Atmospheric Environment (v.34, #21)

Hydrogen peroxide and organic hydroperoxide in the troposphere: a review by Meehye Lee; Brian G. Heikes; Daniel W. O'Sullivan (3475-3494).
The current knowledge of gas-phase hydrogen peroxide and organic hydroperoxide in the troposphere is reviewed: chemistry, properties, measurement methodology and tropospheric distribution.
Keywords: Methyl hydroperoxide; Hydroxymethyl hydroperoxide; Peroxide; Radical; Henry's Law constant; Collection efficiency; Synthesis; Standardization; Measurement; Tropospheric distribution;

Spectral analysis of air pollutants. Part 1: elemental carbon time series by Thomas Hies; Renate Treffeisen; Ludwig Sebald; Eberhard Reimer (3495-3502).
An effective method to analyse different air pollution sources in an elemental carbon time series is presented. As a second feature, this technique allows a fast and efficient classification of monitoring sites. Time series of daily elemental carbon measurements at various urban locations have been evaluated with the corresponding power spectra. Typical and well-known periodicities caused by anthropogenic and meteorological influences have been identified using coherence and phase spectra. It will be shown that domestic heating by coal combustion appears as a 365 day periodicity, traffic contributes 3.5, 4.6 and 7 day peaks in the spectrum and elevated long range elemental carbon can be identified as characteristic peaks with periodicities in the range from 13 to 42 days. As the relative amplitudes of the various influences vary depending on the location of the measurement site in the urban area, the use of estimated power spectra helps to find the influence of traffic, domestic coal-heating and long range transport on the elemental carbon concentration.
Keywords: Elemental carbon; Spectral analysis; Source characterisation;

Spectral analysis of air pollutants. Part 2: ozone time series by Ludwig Sebald; Renate Treffeisen; Eberhard Reimer; Thomas Hies (3503-3509).
Spectral analysis is used to investigate the troposheric ozone formation and decomposition processes. Although ozone is a reactive, secondary air pollutant, a similar method of examination is used for elemental carbon which can be applied to ozone time series. To detect the reason for extreme high ozone concentrations in summer, the data was divided into low-frequency seasonal component and high-frequency component. The latter has been evaluated by using the corresponding power density spectrum. It is shown that meteorological large- and synoptic-scale fluctuations affect the ozone concentrations at all monitoring locations. In the data set of 1993–1995 this influence contributed peaks at cycle durations of 16–35 day peaks in the power density spectrum estimated from the ozone data. The level of the seasonal component at a particular site depends on the level of local influence by traffic and industry emissions. The power density spectrum of the mean early morning ozone concentration (2–4 a.m.) at a measurement site situated 324 m elevation is very similar to the power spectra of the ozone data at all other monitoring locations in 1995. This affirms the dominance of spatial homogenous fluctuations and indicates a relative uniform residual layer over a large region.
Keywords: Ozone; Spectral analysis; Residual layer; Large-scale meteorology;

A field study was conducted in September 1995 to measure the ambient atmospheric concentrations of methyl bromide (MeBr) in the Salinas Valley, California. Air concentrations of MeBr were measured at 11 sites located on the adjacent mountains, valley floor and at the Pacific Ocean coast over a 4-d period. The concentrations ranged up to 8.98 μg m−3. Industrial Source Complex Short Term 3 (ISCST3) and CALPUFF dispersion model simulations were performed with several fumigated fields serving as sources, using two estimates of source strengths from published flux values. CALPUFF was driven by 3D meteorology from CALMET. With the lower of the two estimates, the ISCST3 model underpredicted concentrations for 76% of data and averaged 66% of measured, and the CALPUFF model also underpredicted concentrations for 67% of observations and averaged 84% of measured. With the higher of the two estimates the ISCST3 overpredicted by a factor of two for 67% of data, and CALPUFF overpredicted concentrations by a factor of 1.6 for over 50% of data. Between the model predicted and measured concentrations, the coefficient of determination, R2 , was ≈0.7 for both source strengths with ISCST3 model. The R2 with CALPUFF model was 0.55 and 0.82 with source strength estimated from two prior flux studies. The margin of exposure (MOE) for the population of the city of Salinas was calculated based on the measured ambient concentrations and compared with the current benchmark used by US-EPA and California Department of Pesticide Regulation for acceptable human health risk. Based on the models predicted worst-case exposure concentration, the MOE for acute effects was approximately 10,000. For chronic effects it was approximately 100, indicating a need for attention to exposure to MeBr in areas of intense methyl bromide use.
Keywords: Methyl bromide; Salinas valley; ISCST3; CALPUFF; CALMET; Exposure assessment;

