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A new actinic flux 4p-spectroradiometer: instrument design and application to clear sky and broken cloud conditions

A new 4p-spectroradiometer was developed for measuring actinic flux especially under cloudy conditions based on a fixed grating imaging spectrograph and a CCD-detector leading to a simultaneous measurement of the spectrum. The new instrument incorporates a novel optical head with a 4p-field of view independent of angle of incidence. Comparisons with the actinic flux spectroradiometer of the Institute of Atmospheric Chemistry of Forschungszentrum Jülich showed a very good agreement within the limit of the uncertainties of the two instruments. Our spectroradiometer was applied to investigate the effects of broken clouds on the actinic flux and photolysis frequencies on the ground during the BERLIOZ campaign. Reductions as well as enhancements compared to the clear sky case were seen, both effects are larger in the UV-A than the UV-B spectral region. Furthermore the new instrument was used for simultaneous measurements in different altitudes on a tower to study the transmission and attenuation of actinic flux in low clouds. A correlation of attenuation with the simultaneously measured liquid water content of the cloud was found.
Impact of large solar zenith angles on lower stratospheric dynamical and chemical processes in a coupled chemistry-climate model

Actinic fluxes at large solar zenith angles (SZAs) are important for atmospheric chemistry, especially under twilight conditions in polar winter and spring. The results of a sensitivity experiment employing the fully coupled 3D chemistry-climate model ECHAM4.L39(DLR)/CHEM have been analysed to quantify the impact of SZAs larger than 87.5º on dynamical and chemical processes in the lower stratosphere, in particular their influence on the ozone layer. Although the actinic fluxes at SZAs larger than 87.5º are small, ozone concentrations are significantly affected because daytime photolytic ozone destruction is switched on earlier, especially at the end of polar night the conversion of Cl2 and Cl2O2 into ClO in the lower stratosphere. Comparing climatological mean ozone column values of a simulation considering SZAs up to 93º with those of the sensitivity run with SZAs confined to 87.5º total ozone is reduced by about 20% in the polar Southern Hemisphere, i.e., the ozone hole is "deeper' if twilight conditions are considered in the model because there is about 4 weeks more time for ozone destruction. This causes an additional cooling of the polar lower stratosphere (50 hPa) up to -4 K with obvious consequences for chemical processes. In the Northern Hemisphere the impact of large SZAs cannot be determined on the basis of climatological mean values due to the pronounced dynamic variability of the stratosphere in winter and spring. This study clearly shows the necessity of considering large SZAs for the calculation of photolysis rates in atmospheric models.
Measurements of photo-oxidation products from the reaction of a series of alkyl-benzenes with hydroxyl radicals during EXACT using comprehensive gas chromatography

Photo-oxidation products from the reaction of a series of alkyl-benzenes, (benzene, toluene, p-xylene and 1,3,5-trimethyl-benzene) with hydroxyl radicals in the presence of NOx have been investigated using comprehensive gas chromatography (GCxGC). A GCxGC system has been developed which utilises valve modulation and independent separations as a function of both volatility and polarity. A number of carbonyl-type compounds were identified during a series of reactions carried out at the European Photoreactor (EUPHORE), a large volume outdoor reaction chamber in Valencia, Spain. Experiments were carried as part of the EXACT project (Effects of the oXidation of Aromatic Compounds in the Troposphere). Two litre chamber air samples were cryo-focused, with a sampling frequency of 30 minutes, allowing the evolution of species to be followed over oxidation periods of 3-6 hours. To facilitate product identification, several carbonyl compounds, which were possible products of the photo-oxidation, were synthesised and used as reference standards. For toluene reactions, observed oxygenated intermediates found included the co-eluting pair a-angelicalactone/4-oxo-2-pentenal, maleic anhydride, citraconic anhydride, benzaldehyde and p-methyl benzoquinone. In the p-xylene experiment, the products identified were E/Z-hex-3-en-2,5-dione and citraconic anhydride. For 1,3,5-TMB reactions, the products identified were 3,5-dimethylbenzaldehyde, 3,5-dimethyl-3H-furan-2-one and 3-methyl-5-methylene-5H-furan-2-one. Preliminary quantification was carried out on identified compounds using liquid standards. Comparison of FTIR and GCxGC for the measurement of the parent aromatics generally showed good agreement. Comparison of the concentrations observed by GCxGC to concentration-time profiles simulated using the Master Chemical Mechanism, MCMv3, demonstrates that this mechanism significantly over-predicts the concentrations of many product compounds and highlights the uncertainties which exist in our understanding of the atmospheric oxidation of aromatics.
Stratospheric water vapour as tracer for Vortex filamentation in the Arctic winter 2002/2003

