastal waters compared to other sources on the central Pacific coast. Total mercury (HgT) ranged from 10 to 88 pM, with a sample mean ± standard deviation of 33 ± 22 pM (volume-weighted average 29 pM). MMHg concentrations ranged from 0.12 to 2.3 pM with a sample mean of 0.7 ± 0.5 pM (volume-weighted average 0.68 pM). Reactive mercury (HgR) concentrations ranged from 0.87 to 47 pM, sample mean 7.8 ± 8.3 pM (volume-weighted average 6.1 pM). Acetate concentration in rainwater, measured in a subset of samples, ranged from 0.34 to 3.1 μM, and averaged 1.6 ± 0.9 μM (volume-weighted average 1.3 μM). Dimethylmercury (DMHg) concentrations were below the limit of detection in air (<0.01 ng m−3) and rainwater (<0.05 pM). Despite previous suggestions that DMHg in upwelled ocean waters is a potential source of MMHg in coastal rainwater, MMHg in rain was not related to coastal upwelling seasons or surface water DMHg concentrations. Instead, a multiple linear regression analysis demonstrated that MMHg concentrations were positively and significantly correlated ( p = 0.002, adjusted R2 = 0.88) with those of acetate and HgR. These data appear to support previous suggestions that the aqueous phase methylation of Hg(II) by acetate may be the source of MMHg in rainwater, but imply that acetate concentrations in rainwater play a more important role relative to HgR than previously hypothesized. However, the calculated chemical speciation of Hg(II) in rainwater and the minimal predicted complexation of Hg(II) by acetate suggest that the aqueous phase methylation of Hg(II) by acetate is unlikely to account for the MMHg found in precipitation, or that the mechanism of this reaction in the atmosphere differs from that previously reported ().
Keywords: Mercury; Rainwater; Precipitation; Acetate; Methylmercury
Formation of organic tracers for isoprene SOA under acidic conditions by Mohammed Jaoui; Eric W. Corse; Michael Lewandowski; John H. Offenberg; Tadeusz E. Kleindienst; Edward O. Edney (pp. 1798-1805).
The chemical compositions of a series of secondary organic aerosol (SOA) samples, formed by irradiating mixtures of isoprene and NO in a smog chamber in the absence or presence of acidic aerosols, were analyzed using derivatization-based GC–MS methods. In addition to the known isoprene photooxidation products 2-methylglyceric acid, 2-methylthreitol, and 2-methylerythritol, three other peaks of note were detected: one of these was consistent with a silylated-derivative of sulfuric acid, while the remaining two were other oxidized organic compounds detected only when acidic aerosol was present. These two oxidation products were also detected in field samples, and their presence was found to be dependent on both the apparent degree of aerosol acidity as well as the availability of isoprene aerosol. The average concentrations of the sum of these two compounds in the ambient PM2.5 samples ranged from below the GC–MS detection limit during periods when the isoprene emission rate or apparent acidity were low to approximately 200ngm−3 (calibrations being based on a surrogate compound) during periods of high isoprene emissions. These compounds presently unidentified have the potential to serve as organic tracers of isoprene SOA formed exclusively in the presence of acidic aerosol and may also be useful in assessments in determining the importance and impact of aerosol acidity on ambient SOA formation.
Keywords: PM; 2.5; Secondary organic aerosol; Isoprene; Organic tracers; Acidic sulfate; SO; 2
A global transport model of lead in the atmosphere by T. Niisoe; E. Nakamura; K. Harada; H. Ishikawa; T. Hitomi; T. Watanabe; Z. Wang; A. Koizumi (pp. 1806-1814).
A global atmospheric transport model is used to calculate lead concentrations in the atmosphere. The model performance is evaluated through comparisons with observations in Europe. The model results of lead concentrations in surface air were compared with measurements in East Asia. The detailed comparisons showed generally good agreement for recent decades, although systematic underestimation was found in China. Anthropogenic lead emissions in China are estimated from economic statistics to be 56 000 t yr−1, which is not small considering the economic scale of China. The underestimations suggest a hidden source of lead emissions. The emissions in Japan and Korea are derived from optimization by the model. The magnitude is about 2000 t yr−1, which is much greater than that reported by the Pollutant Release and Transfer Register in Japan and Toxics Release Inventory in Korea.