Passive sampling of atmospheric SOCs using tristearin-coated fibreglass sheets by Jochen F. Müller; Darryl W. Hawker; Des W. Connell; Peter Kömp; Michael S. McLachlan (3525-3534).
Tristearin-coated fibreglass sheets were evaluated as passive samplers for semivolatile organic chemicals in ambient atmosphere. Kinetic and equilibrium experiments were set-up to independently determine model parameters. The results demonstrate that the tristearin/air partition coefficients (K SV ) can be related to the octanol/air partition coefficients (K OA ). Furthermore, a linear relationship between the time constant of the clearance kinetics and K SV suggests that uptake of gaseous polycyclic aromatic hydrocarbons (PAHs) in the sampler was controlled by the air side resistance. A field evaluation of the predictive ability of the sampler was set-up at sites in Brisbane, Australia and Bayreuth, Germany. The results demonstrated that this novel sampler provides a reproducible means to collect gaseous PAHs in the environment. Furthermore, the results of the field evaluation in Brisbane are in good agreement with air samples collected with a filter/adsorbent high-volume air sampler at the same sites. In contrast the predicted results obtained from the field evaluation near Bayreuth were less satisfactory. This might be due to sampling artifacts which occurred with the passive samplers, caused by PAH burdened particles eroded from adjacent needles and leaves.
Keywords: Tristearin/air partition coefficient; Ambient air sampling; Semivolatile organic chemicals;

Canopy and leaf level 2-methyl-3-buten-2-ol fluxes from a ponderosa pine plantation by Gunnar W Schade; Allen H Goldstein; Dennis W Gray; Manuel T Lerdau (3535-3544).
Canopy and leaf level 2-methyl-3-butene-2-ol (methylbutenol, MBO) emissions were measured from a ponderosa pine plantation in the Sierra Nevada mountains from July to October 1998. Canopy scale fluxes were measured using a gradient approach, leaf level fluxes used a flow-through chamber. Emissions were dependent on ambient light and temperature levels and showed a diurnal cycle very similar to isoprene in deciduous forests. Maximum fluxes occurred between 1000 and 1700 h with an average of 2 mg C m−2  h−1 in July and August, which equaled approximately 0.3–0.5% of the simultaneously measured gross carbon uptake. MBO mixing ratios and fluxes over the pine plantation were also measured with a relaxed eddy accumulation (REA) system operated during part of our measurement campaign ( Journal of Geophysical Research, in press). Mixing ratios measured by both systems were in good agreement but the gradient approach estimated MBO fluxes twice as high than the REA approach. Leaf level investigations revealed a seasonal cycle in basal emission rate (emissions at 1000 μmol m−2  s−1 PAR and 30°C) with a maximum in August, decreasing towards the end of the season. We developed an emission model to scale MBO fluxes from the leaf level to the ecosystem level based on the well-known isoprene algorithm. The model estimates are substantially lower than our gradient flux measurements, and match better with the REA flux measurements, and we conclude that the gradient approach overestimated MBO fluxes at this site. Comparing seasonal changes of measured with modeled fluxes showed a pattern of basal emission rates similar to those observed at the leaf level, and the basal emission rate was related to daytime air temperatures. While MBO contributes substantially to local photochemistry and its oxidation contributes significantly to the regional acetone budget, the latter probably does not represent a significant global source of atmospheric acetone.
Keywords: Volatile organic compounds; MBO; Emission model; Seasonality; Acetone source;

