Atmospheric Environment (v.34, #18)

A detailed case study of isoprene chemistry during the EASE96 Mace Head campaign by N. Carslaw; N. Bell; A.C. Lewis; J.B. McQuaid; M.J. Pilling (2827-2836).
A detailed chemical mechanism has been used in conjunction with high-quality field measurements in order to assess the potential role of isoprene at a clean coastal Northern Hemisphere site. The importance of isoprene as a source of peroxy radicals, formaldehyde and ozone is discussed in some detail for a case study day (July 17 1996). Between 10:00 and 16:00 h GMT, isoprene accounts on average for 15% of the OH removal, yet is responsible for the production of about 30% of the HO2 radicals. Isoprene is also responsible for between 40–60% of the HCHO formation, and 20–40% of the 2 ppb h−1 conversion of NO to NO2 by peroxy radicals. We discuss the significant time lag between HCHO formation and destruction, a time lag that must be accounted for in order to accurately calculate the rate of HO2 production from isoprene degradation, and the implications of the isoprene oxidation for atmospheric chemistry.
Keywords: Oxidation; Peroxy radicals; Ozone; Formaldehyde; Box model;

Recent experimental studies have identified cis-pinic acid (a C9 dicarboxylic acid) as a condensed-phase product of the ozonolysis of both α- and β-pinene, and it is currently believed to be the most likely degradation product leading to the prompt formation of new aerosols by nucleation. The observed timescale of aerosol formation appears to require that cis-pinic acid is a first-generation product, and a possible mechanism for its formation has therefore been developed. The key step in the proposed mechanism requires that the isomerisation of a complex C9 acyl-oxy radical by a 1,7 H atom shift is able to compete with the alternative decomposition to CO2 and a C8 organic radical: ▪Thermodynamic and kinetic arguments are presented, on the basis of semi-empirical electronic structure calculations, which support this proposed mechanism, and thereby the competition between the two pathways. The transfer of the labile aldehydic H atom is shown to be especially facile in this case because it occurs though an unstrained transition state; this feature can in turn be attributed to the cis-substitution of the four-membered ring, which enforces the steric proximity of the acyl-oxy and aldehyde groups. The mechanism can explain the formation of cis-pinic acid from both α- and β-pinene, because the acyl-oxy radical is likely to be formed following the decomposition of excited Criegee biradicals formed in both systems. It is also possible that a similar isomerisation reaction of a complex C10 α-carbonyl oxy radical by a 1,8 H atom shift might explain the very recently observed formation of cis-10-hydroxy-pinonic acid from α-pinene ozonolysis, and this possibility is also explored. An existing detailed scheme describing the degradation of α-pinene (part of the Master Chemical Mechanism, MCM) is updated to include the proposed cis-pinic acid and cis-10-hydroxy-pinonic acid formation mechanisms, and the values of several uncertain parameters are adjusted on the basis of reported yields of a series of organic products from the ozonolysis of α-pinene. The updated degradation scheme is incorporated into a boundary layer box model, and representative ambient concentrations of the organic acids and other oxygenated products are calculated for a range of representative conditions appropriate to the boundary layer over central Europe. The simulated concentrations of the organic acids in general, and cis-pinic acid in particular, are strongly dependent on the level of NO X , and suggest that new aerosol formation from the oxidation of α-pinene is likely to be more favoured at lower NO X levels.
Keywords: Monoterpenes; α-Pinene; β-Pinene; Secondary-organic aerosol; Pinic acid; Carboxylic acids; Acyl-oxy radicals; Tropospheric oxidation chemistry; Master chemical mechanism (MCM);

This paper investigates the solution of a 3D atmospheric dispersion problem using a time-dependent adaptive gridding technique. A cell-vertex finite volume scheme based on tetrahedral elements is used to solve the atmospheric diffusion equation. Preliminary studies of dispersion from a single source in stable, unstable and neutral boundary layers have been carried out. The results show the efficiency of using adaptive grids to represent accurately the structures of plumes in the boundary layer and also the advantage of this method compared to fixed methods for mesh refinement. Some comments about the interpolation of input data such as wind fields onto unstructured meshes are also made.
Keywords: Adaptive meshes; Dispersion; Plume; Boundary layer; Unstructured;