Keywords: Transport model; Atmospheric lead; Heavy metal; Particulate matter
Atmospheric mercury in the marine boundary layer along a cruise path from Shanghai, China to Prydz Bay, Antarctica by Chonghuan Xia; Zhouqing Xie; Liguang Sun (pp. 1815-1821).
The total gaseous mercury (TGM) measurements were performed using an automatic Mercury Vapor Analyze (model 2537B) aboard the Chinese research vessel (R/V) XueLong during the 24th China Antarctic Research Expedition from Shanghai, China to Prydz Bay, Antarctica in 2007. TGM ranged between 0.302 and 4.496 ng m−3 with an average of 1.536 ± 0.785 ng m−3 over the entire period. Geographically, TGM in the Northern Hemisphere and the Southern Hemisphere along the cruise path were 1.746 ± 0.513 and 1.471 ± 0.842 ng m−3 in average, respectively. Higher TGM concentrations were observed in the coastal regions outside the polar region due primarily to air masses transported from the adjacent mainland reflecting the contribution from anthropogenic sources. The pronounced episode was recorded when ship passed through Sunda straits, which should be ascribed to the volcano plume and/or biomass burning contamination. In the maritime Antarctic TGM level was in agreement with the values by land-based observation, presenting a diurnal cycle with the maximum around midday and minimum at night. Atmospheric mercury destruction events dominated by the oxidation of atmospheric Hg0 were apparently observed in this region.
Keywords: Mercury; Atmosphere; Marine boundary layer (MBL); Antarctica
Interaction of nitrogen dioxide (NO2) with a monolayer of oleic acid at the air–water interface – A simple proxy for atmospheric aerosol by Martin D. King; Adrian R. Rennie; Christian Pfrang; Arwel V. Hughes; Katherine C. Thompson (pp. 1822-1825).
The reactions between atmospheric oxidants and organic amphiphiles at the air–water interface of an aerosol droplet may affect the size and critical supersaturation required for cloud droplet formation. We demonstrate that no reaction occurs between gaseous nitrogen dioxide (1000ppm in air) and a monolayer of an insoluble amphiphile, oleic acid ( cis-9-octadecenoic acid), at the air–water interface which removes material from the air–water interface. We present evidence that the NO2 isomerises the cis-9-octadecenoic (oleic) acid to trans-9-octadecenoic (elaidic) acid. The study presented here is important for future and previous studies of (1) the reaction between the nitrate radical, NO3, and thin organic films as NO2 is usually present in high concentrations in these experimental systems and (2) the effect of NO2 air pollution on the unsaturated fatty acids and lipids found at the air–liquid surface of human lung lining fluid.
Keywords: Oleic acid; Nitrogen dioxide; Aerosol; Monolayer; Nitrate; NO; 2; Film
Trace metals in soil and leaves of Jacaranda mimosifolia in Tshwane area, South Africa by J.O. Olowoyo; E. van Heerden; J.L. Fischer; C. Baker (pp. 1826-1830).
Plant and soil have been identified as major sink of pollutants in the environment. We evaluated the reliability of biomonitoring of heavy metals in Tshwane area with the use of leaves of Jacaranda mimosifolia. The concentrations of heavy metals such as Ca, Mg, Fe, Pb, Zn, Cu, Sb were measured in leaves of J. mimosifolia and soils collected from 10 sites in the city of Tshwane during two sampling periods. The metals were analyzed with the use of ICP-MS. The result shows significant differences in the concentration of trace metals in all the sites ( p < 0.01). The differences between the two sampling periods were statistically significant ( p < 0.01). Concentration of metals from high traffic and industrial sites were significantly higher than in the residential areas ( p < 0.01). Concentration factor suggests that translocation of metals from roots to leaves could be relevant only for some metals such as Ca, Mg and Sb. The study reveals an anthropogenic source for the trace metals. Leaves of J. mimosifolia were found to be a useful biomonitor of the determined trace metals.
Keywords: Leaves; Trace metals; Air pollution; Traffic density; Soil