Measurements of ozone throughout the troposphere clearly show an annual cycle. Over the last couple of decades it has become apparent that the measured annual cycle of ozone in certain locations shows a distinct maximum during spring and the magnitude of the maximum seems to have increased. There has been much debate as to the origins of this phenomenon. There is broad agreement that much of the ozone found in the troposphere is of photochemical origin. In contrast, there is still no over-arching consensus as to the mechanisms that lead to the formation of the spring ozone maximum. Part of the problem would seem to lie in the interpretation of measurements and the interactions of processes occurring on differing scales from the local to the global scale. This paper reviews both the experimental evidence concerning the origin of the spring ozone maximum and the supporting modelling studies. The roles of stratospheric–tropospheric exchange and photochemistry in the appearance of the spring ozone maximum are discussed; the evidence for various mechanisms for accumulation of ozone and its precursors are considered. The paper concludes with a summary of the state of the knowledge with respect to the spring ozone maximum and some possible areas for future consideration. The spring ozone phenomenon may well be a proxy for the continuing changes to the atmospheric composition owing to man's activities. Understanding the appearance of the spring ozone maximum and the mechanisms that lead to its formation therefore remains an issue fundamental to tropospheric chemistry.
Keywords: Ozone; Photochemistry; Stratosphere–tropospheric exchange; Reservoir mechanisms; Precursors; Measurements; Models;

We consider the dispersion from a point source in the neutral atmospheric boundary layer. The emission from the point source consists of a species A which reacts with an ambient species B present in the boundary layer. The study is carried out by means of large-eddy simulation (LES). The main emphasis lies on the effect of turbulent mixing on chemistry in determining the concentration levels in the plume. First, we consider a relatively slow reaction which is characterized by a reactive time scale that is much larger than the turbulence time scale. Wind-tunnel experiments exist for this case (Builtjes, 1983. In: Wispelaere, C. (Ed.), Air Pollution Modeling and its Application II. Plenum Press, New York, pp. 59–84) and with these experimental data we validate our LES model. A comparison between computational and experimental data shows excellent agreement for the mean concentrations on the plume centreline. Second, we consider the case of a relatively fast reaction which is characterized by a reaction time scale smaller than the turbulent time scale. In this case the reaction is limited by the turbulent mixing. First we consider the effect of mixing by the resolved scales obtained from the LES. The simulation results demonstrate that the total reaction rate is reduced considerably compared to the ideal case in which species are completely mixed. Next we consider the influence of subgrid scale mixing. To account for this influence a subgrid model for the chemistry is adopted. The simulation results show that the subgrid scale mixing plays a prominent role close to the source.
Keywords: Turbulent reacting flows; Large-eddy simulation; Plume dispersion; Neutral boundary layer;

Turbulence parameterisation for PBL dispersion models in all stability conditions by G.A Degrazia; D Anfossi; J.C Carvalho; C Mangia; T Tirabassi; H.F Campos Velho (3575-3583).
Accounting for the current knowledge of the planetary boundary layer (PBL) structure and characteristics, a new set of turbulence parameterisations to be used in atmospheric dispersion models has been derived. That is, expressions for the vertical profiles of the Lagrangian length scale l i and time scale T i and diffusion coefficient K i , i=u, v, w, are proposed. The classical statistical diffusion theory, the observed spectral properties and observed characteristics of energy containing eddies are used to estimate these parameters. The results of this new method are shown to agree with previously determined parameterisations. In addition, these parameterisations give continuous values for the PBL at all elevations (z0⩽z⩽h, zi) and all stability conditions from unstable to stable, where h and z i are the turbulent heights in stable or neutral and convective PBL, respectively, and L is the Monin–Obukhov length. It is the aim of this work to present the general derivations of these expressions and to show how they compare to previous results. Finally, a validation of the present parameterisation applied in a Lagrangian particle model, will be shown. The Copenhagen data set is simulated. This data set is particularly suited for this validation, since most of the Copenhagen tracer experiments were performed in stability conditions that are the result of the relative combination of wind shear and buoyancy forces. As a consequence, a parameterisation scheme, able to deal contemporary with neutral and slightly convective condition, is to be preferred.
Keywords: Planetary boundary layer; Turbulence parameterisation; Statistical diffusion theory; Turbulent velocity spectra; Dispersion models;