Laboratory experiments are described where the uptake of SO2 gas by water drops containing H2O2 is investigated where the taken up S(IV) is quickly converted to S(VI). During the gas uptake the drops were freely suspended at their terminal velocity by means of the Mainz vertical wind tunnel. Two series of experiments were carried out, one with a laminar air flow in the wind tunnel, one with a turbulent air flow in the wind tunnel. Afterwards, the experimental results were compared against model computations using the so-called fully mixed convective diffusion model. The experimental results for laminar flow conditions showed that the fully mixed convective diffusion model for the uptake of SO2 by water drops is not only applicable to large water drops of millimeter size, as demonstrated in our previous papers, but also to small water drops of some hundreds micron size. Our experimental results for turbulent flow conditions allow, within the experimental error, to conclude that turbulence in the air surrounding falling drops has no measurable effect on the uptake of SO2 by freely falling water drops.
Keywords: Water droplets; Gas uptake; Sulfuric acid; Laminar air flow; Turbulent air flow;

This study investigates a mesoscale modelling of sea breezes blowing from a narrow strait into the lower Fraser valley (LFV), British Columbia, Canada, during the period of 17–20 July, 1985. Without a nudging scheme in the inner grid, the CSU-RAMS model produces satisfactory wind and temperature fields during the daytime. In comparison with observation, the agreement indices for surface wind and temperature during daytime reach about 0.6 and 0.95, respectively, while the agreement indices drop to 0.4 at night. In the vertical, profiles of modelled wind and temperature generally agree with tethersonde data collected on 17 and 19 July. The study demonstrates that in late afternoon, the model does not capture the advection of an elevated warm layer which originated from land surfaces outside of the inner grid. Mixed layer depth (MLD) is calculated from model output of turbulent kinetic energy field. Comparison of MLD results with observation shows that the method generates a reliable MLD during the daytime, and that accurate estimates of MLD near the coast require the correct simulation of wind conditions over the sea. The study has shown that for a complex coast environment like the LFV, a reliable modelling study depends not only on local surface fluxes but also on elevated layers transported from remote land surfaces. This dependence is especially important when local forcings are weak, for example, during late afternoon and at night.
Keywords: Mesoscale modelling; Sea breezes; The lower Fraser valley; Mixed layer depth; CSU-RAMS;

Assessment of a relaxed eddy accumulation for measurements of fluxes of biogenic volatile organic compounds: study over arable crops and a mature beech forest by M.W. Gallagher; R. Clayborough; K.M. Beswick; C.N. Hewitt; S. Owen; J. Moncrieff; K. Pilegaard (2887-2899).
A relaxed eddy accumulation (REA) system, based on the design by Beverland et al. (Journal of Geophysics Research 101 (D17) 22, 807-22, 815), for the measurement of biogenic VOC species was evaluated by intercomparison with an eddy correlation CO2 flux system over a mature deciduous beech canopy (Fagus Sylvatica) during the FOREXNOX program. Measurements from a site where winter wheat and barley (Hordeum Vulgare and Triticum Aestivum) were being harvested are also presented. The system was inter-compared with two different eddy correlation systems for measuring CO2 fluxes. Good results were obtained with correlation coefficients for the REA system ranging from 0.71 to 0.82, lending further confidence in the use of this technique. Daily averaged biogenic emissions from the wheat and barley canopies were significantly larger than expected, likely a result of harvesting. Fluxes measured over the beech canopy were also larger than might be expected from northern latitude deciduous forests.
Keywords: Volatile organic hydrocarbons; Relaxed eddy accumulation; Gas flux measurements; Carbon dioxide;

Air quality modeling is of seminal importance to the assessment of air pollution control strategies. Traditionally, these models include four basic components: meteorology data, emissions data, transport mechanisms, and chemistry. The gas-phase chemistry portions have been based on kinetic and product laboratory studies in relatively pristine gas-phase environments. However, given the abundance of particles in the atmosphere, it is possible that aerosols can have catalytic effects on gas-phase reaction kinetics. These studies focus on elucidating those effects. Ultimately, the data gained can be used to improve regional air quality models. Relative rate studies of the OH radical initiated reactions of n-hexane, p-xylene, and 1-propanol were conducted in a Tedlar bag in the presence of ammonium sulfate aerosols. Results show that ammonium sulfate aerosols promote the reaction of 1-propanol with OH radicals compared to the reactions of n-hexane with OH radicals and p-xylene with OH radicals. The relative rate of the 1-propanol/·OH reaction versus the n-hexane/·OH reaction increased from 0.85±0.05 in the absence of (NH4)2SO4 aerosols to 1.07±0.05 in the presence of aerosols. Also, the relative rate of the 1-propanol/·OH reaction versus the p-xylene/·OH reaction increased from 0.45±0.03 in the absence of (NH4)2SO4 aerosols to 0.56±0.02 in the presence of aerosols.
Keywords: Ozone; Relative rates; 1-Propanol; n-Hexane; p-Xylene;