Balloon-borne frost point hygrometers measured three high-resolution profiles of stratospheric water vapour above Ny-Ålesund, Spitsbergen during winter 2002/2003. The profiles obtained on 12 December 2002 and on 17 January 2003 provide an insight into the vertical distribution of water vapour in the core of the polar vortex. The water vapour sounding on 11 February 2003 was obtained within the vortex edge region of the lower stratosphere. Here, a significant reduction of water vapour mixing ratio was observed between 16 and 19 km. The stratospheric temperatures indicate that this dehydration was not caused by the presence of polar stratospheric clouds or earlier PSC particle sedimentation. Ozone observations on this day indicate a large scale movement of the polar vortex and show laminae in the same altitude range as the water vapour profile. The link between the observed water vapour reduction and filaments in the vortex edge region is indicated in the results of the semi-lagrangian advection model MIMOSA, which show that adjacent filaments of polar and mid latitude air can be identified above the Spitsbergen region. A vertical cross-section produced by the MIMOSA model reveals that the water vapour sonde flew through polar air in the lowest part of the stratosphere below 425 K, then passed through filaments of mid latitude air with lower water vapour concentrations, before it finally entered the polar vortex above 450 K. These results indicate that on 11 February 2003 the frost point hygrometer measured different water vapour concentrations as the sonde detected air with different origins. Instead of being linked to dehydration due to PSC particle sedimentation, the local reduction in the stratospheric water vapour profile was in this case caused by dynamical processes in the polar stratosphere.
Problems regarding the tropospheric O3 residual method and its interpretation in Fishman et al. (2003)

In this article we will show that the Total Ozone Residuals (TOR) method as presented by Fishman et al. (2003) contains an erroneous assumption, due to which the TOR as presented in this article reflects mainly a tropospheric O3 climatology (Logan, 1999), scaled to the layer between the surface and the tropopause height, rather than a satellite measured TOR. We will show that it is possible to obtain a tropospheric O3  column that is very similar to what is being presented in Fishman et al. (2003), solely based on the Logan (1999) tropospheric O3  climatology and an estimate for the tropopause heights without using satellite data. Furthermore, we will show that one of the interpretations of observed O3  variability (high TOR values over Southeast Asia) is not the result of pollution, but primarily of tropopause height variations. We suggest adjusting the method in such a way that the final retrieved TOR product becomes independent of the actual values of the Logan (1999) O3  climatology. The proposed adjustments are also more in line with the original methodology suggested by Fishman and Balok (1999) in a previous paper. We furthermore show that the assumption that the stratospheric O3  column is nearly constant within a 5-day period is not always valid. This can introduce errors in the final TOR product, although we recognize that these errors may become small when averaging the TOR product over longer periods of time. However, we feel that a detailed study on this assumption is also needed.
Heavy hydrogen in the stratosphere

We report measurements of the deuterium content of molecular hydrogen (H2) obtained from a suite of air samples that were collected during a stratospheric balloon flight between 12 and 33 km at 40º N in October 2002. Strong deuterium enrichments of up to 400 permil versus Vienna Standard Mean Ocean Water (VSMOW) are observed, while the H2 mixing ratio remains virtually constant. Thus, as hydrogen is processed through the H2 reservoir in the stratosphere, deuterium is accumulated in H2 . Using box model calculations we investigated the effects of H2 sources and sinks on the stratospheric enrichments. Results show that considerable isotope enrichments in the production of H2  from CH4 must take place, i.e., deuterium is transferred preferentially to H2 during the CH4 oxidation sequence. This supports recent conclusions from tropospheric H2 isotope measurements which show that H2 produced photochemically from CH4 and non-methane hydrocarbons must be enriched in deuterium to balance the tropospheric hydrogen isotope budget. In the absence of further data on isotope fractionations in the individual reaction steps of the CH4 oxidation sequence, this effect cannot be investigated further at present. Our measurements imply that molecular hydrogen has to be taken into account when the hydrogen isotope budget in the stratosphere is investigated.
DRIFTS and Knudsen cell study of the heterogeneous reactivity of SO2 and NO2 on mineral dust