A three-dimensional regional scale atmosphere-chemistry model has been developed to contribute to an improved understanding of atmospheric photochemical processes. This on-line model determines meteorological processes directly together with tracer transport and photochemistry. Usually, off-line chemistry-transport models are applied which use archived data from a meteorological model as input information. However, a number of disadvantages result from the seperation of meteorological and photochemical modelling: only a part of the whole meteorological information is available in the sampled data sets and only in distinct time intervals. The availability of the whole meteorological information including subgrid scale dynamical motions like turbulence and moist convection at every model time step is the main advantage of the on-line procedure. In addition, on-line modelling allows to consider chemical–dynamical feedbacks which is not possible in the off-line mode. For validation and evaluation studies a 10-days simulation of a summersmog episode over Europe in July 1994 has been carried out with the first version of the on-line model. The results are compared with observations and with an off-line model simulation. The on-line model is able to reproduce measured near-surface concentrations in much better agreement than the off-line model does. The reason is an improved representation of tracer transport in convective clouds in the on-line model. It is responsible for the upward mixing of short-living precursor substances of photooxidants from the planetary boundary layer into the free troposphere, with a significant impact on photooxidants concentrations in the planetary boundary layer as well as in the free troposphere.
Keywords: Three-dimensional modelling; Moist convective transport; Ozone; Oxides of nitrogen; Process analysis;

A skewed meandering plume model for concentration statistics in the convective boundary layer by Ashok K. Luhar; Mark F. Hibberd; Michael S. Borgas (3599-3616).
The meandering plume technique, which assumes that the total plume dispersion can be split into independent meander and relative dispersion components, is especially suited for modelling concentration (fluctuation) statistics in the convective boundary layer (CBL) with its large-scale turbulent motions. We develop a simple and practical meandering plume model for CBL applications that accounts for the skewed and inhomogeneous turbulence characteristics of the convective flow. The meander component is derived from a one-particle Lagrangian stochastic dispersion model by requiring that the meander and relative dispersion components correctly balance the first two total dispersion moments. Balancing of the third total moment implies a skewed relative dispersion, for which a bi-Gaussian distribution is used. The relative dispersion variance is parameterised with an extended asymptotic formulation for travel times much smaller than the Lagrangian integral time scale. For large travel times, the relative dispersion variance approaches the total dispersion variance. The in-plume fluctuations in the relative coordinate system are accounted for via the gamma probability density function. Laboratory data and large-eddy simulation results on total, relative and meander spreads are used to examine the model parameterisations and results. A requirement of the meandering plume model, that it should give the same mean concentration distribution as that obtained by the one-particle Lagrangian approach, is virtually fulfilled. Comparison of the model predictions of the concentration fluctuation intensity with existing laboratory data highlights the important contribution of in-plume fluctuations, which are normally neglected in meandering plume models. The paper also describes limitations of the new model and indicates the scope for further refinements.
Keywords: Turbulent diffusion; Relative dispersion; Particle models; Concentration fluctuations; Air quality models;