In this paper we investigate the important role of the biogenic species isoprene on tropospheric chemistry using a land surface model (LSM) and a three-dimensional (3-D) tropospheric chemistry transport model (CTM). An efficient and conservative coupling scheme is used to couple the LSM to the 3-D CTM. Annual integrations of the coupled model have been performed and the results compared with other estimates. The comparison shows that the annual global isoprene flux from terrestrial vegetation is 530 Tg C yr−1, which is in good agreement with 503 Tg C yr−1 estimated by a high-resolution (0.5°×0.5°) vegetation model of Guenther et al. (1995, Journal of Geophysical Research 100 (D5), 8873–8892). Comparison of the seasonal variations of the surface emission distribution between the coupled model and Guenther et al. (1995) also shows close agreement. The potential impact of isoprene on the levels of tropospheric species is studied by running the same coupled model for the period of June–December but without biogenic isoprene emissions included, and the results are compared with the run which includes biogenic isoprene emissions. Our comparison indicates a significant difference in O3 and PAN for both hemispheres. The discrepancy between the run with and without isoprene is predominantly governed by the spatial and temporal variations of terrestrial vegetation. The largest difference is seen in the summertime northern hemisphere at locations with extensive terrestrial vegetation (e.g. North America, Europe, east and southeast Asia, South America and equatorial central Africa). For O3, there is about a 4 ppbv increase over the oceanic areas and about an 8–12 ppbv increase over the mid-latitude land areas. For PAN, a maximum of about one order of magnitude in difference, which increases from 0.01 ppbv (without isoprene emissions) to 0.1–0.3 ppbv (with isoprene emissions), is seen in areas of extensive terrestrial vegetation.
Keywords: Isoprene; VOCs; CTM; Land surface model; Coupling scheme;

Exposure-annoyance relationships for odour from industrial sources by H.M.E. Miedema; J.I. Walpot; H. Vos; C.F. Steunenberg (2927-2936).
Existing data from field surveys have been reanalysed in order to establish relationships between odour annoyance and odour exposure concentrations caused by (bio) industrial odours in a community. The percentage of highly annoyed persons (%HA) was found to have a simple relationship with the logarithm of the 98 percentile of the odour exposure concentrations (lgC98). Pleasantness ratings of the odours were obtained through a supplementary laboratory study with samples from the sources concerned. It was found that the prediction of %HA improves if the pleasantness of the odour is taken into account. The %HA at a certain level of lgC98 is found to be higher when the odour is less pleasant. This indicates that odour standards may improve if they take the odour pleasantness into account. Before doing so the possibility must be ruled out that the effect of pleasantnees on %HA was caused by factors confounded with the pleasantness of odours.
Keywords: Odour; Annoyance; Concentration; Pleasantness; Survey; Community;

Deterioration of concrete structures caused by acid deposition was investigated by laboratory and field exposure of portland cement mortar specimens to acid deposition. Laboratory exposure experiment showed that the dissolved amount of calcium hydrates, which were the major components in mortar, increased with the increase in the acidity of simulated acid rain solution and the decrease in the flow rate. There was little difference in their amount among different temperature treatments after each exposure to the solution with the same acidity, namely left at room temperature, heated at 70°C, and cooled at −2°C. The neutralization progressed more deeply under the heated and cooled condition and was accelerated by even acid rain with pH 4.7 during a long period (90 exposure cycles, which correspond to the rainfall amount of 15 years in Japan). A field exposure experiment for two years indicated that the carbonation of calcium hydrates and the formation of other corrosion products such as chloride, nitrate, and sulfate were limited to the surface of mortar specimens. The neutralization progressed more deeply in mortar specimens sheltered from rainwater than in those washed by rainwater.
Keywords: Concrete; Acid deposition; Calcium hydrate; Neutralization; Corrosion products;