The heterogeneous oxidation of SO2 by NO2 on mineral dust was studied using Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) and a Knudsen cell. This made it possible to characterise, kinetically, both the formation of sulfate and nitrate as surface products and the gas phase loss of the reactive species. The gas phase loss rate was determined to be first order in both SO2 and NO2. From the DRIFTS experiment the uptake coefficient, g, for the formation of sulfate was determined to be of the order of 10-10 using the BET area as the reactive surface area. No significant formation of sulfate was seen in the absence of NO2. The Knudsen cell study gave uptake coefficients of the order of 10-6 and 10-7 for SO2 and NO2 respectively. There was no significant difference in uptake when SO2 or NO2 were introduced individually compared to experiments in which SO2 and NO2 were present at the same time.
On the influence of fuel sulfur induced stable negative ion formation on the total concentration of ions emitted by an aircraft gas turbine engine: comparison of model and experiment

A model which considers the formation and evolution of combustion ions in a combustor of an aircraft engine in dependence on the electron detachment efficiency from negative ions is presented. It is a further development of the model reported by (Sorokin et al., 2003). The model allows to consider the effect of the transformation of primary negative ions to more stable secondary negative ions with a much higher electron affinity and as a consequence a greater stability with respect to electron thermal detachment. The formed stable negative ions most probably are sulfur-bearing ions. This effect slows down the charged particle neutralization rate leading to an increase of the concentration of positive and negative ions at the combustor exit. The results of the simulation and their comparison with the ground-based experimental data obtained within the framework of the project PartEmis (Particle emission, measurements and predictions of emission of aerosols and gaseous precursors from gas turbine engines; coordinator: C. Wilson) at the QinetiQ test facility at Pyestock, UK (Wilson et al., 2003) support the above hypothesis, i.e. the increase of the fuel sulfur content leads to an increase of the ion concentration at the combustor exit.
A revised parameterization for gaseous dry deposition in air-quality models

A parameterization scheme for calculating gaseous dry deposition velocities in air-quality models is revised based on recent study results on non-stomatal uptake of O3 and SO2 over 5 different vegetation types. Non-stomatal resistance, which includes in-canopy aerodynamic, soil and cuticle resistances, for SO2 and O3 is parameterized as a function of friction velocity, relative humidity, leaf area index, and canopy wetness. Non-stomatal resistance for other chemical species is scaled to those of SO2 and O3 based on their chemical and physical characteristics. Stomatal resistance is calculated using a two-big-leaf stomatal resistance sub-model for all gaseous species of interest. The improvements in the present model compared to its earlier version include a newly developed non-stomatal resistance formulation, a realistic treatment of cuticle and ground resistance in winter, and the handling of seasonally-dependent input parameters. Model evaluation shows that the revised parameterization can provide more realistic deposition velocities for both O3 and SO2, especially for wet canopies. Example model output shows that the parameterization provides reasonable estimates of dry deposition velocities for different gaseous species, land types and diurnal and seasonal variations. Maximum deposition velocities from model output are close to reported measurement values for different land types. The current parameterization can be easily adopted into different air-quality models that require inclusion of dry deposition processes.
Investigation of airborne foot-and-mouth disease virus transmission during low-wind conditions in the early phase of the UK 2001 epidemic

Foot-and-mouth disease (FMD) is a highly contagious viral disease of cloven-hoofed domesticated and wild animals. The highly contagious nature of FMD is a reflection of the wide range of host species, the enormous quantities of virus liberated by infected animals, the range of excretions and secretions which can be infectious, the stability of the virus in the environment, the multiplicity of routes of infection and the very small doses of the virus that can initiate infection. One of the mechanisms of spread is the carriage of droplets and droplet nuclei exhaled in the breath of infected animals. Such spread can be rapid and extensive, and it is known in certain circumstances to have transmitted disease over a distance of several hundred kilometres. During the 2001 FMD epidemic in the United Kingdom (UK), atmospheric dispersion models were applied in real time in order to assess the potential for atmospheric dispersion of the disease. The operational value of such modelling is primarily to identify premises which may have been exposed so that the human resources for surveillance and disease control purposes are employed most effectively. The paper describes the combined modelling techniques and presents the results obtained of detailed analyses performed during the early stages of the UK 2001 epidemic. This paper investigates the potential for disease spread in relation to two outbreaks (Burnside Farm, Heddon-on-the-Wall and Prestwick Hall Farm, Ponteland, Northumberland). A separate paper (Gloster et al., 2002) provides a more detailed analysis of the airborne disease transmission in the vicinity of Burnside Farm. The combined results are consistent with airborne transmission of disease to livestock in the Heddon-on-the-Wall area. Local topography may have played a significant role in influencing the pattern of disease spread.