A computationally efficient hybrid approach for dynamic gas/aerosol transfer in air quality models by Kevin P. Capaldo; Christodoulos Pilinis; Spyros N Pandis (3617-3627).
Dynamic mass transfer methods have been developed to better describe the interaction of the aerosol population with semi-volatile species such as nitrate, ammonia, and chloride. Unfortunately, these dynamic methods are computationally expensive. Assumptions are often made to reduce the computational cost of explicit dynamic calculations, including instantaneous equilibrium and/or use of bulk-aerosol composition. A novel approach to the modeling of the mass transfer of semi-volatile species is presented. A hybrid method is developed that utilizes equilibrium assumptions for the fine aerosol mode (particles with diameters less than 1 μm) and the dynamic approach for the coarse aerosol mode. A comparison among three methods (equilibrium, dynamic, and hybrid) is made for varying conditions of aerosol acidity, dry and wet particles, and marine and urban environments. Results show that the hybrid method maintains most of the predictive ability of the dynamic approach and is 50 times more computationally efficient for our test scenario. Sensitivity of the hybrid method to the equilibrium cut-off diameter and to the frequency of the evaluation of the equilibrium portion of the aerosol distribution is also discussed.
Keywords: Modeling; Aerosol dynamics; Aerosol thermodynamics; Nitrate; PM2.5;

A method was developed enabling the simultaneous sampling and separation of ice crystals and supercooled cloud droplets by means of an inertial impaction system. The principle of separation is based on the lower sticking coefficient of ice crystals compared to supercooled cloud droplets. Supercooled cloud droplets freeze upon contact with the corresponding stage of the impactor whereas ice crystals bounce off because they do not stick to the impaction surface. In order to examine the separation principle laboratory studies were performed. Furthermore, the applicability of the newly developed method was tested in field experiments and has proven to be a useful tool to investigate scavenging processes of ice crystals.
Keywords: Ice crystals; Cloud drops; Ice nuclei; Cloud condensation nuclei; Chemical composition;

Soils contain more than one activity consuming carbonyl sulfide by Ralf Conrad; Katja Meuser (3635-3639).
Exchange rates of carbonyl sulfide (OCS) were measured at 25°C in the laboratory in soil samples from a forest (PBE) and a former rape field (RA) over a range of OCS concentrations (250 – 120,000 pptv). The exchange of OCS changed from net production to net consumption above the OCS compensation concentration which was 785 pptv for PBE and 1470 pptv for RA. The OCS uptake rate constants were 0.17 and 0.14 l h−1  g−1 dry weight soil, respectively. However, at OCS concentrations higher than approximately 5000 pptv, uptake rate constants in RA became smaller suggesting saturation, and increased again at >50,000 pptv suggesting the existence of a second OCS consumption activity operating at elevated OCS concentrations. In PBE, the rate constant of OCS uptake increased at OCS concentrations higher than about 4000 pptv, also suggesting a second activity. Thus, our study indicated that OCS at close to ambient concentrations was consumed by a different activity than OCS at higher concentrations. Below the compensation point, the soil samples acted as a source rather than a sink for atmospheric OCS, stressing the necessity to measure OCS flux as function of OCS concentration to obtain reliable source or sink data for atmospheric budgets.
Keywords: Carbonyl sulfide production; Compensation concentration; Consumption kinetic; Microbial activity; Laboratory study;

The loss rates of PAN in several dilute aqueous salt solutions (NaBr, Na2SO3, KI, NaNO2, FeCl3, and FeSO4) and in sulphuric acid were measured at 279 K with a simple bubbler experiment. They are little different from that in pure water. For 5 M sulphuric acid hydrolysis and solubility were determined in the temperature range of 243–293 K. The hydrolysis rate k h=3.2×10−4  s−1 at 293 K is close to that in water. The observed temperature dependence of the Henry's Law constant H =10 −6.6±0.6 exp((4780±420)/T) M atm−1 leads to enthalpy and entropy of solvation ΔH solv=−39.7±3.5 kJ mol−1 and ΔS solv=−126±11 J mol−1  K−1, respectively.
Keywords: PAN; Liquid-phase reactions; Henry's Law constant; Hydrolysis;

Correspondence (3645-3646).

Correspondence (3647-3648).