Emissions of nonmethane organic compounds (NMOCs) from grassland vegetation were collected in Summa® passivated stainless-steel canisters with a static enclosure technique and were analyzed by high-resolution gas chromatography with flame ionization and ion trap mass spectrometric detectors. Approximately 40 NMOCs with 6–10 carbon atoms were observed in samples analyzed by high-resolution gas chromatography with the flame ionization detector. Nineteen NMOCs in this molecular weight range (6 aliphatic oxygenates; 1 aromatic hydrocarbon; and 4 acyclic, 5 monocyclic, and 3 bicyclic monoterpenoids) were identified by ion trap mass spectrometry. Mass spectrometry was particularly useful for identifying myrcene and cis-3-hexenylacetate, which coeluted on a fused-silica capillary column coated with a 1-μm-thick film of polydimethylsiloxane. An evaluation of the reactivity of the grassland emissions revealed that the aliphatic oxygenates have lifetimes of a few hours with respect to oxidation by OH and O3 in the atmosphere. This value is similar to the lifetimes of the bicyclic monoterpenoids. The expected lifetimes of the monoterpenoids with respect to oxidation by NO3 are only several minutes.
Keywords: GC/MS; Monoterpene; Nonmethane organic compound; NMOC; Oxygenated hydrocarbon; OxHC; Grassland; Biogenic emission;

The trajectory-grid method (Chock et al., Atmospheric Environment 30 (1996) 857–868) for solving the transport equation in a grid model can be easily adapted to solve the condensation/evaporation equations of internally mixed multicomponent aerosols based on the sectional approach. Compared to the Bott scheme (Monthly Weather Review 117 (1998) 1006–1015, 2633–2636) often used for solving these equations, the method is significantly more accurate, of comparable speed, but gaining in speed as the number of aerosol components increases. Most notably, there is no mismatch in size change of the different components of the same aerosol. The method is quite flexible, can handle an abrupt change in aerosol size due to evaporation of fog particles, and is ideal for multiply-parallel processors.
Keywords: Aerosol modeling; Particulate matter modeling; Condensation equation; Internally-mixed aerosols; Air quality modeling; Advection equation;

Ozone production efficiency is a parameter used for evaluating the effect of anthropogenic NO x on tropospheric ozone. For this work, zero-dimensional simulations were run to compare the differences in ozone production efficiency when air masses of different origin were separated and when they were merged. The purpose of the simulations was to estimate whether coarsely resolved models might under or overpredict ozone production due to their blending of air masses of different origin. Cases were run for several combinations of air mass origin, different latitudes, times of year, air mass dilution ratios and initial gas concentrations. The main result of this study is that integrated ozone production may be overpredicted by as much as 60% in coarse-model grid cells exposed to different air masses. Under certain conditions, such as in rather finely resolved urban airshed models, ozone production may actually be underpredicted by about 20% in mid-latitudes during summer. The results imply that large-scale global models may have a difficult time correctly predicting ozone concentrations near, for instance, urban/free tropospheric boundaries, a conclusion supported by other studies examining parameters other than ozone production efficiency.
Keywords: Ozone; Air pollution modeling; Photochemistry; Atmospheric modeling;

Measuring and simulating particulate organics in the atmosphere: problems and prospects by Barbara J. Turpin; Pradeep Saxena; Elisabeth Andrews (2983-3013).
Although organics constitute approximately 10–70% of the total dry fine particle mass in the atmosphere, their concentrations and formation mechanisms are less well understood than those of other components such as sulfate and nitrate. This is because particulate organic matter is an aggregate of hundreds of individual compounds whose concentrations cannot be characterized by a single analytical technique; more than half of the organic carbon mass has not yet been identified as individual compounds. Moreover, the collection process itself can alter the gas–particle equilibrium of a number of condensable organics resulting in both positive and negative sampling biases. The incomplete characterization of particulate organics coupled with the complexity of the photochemical reactions that produce particulate matter from volatile organic emissions has prevented the development of a first principle simulation approach. These limitations are providing an impetus for numerous scientific studies, proving organics to be the next frontier for particle characterization and simulation. This paper reviews the current state of organic aerosol sampling, analysis, and simulation, examines the limitations of the current technology, and presents prospects for the future. The emphasis is on distilling findings from recent atmospheric, smog chamber, and theoretical studies to provide a coherent picture of what has been accomplished, especially during the last five years, and what problems are ripe for further investigation.
Keywords: Organic aerosol; PM2.5 characterization; Aerosol sampling artifacts; Organic analysis; Atmospheric modeling; Denuders;

The well-known carbon bond IV (CBIV) chemical mechanism (33 species, 81 reactions) is compared with an adjusted carbon bond mechanism (ACBM) (109 species, 233 reactions) and a more explicit master chemical mechanism (MCM) (1427 species, 3911 reactions) in tests of their predictions of O3, NO x (=NO+NO2), HCHO, HNO3, H2O2, and peroxyacetlynitrate (PAN). The ACBM was developed from a fourth mechanism, the expanded carbon bond mechanism (CBM-EX), by explicitly including the decomposition of C2H6, C3H8, and C3H6. All three mechanisms tested were updated with the inorganic chemistry from the ACBM and implemented into the sparse-matrix, ordinary differential equation solver, SMVGEAR II. Sparse-matrix treatment in SMVGEAR II reduced the number of calculations during matrix decomposition for the MCM by a factor of 15,000 (99.995%), or from an estimated 154 h to 37 s of cpu time per simulation day in one grid cell on an SGI origin 2000, in comparison with a full-matrix solution. Computer time for each mechanism was linearly proportional to the number of species in the mechanism. It is shown that the three mechanisms agreed closely when aromatic concentrations were initially low in comparison with alkane, alkene, and aldehyde initial concentrations. When aromatic concentrations were initially high (higher than that observed in urban air), the yields of O3, HCHO, and PAN differed significantly among the three mechanisms although the daily maximum concentrations of these species agreed better. The aromatic representation in MCM appears to lead to systematic overprediction of ozone, according to a comparison with smog chamber data. For initial conditions taken from measurements at nine sites in Los Angeles, the daily maximum concentrations of O3, HCHO, PAN, and H2O2 predicted by the three mechanisms differed by 30–50%, 10–40%, 15–40%, and 60–80%, respectively. The relative differences between the daytime series of O3, HCHO, H2O2, and PAN predicted by the three mechanisms were 7–68%, 7–46%, 35–150%, and 10–64%, respectively. The use of the aromatic scheme of ACBM in MCM significantly reduced the disagreement with respect to ozone. The measurement of H2O2 in smog chamber experiments would be useful in validating chemical mechanisms.

Seasonal variations of isoprene emissions from deciduous trees by Zhang Xiaoshan; Mu Yujing; Song Wenzhi; Zhuang Yahui (3027-3032).
Isoprene emission fluxes were investigated for 12 tree species in and around Beijing city. Bag-enclosure method was used to collect the air sample and GC-PID was used to directly analyze isoprene. Ginkgo and Magnolia denudata had negligible isoprene emissions, while significant emissions were observed for Platanus orientalis, Pendula loud, Populus simonii, and Salix matsudana koidz, and other remaining trees showed no sign of isoprene emission. Variations in isoprene emission with changes in light, temperature and season were investigated for Platanus orientalis and Pendula loud. Isoprene emission rates strongly depended on light, temperature and leaf age. The maximum emission rates for the two trees were observed in summer with values of about 232 and 213 μg g−1  dw h−1, respectively. The measured emission fluxes were used to evaluate “Guenther” emission algorithm. The emission fluxes predicted by the algorithm were in relatively good agreement with field measurements. However, there were large differences for the calculated median emission factors during spring, summer and fall. The 25–75 percentiles span of the emission factor data sets ranged from −33 to +15% of the median values.
Keywords: Isoprene; Emission factors; Emission rates; Diurnal variation;

Emissions from the combustion of peat: an experimental study by T.R. Muraleedharan; Miroslav Radojevic; Allan Waugh; Anthony Caruana (3033-3035).
The first-ever experimental study of gaseous emissions from tropical biomass in southeast Asia is reported. Forest fires have been responsible for regional haze episodes in recent years, and most of the fires were in areas where peat is the dominant biomass fuel. Samples of peat were combusted at temperatures typical of smouldering combustion (500–600°C) and the combustion products were analysed for CO, CO2, CH4, C2H6, C3H8, C4H10, C2H4, HCHO, CH2CHCHO, and PAHs. The most abundant carbon-containing species was CO2, followed by CO, and CH4. Emission factors were quantified for the gases that were detected. No aldehydes nor PAHs were detected in the combustion products, however, these may have been present at levels below the method detection limits. Aldehydes and PAHs identified in field studies during biomass fires and haze episodes may also result from atmospheric reactions of primary emissions.
Keywords: Peat; Haze; Combustion; CO; CO2; CH4; PAH;